Rules of thumb for boat and yacht design – are they legitimate?

A creak of leather as he climbs on his hobby horse

I sortof have a mission against using rules of thumb.

The problem is that often they have been stripped of the original context. A particular type of boat or a particular situation.

I haven’t really seen any that hold up in all situations – or at least very few.

Example 1 – Sail Area to Keel area

One rule of thumb that crawls around the net is that the centreboard area should be a particular proportion of the sail area. It is set up to work for yachts, but by the time you get down to small boats they travel so much slower that you need a lot of extra area.

My take is that for dinghies and other light boats (take that as missing context in the original) the centreboard or leeboard should be getting up toward 3ft deep and 10 inches wide. If a boat carries a trapeze or leaning plank or is flared widely the area will need to be tweaked up more the closer you get to 16ft.

Preliminary sketch for Son of Goat designed by Michael Storer

Inevitably if I write this often enough it will become a rule of thumb itself – but will be presented as an absolute size as the contextualising information is gradually stripped away.

The original “rule of thumb” is even wrong for yachts as we know now that the depth of the centreboard, leeboard or keel is much more important than the area.

Example 2 – that you can stretch or shrink an existing boat by X%

One “rule of thumb” for boat design that continually comes up – and I got bitten by it once when a customer, after talking to a shipwright came away with the impression it was OK … which resulted in a boat that wouldn’t sail at all because it was too unstable and a customer unhappy with me.

A builder contacts me and says “something wrong with the boat” he built to my plan. Later I found that the 15.5ft Goat Island Skiff had been shrunk to 14ft.

That “rule of thumb” that is quoted is that you can generally shrink or expand a boat 10 or 15% with little problem – this one is so wrong that it is a real danger.

15% reduction in overall dims and stability is halved.

The source often quoted for this is Bolger – who once said that a cargo boat could be increased in length by adding a 10 or 15% section in the middle – with a possible improvement in fuel consumption. Completely different – not only is the body of a cargo boat mostly parallel sided in the middle, making the mod easier, but Bolger was tempering it with his vast experience.

He mentioned it only as a “notion” and certainly didn’t just add a bit in the middle – but drew it out carefully and properly as a new design.

Example 3 – the mast on the small boat should be three times the beam

Now for the Chappelle comment that mast length being 3 times the beam. Clearly he is talking about stability. So when he is talking about a specific model and size of boat he might be right – but what rig is he talking about – does he mean the peak/head of the sail?  A lug sail could almost have double the area of a triangular sail under this “rule of thumb”.

These three boats have about the same mast length.  The lug is 105 sq ft and the triangular sails are 82 sq ft.

Three masts about the same size.

But there is a more serious part to the argument.

In general if you double the size of the boat the stability goes up by a factor of 16 but the heeling forces go up by a factor of 8. This is why a maxi yacht can either have a lighter keel – or like the modern ones be somewhat narrower than a smaller boat of roughly the same proportions – note the really long keels on model boats.

This pic shows a keel depth about half the length of the hull – the full size boat would need a keel 35ft (10m) deep – not necessary though

A model boat needs a much longer keel than a full size boat - boat design

The best guide is looking at existing boats built to a similar purpose to see which ones are good sailors and keeping to the same rough proportions.

Beyond that you can make sensible conclusions – like a goose is more stable than a pointy nosed boat and put a bigger mast and sail in it. That is assuming it has a similar position of the centre of gravity.

Or that a new design will sail a little faster so maybe the centreboard can be made a little narrower.

Howard Chapelle knew exactly what he meant by the comment, but it has been pulled out of context by being turned into a “rule of thumb”.

Legitimate rules of thumb?

There are a couple of areas where I use rules of thumb.

Francis Herreshoff once said that the rake (angle back) of each mast on a boat needs to be 2 degrees more than the mast further forward.  This is to simply prevent the masts look like their tips are converging.  It is simple, it works and is not very open to interpretation.  It also doesn’t have any effects beyond its brief.

Another place I use rules of thumb is in construction.  For plywoods under 9mm an epoxy fillet needs to have a radius three times the plywood thickness.  Same if you use a piece of timber in the join – the glue cleat needs to be have three times the gluing area to the ply of three times the ply thickness.

This is another rule that falls apart if taken too far.  Testing shows that the fillet radius actually needs to be about 2.2 times the ply thickness, so there is a safety factor already applied (someone is always ready to “add a bit” for safety.  But as the ply thickness gets bigger the rule falls apart as you need to add glass cloth to get enough strength – and by that stage if you rely on fillets only then the fillet has so much volume that it has become crazy expensive.

So, I would suggest that rules of thumb are very useful for professional designers because they know the ramifications – it is easy for Boger or Chapelle to know what they mean and know when the “rule” breaks down.  But a better guide for amateur designers is to look closely at similar sized boats to see which ones perform well in terms of meeting the required use patterns and which ones do not.

Cautions on copying current practice – beware of creep

A problem with looking at current practice as a standard is that it can go badly wrong if you just look in one place for information.

One example is plywood canoes and kayaks.

When I got into wooden boat building in the late 1980s it was common for a stitch and glue kayak to be built of 4mm (3/16″) ply and glass taped with 50mm (2″) tape inside and outside the seams.  This was also common for plywood racing dinghies in Australia and New Zealand – which are subject to very much greater loads.

By 2000 the standard way was to use glass tape on the inside of the boat but do the whole outside of the boat in 4 or 6 oz (125 or 200gsm) glass cloth.

By 2010 several major manufacturers of kits are saying you have to build the boat with the boat completely sheathed with glass inside and out.  As any builder knows – glassing the inside of a built hull is a cow of a job.

But it adds significantly to the expense, the cost and weight of the structure.  Some say that glass has negligible weight – they obviously haven’t had to lug a boat very far!

Others say it is to get an “I-beam” or “composite sandwich” effect.  This is misunderstanding how materials work.  ALL materials loaded in bending, whether by having weight in the middle, or by water pressure work by the same method.  It just has to be strong enough for purpose.

If you are expeditioning and doing rock shore landings it might be worth going to glass inside and outside – but this is not how 95% of the market is going to use their boats.

One last parting shot … this is a Jarcat – a very clever range of trailer catamarans designed by Ross Turner in OZ.  There are hundreds launched.

Jarcat proves most plywood kayaks are overbuilt. Classic Ross Turner design.


