Designing a performance small trimaran

Last year I built a tacking proa of my own design which was 18ft. long and lightweight for off the beach use. This year I wish to convert it to a trimaran which will make it more stable, therefore less likely to capsize.
I already have the centre hull and because of storage restrictions am limited to an overall beam of 9 feet 6inches. I already have carbon tubing for use as cross beams and intend using the rig built for the proa.
What still needs designing and building are the floats. I do not intend using the proa float as its buoyancy was only enough to support one adult. As the trimaran is a two person craft the new floats should have buoyancy to support two adults at least.
I have considered building to a similar shape as the narrow hulled International Moth but with less freeboard and more rocker.
The flat bottoms if not heavily loaded potentially would plane, giving additional righting moment at speed.
Any one had any experience with this sort of sailboat? Alternately if you might have any good ideas I would welcome your opinion.
By the way to view the International Moth Hull shape click onhttp://www.moth.asn.au/design_guide.html and check out the "Hungry Tiger" design.

 

small tri

Is there no way you can come up with a way to fold or "plug in" the crossbeams to extend the beam to something like square? Folding would add to the height requirement for stowage but properly engineered "plug ins" would allow increased beam w/o any increased stowage requirement for height. Beam on an 18' tri limited to 9' is quite a restriction in my opinion. I haven't laid it out but just off the top of my head it seems like it would limit flying the main hull to pretty high heel angles.
I would suggest keeping the outside hulls at a beam to length ratio at or above 14/1(Moth is 10-/11/1, generally)-the higher the better. Keeping the ama's narrow and at minimum wetted surface would help in light to moderate wind and using "banana board" daggerboards for additional dynamic lift would probably be more efficient than developing that lift on a narrow,inefficient planing surface. Banana boards don't require an altitude control system since they are just "foil assist"(and can be retracted)-but they probably wouldn't be too effective under an overall beam to length for the whole boat of 1/1 or square.
With that narrow a beam ,if it can't be solved by either suggestion above, you might consider a configuration like Bethwaites "HSP"-with the ama's designed to rarely touch the water.
What is the beam to length ratio on your main hull only ?
I designed and built a 20' tri with an 18' beam that used a 6.5/1 beam to length ratio on the main hull and 14/1 on the ama's. The point was to allow the main hull to plane which it did in quite light air and also to increase room. I wouldn't do it that way again but it was a fun ,powerful boat.I did a 14 with a main hull beam to length ratio similar to a Moth with very small 10/1 beam to length ama's that were there just for non sailing stability/ emergency back up.
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This is a rather radical ,experimental solution to the use of planing hulls on small(or large) trimarans-the geometry has been tested on a small non saiing model but the concept remains just an idea for the time being:
Rotatable Hull(ROH) Multifoiler - Boat Design Forums
Address:http://www.boatdesign.net/forums/showthread.php?t=10267&highlight=ROH Multihull
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I'd be interested in the solutions you finally adopt(and why)-good luck!

 

Sounds like a fun project. Instead of building hulls, have you cosidered using H16 hulls and use the plug in method. Wouldn't need to worry with daggers. Sounds like a 12ft beam would be the ticket.

 

Thanks Doug and Delane for your advice. Two of the requirements I have is that the overall weight of the rigged trimaran can be picked by two men and carried in and out of the water, also to assist high performance with the existing rig. Mainsail around 110 sq. ft. jib around 48 sq ft. assymetric kite around 190 sq. ft. Also the engineering required for a fold up crossbeam arrangement is complex therefore potentially breakable in typical Perth Australia summer strong sea breezes. (20 knots plus). I already have bought the 9ft. 6in tubes from CTS composites at considerable cost.
Note that I dont require the tri to lift the main hull as this will require floats that have around 160% buoyancy minimum which would make them larger and heavier than I want. The main hull has length/beam of 14 and weighs something less then 80 lbs. I expect the floats might be 12ft. by 10 in beam at waterline with flat bottom and rounded off chines (like a current Moth hull). This is also a length/beam of just over 14. Expecting to build the floats at a weight of less then 20 lb. each using 200gsm carbon over 5mm klegecell.
In previous sailing in proa mode with only the main hull in the water it was clearly planing in around 12-15 knots of wind, as the main hull shape is vertical topsides and the bottom arc shaped, the curvature of which is greater near the bow and less near the stern. Also the chine is sharp over the entire length.
The banana boards (is this also known as a Bruce Foil?) is excellent in theory but needs a much wider beam and a lot more engineering than I want to get into.
From my water skiing experience I know that a typical single ski of around 2.5 sq. ft. planing area will support a 150 lb. skier at around 25 knots of speed. If the planing area of my float might be around 8 sq. ft. then if correctly orientated (flat when the tri is slightly heeled) than 200 lb. of hydrodynamic lift should be realistic at the sort of speeds the tri should acheive in 20 knots of wind. If this is so then combined with trapeze(s) for crew I should be able to negate the need for much wider beam.
Any opinions on all this?

