“Planing” is Bunkum – Myths about Planing, Displacement and semi Planing

Often on websites you find something like the following …

  • Displacement hulls – will not exceed hullspeed or not exceed it by much.

  • Planing hulls – can exceed hullspeed by lifting over the water and planing at speeds well exceeding hull speed.

  • Semi Displacement hulls – they can exceed hullspeed by some degree but at the cost of some excess power and fuel consumption.

  • Catamarans and other multihulls are rounded shapes and narrow so they cannot plane like a flat bottomed boat.

  • There are two modes a boat can have – displacing up to hull speed then planing mode above.

“Hull Speed” is the point where a boat reaches a point of extra drag because of the position of the waves it generates as it travels.  It is generally quoted as 1.34 x (square root of waterline length in feet) is the speed in knots..   For a 16ft boat the hull speed is a bit over 5 knots.

These are all demonstrably bunkum.  They are illogical and arbitrary explanations of natural phenomena.  They belong with “jib slot effect” and “wings develop lift because the air has longer to travel on one side”.

The Questions are formed about planing and displacement.

The normal explanations on the net are cloudy thinking and the more you think about it … the more absurd they become.  They just don’t fit the evidence and the explanations get weirder and weirder the more you ask questions.

I finally put this together on a drive with and old school friend, Graham Murray, on a trip to see the Newick designed trimaran he’s been building for a few years.  As high school students we had both drafted a letter to Alan Payne, the great Australian yacht and boat designer about becoming boat designers.  Well Graham did everything actually.  Armed with Alan’s reply I headed off to study Chemistry and Graham became an airline pilot.  We ran into each other on the net years later and I went to see the boat he was building up the coast.

Anyway the question that came up in a car about how monohulls have “planing” as a way to exceed this “hull speed, but multihulls have their “slenderness”.

It bugged me really badly … because the natural world should not have TWO explanations for one phenonenon.

The second part that bugged me is that there was on one hand this idea of “displacement sailing” and at some particular speed it became “planing” .. er, except for catamarans and trimarans which “don’t plane”.  Why should displacement suddenly turn into planing and ignore multihulls all together?

So … I do what I do and thought about it for a few months.  That’s what I do – when I see an illogicity I have to bother it.  It took me years to get some basic grasp on Quantum theory 🙂   The result of my thinking is out on the net, but it is down in the corners and not put together in a concrete way.  Unlike the crappy normal explanation which is widely repeated with authority.

Ok … I am an iconoclast – on a bad hair day anyway.  So the results of my thinking …

Planing and Displacement add together to equal One.

It seems to me a boat … as soon as it is moving is developing some degree of lifting forces … either in a positive or negative direction.  it seems absurd that “planing” forces that “appear” when the boat reaches a certain speed.  Surely the logic is that the same rules apply from the very beginning of when the boat starts moving.  So logically there are lifting forces as soon as the boat is moving at all.  For scientific objectivity we can say that their total might be positive or negative.  That they will either tend to lif t the boat or they might pull it down into the water.  The reality is that the lifting forces along a hull tend to be a mix of positive and negative – and at any speed they will add up to make a non zero total … they will have a small influence at low speed and a growing influence as the boat goes faster.

Also that if a boat is 100% planing …. that means it can’t displace anything at all.  It actually cannot be touching the water … which means it cannont develop forces to plane at all.  It cannot interact with the water at all. It is all mutually contradictory.

I would tend to say that lifting forces … or better word “vertical lift” is made up of two components.
Immersed volume of  the hull plus dynamic lift and added together they equal the weight of the boat.

Planing lift + water displaced = weight of boat, gear and crew.

So how do you know the boat is generating lift? And how all multihulls probably plane.

Generally there is a depression behind the boat. The weight of the water in that depression equals something the lift from the dynamic lift.

Here it is behind a motor boat.

Powerboat shows lifting forces on hull by evidence of the hollow behind the boat


And just to show that the categorisation of multihulls into non planing is also arbitrary and incorrect … I offer this picture … if you see a smooth hollow behind ANY boat … the force that pushes the water down behind the boat equals the force that pushes the hull up.

Multihull demonstrates lifting forces ("planing" is an illogical term for any boat - read the article) by the depression in the water behind it.


That leeward hull is getting significant lifting forces.

Hull speed is not a place where the boat goes from “displacement mode” to “planing mode”.

There is something theoretical called “hull speed”. but every boat can exceed the hull speed by some amount with adequate power. The exception might be a perfect box with the bow end immersed and pushing a big bow wave … but move the weight back a bit so you have a clean entry and …

Even a stupid shape can plane - read article on how planing doesn't really exist!  Storerboatplans.com


In the case of a hydrofoil boat there is a hollow in the water surface over the foil/s. Just like if you fly an airplane real low you get a depression in the water underneath it. There is some up force so the water surface makes the compensating down force visible.

