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A “Smaller Trimaran” Discussion Continued…

(Publisher’s Note: The following discussion by Frank Smoot, from www.diy-tris.com, is really a continuation from the previous post about the new Uffa 10 trimaran. As you can see, that thread generated quite a bit of discussion in the comments area. Frank offered to add some photos to the continued discussion, so I’ve created a separate post here for him to do just that. To understand fully what Frank is writing about here, you should read the previous post, especially the comments contributed by Ian M and Stefano).

Smaller Trimaran” Discussion Continued…
by Frank Smoot

Great idea about teaching school kids to build boats, Stefano, especially here in boat-crazy Florida. Maybe I can help de-program them from a life of monohull monotony…

As usual, Ian, your post is insightful and informative. I couldn’t agree more about the wasted energy in those bouncy little dinghys. That’s the complete opposite of what I’m looking for in my sailing experience.

And re planing hulls and amas, I kind of hate to open up this can of worms, but here goes:

“What I’ve Learned (So Far) About Planing Hulls And Amas”

Since I don’t know anybody else who has tried half a dozen different ama shapes and styles on similar boats (my various 16′ tris) and in similar circumstances (good old Terra Ceia Bay and it’s ultra-fickle sailing conditions), I feel (borderline) qualified to share some of my observations conclusions with the elite sailors who read this fine column:

– Pros And Cons of Planing Main Hulls:

My 2-year-oild boat, “No Commotion,” has ended up being the default baseline for all my comparisons, mostly because it turned out to be so much better of a sailing experience than all my other efforts. It is fast enough (13 mph so far on a single 102 sf sail on an unstayed mast), quiet, smooth, and dry.

Trimaran Named - "No Commotion" with 102 Feet of Sail

I wanted to see if I could go faster, so I built a main hull that was specifically designed to plane. I probably overdid it, as the aft half of the hull tapered from a shallow V to nearly-flat (like the planing dinghys). While this hull was reasonably quick, it took a LOT of power to get it there. It was also noisy, wet, bumpy, and sluggish.

I actually recontoured the entire first ½ of this hull to make a smoother, finer entry,, but all that really accomplished was to move the noise, turbulence, and splashing farther aft. It was still too wet, too noisy, etc. By contrast, both No Commotion and my wife Laura’s tri were much quicker and quieter under virtually all wind and wave conditions.

My conclusion is that it just takes too much power to get a planing hull this size up on a plane. You need a very windy day, a big sail, and fairly flat water — a difficult combination to come by.

Of course, on the plus side, the shallow hull floated on almost zero water and turned on a dime. But like Laura’s flat-bottomed boat, it didn’t track worth squat or go where you pointed it if you couldn’t get at least a foot of the leeboard down into the water.

So on balance, I think a moderate V-hull of about 30-35 degrees dihedral makes for the best overall boat. Also, I’m not 100% convinced that a transom is really necessary on these boats, so I’m now building deeper V (45 degrees) double-ended, dory-type hull to see what that’s like.

– Pros And Cons Of Planing Amas:

Fortunately, my planing amas have proven a much more enjoyable experience than my planing main hull. I built them because my original amas (tortured ply) went underwater too readily (or so I thought) when I was trying to go really fast.

Even though those old amas had about 400 lbs of flotation each, it wasn’t hard to bury them when I was using the “big rig” (124 sq. ft. from a Laser II). And when they went deep enough for the akas to hit the water, it was like slamming on the brakes in your car — a jarring shock to me, and to the equipment as well.

The planing amas definitely eliminated that problem. Not only do they each have 800 lbs of buoyancy, they also have “powerboat” planing entries and V-bottoms that provide more lift the faster you go. So in that regard, they work very well indeed.

The downside is that the high-flotation planing amas make for a rougher and bumpier sailing experience precisely because they don’t want to go under. And when you want to go parallel to the troughs — where hearty beam breezes (and maximum speeds) are found, the boat pitches rather violently as the waves roll by. Ditto for when you encounter a powerboat wake.

