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Unique Trimaran Dinghy – Construction & Launch (Part Two)

Here is the sea trials report of David K’s newly launched trimaran dinghy in Ohio. Be sure to check out the first post, which talks about how this dinghy trimaran was constructed.


Dingy Trimaran Sea Trials
by David Kagan

Day One — Trimaran Dinghy Motor Launch:

The first launch of my new dinghy trimaran was a “motoring launch” in the St. Croix river, a national scenic riverway. It was deep shore to shore with sandy beaches everywhere.

Ready to Launch the Dinghy Trimaran

This motoring launch was pretty successful. The boat sat nicely in the water and moved great.

I thought I might have to mess with the strut length from beams to outriggers, but these are good to go. It’s stable side to side and moving around impacts fore and aft trim only a little. Not bad for an 8 footer.

Day Two — Trimaran Dinghy Sailing in Very Light Airs:

Sailing went well for our trimaran dinghy. The boat sailed all points of sail. And it was responsive to tiller with fingertip pressure.

Sheeting in sail while ignoring the tiller causes the boat to head up into the wind. I was hoping for this weather helm, but you never know until the boat gets wet. Better than lee helm.

Light wind today, 2 to 5 mph with some brief “gusts” at 8 mph or so; thus it was hard to see how it will really move. Bows had no tendency to dive. Very comfortable seating. Roomy for an 8 footer!

Very solid. No creaks, groans or things that need beefing up.

Mediocre tacking. Took 1 or 2 tiller pushes to complete tacks. Maybe this will be better with more wind and a block system so I can sheet the main in tighter as the tack progresses.

Not sure I’ll be able to make the footwell self draining. The waterline might be too close to the sole. Very dry ride, though. No water got into footwell or even splashed up through seats.

Never had enough wind to lift the windward outrigger. The 70 ft2 sail moves the boat nicely. Something like a 9.5 or 10.5 windsurfing rig might make this really go.

I’m also amazed at how comfortable this little dinghy is to sail for an 8 foot boat. People loved looking at it when I was at the launch. They wondered who built it and where you can buy one :-)

Dinghy Trimaran Saiing Under Very Light Wind


  1. Neat job! I love the transparent sail and would appreciate more detail on the mast foot /ama connection.

    Love the entirely transparent sail !
    As for improving tacking, in such a short boat, try moving weight forward so to immerge the bow instead of the stern, this should improve things.

    Cheers Stefano

  2. Stefano:

    Thanks for your comments and suggestions on tacking. I like the transparent sail, too, but don’t like the windsurfing rig. So, this week I have built an alternate rig using transparent polytarp trimmed with red on the edges and corners and a blue stripe along the reef points. The new rig provides a little more than 100 ft2 of sail area. The stub mast assembly is totally modular, so I took it off and replaced it with a modular mast box to support a 20 foot wood mast (built in June) for the new rig. The mast box is crude because I want to make sure it works before taking time to make one that looks good. In the meantime, I bought components from McMaster-Carr so that the cross beams will fold to provide a folded width of 7 feet. Those components have not arrived yet. It’s dead calm again this morning (there’s been no wind since I launched!) so I am waiting for some wind to try this new rig out. The beams are very, very strong. The only drawback I see with them is that paint chips off them easily. I’ll be happy to send pictures of the mast and ama details. Let me know how you’d like to handle this. Perhaps I can let Joe F. know it’s okay to provide you with my email or vice versa.

  3. Stefano’s suggestion is a good one for a boat of this configuration…sadly, one big drawback of light boats with no keel or boards is that they tend to mush out sideways when tacking and lose a lot of momentum in the process- especially multihulls and especially when there’s no jib to help pull the bow around (Anyone who has ever sailed a Hobie 14 in less than strong winds will know what I’m talking about). My guess is that with more wind a lot of the problem will correct itself to some degree, but in the end the lack of any point to pivot around can make for less than optimum turning accuracy…it’s kind of the same problem hovercraft have.

    Personally I’d wait until I had the thing out in heavier airs before going too crazy with the sail area…100 square feet is a massive amount of sail for a boat this size and while it may handle it as far as keeping the thing flat, that much power in such a small boat presents the very real possibility of sailing it under or overstressing things when an ama buries or the boat pitches or rolls more than usual due to something like a gust or wave/wake.