The Jarcats in the 16ft range is built of the same 4mm ply – with 20z glass on the outside only.  The cabin and other structure still needs to go on this one, but you can see how much bigger than a kayak it is but built out of lighter materials than a standard kayak is.It is a case of “creep” a small step by step change which is not perceptible but ends up a long way from the original point with little real justification.

Either the kayak designers or the Jarcat is right – the other wrong.  With 500 or more Jarcats on the water …

I only found this out in the last couple of years – despite the Jarcat being around for a couple of dozen – my bad.  It has made me think that for most boats in dinghy size that are going to be glassed that some glass is more important than the actual weight per metre of the glass – and also to think WHERE the glass is really needed.  Alternatively the earlier practice from the 60s and 70s might indicate that glass taping is all that is really needed for most boats and most normal uses.

So it is sensible for amateur – and all designers – to look broadly and find out what is the actual best practice across a wide range of boats rather than accept things that are touted on the internet.  Do your own research and share your conclusions.

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29 thoughts on “Rules of thumb for boat and yacht design – are they legitimate?

  1. I think the biggest problem to using these kinds of rules of thumb has been over specialization. You have three competing goals that have to balance out: the Designer, the Engineer, and the Builder. Each of them off isolation make bad decisions for the others. In a modern context, they are all different people and may not even normally work together. The Herreshoffs are a perfect example of when it worked well. They were a one stop shop. You got the designers, the engineers, and the builders in one package that all worked together and understood their roles and were able to accommodate each other.

    The Engineer wants to build in more and more reliability and the overall design and build considerations are secondary. They want to calculate everything, whether it can even be calculated or not, and are resistant to making extraordinary leaps by deviating from standard practice.

    The Designer wants to make their ideas real. Let the other people do all the real calculations and figure out how to make it. There is a lot of “eventually the technology will be available.”

    The Builder is looking at cost and how to actually put things together. They know what the materials really cost and understand the reality of what the materials can do in a practical sense rather than a theoretically.

    In the limited context of boat building, the Engineer is in trouble. The environment is dynamic and chaotic. Only recently with high performance computing has it even been thinkable to try to accurately model what forces are involved even in one static scenario let alone in all conditions. Sometimes all they can do is make a guess and see what has worked before and maybe try some tiny deviation.

    The Designer always has the handicap that they are trying to divine what the customer wants and it can end up being akin to working with an Ouija board. They tend to follow what everyone else is doing. Kinda like the car designers putting fins on cars.

    The Builder is constantly confronted with new materials that they do not yet understand and the rest of the materials are full of flaws and variations. Many of the newest materials are so unknown that about all you have to go on is the advertising copy from the manufacturer and they are afraid to put down hard information because they think everyone will sue them. They also get the blame with things go wrong. The Designers and Engineers just fold their arms over their chest and say they were right and the Builder just didn’t follow their specifications.

    About the only time these three competing egos can agree is when they are the same person. Even better if they are also the customer and will take responsibility when they get it wrong. The real magic happens when these conflicting entities balance out and create a really special outcome.

    • Nice essay in reply David,

      I think the problem is more with the idea that there are simple solutions that anybody can use. There are some, like the gluing area example I gave that are highly useful to anyone building a boat. But then most go too far.

      For example I just found out that the figure that the centreboard has to be 4% of the sail area is from Marchaj – a very respectable source – but experimentally based – but I know from experience it is wrong as a general rule. You just have to look at model yachts compared to maxi yachts to see that it is not a practical solution.

      I will have to have a closer look at Marchaj and see the exact wording and methodology that he used before commenting further.

      Part of the reason the model yachts have a deep keel is that they have less stability and the weight needs to be placed lower down – but it doesn’t explain how wide the keels are in comparison with real yachts – or how a very good bunch of sailors can make a boat with a tall skinny keel fly really well, but a bunch of amateurs make it sail like an old sock! I know why – it is idea of keeping up speed so the narrow keel can “fly” all the time – never try to sail high before hitting critical speeds for the windspeed and sea conditions.

      The practical differences can also be practically observed in the difference of behaviour between an 8ft boat and a 16ft boat. The 8 footer needs much more area in the centreboard/leeboard in comparison and the sail area is only a secondary effect.

      Anyway – going to your points – it is all about knowledge. The “Engineer” and the “Designer” will agree with the above and tell the “Builder” what to do.

      Like you say the problem comes for the “Builder” when they have no access to the thinking of the “Engineer” or “Designer” – and that of course is where a good plan comes in very useful – something I try to use as a tool to increase the knowledge of the builders so they know WHY the “Designer” and “Engineer” tend to be relatively inflexible about the big things. This might seem like a restriction, but if the builder accept the knowledge presented then they can make educated steps of their own in their own future projects. That is something I work rather hard on in my forum, emails and general internet activity. If people UNDERSTAND they will do brilliant things on their own.

      I would be the last to suggest that builders should not experiment, or that inexperienced people should not design their own boats – but they do need to be very objective if they want to learn.

      The best thing for them is the objectivity of entering some races.

      There are many websites now that say how easy it is to build a carbon mast, or build a solid wingmast for a little boat or using carbon and plywood to make a stiffer hull “sandwich”

      They all say how good their boats are. However with examination we find out their carbon mast is heavier than a wooden one because they started with too small a diameter, or the wingmast boat can barely sail out of its own way because the hull design is so bad and the wingmast so heavy or that the carbon/plywood sandwich is less stiff and heavier and WAY more expensive than using the next plywood thickness up.

      Great things to have on the net – but not an objective point of view – and they become some sort of standard because they are widely read and discussed.

      For example I would trust information from the Moth or UK Cherub websites about mast making – because of the sharing of knowledge and continuous improvement in racing classes, but very much more doubtful about an amateur working at home in isolation and appearing to have done little research about how much a timber mast or a carbon mast SHOULD weigh.

      That’s where my comments are directed – a certain reality comes by observing the scene in a broad way – to try to make generalisations about the “best practice” and getting your boat out into company to see how it goes.

      I will leave with a pic of a rig developed my friend in Texas, John Wright. He is an experienced sailor and amateur builder – and a great seat of the pants designer. He developed this rig from a standard telescoping pole and all bits for the sail and rig fit in a 4ft long bag. The boat itself is a ply/foam sandwich – a brilliant bit of industrial design – that weight in around 35lbs (16kg).