 

Performance tri

Frosh, "banana boards" were curved daggerboards that that were used (and named by Steve Clark) on Patient Lady in concert with rudder t-foils as "foil assist" to reduce the displacement of the hull and therefore wetted surface. A benefit of the type is that it doesn't require an altitude control system. The most "famous" use of a curved daggerboard to develop lift is on the Orma Tris's-there they are curved so that when they are retracted they don't exceed the beam limit of the boat while still creating upward lift when deployed. A "Bruce foil" is ,generally, a low aspect foil used at the end of an outrigger and angled to provide lateral resistance and lift up helping with stability(see foils.org).
You could use a t-foil instead of a curved board but you wouldn't be able to retract them 100%. I'd still bet that developing the lift you want using foils would be less draggy than doing it by long narrow ,very inefficient planing surfaces. The main reason Parlier went with stepped hulls was to utilize a more effective aspect ratio for the planing surfaces but he said that they were really draggy from about 8 knots boat speed to 20 or 22. But above that the drag compared to a "normal" ORMA hull went way down. That's why I came up with the idea of the ROH multi-to be able to literally shift "gears" from a high beam to length ratio displacement hull to an effective planing hull using each in the speed range they are most effective. But that's probably a bit much for your boat-my idea was to come up with a high performance tri that would smoke a beach cat by relying on the huge RM to be gained by an 18' square configuration. Foils are primarily used in the ROH concept for pitch control....
My planing tri's worked well and were alot of fun and yours should be too.
I guess you have to decide what level of performance you're trying to achieve: less than or equal to a beach cat or faster than a beach cat. For instance, do you envision your boat beating an F18? Set your target and then look carefully at all the variables.
By the way, I got my carbon tubes for my 16' foiler from ICE in the Pacific Northwest US and they made up "ferrules" that allowed joining the tubes. Maybe you could get the same thing from the company you did business with and just maybe -depending on your target- it would be worth the added performance. The bad news is that if you powered up like I'm suggesting your current rig would be very inadequate.
But I hope it works the way you want- you're bound to have a blast getting from here to there!

 

Doug, thanks for the additional info regarding foils and dynamics of planing surfaces. I am only doing this for my own satisfaction not to smoke everything else around the same size. I have no intentions of building centreboard cases in the floats as the construction gets more complex and then they need to be operated while sailing, not to even consider the additional wetted skin friction drag. I already have a decent centreboard in the main hull. If I can get Hobie 16 speeds on average I will be happy with that. My mast and boom are 100% carbon and shorter, so less than half the weight of typical H16 alloy ones. Also I am aiming at a fully rigged weight of around 160lb. which I am fairly sure I can acheive. An F18 weighs in excess of 380 lb. Also my hull sections are more efficient than the typical beach cat as previously the main hull has visibly lifted when planing, sitting on the about the rear two thirds, very unlike any cat hull I have seen. With a saving in weight of around 220lb, lower wetted surface, compared to an F18, more sail power wont be an issue especially as most of our summer breezes are around 20-22 knots. I also dont expect a lightly loaded 12ft. x 10 in. hull to be very draggy, judging on the experience in Moths and A class cats. Planing can happen at low or high speeds, depending a lot on length/beam of planing surface and weight loading per square foot of planing surface. Low L/B ratio surfaces work better when there is an excess of driving power, eg 1 meter wide formula sailboards with huge sails, (however a formula board is completely useless in sub-planing conditions), but at higher ratios much less power is needed due to low wave making drag, even though planing will not happen as at as low a speed. There seems to be little info. on planing sailing multihulls, and I think it is about time that this very interesting area of design was more fully explored. You also want good performance in light winds as well, so a design should be optimised for light winds and strong winds, although I realise that there will always be the need for compromises.