The old tortured ply amas, on the other hand, have a very deep V-bottom and a very gentle entry, to they never cause a hard ride. They seem to split the water smoothly, like a knife, so they are also quieter. But perhaps most importantly, the tortured ply amas (with their nearly vertical sides) add significantly to the rig’s overall lateral resistance.

This was actually Laura’s discovery. She noticed how much better her boat pointed and tracked upwind when using the tortured ply amas I originally made for her boat vs. the round-bottom foam & fiberglas planing amas I later made for her boat. In fact, she now refuses to sail with the planing amas, saying they just compromise performance too much. And with the flattish bottom of her boat, the contribution to lateral resistance made by the tortured ply amas becomes all the more important.

If I want to ever take my own boat into bigger water with bigger waves, the high-flotation planing amas will be the first thing to go. Not only would the ride be too rough, but the stresses on the boat would be much greater.

However, for the kind of sailing I actually do, the non-planing hull of No Commotion plus the planing amas (currently attached to my new sliding aka system) makes for an ideal combination. Laura’s rig is almost the opposite — a mostly flat-bottom main hull and deep-V tortured ply amas — but that’s what works best for her and her boat.

Bottom line on all of this: Planing hulls and amas do have some advantages, but also some drawbacks, and anyone designing, building, or buying a trimaran needs to be aware of them.

Tris are not at all like monohulls, and not very much like catamarans. They are unique sailing vessels and play by a different set of rules. So prospective tri pilots need to know, first and foremost, where they plan to do most of their sailing, and what they want their sailing experience to be like.

How can you find out what combination works best for you? If possible, build two very different main hulls and two very different sets of amas, and go sailing. And let me know what you discover!

Cheers – Frank


  1. Hi Frank,

    I can’t agree more with this-

    “prospective tri pilots need to know, first and foremost, where they plan to do most of their sailing, and what they want their sailing experience to be like.”

    it applies to all boats really, but becomes critically important when the subject is designing a boat, and even more so when you are really trying to take any aspect of it to an extreme, whether that be flat out high speed, or ultimate comfort, or maximum safety…there are always tradeoffs and even a boat designed for very localized conditions won’t be perfect in every situation.

    The realities of planing under sail power present a fairly rigid set of building and operational requirements that are quite often just too detrimental to other qualities that are must-haves for a lot of people- for the most part, speed potential is the biggest advantage, but the more you optimize the boat for flat out speed and quick planing, the more you suffer in terms of seahandling and ease of operation, not to mention structural issues that arise when you push the boundaries of light weight.

    But I still think there’s a lot of room for better solutions than what we’ve seen so far… one avenue that hasn’t been explored much from what I’ve seen is the Tanzanian “ngalawa” concept that uses very low volume ski-like amas and adjustable crossbeam connections that allow for the planing surfaces to be angled for different conditions and also flex enough to dissipate some of the inevitable pounding stresses of a flat planing surface in operation…

    when I saw this-


    my immediate thought was this-


    when I see those 12′ skiffs bouncing around, I wonder how being configured like this would even things out-


    -one other thing with those skiffs is that the asymmetrical spinnakers they are flying are certainly generating a lot of lift, but being so far forward it just levers the bow up and screws up the hull trim…if that lift was more centered over the hull, it would effectively make the boat lighter.

    My proposed rig for a small planing tri is an over the top tacking delta/crab claw hybrid that is essentially a tethered kite joined to a short stub mast with a universal-type joint; the point of the delta is held in place by a line and the sheets attach about 2/3 of the way back on the spars…this drawing shows one set up on a ten foot-ish tri for heavy weather conditions-


    here’s a not so great photo of a crude model with this type of rig installed- the point of the delta is lashed down in this photo but in real world conditions is tethered to the foredeck and everything can pivot around the spreader arm/stub mast connection…hard to describe it in operation, but a huge advantage is that as you let the boom out in heavier airs, the thrust of the air spilling off the thing is pushing the sail up like a kite-


  2. For anyone who was interested in the “Wedgie” class rule stuff I posted in the other thread, here’s a lines plan of one variant I drew up…somewhere along the way the dimensions got inflated a bit on Freeship, so disregard the figures or adjust them mentally to fit the 10″ rule-


    here’s another crude paper model based on this plan-


    this variant uses the two allowed ply sheets to cover the main hull, one on top and one on the bottom, with the long edges oriented to make the chine connection…it’s a flatter version designed to plane or at least surf easily.

    the model shown in my last post is another variant with the ply sheets oriented so that the long edges make up the deck centerline and keel seams…it’s the “tall” main hull version that would probably be better suited to upwind and light air conditions.