    That’s one area where someone building a boat like this as its own entity rather than as a quick-build prototype could really dial in performance while maintaining some degree of predictablity in big airs/seas, by religiously keeping the thing as light as possible so it can be driven with less sail to begin with…typical sail trainers of this size like the Sabot and El Toro dinghies use 36-50 square foot sails and it would seem that as long as the weight was kept in the same range, a multihull of this size with a far more easily driven hull configuration than a pram couldn’t help but outperform those designs using the same sized rigs and could *really* outperform them with a more modest boost in sail area than more than doubling theirs.

    All that said, I’ve done stuff like put Laser and Sunfish rigs on 8′ Sabots just for the hell of it; even rigged up a 25 HP Yamaha outboard on one once, lol…that kind of thing can be a lot of (wet) fun so have at it; just try not to stop too fast :)

    Also if Joe doesn’t mind, I’d love it if he could help us connect off list as well, as I have been playing around with similar methods of sheet based hull development for this type of boat and would like to compare notes and pass along some pics and drawings to you as well.

  4. Ian, I’m working on a way for everyone here to connect. Stay tuned. I’ll try and email the details to everyone, especially you guys, this week!

  5. Ian:

    Funny that you mention your 25 hp Yamaha, as I thought about putting a 15 hp outboard on this and heading out. The beams are too high above the water for the shaft, though. I don’t even think a 25 inch shaft could reach the water from the beams. The electric trolling motor you see in the pictures has a long enough shaft, though.

    This boat definitely could be lighter. I think the whole thing including rig weighs around 200 to 220 pounds. Other tris in this class seem to weigh about 130 to 150 pounds built using lighter scantlings. All these seem absurdly heavy for a dinghy, but then again there are 3 hulls. In one respect, these boats have 24 to 30 feet of hull, so 130 to 220 pounds is not so ridiculous from this perspective.

    One of the things I wanted to do with this boat was to use the same ply materials available locally that I would use in a bigger version. This would allow me to see how these ply products held up. If they failed on a small scale, then I’d want to know that on a small scale. In particular, a local store has 5 ply baltic birch with exterior grade glue and no voids inside that I have ever seen. This ply is 6 mm. It’s heavier than okuome (but maybe comparable to other heavier marine plywood) marine plywood, but it’s also very, very strong and impact resistant. You can whack it with a hammer without damage whereas underlayment and other ply would fracture. It finishes very nicely as you can tell from the reflections in my finished hulls. It’s also under $25 per sheet apart from being available locally. Lightly glassed on the outside (a drawback of birch I’ve read is that it needs to be at least lightly glassed or otherwise protected) and sealed on the inside, I want to see how it holds up to assess if it will work for the larger version. I used the same stuff but 12 mm thick for bulkheads, as this is what I’d use for bulkheads in a larger version.

    To see how the baltic birch ply holds up with different surface treatments, one of the outriggers is glassed and painted with Brightsides and the other is only glassed at the keel and then epoxy sealed and painted with Brightsides. One side of the main hull is glassed entirely. The other side is glassed only at the joints. This way, I can see if the epoxy/paint alone protects the birch or whether glass is needed. From what I’ve read, birch ply can turn black on the surface when unprotected and exposed to water. But others have used this ply successfully with fiberglass sheathing.

    This is embarrassing, but I can’t recall which outrigger has glass and which doesn’t. If one of the hulls turns black, the mystery will be solved. If both hulls stay good, then I’ll also get useful information. Right now, everything looks good. I’m leaving the boat outside in the elements, so I’m not coddling it.

    Prior to building, I tested two underlayments that were said to be waterproof. These also had no voids that I have ever seen. But the underlayment delaminated extremely quickly in boil tests (a few tens of seconds) and the surface plies delaminated in short order (overnight) with mere soaking tests. I avoided these.

    Interestingly, the steel beams don’t weigh much and are a small part of my weight premium. It’s comforting to have such strong beams as well. There are fiberglass versions of these beams that weigh 30% less. The lighter fiberglass versions give up a chunk of strength in exchange for lighter weight, but the specs for the fiberglass beams are still pretty good for a little tri dinghy and might work. This would shave about 9 pounds. I think the larger version, e.g, 18 foot, would need the strength of the steel beams. But I’d have to take a closer look at the specs. I would worry that the fiberglass beams would shatter suddenly under failure, whereas the steel beams would not fail in this way.

