Shas Cho on my forum wrote a number of questions to do with outriggers – very general questions – which are often the most interesting type.
Please excuse me if this has already been discussed (I’m reading the archived articles as fast as I can!) ; I’ve been looking at commercial and home-built outriggers for canoesand am puzzled at the apparent lack of either rhyme or reasonregarding the optimum size of the equipment.
There are very many more poor designs of ama than there are excellent ones, but there are a bunch of reasonable compromise ones too.
The amas (dismissing the crab pot traps and the swimming pool noodles) appear to be anything from one quarter the length of the boat to the full boat length.
Great observation. For excellent speed the best solution is for something like the length of the middle hull. But this can be too complicated and expensive and hard to move around. So often a smaller size is chosen. Amas, single or double, add two things to the boat and have to be analysed from both of those directions. 1/ a slender low wetted surface hull compared to the main hull 2/ lots of stability This doesn’t really differ from normal multihull design – all the standard texts will fill in details.
So if you push the ama down in the water to deck level – it will have a maximum displacement. My drop in outriggers have a displacement around (I can’t really remember) – but maybe 140 t0 200 lbs. This is my version Lets do the Maths. Main hull – 45lbs amas 2 x 10 lbs = 20lbs crossbeams 2 x 10lbs = 20lbs Rudder, spars and stuff – 30lbs maybe Person – use a 170lb average. That all comes to 300lbs. So what is going to happen when the boat catches the wind and starts heeling?
The weight is gradually transferred to the leeward ama. It will disappear under the water when the main hull is half lifted out of the water. This will slow the boat down but also it provides LOTS of warning that the boat is loading up and the rudder cannot come out of the water as the main hull cannot lift.
This is termed a submersible ama design. It doesn’t use all the available power from the sails and it tells you to ease the sheet or start climbing out to windward yourself
The event is on every year and had (appropriately) multihull designer Richard Woods, Small boat adventurer Howard Rice, Skin on frame designer Dave Gentry, polymath designer/instructor David Nicholls and Michael Storer (er, me)
Ok … cutting to the chase. These hulls are double the length also wider and deeper so you would have to guess that the volume was around 400 to 500lbs per ama. If the wind loads up the sail there might be enough volume in the hull to allow the main hull to lift out of the water
But at that point the sails will have to be developing twice the power of my outrigger. As the main hull lifts there will be an increase of speed as wetted surface is reduced, then a progressive loss of control as the rudder is pulled up out of the water.
The same smart helmsperson that sailed my hypothetical submersible hull boat would be waiting for any slushy feeling in the rudder and ease the sheet whenever he or she felt it. Some ocean going trimarans have rudders in all three hulls so they can keep pushing hard with the main rudder out of the water. Yet another way is sometimes seen on Proas and some indigenous boats. Dave Gentry’s super elegant outrigger at the same weekend has this setup.
This gives resting and low speed stability to a very narrow hull. It can be used for sailing if the crew transfer their weight to windward (a windward ama, platform, leaning plank or this can be to windward as a Pacific type Proa)
This has little ability to support much of the boat weight at all but is there to slow the roll of the boat as the load comes on. A sharp crew notices this and adjusts sails, their position or other factors to either provide the stability needed or reduce the heeling force.
A nice case study – amas for foiling multis
An interesting variant of this idea is the record sailing boat Hydroptere.
The foils out on the amas provide all the righting moment in the photo, but to get some lift from them the boat needs to be moving.
But it can’t get moving without any stability to it has small buoyant amas – not enough to get the boat to much speed but enough to start the hydrofoils developing the full sailing lift.
The forces are pretty much similar to a non submersible ama in practice as it is difficult to lose lift – if you are moving fast the foils can support. But if the boat goes slow in lots of wind it will overpower those little amas easily. Like the very tiny ama of the skin on frame boat.
I have not seen any reference to the length of the floats produced by your plans (though your pdf file “outriggers drop in Eureka version b” shows the ama as about one half the boat length) … nor have I seen the function of ama length discussed anywhere.
Is there no formula for the relative lengths of boat to amas?
And aside from the effect on bouyancy, does the width and depth of the amas affect sailing performance? What about the effective beam of the outriggers relative to the beam or length or displacement of the ‘host’ boat? And we haven’t even touched on the fore-and-aft placement of the amas and, separate but related, the akas.
Ama volume, volume distribution, aesthetics – no rules but common sense bounds
Relationship between main hull volume and centres of ama volume is a big area. if the ama volume is too far aft the bow of the main hull will be forced deeper into the water. Not good for steering – also the trim pushes the bow of the ama under.
Crazy far forward and the stern of the main hull will drag as the boat heels. So something similar in volume position of the main hull. But remember that heeling forces are forward and to leeward too so maybe it makes sense to have the volume of the amas slightly forward of the volume of the main hull. Ok – back to my drop in outriggers .. my starting criteria were that…
- The hulls had to be shorter than an 8ft piece of ply. Then, after that…
- Second criterion was to try and get two decent volume amas (as we have seen volume and beam are what create power) out of a sheet of ply. It wasn’t, but some other designer’s amas have this feature.
- Then I had to choose how much volume. Slenderness ratio of beam to length is much less important than it was once mooted to be. You can have very skinny hulls like a Crowther Twiggy or a very wide main hull like a Farrier – there is much more choice here than people realise – within reason of course.
- So reviewing the simplified cross section there was an aesthetic choice too – that it had to match a reasonable number of potential main hulls.
Decision Points – working up through the grades
I did see a brief mention of fine vs blunt ama entry when someone considered scaling down his Quick Canoe plans to one-third for use as amas, but I don’t recall any conclusions being drawn, other than a mention of lateral resistance when the ama is submerged (No doubt one must consider the intended purpose of the outriggers, whether they are part of the sailing equipment or merely ‘training wheels’).
Yes … from the training wheels of minimal volume amas To the moderate power of the submersible ama – remember that the volume of the ama is about 150lbs – the weight of a person and that person is 5ft from the middle of the boat pushing upwards hard! Much more power than a single person trapezing on a performance dinghy To the full power of non submersible amas.
All of it is a game to balance performance in terms of increased stability pushing towards maximum volume and maximum boat width. as one factor against
- required stability for intended performance.
- acceptable amount of bulk
- building materials cost
- encumberance for the main hull
- trailer/roofrack weight
- boatshed storage or building space.
Beam and volume add to performance, all the others tend to reduce it. More you can check out design books – this is a link on Amazon.
I remember the Chris White book is pretty reasonable. The Norwood book which I have in storage is rather nice too.
Any back issues of Multihull magazine have articles from reviews on boats but also a good ongoing discussion about design criteria and practicality that goes back four decades. You can also see a whole bunch of articles on the net by looking for specific things. Maybe readers can put some notes to articles that are more specific in the comments below.
Here are the photos of different boats at sail Oklahoma.