  3. Point of clarification for Ian and others:

    when trying to explain why 10 ft is the development rule here in Europe (3.05 metres hull length) and by the way, also in Australia ( see 3 metre trimaran rule, which is 3.00 metre), I was pointing out that having ply sheets 5 by 10 ft, this makes it easy to build sides and bottoms of narrow hulls without joining, scarfing etc the ply sheets.

    Considering that available measures in the USA are – I gather from Frank – rather 4 by 8 sheets, you can think of anything that need only one join to build a side or bottom panel, so you can come close to a hull length of close to 16 ft. This would make a more substantial boat, and in my experience, still car toppable ( especially with USA size cars) in the 200 lbs range total weight, with about 70 lbs for the main hull so to allow even one person alone to load it on top of a car (this is with some ropes and roof rack rollers).

    The total sail surface could be limited to some locally available dinghy. I would set this limit for my experience to a 10 sqm sail, one or even three sails iy you wish, and one spinnaker or gennaker of perhaps 10 or 12 more.

    Even the 10 square metre should be more than enough to switch to planing mode in a breeze comprised between 3and 4 Beaufort. A spinnaker would set this planing speed mode definitely at around 10 knots of true wind…

  4. Tom Raidna, from http://www.buildboats.com just sent me the following webpage. It contains some great photos, taken during the “Sail OK 2011” event this past summer.

    There are a couple of trimarans there, including a very small one built in strip cedar. You’ll find them at Jim Michalak’s Boat Designs site – http://www.jimsboats.com/

  5. Stefano wrote to me directly with the following idea …

    Hi Joe !

    As for the development class … looks like you have the equivalent in the US – http://www.nwmultihull.org/3meter/3mtr.htm

    You could cover this with an interview and perhaps launch a national building programme with schools involved.

    The only stupid thing I see here is a minimum 380 pounds. This is silly… My boat weighs 90 kgs fully equipped and I am 73 kgs … the boat is 4.2 metres, i.e. 50% longer than 3-meters tris.

    All up weight would be 163 kgs which is 355, for a much larger boat … 380 makes no sense but there could be local derogations to the rule.

    And here is the Aussie 3 metre trimaran – http://www.teamscarab.com.au/Aussie%203%20m/design.html

    Also, this discussion has been going on – http://www.boatdesign.net/forums/multihulls/international-3-meter-19222.html

    I would probably set the rule at 12 or 14 ft, crew of one or 2, and other box rules but not the damn weight and the sitting int the boat for safety and people being handicapped by weight and other issues.

    I definitely care that these people have an option to sail, but others have to have a chance to sit where in the world they prefer, move around, and trim the boat and steer at their will.

    ciao, Stefano

  6. Hi Stefano,

    4×8 is indeed a standard size for most building related sheet goods in the US, but larger sizes are available; 4×10 sheets are pretty easy to order for most quality plywood, it’s just not a stocked item at most home improvement stores (where all the great yacht builders go).

    My decision to use the ten foot size still allows for hull sheathing to be scarf-less, which I think is a big advantage for non-boatbuilders who might attempt a project like this, and cuts down greatly on waste, workspace and tool requirements and associated costs.

    I also consciously chose this size boat for the fact that it’s a more reasonable size for a first timer sailor or kid…even a ten foot tri is a lot of boat with the increased width over a similar length mono, and the quick acceleration of tris adds another element that might make something even 14′ long just a bit much for an inexperienced junior sailor.

    My personal goal with these ideas is to establish designs that are something of a trimaran analog of the Sabot or Optimist prams…easy and cheap to knock together and get on the water and suitable for turning your ten year old loose in.