    The other scantlings I used are perhaps heavy for a boat this size. I think I could save at least about 60 pounds next time around without having to do too much noodling using 4 mm ply for the hull and perhaps the same or 6 mm for the bulkheads. I estimate this might save about 40 pounds in the main hull and the balance in the outriggers. My footwell sole is 3/4 inch and is supported by two cleats at the ends and three bulkheads underneath. This certainly could be 1/2 or perhaps even 3/8. I might not go as thin as 1/4 or the risk of fracture becomes high. But the area of the sole is small and weight savings to be had here are small by switching to thinner stuff. It’s nice to have a sole that an elephant can jump up and down on without a hint of cracking or groaning. I didn’t not want the footwell to be a source of leaks or failure.

    But if you go lighter on the scantlings, you could sacrifice some ruggedness. I’ve needed the ruggedness on two occasions thus far. I’ve had an Alumacraft run into me at the launching ramp/dock with no damage. The guy did not even apologize nor did his beer. And to think I used a white and red paint scheme so the boat would be highly visible.

    Also, first time on the trailer in my back yard, I tried to get the boat onto the trailer myself using one of those “I-can’t-believe-he’s-doing-that-and-he-will-end-up-on-Youtube” jigs that failed, of course. The boat fell from about 4 feet onto one of its outriggers. All the weight was focused onto the midship keel region of the outrigger that hit the ground. Fortunately, there was no damage or cracks that I could find then or since. This is the first time that I realized that this boat is pretty strong. As the boat fell and before I inspected it, I thought for sure I was going to end up with a proa. The next day, 4 of us lifted the boat onto the trailer pretty easily which should have happened in the first instance if I had more patience.

    A very, very light version of any of the trimaran dinghies could be made using techniques taught by Sam Rizetta in his relatively new book on composite kayak and canoe construction. He’s made 18 foot kayaks with decks that weigh under 25 pounds and 12 footers that weigh under 15 pounds, so conceivably his techniques could produce a tri dinghy that weighs well under 100 pounds. His boats also are said to be durable and impact resistant. But the cost of the Kevlar, glass, and resin to make this happen could be prohibitive for three hulls of a trimaran. But maybe not when you take ply cost, fiberglass cost, and resin cost into consideration for a ply hull. I’d have to do a cost comparison. If the difference were small, it might be worth trying. It would be interesting to have 15 pound outriggers and a 50 pound or less main hull not including rig or beams.

    Sail area is an interesting area for discussion. I have an 8 foot pram that sails nicely on 48 ft2 of sail. I tried 70 ft2 on that little pram and that was really too much in most conditions. I don’t think a small 48 ft2 sail would move this trimaran dinghy very well, though. Indeed, looking at other small trimaran designs on the web, such as those in the 3m class such as the Seaclipper 10 or the new 3m boat from Team Scarab, most use rigs having 65 ft2 to 90 ft2 of sail. Kurt Hughes also has a 12 foot tri dinghy in this regime. Almost all of these weigh substantially less than my dinghy (50 to 70 pounds less) and have a narrower beam. Also, the volume of their outriggers is less. In contrast, when I looked at other boats weighing the same as my tri dinghy, they used more sail area, e.g., 120 ft2 to 160 ft2, but they were monohulls and usually 12 to 16 foot in length. So I thought I would try 100 ft2 and see what happens. I do have a concern that this much sail area could submerge the lee outrigger bow in some wind or chop conditions. This is one important reason why I made the center hull so tall: so the hull had enough volume to substantially avoid being a submarine, particularly when running downwind. I can stand on the outside gunwhale of an outrigger and the boat overall sinks a few inches at most. A lot of this stability is the center hull doing one of its jobs.

    Also, even though the new modular mast box built this week is stabilized fore and aft as well as side to side, I’m guessing this could be the first place where a structural weakness might show.

    Consequently, the sail has two lines of reef points (i.e., two rows of grommets) so that I can take out sail area in about 25 ft2 chunks down to 50 ft2 if needed. Also, this sail can be lowered and stowed much more conveniently than the windsurfing rig with plenty of room left to sit, so I can always motor back. Take the windsurfing rig down, and there’s no place left to sit unless you take the rig apart and roll the sail up on the water. Not convenient and the inability to modify the rig via reefing or dousing is one reason that I don’t favor it after having used it.