But what we have is different boats that can move through the hull speed more or less easily. The limit they are up against is wave drag … as opposed to wetted surface drag and some induced drag.

The wave drag increases quickly as the boat gets to hull speed – so that part is true.  But it seldom or never limits the speed to hull speed – most boats can exceed it to some extent

Why do Some Hulls exceed “Hull Speed” more easily.  One explanation for all hulls.

There are two traditional methods of beating the wave drag.  They are often said to be separate.

  1. One is make the hull very slim with the displacement well spread out. This way the wave drag doesn’t grow as quickly around hull speed.  The boat doesn’t “push” the water far at an point as it moves around the hull so no large waves are created.

  2. The other way is to develop lift forces that reduce the displacement. The displacement is what causes wave drag. So if you reduce it … you reduce the wave drag.

There is also a generality about what sorts of boats can get past hull speed more easily.

Basically what happens is that at any speed there is a wave at the bow and according to Froude there will always be a wave train behind that. The distance between the the peaks of the waves (wavelength) varies with the speed of the boat .. or the speed of the wave train if there is not boat involved.

As the boat goes faster the second wave after the bow wave moves further back.

The maximum drag occurs when the bow wave … is at the bow and the second wave is at the stern. The reason for this extra drag is that the boat now has a trough under most of the hull … it is the also the part of the hull that is supporting the weight. So the boat sinks deeper in the water.

If the boat carries a lot of its weight in the mid section … like a heavy commercial boat or older yacht … it sinks down substantially.

An accurate painting by artist Alexander Creswell makes the point

A deep hull develops a deep trough in the body of the hull so it sinks deep and has limited speed

Or a tug being driven fast.


So why doesn’t a light dinghy or powerboat or multihull bog down and why can it beat this barrier.  And can it be explained without breaking monohulls and dinghies into two groups.

Two reasons …

  1. first of all all these boat types are not that deep so cannot develop a deep wave.

  2. Second … at the peaks of the waves at each end of the boat (where the  … the hull has enough volume towards the ends not to sink down too much. Unlike a heavy boat which will have more volume in he middle so is more open to being undermined as a trough is created around the boat.

An example of how a “displacement hull” can go much faster than “hull speed” proves the new explanation.

The original theory would say the “DISPLACEMENT” hull can’t go much faster than hullspeed “EVER”.

But here we have an exception.

What about when a truly displacement boat surfs on a big ocean wave.

It can go much faster than its hull speed … why? . What is happening there … in effect the wave the boat is on PLUS the boat’s own wave at hullspeed end up supporting the middle of the boat better so it doesn’t sink down as much. The peak of the ocean wave is holding the boat up and preventing it from sinking. The traditional theory falls apart and the one I am putting forward explains it beautifully!

If it was a matter of “displacement hullshape this just couldn’t happen.  But here is a photo from a famous event … to Australians anyhow!  Not to mention old timers at the New York Yacht Club.

Photo by Stanley Rosenfield … see many more photos at mystic seaport http://www.rosenfeldcollection.com/inde … rtRow=2661

This is the photo of the Australian 12 metre “Gretel” that took the first race off the American’s in the Americas cup for decades. She caught a monster wave. 12-metres are classic “displacement boats. But here we have a wave preventing the boat from sinking down and she can greatly exceed her normal speed. This photo is a moment of some pride for sailing Australians. I used to slip and unslip that boat years later – it was pretty cool!

The ocean wave cancels out the trough that would normally be under the hull.  The interesting thing from the photo and for sailors is that for an old heavy boat like this it will never start planing down the face of the wave like a surfboard.  The wave comes up from behind and the boat just ignores the point where a surfboard or racing dinghy would start surfing down the face.  The big heavy boat only starts moving fast when the crest of the wave is UNDER THE BODY OF THE HULL.  I would expect that happens because the gravity effects of the boat falling down the face are much less than the effect of the crest of the wave supporting the boat.

Remember that the classical theory of displacement hulls cannot explain this behaviour at all – it says REGARDLESS of the amount of power you put in the boat will never go much faster than hull speed.  But in reality it happens all the time.

What happens to the stern wave when you are going faster than hull speed?

Lots of sailors know the answer to this one.  The reality is that there is a bow wave and then the second wave will be the right distance behind the first according to the equation by Froude.

So at slow speeds the second wave will be a very short distance behind the bow wave and as you go faster the second wave moves further back.  At hull speed it is near the stern of the boat.  So at higher speeds it is behind the boat.

If the boat is doing around 5 knots the second wave will be about 16ft behind the bow wave.  So a 16ft boat would be at hull speed, an 8ft boat would be truly “planing” (it’s ok to use the word descriptively, but not technically) and a 25 footer would be well below hull speed.

Here is a picture of my sailing a Goat Island Skiff at Mooloolaba.  See that the stern wave is about 2/3 of a boatlength behind the stern.

Goat Island skiff planing - see second wave (incorrectly sternwave) is about 2/3 the boat length behind the transom.