      He developed this rig – which most won’t realise, but it is actually closer to the classical “Junk Rig” than anything else. But he sailed it with the other Puddle Ducks at the Georgia world a couple of years ago. Here he is racing it around the course with the other puddle ducks – a great test! He ended up finishing mid fleet after three races and was improving with more time with the rig. It’s never going to be a world beater, but nobody else could put their entire rig in a bag without use of an axe!


      This kind of stuff is brilliant – and it makes sense to choose a simple platform of similar boats like the ducks to experiment with.

      Best wishes

      • I think that another problem that you touch on is the infatuation with seeing some materials as more glamorous. Similar to the carbon fiber mast versus a wooden mast example, one really eye opening experience was watching a video by Matthias Wandel demonstrating his home built bandsaw design ( ). He hooked up a dial indicator and hung his weight on the arm of the wooden home built saw and did the same with his cast iron saw. The wooden bandsaw had about half the deflection as the cast iron version.

        Which do you think is a bigger problem, the people that jump into new materials just because they are new and sexy or the people that hang onto more traditional materials just from nostalgia?

        A good example would be the short reign (in a historical sense) of metallic rigging on boats. For most of history, it was fiber lines, then everything converted to steel, and now we are back to fiber lines with dead-eyes being the avant garde.

        I know the history of why the plywood tradition was developed so strongly in your part of the world. For the rest of the world, do you think plywood will overcome it’s stigma? Do you think that fiberglass hulls will follow steel rigging into history or will the ability to build “good enough” boats with relatively unskilled labor blowing it into molds will keep it going?

      • Howdy,

        I think the answer to excessive nostalgia or progressiveness is to keep up to date about the past, present and future and be able to see what each can do.

        For example the state of the art masts for the Moth Class boats are made of carbon and have a diameter of slightly less than 40mm. In the days of aluminium masts the diameter would have been about 50 mm.


        Timber spar diameters are generally about the same as alumimium but with a much greater wall diameter. I only know this because of experience of all three including design and building. The moth experience is very recent – just a couple of weeks ago. I had an invitation from a Goat Island Skiff builder to go and view the Moth Worlds at Lake Macquarie. This sort of experience of seeing state of the art information applied in a competitive environment is hugely useful.

        The moth pics I took are quite detailed about some of the bits – I was quite surprised at the mast sizes though. This is my car key beside the mast.


        The 2011 Moth World photos are here.
        Another great resource is the UK Cherub website.

        I know from our database of spars for Goat Island Skiffs that a properly designed wood spar is about 16lbs but a carbon spar is about 6lbs. This would be a guideline for comparison between any other freestanding mast – that the carbon should be about 1/3 the weight of a timber mast.

        The general idea is to observe and be able to do a bit of math.

        The plywood tradition here is actually fairly dead now except for those of us who were involved. When the racing classes finally allowed foam (and other) sandwich constructions then ply was outclassed – in some classes very eventually – but it happened eventually.

        The low hull weights we were getting with ply (8 to 10lbs per hull foot – or Moths in the wood era an amazing 3lbs/foot) are still maintained but the sandwich allows greater panel stiffnesses – but has lost the simplicity of single skin glass construction – which was the big advantage of the technology.

        I do think there are places for boats of all constructions. People who are really interested in materials will attempt to be quite objective and take the maximum from every opportunity they get to learn more.

        One recent one for me was the Goat I watched the Moth Worlds in had a hull weight of 105lbs (6.8lbs/foot) because the builder had used Paulownia timber for a lot of the framing. It is between cedar and balsa in weight and there was a little concern about whether using it to hold the ply would subject it to too high splitting loads, but Bruce has sailed his GIS is all sorts of nasties in the last 6 months and the boat is absolutely fine.

        It stood up to a full planing crash onto a sandbank with two aboard which threw both of them over the front of the boat with only tiny damage to the centrecase – a great indicator of just how strong the daggerboard setup is.

        So I have learned even more!

  2. I am curious though. I wonder how the total weight of the mast and standing rigging is compares to a free standing rig. I realize that these Moths are under a different set of constraints (and not designed for all around sailing.) They are cool in their own technologically interesting way but pretty much just an obscure offshoot much like hovercraft.

    You can get one person moving faster across the water with a wind surfer or a sail board and not spend 18K. I think that what you make with your high performance boats such as GIS (although it is cunningly disguised as a “Traditional Boat”) makes a lot more sense. Lower cost barrier for entry. Sails in a wider range of conditions. Doubles as a party boat when you put more people aboard.

    I think much more interesting than the Moths are the little PaperJet 14 from Dudly Dix ( The three rig options make it something you can grow with or play with different groups. I know there are others in that genre but these are cheap and readily available. Under 1K in kit form is much more reasonable than the 18K for the Moths. Could possibly have it on the water under 3K.

    I still think the GIS is a nicer option since you can also carry people and not just a 1-person toy. Not to mention simpler rigging.

    • Howdy David,

      The Paperjet is a fine little boat, but it doesn’t do anything particularly new. It represents something similar to the plywood boats in OZ from about the 1980 era – though the Paperjet construction method is much, much better sorted out. I would still recommend it as a boat to consider for many sailors.

      Since that time there have been very big discoveries about what makes small boats fast but easy to handle. Starting from the mid 1980s in the Moth class where they found the scow type would eliminate nosediving if they made drastic changes to the shape of the stern – before that they thought they had to do something at the BOW to stop it from digging in. This happens to be true for multihulls as well – adjust the back half of the boat and you can prevent much nosediving.

      It is the same reason canoe sterned boats have a better reputation for seaworthiness – about distribution of hull volume, rocker (and in a yacht the aft buttock lines.

      From the scow moths it went to the skiffs and then restricted classes like the NS14 and Cherub. The Goat incorporates that thinking and it can heel to very high angles and still steer reliably – something the pre 1980 hullforms won’t do.

      A second area is planing behaviour – the experience of these new hullshapes can be a surprised to very experienced sailors (unless they are sailing an Oz or NZ derived design like the Olympic 49er. This is from Michael McNamara who makes sails for some of the most competitive classes in the UK about sailing the Goat. This is what he wrote to Ralph Gersson in the Netherlands.

      I had a nice long sail in Richard`s Goat Island Skiff today and had a lovely sail.

      He has made a great job of the boat and it went like a dream.

      I was very impressed with how simple the controls were and how quickly the boat went. It was also beautifully balanced.