 

Small Performance Trimarans

Frosh, here is a thread that I just ran across ; you may enjoy some of the discussion:
small trimarans - Boat Design Forums
Address:http://boatdesign.net/forums/showthread.php?p=36354
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Just for fun this weekend I've been looking at a 12-14' square(or more) tri designed with a 240-250 pound crew in mind. The crew would sit inside the main hull or on small seats either side of a cockpit that would be set up for max comfort. I was thinking of using my 182 sq.ft foiler rig since there is plenty of RM and using foils that would fly the main hull in relatively lite air-before it would fly due to heeling force. The foil on the daggerboard would use an altitude control system so that it would pull down as necessary as the wind picks up. The ama's would be small ,maybe rotatable with a foil, foil assist and /or a planing surface for higher wind speeds.The ama's could be quite small using this system.
Just looking at the numbers there seems to be huge untapped potential in a small tri around this size; what do you think?

 

Small high performance trimarans

Hi Doug and other interested yachties. I have already decided that on my tri which is well on the way, construction wise, that (1) RM will be acheived by crew on trapeze and hiking (like Bethwaites HSP) and that amas will not be of sufficient volume to fly the main hull. My reasoning is that if flying the main hull was required the ama would be double the volume, double the waterline beam, and more than double the weight. I also have doubts that a small tri could plane on it's ama at all if the entire weight loading was on that ama. For this reason I intend the main hull to be carrying perhaps 70% of the weight and the remaining 30% on the ama. There is also the load on the leeward ama due to heeling force which it will resist by planing. Therefore I know the main hull already planes readily, and with the leeward ama planing also, it should be minimally immersed ( like a waterski at speed). This should provide a high top speed in medium and strong winds.
(2) Going for a square length to beam configuration apart from the inconvenience, requires much more than the two carbon tubes 60mm outside diameter of total weight for 2 of less than 10 lbs. The engineering and weight of 18 ft. crossbeams is far more than I want to contemplate.
My concept will also have far superior light air performance due to signicant weight saving, and low drag narrow amas.

 

small square tri

Frosh(or any interested sailor/designer), not trying to change your mind; just curious if you looked at the possibilities in terms of power to carry sail in a 12-14 foot square tri as I mentioned in my last post?
A power to weight ratio better than the lightest Moth could be achieved and by using a foil(or maybe planing surface in higher winds) the ama's could be quite small. With the Moth beating many multihulls under 20' it seems like a concept like this could pump some speed back into a boat with more than one hull and could finally put the trimaran in it's rightfull place. What do you think from a theoretical and practical standpoint?

 

Small square tri

Hi Doug, It is not what I want but I am happy to discuss the concept further.
Firstly it would be a singlehander and I would use a main hull similar to the M4 and amas perhaps similar to moth hulls but scaled down to 9ft. length and more rocker and more transom width on the waterline for planing. Assuming the same hull weight as the M4, 20 kg and amas at 6 kg each. Carbon tubes for cross beams and rig, something around main sail 14 sq m and assy kite 18 sq m. overall weight could be around 60 kg. Pitchpoling would be your main enemy, so fine bows with V or U sections would be out. I suggest arc shape bottoms all the way to the bows to provide lift forward at planing speeds. As far as speed against a moth is concerned, it would probably equal the low rider until the kite was up then it should leave it way behind. The additional cost and complexity over the Moth would be compensated by the much wider spectrum of sailors who could master the tri to a competent level. As we know the number of sailors that can really master a skinny moth is really a very tiny percentage.