    This boat is strong, thankfully, as I’ve had the chance to see from the two accidents mentioned above as well as from just from sailing it. I’m glad to see this as I’m also testing using low VOC PL construction adhesive and three layers of drywall fiberglass tape to make inside fillet joints in locations that can’t be seen such as the bulkheads. I want to see how strong these are in contrast to epoxy/fiberglass fillets. The cost is about the same as epoxy and glass fillets in terms of material usage, so why do this? It’s really fast. It doesn’t take long to run fillets of caulk using a caulking gun and then embedded the drywall fibeglass tape down into the fillets.

    But, these joints almost didn’t make it to the water. 24 hours after I made the first ones, I checked on the joints. They seemed unsatisfactory. Too weak. I could have pulled the bulkheads out by hand. But, I had to get to work and it was 5 days before I got back to the shop to take them out. When I finally got back into the shop, and now that the glue had some time to cure, it was a totally different story. The joints were very strong. I grabbed one of the outriggers by a bulkhead and shook it hard. No creaks or groans. A few weeks later, they were even stronger as the fillets were now very hard yet resilient and hard to mar even with tools. So, it seems if you make one of these joints and test it 24 hours, it will fail. But 24 hours is not enough time for this kind of glue to cure up. You need to let the joint set up for a handful of days to get a better picture of whether the joint is good or not. I’ve since found out that the glue hardens somewhat fast on the outside but can remain gooey inside for an extended period. but when the whole thing hardens, the stuff is strong. So, for a small dinghy, I think this kind of joint works well but I ended up doing the fillets in two stages and letting the first stage have several days to cure before doing the second. While letting the first stage cure, there’s other building tasks to tend to so that progress still can be made. Even with making these in two stages, it’s still a fast way to make fillets that are very strong if given enough time to cure.

    The drawback of the adhesive is odor. The low VOC stuff is better than the regular stuff in that the regular stuff stinks way, way more and foams and bubbles way more. The regular stuff is about half the price of the low VOC stuff, but it really stinks for a long time. I would not use the regular stuff for this purpose for these reasons. Even with the low VOC stuff, I had to leave the shop door open during the day for several days to let fresh air in. I probably would not want to use this stuff without adequate ventilation, and I wore a respirator mask in the shop when using it and until the stink was totally gone and then some. This is probably not something to use in the cold winter when the shop has to stay buttoned up. The adhesive is also thick. A cheap $3 caulking gun is not suitable. A better unit that sells for about $19 at Home Depot works well. Also, if you don’t use all the glue in your cartridge, just set the cartridge on the shelf. A small plug near the nozzle outlet hardens to protect the inside contents. Next time you use the cartridge, you can squeeze out the small, hardened chunk and have fresh glue to dispense again. I’ve left cartridges sit for weeks that still worked again this way. So, there’s little waste.

    Would I use the low VOC stuff again? Definitely if it’s not the middle of winter so I can air the shop out. And if I can work on other tasks for a while, perhaps a month or so, before closing up the compartments with a deck or the like. Will it be strong enough to work on a bigger boat? Unknown. I might try using this glue to make fillets inside one of the outriggers on the bigger tri version. If the glue fails, only one outrigger is compromised. I think it will be strong enough, though.

  6. Yes indeed Joe can provide you with my mail address.

    100 ft of sail seems a lot, but if you can reduce it, perhaps it’s manageable. I’m putting a maximum of 24 square metres on a 14 ft trimaran which is close to 258 sqft, but the sail is split in main, jib and gennaker. The main can be reefed by one third, the jib rolled and the gennaker lowered in very little time, leaving a mere 6 sqm, i.e.25% of maximum surface. To me it works.

    I have just seen this week a batten-less windsurfer rig. It could be made reefable along the mast axis like in hobie mirage adventure tri and /or in cruz ketch topper dinghy masts. Check on those, very handy system to be out with too much sail and reduce in very little time.

  7. Stefano:

    Where did you see that batten-less windsurfing rig? That would be interesting. The Hobie Bravo has an interesting roller furling main with battens that actually roll up as the main sail is furled. The sail is just under 90 ft2, which I bet would be very suitable. The Bravo weighs 195 lbs (88kg) and has a beam of 4.5 feet (1.3 m) as a reference to how sail area is matched to dinghy size. I looked for a used rig of this type but was unable to find rig alone. I’ll have to check out the cruz ketch topper dinghy system and the mirage tri system. Thanks for the info.