      As you could expect the boat felt better when it was slightly heeled when beating. Off wind it felt fast but didn’t create a planing sort of wash…it just went faster and faster.

      When tacking I found that I had to be quite forceful in pushing it into the wind and it was definitely better to “roll” it coming out of the tack.

      The highlighted words are exactly what has been worked on in OZ – playing with rocker, volume distribution and effective hull bottom width.. Looking at the way the National 12s are sailing in the UK – they are using the same methods and it is spreading to other classes where there is freedom to change the hull shape. Interestingly the big high speed yachts like the supermaxis and Open60s are still oblivious to how much better their boats could be. You can keep up higher speeds in marginal planing conditions because the boat transitions through to planing much more easily and you can push closer to the edge and know you can keep control with much less rudder effort.

      OK – Moths.

      The thing about the moths is their EFFICIENCY. Sailboards are not even close.

      Years ago on Sydney Harbour they had a race between C-class Catamarans (the full wing Miss Nylex), Tornados and 18ft skiffs. The C-class cat came first, the 18ft skiffs came second and the Tornados were shocked to find that they came third.

      A sailboard would not be in the hunt because their upwind performance is so poor and their dead downwind performance requires much longer distances than the others. The others can go deep. On a reach a sailboard comes into its own, but upwind and down or in rough water they are nothing special. Actually, I exaggerate – they ARE special, but are let down by performance in particular areas.

      I agree partially with your argument about cost – but by the time you put a sailboard together with the different rigs and accessories it is going to be a good proportion of the $18K for a Moth. And its never certain whether you need the longboard or a high volume shortboard for light winds or if you need the low volume shortboard for over 10 knots. Might end up being much more expensive than the Moth if a sailboard sailor is looking for best performance in all winds.

      I will back this up with data.

      So lets look at performance of a few boats around a regular racecourse. The Portsmouth Yardstick is based on actual performances averaged around real racecourses using real boats.
      Check the current Portsmouth yardsticks (PDF) (note the Moth has two figures – foiling and non foiling versions)

      Tornado – two person catamaran – 20ft – 643
      A-class cat – 1 person singlehanded – 18ft – 690 (very highly developed because of a lack of restrictions in design)
      Hobie 16 – two person beach cat – 802
      505 – International trapeze racing dinghy – 16ft – 902

      There is no official number for the sailboards I could find, but from pursuit races in the UK the sailboard people themselves were giving numbers close to the 505 – which is pretty good – perhaps a little optimistic, but let’s accept their figure…
      Raceboard class – 902 approx

      Moth – 690
      This is a serious breakthrough – around a course 30% faster than the best normal dinghies and sailboards and equal with the best singlehanded cats.

      A sailboard can go faster in a 50ft wide trench down in the tropics with wind from the perfect direction – but put the same board on an upwind downwind course in a normal chop and it won’t be able to get to the top mark – too specialised with a narrow band of efficiency. Moths have full sailing performance with efficiency on all points of sail. Even non foiling they have close to the upwind speed of the Goat Island Skiff, sailed hard and 4.5ft longer.

      I agree the particular solution is not for everyone!

      In some ways it is still early days for Moths – but the intensity of the racing (over 100 boats at the worlds) will probably mean more big jumps – extending the takeoff speed ever downwards and reducing the “crash and burn” in really rough water. The price at about half of a high performance two person dinghy has meant a serious growth in the fleet and the countries sailing. Ten years ago there would have never have been participants in the Moth from the USA, China, Sweden.

      It really is a big jump in many ways. But as you say – not for everyone!

      Best wishes

  3. Well, before this thread gets too long…

    In general, I get the feeling that you enjoy the open class developments. The experimental stuff that may be a huge step forward or a non-starter but still a valiant effort.

    I’m quite sure that when the first Moths started flying, they were just a strange oddity until they got everything sorted out. But one huge difference is that they continue while the other foilers such Hobie Tri-Foiler and the Windrider Rave just make a curious historical footnote. Do you think that the closed nature of just being a pre-packaged solution and not open to tinkering killed them off or just the fact that they were too expensive to get adopted on a larger scale?

    Is it that someone gets a good start but without the gestalt of other people picking and improving, it stagnates?

    Why do you think that some designs fall flat? A perfect example in my opinion is the Adventure Trimaran 24. I look at this video

    and see it as just stunning as it ghosts along. But in the other videos out in real wind in pretty modest sailing conditions, it does fine but nothing to particularly commend it over any of the other multi-hulls.

    • Very well observed David – and I have no fear of this discussion getting too long – it is great.

      The Moth has moved a bit away from its roots. It was a constructor’s class until recently with teams of expert and less expert builders making boats in the off season out of sticks and glue.

      It has few restriction.
      Hull Length 11ft
      Sail area 85sq ft (sorry – I am not metric with sail areas)
      Maximum height of sail above deck – 18ft
      No hollows in the hull shape (to prevent catamarans)
      No minimum weight, minimum beam

      During those days the moths were very potent for their cost and size.

      The scow moths of Oz – this one was the only one in the 2011 worlds in Oz – and is just stooging around before the start – it is a late ’70s design, before the transoms went narrower for better downwind control (mentioned in the comments above).

      And the skiff moths of the UK (nonfoiling at that stage) – the pici is of John Claridge – one of the guys who made the skiff moths very competitive with the Australian Scows (finally).

      … fought it out for dominance for decades – and resulted in the most sophisticated lighweight construction – 0.7mm (1/64″) hull skins and bulkheads and the cockpit out of 1.2mm with very light glass under. This was aircraft type construction – true monocoque skin supported by light western red cedar stringers.

      Not for everyone either – but it showed what is possible. The scow hulls were about 34lbs without fittings. They ended up with a performance yardstick similar to the International Contender – 50% longer and 50% more sail.

      This is the Contender – a puny 11ft boat can sail equal with that – it is pretty similar to the Paper Jet we discussed earlier in term of hullshape technology. Don’t get me wrong – the Contender is way fun to sail too.

      A triumph of efficiency over sail area and length.

      In the end the skiffs went really narrow – and that’s when they started to become superior to the scows. They tended to nosedive though – so inverted T foils were put on the rudder – so when the boat goes nose down the forces developed by the rudder T foil pull the stern down so the boat stays in trim automatically.

      The next step was the trifoiler – think Hobie Trifoiler but with the weight and complication and expense wrung out. The minimal approach really defines the Moth.