    I have looked at the US three meter class rules and had the same reaction you did to the weight and cockpit/operational requirements…they simply don’t work for the type of sailing I want to do now, and for the type of sailing I remember doing as a kid learning how to sail in Sabots, when comfort was the last thing from my mind and capsizing was something you did for fun :)

    The whole trim/ballast thing is another area that seems to have been overlooked in most discussions of planing tris- one aspect of the high speed planing mono skiffs that is a constant is sailing the things as flat as possible using hiking straps, wings and/or trapezes for leverage. In the typical sit-to-steer tri setup, you are stuck with using the ama for righting moment and have to accept that added drag/wetted surface, along with the weight required to have a structure substantial enough to carry the associated loads at all times.

    I personally think it’s short sighted to accept as a given the notion that a small tri *must* be configured to have the leeward ama in displacement mode or even in contact with the water at all times…like I said before, if you look at the Uffa 10 with no amas mounted it looks like a typical mono skiff with wings, so why can’t a small tri be sailed like one, using the windward wing area for leverage via live ballast and the leeward ama staying mostly free of the water but still there to add more righting moment and planing surface when needed?

    In other words, rather than the typical tri that is sailed like an atlantic proa with the smaller ama to leeward and providing righting moment, it becomes more like the traditional south pacific proa that is like a monuhull with a windward wing/ama used to stabilize it.

    This is where I have to respectfully differ with Frank’s statement that “Tris are not at all like monohulls”…in the case of a small dinghy type vessel, the biggest traditional differences- lack of static ballast and a fixed keel on the main hull- simply aren’t there.

    So then you are left with the wing structure and amas- a wing structure is old hat on mono skiffs so that’s no difference, and if you sail a suitably configured tri flat enough via live ballast, the leeward ama becomes immaterial for a large part of the time.

    Frank is absolutely correct on his other points about seahandling, noise, pounding, wet ride, light air performance etc. but other than seahandling, when I sail fast just for the joy and thrill of sailing fast, I really don’t care much about that other stuff…

    and a lot of the seahandling stuff related to planing amas that Frank mentioned I believe can be alleviated when you don’t rely on the leeward ama for flotation when at speed…specifically the negative wave interaction, tendency to hobbyhorse or catch the ama bow and pitchpole can be reduced by using shorter amas with low displacement in the ends and moving them aft.

    So with all due respect to Frank and the people who developed the US 3m rule and all the others who favor the sit-in concept…more power to them and I *really* dig your boats, but I already have a very nice Eames lounge chair knockoff…I want to do THIS, just minus the dangerous trapeze and the trim instability at speed and the static instability and the barely manageable rig and the inevitable crash-


  7. For what it’s worth, here’s the lines plan of the ama shape I used on the “wedgie” class boats, that employs some of the concepts discussed in the last post…

    it is developed from a single sheet of 4×8 material, tortured and partially folded into shape…the aka recievers are simple tubes that the sheet ends fold over and are clamped/glued or otherwise fastened to. I’ve yet to do one full scale, but I’ve done countless models in all kinds of materials from paper to styrene to foam and it is simplicity itself- two identical but mirrored convex curves on the long sides of the sheet form the bottom rocker, two perforated and folded/glass taped curved lines form the chines…stitch the bottom, fit the aka reciever tubes, glass it up and you’re done.

    The shape is essentially that of a surfboard, just puffed up for increased volume…anyone familiar with old surfboards will no doubt recognize the various waterlines as being very reminiscent of standard longboard templates, and you also need to consider that these are static waterlines- with a slightly stern down trim like you would sail a small skiff at speed, these lines morph to something more like flat -sterned deep-V power skiff and the lifting area moves way aft where you need it.

    Like a surfboard, it’s a planing semi-submersible wave piercing design, just with the knife edge bow oriented 90º from where we’ve been conditioned to expect it on a performance tri ama…when properly trimmed this surfboard-like shape can be buried pretty deep going forwards and still not trip and pitchpole ( what surfers refer to as “pearl diving” or “pearling”), and the crowned deck, flat bottom and narrower bow make it naturally prone to want to correct upwards and forwards when the form is sunk and the buoyant part kicks in.


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