  8. Hi again David,

    Thanks for the detailed response, it’s very enlightening and you seem to be well aware of the various potential issues involved with the various experiments you are trying, so it’s all appreciated as good general knowledge for any builder of this type of boat.

    While many traditionalists might dismiss some of your methods out of hand as far as the hull shape development and minimal framing methods you are using, I have come to the conclusion that while you don’t necessarily want to throw out the entire history of boat building technique, when designing modern multihulls it is well worth considering just how much of the traditional methods is beneficial and what tradeoffs are involved…

    Your heavy duty footwell sole is a good case in point- having a solid spot to jump to/from makes all kinds of sense, but in the case of something like an ama on a tiny tri like this, traditional deck strength considerations might become somewhat moot if the thing is going to sink with little resistance the second someone steps on it…same goes for the ama deck shape itself; fairly flat decks are pretty crucial if you are going to walk on them but a highly crowned one might not only save build time and framing complexity on a boat this size developed from sheet goods, it might also help to discourage people from jumping on it in the first place if it is built very light.

    This is of course how things like skin on frame kayaks and ultralight rowing shells get away with using paper thin hull and deck sheathing- they don’t need more than a cover to keep water out as nobody is going to walk on the deck in normal use…and in the case of the eight man shells, with carbon fiber hulls they are weighing in in the 210-220 lb range for a 60 foot long boat…even a shell in FRP like the Alden Star has a 21′ hull that weighs 45 lbs all up, so there’s obviously a lot of room for extreme weight savings without getting too exotic materials-wise, if a person is willing to exercise some degree of caution in use.

    In the case of a tiny tri, a skin on frame (or hybrid of SOF and sheet panel) approach could yield some pretty sophisticated shapes for amas and even main hulls at minimal cost, and could be set up to use the internal framing that the stringers bend on as a major part of the crossbeam/hull/ama/standing rigging connecting structure and could thus create significant weight and material cost savings by not requiring major parts of the hull and deck sheathing to do much more than keep water out.

    Interestingly enough, this type of idea is at the heart of not only ancient kayak design but a primary concept in aeronautical engineering as well, which gets to the point that in some ways, modern trimarans share more engineering considerations with aircraft than they do with the traditional wood planked hulls that guided the development of traditional boat framing…one of the biggest ones being that excess weight not only hampers general performance, it can actually create a craft that is weaker in use than something that is lighter and might intuitively seem less robust.

    As far as crossbeam materials, there are some aircraft aluminum alloys that are not only very strong and resistant to deflection but can sometimes be had for a fraction of their retail cost at places like industrial salvage yards and as scrap at aircraft manufacturing facilities…you might actually *want* to introduce some flex to the structure to help dissipate spikes in stress loads generated in a turbulent seaway…again, something the Eskimos figured out pretty early on and that keeps the wings of airliners from snapping off.

    Another interesting avenue for anyone building a larger boat would be repurposing some street light pole or traffic signal arms (aka truss or mast or davit arms), many of which come in composite and aluminum and are built for both high strength/stress loading and 24/7/365 exposure…and are often designed with very pleasing tapered curves that look for all the world like someone swiped them off a high performance trimaran-


    I doubt they are very cheap brand new, but I’ve seen them around on the salvage market-


    food for thought…


  9. Hello David,
    very nice boat! Is the LOA 8 feet?
    I am looking for a building plan of a 8′ trimaran.
    Do you sell plans of this design?

  10. Michael:

    Thanks for your comments. The LOA is just under 8 foot, as the length is one sheet of plywood following a curve that is 8 feet long along the curve. There are no plans as I pretty much built it as I went, having just the paper models to work from along with an idea of bulkhead placement, deck beams, etc., in mind. I have not yet had the boat and any rig out in any real wind, so I can’t fully advise yet if the performance is what I’m hoping for or whether I would recommend the design to others for sailing.

    The new rig is finished, so I hope to be on the water this weekend to test it out. If I have good wind, I might go out with both the new rig and the windsurfing rig for a comparison. One thing that I can say, that I also said above, is that the side seats are very comfortable and make the boat seem bigger than it is.

    The Seaclipper 10 is a proven excellent design that would be a good small tri to build.

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