      Hobie trifoiler followed by a trifoil moth pic

      That was the mistake of the Hobie trifoiler – it was big, complex and expensive. The little trifoiler moths were quite easy to sail (apart from the occasional crash off the foils) and represented a rooftop package weighing little more than 90lbs all up. However they were considered too easy to sail. Suddenly a class that was considered to put a premium on handling and skill could be sailed by geriatrics. They were in a quandary.

      One of the main drivers of the DIY wooden moth development was a Sydneysider – Ian Ward. He demonstrated a bifoiler (two foils only) after being told it was “impossible to balance” – two foils only concept – was possible to sail by parading up and down in front of the sailors at the Seaforth Moth Club (I think that is the story). The foilers are easier to sail than the non foiled narrow moths, but very difficult to sail well.


      Originally the hydrofoil moths would race amongst the conventional boats at the worlds – would win one or two races by miles but the rest of the time not look very special. This is exactly had happened when the very narrow skiff moths were developed too (in the UK). The first generation bicycle configuration Moths would only fly reliably in 10 knots of breeze.

      I saw some racing from a distance at the beginning and you would see one of the few foilers jump out of the water and disappear into the distance, then drop back into the water and the other boats would catch up.

      But week in, week out racing has changed all that. They now fly in quite light breezes – about 5 knots of breeze. This is not HALF of 10 knots – the power available in a 5 knot breeze is only 1/4 of that in 10 – showing how far the boats have come. All sailors gybe without falling off the foils and some of the better sailors can tack without falling off too.

      So to the trifoiler (and windrider) – it was too big, too heavy, too complicated to rig and unrig and very expensive. The complexity led to unreliability of the structure. How could it succeed? However it pointed the way. I was gobsmacked to see one on TV just turning circle after circle. Prior to this hydrofoils (apart from the little speed week cat “Mayfly” – another important step) were touchy and difficult – But Russell Long brought something new into creation – you could sail in circles all day and anyone could sail it. But every step of production added more and more complexity.

      You need some pressures applied externally to be minimal – the moths are restricted by maximum sail area (it is not big) and no minimum weight – so huge pressures to simplify.

      The pressure on the Moths from the start has been to do things in the simplest, most minimal way – you just need to look at the fittings on the boats to see that. Bits are “lashed on” with carbon roving soaked in epoxy. They have lost the low cost of the wood boats – but some of that thinking still holds on – but I think they are still justified because of the almost harrowing speed of development.

      Here is a brochure for a 420 dinghy – a standard fibreglass racing dinghy used for international competition. It is great for regattas, but its performance is a bit “old hat”. The full price to get one on the water is similar to the cost of the Moth – $16K compared to $18K. I know which one I would have been sailing at that stage of my career.

      They start looking a bit sick when you realise that wooden moths could be built for about $2000 now – still very fast boats for their size.

      I do regret the “build a boat during winter” approach has been lost – the loss of that might mean that Moths head for more and more expense, rather than just costing a plywood, epoxy and timber budget – and 35lbs of that doesn’t cost much. So far they are reasonably priced as a full carbon production thing compared to boats of similar performance and many of much less performance. – but that will tend to stagnate development – instead of 20 home builders creating slightly different boats, we have to wait for two or three manufacturers to do some development.

      Moths would have been impossible without “Mayfly” or the “Trifoiler” so give credit where credit is due. But the open-ness of the class and the rule pressures to make things minimal have been a potent cocktail to make Moths the amazing boats they are.

      photo sets on Flickr

      And here is the Moth’s own viewpoint of their development

      • Michael,
        Could you provide or tell me where to find some more information on the trifoiler Moth? I am very interested in constructing one as it seems much simpler to construct and sail than a “normal” foiling Moth.

        Thank You.

      • Hi Dewey,

        I would make enquiries of the Moth Association in Australia to see if they can direct you to someone who can give you some information.

        They also have a good website and some of the state websites and their old website has considerable information … maybe you can join their chat forum (ask them) and see if anyone has info they can share.

        I am pretty sure most of the trifoiler development was manufactured stuff. I don’t know if there might even be some parts for sale.

        Main thing is that whatever it is attached to has to be pretty light – I think Moths were around 80lbs all up in that era, and also a fairly fast intrinsic hullshape so that when you crash off the foils the hull can kinda keep on sailing.

        HOpe this helps

  4. I think you could go broke trying to sell reef points in OZ.

    Maybe some day they will make something with lifting foils for us old folks just to make for a nice smooth ride instead of banging into chop.

    • Haha – we are an embarrassing lot really!

      That’s the reason we put so much sail on the OzRacer/OzPDRacer – as a sort of antipodean joke.

      And were we surprised when we found the boats had enough stability to carry it easily!

      I think there is space in the market for something simple to sail but foiling. The company that makes the ’49er class dinghy, Starboard Products, has produced some very fast experimental boats, but they have been worried about putting them on the marketplace because they are not sure that having inexperienced sailors zooting around the harbour at 25 knots is such a great idea!

      F=mv^2 !!!

      Your point about bangin’ into chop is a great one. Have a look on youtube at some of the big multihulls going fast in a seaway and then compare to Hyroptere – the similarly sized foil tri – it looks as smooth as silk in comparison – though every one is on edge.


  5. Well, as far as them going around the harbor at 25 knots, it couldn’t be worse than the jetskis at 50 knots 😉

    Actually I have thought at times about taking one of the square boats like the Oz Racer or the Goose and adding foils. Those nice square corners just cry out as attachment points for foils…..

    Of course to complicate matters, I would want to make foils that are tubular much like ducted fans but no mechanicals. Either that or the ladder foils that loose lift as they emerge to focus on stability rather than speed.


    • Haha – any mindless fool can BUZZ BUZZ around the harbour on a jetski – but when it comes to it – they drive similarly enough to a car with some sideslip. But a sailing boat – I can imagine someone thinking they can blast across in front of a Manly Ferry, not realising that they are gradually pinching the boat and find they are dead stopped in front of the Ferry. Or cannoning into another boat when a gust hits.


      The thing about foiling is that every contrivance will crash to earth occasionall. With the big flat area of the PDR I would recommend getting your foiling photos early in the piece incase of a fatal crash later. Fatal to the big flat sections of the PDR!

      The ladder foils would be more tolerant as there is more that has to ventilate for you to lose lift completely – whereas when the T foils suck air down from the surface they just lose the lot.