 

Tri

I'm thinking of more like 16m² upwind in a high speed rig on a 12- 14 foot singlehander. Pitchpoling would be virtualy eliminated by foils; I'm thinking of a small multihull that could actually beat a Moth on foils. That would be extremely difficult to accomplish but I think it may be possible.
But the Moth or any really well designed monofoiler will still have the edge in bang for the buck since the tri will be substantialy more expensive. I'm guessing the tri would be somewhat easier to sail....
PS- I'm not sure this type of tri would benefit a whole lot from a spinnaker-maybe a screecher?

 

Tri foiler?

Hi Doug, arent you sort of reinventing the Hobie trifoiler? We know that this has acheived somewhere in the vicinity of high 20's (knots). This is more than than the Moth is currently capable of. Carrying 16 sq.m mainsail and avoiding pitchpoling on such a short craft would require a trifoil arrangement with the mid-foil under the stern of the main hull and lateral foils under the bow region of the two floats, to my way of thinking.
As far as carring an assy kite, Tornados do, and probably approach 30 knots flat out.
I think that the craft you envisage is becoming too technical and titchy for anyone but a very experienced speed freak.
I was thinking more towards something that was a good around the buoys racer, and accessible to any reasonably experienced sailor. i.e. If you can competently sail a Hobie 14 you can sail this tri. Is that what you had in mind or not? :?:

 

Tri speed

Frosh, the princible difference(and a big one) between my idea and the Rave or Hobie multifoilers is that they use the power of the hydrofoils to develop all the RM as well as lift. My boat would primarily use the foils only for partial lift developing RM by using the weight of the boat. It would have a foil on the daggerboard that would be altitude controlled using a wand-the idea being to lift the main hull fairly early(and maybe to pull down if necessary in gusts).And a foil on the rudder-both retractable. The ama's might use a small foil for "foil assist" like the ORMA 60's so the ama could be kept somewhat small and probably use a rotatable hull so that at a certain speed the ama's could be rotated to get rid of the low speed displacement hull replacing it with a stepped planing hull- I would definitely want to experiment with that concept. At this point it's just an idea for something for me personally(down the line if ever) but I would think that a production version could be very easy to sail- though not cheap. I would want to go after the Moth in the range of wind speeds of 7-8 knots to 15 where it already is probably the fastest sailboat on the water under 20'. The problem is that the Moth only uses two foils and as you said I would need three and/or a planing surface. The chance the little tri has is it's awesome power to carry sail and a carefully designed foil system that would be very lightly loaded at speed.

 

Faster than a foiler Moth?

Hi Doug, your proposed solution has become so technical that it has moved beyond what might be accessible to the average sailor and could only ever be a one off specialist craft. I dont honestly believe that at a little tri can be designed to be faster than a current foiler Moth in all wind strengths. The tri with its huge RM could probably come close to Tornado speeds in stronger winds therefore being quicker than a Moth, but at 10 knots of wind, No!
Just look at the incredible low weight and low wetted surface of a foiling Moth. No tri can come close in my opinion, and extra sail isnt going to make all that much difference.
To beat a Moth in medium winds you need to design a better Moth with no class restrictions at all. I dont need to tell you what exactly is required, as it is self evident. Problem is that it becomes even more demanding to sail.

 

small tri

Well, I don't know. The tri would have significantly lower "wing loading" than a Moth: 1.7 lb. per sq.ft. SA vs around 2.5 for a Moth-both with 160lb. crew.
The bad news for the tri is that the square feet of SA per sq.ft. of foil of the Moth is about 53 and on the tri about 40.That includes a bit of hull in the water for the tri at speed .
Would the lower weight per sq.ft. of SA offset the Moth's SA/ wetted surface advantage? Would the fact that the main foil and ama foil may be more lightly loaded than the Moth main foil reduce drag enough that combined with the better power to weight ratio the tri could zap the Moth? Or would the small hull interaction with the water's surface in the speed range of 8-15 knots spell doom for the tri? That's not including the significantly better power to carry sail of the tri because that would primarily be beneficicial in higher wind ranges. But the "Veal Heel" of the Moth is a powerful upwind tool that the tri wouldn't have unless I could figure out how to compensate for it.
I don't think it's all that technical and it would be a blast flying the main hull in just 6-7knots of wind. Beating a foiler Moth? Probably not but awfully fast for a small multi just the same.....