So if we do the calculation.  2/3 of the hull is in the water and the second wave is about 2/3 of a boatlength behind the boat.  This is about 20ft.  Square root of 20 is about 4.47.  Multiply by 1.34 (the constant for the dimensions we are using) is 1.34 x 4.47 = 5.99.  So the speed here is 6 knots.  Not that fast for a Goat!  John Goodman and I had his between 9 and 11 for hours each day in the Texas 200!

Heavily laden goat island skiff going fast in Texas 200 event.

Summing up about Planing and Displacing

So as you see … the whole thing is not about different types of hull … it is a continuum. Every boat exceeds hull speed to some extent … but lightness and spreading the hull volume more evenly over the length also help.

Every definition of planing, semi planing and displacement is flawed because every boat has lift from buoyancy and dynamic lift from movement (which can be positive or negative – it actually varies through the length of the hull.

Water displaced + dynamic lift = total weight of boat, gear and crew.
planing force + displacement = total weight

So “planing” is something that belongs in descriptive language rather than being a technical term. Like saying the moon is “golden”. So it can still be used for something you experience … but it is something that just can’t exist in a 100% pure form …

… because if it did … the boat would not be touching the water at all … so could not develop “planing lift” from the water.

More goat pics here … I need to link back to flickr where the images are kept.

2 thoughts on ““Planing” is Bunkum – Myths about Planing, Displacement and semi Planing

  1. On the Goat Island Skiff Facebook Group Archie wrote

    Good one, MIK. I think you’re onto something. One assertion that bothered me, however, was that a shallow hull can’t generate a deep trough in the middle–due to its shallow hull shape.

    I wrote a reply there, but I will put it here too as it may bug some other people too…

    I guess it is a matter of perspective.

    Is it more that a shallow boat is light so that when the trough forms under the ends don’t need to sink down much to compensate? The comment about shallow hulls not being able to create deep waves is from Bolger. Maybe I accepted it roo easily.

    I think it is at least partially right at least. At hullspeed a deep heavy boat will have a deep trough but a light one won’t. It is at least partially due to the effect of the deeper volume. Wavelengths will be the same if course.

    Both those explanations are the same one I think … just from slightly differing perspectives

    • This article seemed to annoy some folks over on some other forums, but they were somewhat mixed up in their thinking – as we all are if we accept the normal paradigm of planing, semiplaning and hullspeed.

      Whatever way you look at it – it doesn’t hold water except as a very broad classification – the boats really don’t behave that differently at all.

      One of the nice emails I got was from Tom Lathrop who designs easily driven powerboats that perform nicely with small powerplants.

      Tom Lathrop low power powerboat

      Tom said

      I read your thoughts on the planing phenomena and agree that the emphasis on “hull speed” is an arbitrary idea and a deterrent to understanding what is going on under a boat. Since all my power cruising boats are intended to operate in the transition range normally assigned to semi-planing or semi-displacement or whatever it might be called, I have spent some time in looking at this area. I collected and published my thought on my website under “Planing Boat Theory”. http://www.bluejacketboats.com

      I look at the phenomena from the aspect of the water rather than from the boat. This disturbs some hydrodynamicists who want to see everything as flow streamlines but it think it allows a clearer look and a better understanding for non professionals like us.

      My reply after reading Tom’s article above

      Darn, Just read your article – really nice. Two main shocks … that the diagram from Elliason (an excellent writer and educator) is silly and the elegance of your mirror image diagram.

      Elliason's strange diagram for boat design and explaining planing.  The plate is above the water surface completely

      With Elliason’s diagram see how the plate is above the oncoming water – it’s flying and would not be in contact with the water to initiate this flow.

      Actually I am a crap counter … three … the same annoying statements prompted you to write the article too and you refuse to label!

      One hesitation only … that there are two responses of a “pea” … that is to approach the plate and then be co-erced into going either forward parallel with the plate of aft. No bounce. Otherwise there would be air gap! 🙂 I remember from first year uni we played with a Pelton wheel – a simple water turbine with very cupped blades. The maximum power was when the velocity of the wheel (in an instantaneous calculus sense) was half that of the water flow as the 180 degree cup extracted double the momentum from the water and dumped the water downwards with little forward or reverse momentum. In the planing plate the water (OK guessing from this point) would impart momentum from the direction change to move parallel with the plate and only a minor amount from a velocity change .. but I have no idea of the proportions.

      I don’t agree at all with Bolgers Pea arguments about what happens at chines for the most part … doesn’t fit observable facts. Never a vortex off a Goat or other efficient boat chine despite the greatly different curvature rates and angle of incidence of side of boat vs bottom. These are unavoidable in lighter displacement boats with low (or no) deadrise. I think it is something about minimising the energy state of the system. The water goes with the flow. Your engineering instincts would tell you if I’m kindof right or kindof wrong on this. I haven’t thought about it enough yet.

      Just want to get people thinking rather than accepting nonsensical stuff.

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