      Doesn’t seem to worry the moths too much – before the foils they would sail happily at 15 knots or so – which is very fast for a monohull. A PDR hull might be a bit at sixes and sevens hitting the water at that type of speed!


  6. As always, Mike, you write with a great deal of knowledge about small boat design. However, I would like to provide some input about windsurfer efficiency.

    Seen in the same terms as a non-foiling dinghy – i.e. their all-round speed around a conventional course in a range of winds – the boards are definitely efficient for their LOA and SA. There are actually yardsticks that are pretty reliable. They rate the Raceboards (3.9m, around 9.5m of sail) at a similar pace to the International Canoe (5.3m-ish, 10m of sail) and from about 8 knots that’s under-rating the pace of the board. In 10-12 knots or so the board is toasting the IC upwind and downwind.

    Similarly, the Division 2 boards (3.9m, 7.4m sails) out-paced the pre-foil International Moths which had fewer rule restrictions. I don’t believe the extra length of the round-bottomed D2 boards was a significant factor as D2 design had moved away from maximising LWL. From about 8 knots the board was considerably faster,from my racing against the world’s best 2 Moths, and again the board is quicker than the Canoe from around 8-12 knots.

    While these speed are definitely not in the realm of the foilers, they are notably quicker than the best of the conventional boats of similar dimensions. Just a minor point but you are so accurate on this stuff I thought I’d mention it.

    • Hi Chris,

      Grrrr, I hate to be wrong, but I’m glad for the correction.

      In a way, my numbers, taken from the yardsticks below agree completely with what you say – so don’t think I was too far out – but certainly my EMPHASIS was out!

      My big sin was, I think this

      The thing about the moths is their EFFICIENCY. Sailboards are not even close.

      If the sailboards are being compared with the pre-spinnaker international canoes – then I think there is some validity in what I say. The canoes are brilliant boats upwind and reaching, but on turning onto the running leg in light or medium weather also became nothing special – just another dinghy. So to compare the sailboard probably admits there are some holes in its performance too. Whatever the canoe or the sailboard might do in the right conditions to a more complex but fast boat like a 505.

      Are the yardsticks reflecting this available, I looked everywhere!

      I did a check on your email address and found activity on the Taser IC and sailboard furums. Useful thing Google!

      Of the sailboard class with the best performance, what is the basic configuration these days? Volume, width, board weight

      Are centreboards still used on raceboards?

      Cheers MIK

      Also where are the competitive Raceboards fleets around Sydney these days?

      Follow up – I had a good look around to see what has happened with sailboard design. It seems that it has largely stagnated with boards like the Lechner and the D2s being quoted as being reasonable examples. The big developments that have been made have been with sail design – with boards being able to carry 9.5m sails through to surprisingly high windstrengths, when I remember a decade or more ago that they were struggling to carry 7.5 square metre sails.

      It might be fair to say that my assumptions, partially drawn from Bethwaite is that the sailboards have been too limited by a lack of righting moment (no trapeze, leaning planks or stilts!) have been overcome to some extent as evidenced by the bigger sails. Though I haven’t read the revised version of his book “High Speed Sailing” since the first editions.


  7. Thanks for replying, Mike.

    I suppose part of the issue is defining “efficiency”. Yes, we both agreed that boards are not as efficient (in some respects) as a foiler Moth. However, as compared to a non-foiling dinghy and when compared against dinghies of similar LOA or sail area, the boards are certainly efficient, as demonstrated by their superior all-round speed over the pre-foiling Moths which were arguably the most efficient dinghy of their day.

    Board development has been stifled by rules, IMHO. Not that I’m against rules, but there’s no really open class left in boards. All the International classes are restricted to production boards and a certain minimum run is required. So backyarders of the type that produced foiler Moths and the original windsurfer are ruled out of the sport, which is bizarre IMHO.

    The Formula Windsurfing boards are producing excellent performance, comparable to a foiler Moth when both are planing or foiling, but FW boards are very short and fat and basically cannot finish a race under about 8 knots of wind. I’d rate them therefore less efficient and slower all-round than a foiler Moth. Dimensionally, the FW board has 50% more sail but about 60% of the LOA and cost.

    The Division II boards are basically dead (the class has gone and they are ruled out of other classes) but remain the fastest under about 8 knots and, according to some experts a few years ago, the fastest around a course from 0 to 30 knots because of their light-wind prowess (going as fast as a Canoe or pre-foil Moth in light winds with 7.4m of sail on a restricted platform is pretty damn outstanding, especially when you move away from the latter two in more breeze).

    The advent of larger sails has improved the performance of Raceboards although the new boards (there are now three in production, a big improvement over earlier years) are basically no faster than Mistral Equipe IIs and other 1990s boards.

    The larger sails may be easier to handle because of superior materials and design that has sacrificed lift per metre for superior handling. Of course, while that allows you to carry a bigger rig, it also means you have to lug that bigger rig through transitions and pay for the bigger sail and spars…. sometimes I think we over-rate the superior performance of modern flatter sails. Interestingly while windsurfer talk about light modern rigs, most sails are at least twice as heavy as the Windsurfer One Design mylar sail and the old-fashioned One Design sail appears to generate much more lift per metre.

    The righting moment issue has been attacked in Formula and shortboards by making them wider, with Formula boards hitting the class beam limit of 1m. That allows them to carry huge rigs and fins. However, in sub-planing conditions the very high drag makes these boards extremely slow.

    A skinny longboard with hiking racks could be a real mover, but since the longboards can already beat any comparable single-sail single-handed non-foiling dinghy you could say that speed is not the issue. Merely adding more RM would not get them to the pace of a foiler Moth around a WW/LW course,as far as I can tell. A foiling longboard would be an interesting beast!

    The yardsticks are at the bottom of the on-line VYC listing but they have the Div 2 boards listed incorrectly.

    There’s only one Raceboard fleet in Sydney (Dobroyd on Iron Cove) and one in Wollongong, with a small scattering in other states.

    BTW on general dinghy design (as discussed above) I’m not sure that ANZAC design has lead the way as much as is often assumed. For example, the National 12 was miles ahead of Australian designs in adopting the U sections seen in most high-performance modern craft. We haven’t had a world’s winning design in 14s, for example, since the Northern Hemisphere and Aussie classes were unified. The mylar-sailed Tasar now seems to be pretty much back on a par with the NS14 in terms of pace (judging from the last two regattas where significant fleets raced) yet the National 12 apparently regularly beats Tasars in UK conditions despite being almost 3′ shorter.

  8. What I should have added, to bring the post back to topic, is that all the stuff I can find out about small-craft design (and I’ve been lucky enough to talk to most of the top names) could be encapsulated in one rule of thumb that there is no one type (and “type” includes nationality) of craft that is much more efficient than the others – if you define “efficiency” as including aspects such as cost, ease of use, versatility, etc.

    • Howdy Chris,

      I guess what I was getting at was that there are boats that can sail very well through all conditions – up and beyond the speeds of comparable boats.

      When the 505 came out, it was one kind of revolution, the Tornado another. 50 years after their design these are still very close to the best standard in terms of all round performance – they have been overtaken, but not by that much!

      Just as a comparison, the International Canoe, pre spinnaker was screamingly fast in some conditions, well able to burn off a 505 – but in other conditions it sails just like a standard sailing dinghy with 10sq metres of sail.

      I guess that this also disqualifies my thesis about the moths! And sailboards – at least if you take one type of sailboard at a time.

      But the wind speed for Moth effectiveness moves ever downwards. It must have a limit of course – there will always be a windspeed where a foiler moth becomes about as much fun as sailing a wet sock.

      Thanks very much for the roundup of sailboard performance – especially the comments of bigger sails set up for better handling. When the first 7.5 sails started appearing with this thinking, I thought it might be the start of a single rig that would do for all conditions.

      I agree completely and in heartfelt way with out about backyarders being slowly eliminated. With two or three main manufacturers of foiler Moths we won’t see the fast development and ability to drop blind lines of development that existed in the days of homebuilding Moths when 8 or 12 smart guys in Sydney would build new boats over each winter. Not to mention the ones in the UK. Or same for the early days of sailboards – you remember those somewhat lumpy styrene cored kevlar and epoxy creations of the ’70s? Lots of smart guys involved there too.

      Pity that they decided to put a minimum wind limit in for international raceboard events. That’s the sort of rule that is often counterproductive to developing better boards. Or like allowing canting keeled yachts that have to run engines – it means that develoopment of more sensible alternatives is not supported.



  9. Hi Mik, I use rules of thumb all the time, but generally I don’t think they are understood by the budding or would be designer.

    A good example of this is a custom design I recently started. It’s not a modern boat, but a “in the spirit of” type of design and a cruiser of about 26′ (7.9 m). In this vain I knew I wanted an SA/D in the 16 to 18 range, but since he would likely spend time in the trades occasionally, I elected to be on the low end of this rule of thumb for a cruiser. She carries a 15.7 SA/D, but has light air hoist options to jack this up to 25 if desired.

    For comfort I elected to go after a moderate D/L, but also to help preformance I stayed on the low side of the rule of thumb and she ended up as a 266 D/L.

    The amount of sail area rule of thumb, based on wetted surface supplies well and under working sails she’s a 2.2, again in keeping with her cruiser status, though under light air sails, this is a much more respectable 2.6, so she’ll fair very well in soft wind strengths.

    As far as the 3 times the beam thing, well this is as absurd as you make it in your piece, but maybe it once had some value. We are also in complete agreement when it comes to scaling up or down a particular design. In fact, this is a very common request for a designer. “Hey, can I have this boat, just 20% longer?” My reply is much like yours and a quickly uttered “nope”.

    As far as keel area, well I didn’t have a problem here as she’s a full keeled boat, though the forefoot is well cut back, partly to optimize the appendage for the rig choices, but mostly to get her to maneuver smartly. Her skeg hung rudder, does fall within the usual rule of thumb for selecting it’s area, but I didn’t use it as a guide. I had only so much room left under her after the sail drive leg, so I filled most of what’s left with the skeg and rudder assembly, of course with sufficient enough separation to make the skeg useful.

    I guess my point is, rules of thumb can be very handy, particularly in the conceptual stages of the design spiral. The important thing is knowing how and when to employ them. For the most part they are useless in all but the beginning of the design stages, more a ball park type of thing.

    Telling someone they should shoot for a 2.45 wetted surface to sail area ratio is meaningless, unless you know what this will bring to the other perimeters and particulars of the design. For example (again) this WS/SA ratio would be too “tall” on a narrow, slack bilged craft or grossly under canvased on a canoe body with fat hind quarters. You have to know where, how and when the apply and most importantly their limitations and/or implications to the rest of the project.

    Rules of thumb don’t govern the design, but do establish general set of target ranges to shoot at while working up the design ladder.

    • Howdy Paul,

      Nice comments!

      I think there is a difference between analysis and “rules of thumb”. But maybe I am clutching at straws?!

      The ratios of displacement to length or Sail Area to Displacement are a progressive estimate to break boats into different classes of behaviour. We know what the numbers should look like for a slow and steady cruiser compared to a mad planing machine and the “cruiser racers and occasional offshore surfers fit between and have their own ranges.

      Those types of analysis ARE very useful because they come from real analysis.

      But simple single statements that are supposed to fit all cases are highly suspect. Like “the area of the centreboard or keel should be 4% of the sail area”.

      The best resource of both good rules of thumb and “stoopid” ones is Gavin Atkin’s page on The Rules (of boat design).

      Gavin has collected a whole heap of them, but I would like to throw away quite a few as complete nonsense and almost completely misleading. Each one is a great starting point for a discussion though – or a talk about boat design. Just to pick out where each is a supportable statement and where it starts falling apart – some sooner than others!

      Some would do this because they are taken out of their original context. Also Gavin has juxtaposed the contradictory ones.

      But the ones that try to be a single simple solution are the most suspect.

      “The beam of a flat bottomed boat should be no more than half the length; if it is to be rowed or sailed, the length should be two and a half to three times the beam – HV Sucher”

      The list is huge FUN though and lead to other thoughts – which is exactly why Gavin posted them – note his quote at the top from Norman Skene – it is an obvious disclaimer to those who read between (analyse) the lines rather than trust the lines themselves (repeat without thinking)!

      Thanks for posting!


  10. Most of us in the industry, use rules of thumb without thinking about them. By this I simply mean we’ve developed such a sense of proportion and understanding about the way things work, that we can guesstimate fairly closely what a prospective boat’s displacement might need to be, to meet the SOR, general hull form shapes, approximate sail area necessary for the preformance envelope envisioned, etc., etc., etc. This is using the rules of thumb, possably on a subconscious level to a degree, as I for one don’t think about it, I just know or have a feel for what’s right, before I run the math. It’s rare for me to be surprised by the “numbers”

    Another example would be a fast daysailor I did not too long ago. She was a narrow V bottom boat, with a fairly flat chine, so she’d be good up wind, if you can hold her down and have less leeward bow wave resistance, then a hull with a dramatic chine sweep at the bow. This naturally meant the sail area would be somewhat less then the ideals for a fast dayboat. So a moderate WS/SA and SA/D are the result. Just looking at these to figures you’d think she wasn’t particularly impressive, but having sailed the prototype at over 16 knots, this isn’t the case. She does all the things I expected, including pound hard going to windward, because of the flat chine choice (see, I didn’t say compromise, even though you know it was).

    Most of the “rules” are as Mik suggests and just a general guide to compare against other, similarly sized craft. Once you generate some experience and more importantly success with design, you harbor favorites, a keenness for certain things and an understanding of the “relationships” with the “numbers”. These become second nature after a while, as you build on successes and learn from not so successful attempts. This understanding then becomes a set of personal rules of thumb. You may wish to test or stretch the rules from time to time, but most of the designs you work on, will usually fall right inside the comfort zone your personal rules create.

    • Completely agree Paul,

      The first para in my article talks about the “context” of the rule of thumb. We all have shortcuts as designers.

      The risk is when the “rule of thumb” loses its original context.

      As designers we have some experience to back up our use of rules of thumb. Often because we have the chance to make mistakes – usually of the smaller kind – like the centreboard on my original BETH sailing canoe being too small. Handling could only be describes as lazy and performance disappointing upwind. I made another board with 9″ more depth and she became pretty good upwind – not quite good enough to stay with a well sailed Laser, but not embarrassingly far off the pace either.

      Or sometimes further experience can show us that the rule of thumb is just wrong. Like the excessive glass recommended for kayaks one or the “you can shrink or expand a boat 10 or 15% with few problems” one.

      Though it is hard to know if these are wrong because of a change of context or loss of context.

      But I would generally think, that if a simply stated rule of thumb appears to be applied universally then it is highly suspect.

      Also a good rule of thumb is if something is made heavier and stronger over the years … then it is very probably being overbuilt or overdesigned.


      In other words there needs to be a realistic justification for it, rather than some sort of nebulous “creep”

      Thanks again!


  11. For what it’s worth Mik, I keep my rules of thumb to myself, if for no other reason, then to prevent it from becoming a generally accepted rule of thumb. In reality, I keep it because I can use this information to advantage.

    For the most part we’re in agreement in that these rules of thumb are worthless, for the novice sailor or budding designer and can cause much more harm then good, if considered more then what they are, which is a very rough and general guide, typically for comparative uses.

    I have a secret rule of thumb for finding a wife, but it’s use hasn’t worked well for others, for their lack of understanding of it’s implementation.

  12. Too much modern stuff — should be a discussion on “rule of digital calculator”….

    The penny (size) of a nail used to fasten a plank on a wooden boat is one penny for every eight of an inch of thickness of the plank. For softwoods, add two pennies.

    The general rule for lower mast diameter was 7/8 inch for every foot of beam.

    Beveling for caulking: the outer edge should be beveled 1/16 for every inch of plank thickness; beveling from the center of the plank.

    • Hi Bill,

      Thanks for the comment.

      I can only agree. Like PAR says above, that going straight in and grabbing the pocket calculator and thinking it is going to work is a mistake.

      I use computers for my design work, but really, it is only for the laying out of the lines and doing the calculations. You still need to know what the results of the calculations mean! 🙂

      Similar with rules of thumb. The middle one is probably correct for a certain type of boat the builder was used to building, but will fall apart for different types of boats. For example a light dinghy 5 ft wide and 13 ft long compared to a classic square metre style yacht which might be about 30ft or longer on the same beam. But kept within its original intention of size and type it probably performs admirably.

      I rather tend to trust constructional rules like your other two, but usually do check them against the result from more modern standards. Maybe not all that modern if we are talking about rivets – maybe I mean I will check a couple of other sources just to make sure I’m not overdoing or underdoing it.

      The first couple of boats I designed were laid out full size on the floor. Because that’s the way I had been building them until that point. As I got used to computer design tools I understood very clearly that it really is the same process as drawing on the floor, except the calculation results are available more quickly and more accurately.

      The computer model has to come back to wood at some point – so if you try to make the lines on the computer do something that wood won’t do when the boat is actually built, it will turn around and bite you quite badly.

      This is probably a good thing.


  13. Hi Mik,

    The only rule of thumb I cling to religiously is that when someone starts flinging around ‘rules of thumb’ too casually, he gets beaten with a stick no bigger than my thumb.

    Seriously, though, this is a great discussion! As a designer of things wooden, I find some of my most exciting failures and successes come from challenging my own developed-over-time patterns and assumptions. “Hmmmmm…. it’s probably crazy, but I wonder what would happen if I ignored reason and…”

  14. I agree and disagree with the rules of thumb position the author states. How committed to the rule are you in finanacial terms, and can you adjust and tune from there? is it an emergency? If you built a boat by rules of thumb and could tune it after the first few runs, you could get a real good boat, that fits your style. If you get dismasted how much sail do you need? broke the rudder? how big was it? how close to the rule of thumb does the deck plate swept oar you made in an emergency come to the rule? My drag racer back ground is all rules of thumbs then tune tune tune. If you jurry rigged a lee board by the rules of thumb then tuned it in practise before you made permanent mounts you would hace s good chance to build a hell of a boat.

    • Hi Mark,

      Good points … you do need somewhere to start from. Whether it is to look at a boat (or drag racer) that is similar or search for some formula, equation or methodology.

      Sometimes rules of thumb can work. And that’s why context is important.

      The difficulty is that the “rules of thumb” are often used as a one shot method as a package solution.

      But if there is a tuner/tinkerer’s culture like drag racing it can be a very successful method.

      However, most boats are built just to use and not to worry about.

      And I’m sure in the drag racing world there are lots of people who have read a bit … and then those who actually have been committed for a long time and actually gone up the practical learning curve of actually competing and seeing how the ideas pan out.

      The first group are not so useful. But the guys up the real learning curve who have tested the rules of thumb and found them worthy .. or unworthy .. their words are like gold.

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