Beam Construction Methods

Would some of the more experienced builders discuss how they construct the cross beams for their Tris? I’m mostly interested in the “arched” style (like ones found on full size racing tris) but would also be interested in “alternative” methods that include some good 'ole around-the-house-engineering :stuck_out_tongue:

Look forward to the ideas… Also - if you are able, please include a pic of your solution!

well, I have pretty close to no building experience, and have not researched tri’s very much. here are some things I have seen:

for the arched, cut out a template(plywood would be best, but maybe unnecessary for only two or so beams. then using the templates and a hot wire cutter, cut out the beams from pink insulation foam. then wrap it with clear packing tape (or not). then carbon fiber it. I believe Dick L. said this is how he build his “one off” tri’s

for the cheap and easy method, use aluminum tubing(found at a hardware store) for the beams. to attach it to the hull, use a bolt built into the hull and a wing nut. the tube has a hole drilled through which the bolt goes through. This method is also easier for transportation. For a better looking version, fiberglass into the hull a tube that the cross beam can slide into. then bolt it in place.

that is my input, hope it helps. hopefully some of the experienced members will elaborate or have a better idea.

For the 1 Meter prototype “IMPULSE” - I used 1/2 inch diameter carbon tubes. They are simply straight tubes, easy to attach (slide into a slightly larger diameter tube) - but don’t meet your visual needs.

For the F-48 “WATER RESIST” design - I copied the front view of the beams full size. Spray glued the copy of the lines onto 3/4 inch underlayment and cut to the desired shape. I then cut about 8-10 strips of 1/8 inch thick x 2 inch wide western red cedar, and laminated them up by forming to the previous cut shape. You have to cut thin, or steam/soak to allow cedar to bend. I bent dry, but lost two strips due to splitting. Photo shows the layup. I used PVC plastic pipe, cut to varying thickness for clamps. These are 5/8 and 3/4 inch wide. The wider the PVC pipe section, the more resistance. Once glued up into laminated beams, they were sanded to shape and epoxied. I experienced about 1/4 to 3/8 inch spring-back when released from the form template. Also they have flex, but if it becomes an issue, carbon fiber tows can be epoxied underneath to strengthen them to remove any flex. These look OK - but most modern trimarans not only have the “gull-wings” but also have a curve forward and/or backwards as well.

I believe Andrew from Australia is in the process of building beams for some of his Mini40 boats.

You can cut the profile from Luan plywood (viewed from front), and then build up using shaped foam with glass covering to give similar look. If you want the compound bends, probably you will need to actually make a foam plug - then a mold and finally layup inside the mold.

Finally, as a German builder did, you can lay up glass, then form over a foam core.

I am part way through making the compound shaped beams. I laminated a wide enough section to cut both beams from 3 layers of high density blue foam (extruded styrene) in my “mould” - which was cut from 150mm thick expanded styrene foam (just happened to have the right size “scrap”)
The plan shape for bith beams was then cut from this lamination and sanded to a tear drop type shape (Yes, I did break them 3 times!). I then glassed one beam with 6oz directional glass and the other beam also 6oz directional glass with an extra stripe of carbon top and bottom. Both beams were placed in a vacuum bag, and put back into the foam 'mould" (the blue foam I used was strong enough not to need the mould, but placing both beams in the one bag caused some distortion problems so I used the mould.

The glass only beam flexes slightly, the added carbon on the other beam has stiffened it considerably.

I am now going to try and attach photos (first time)to this article - went OK after I reduced file size!


Should have given more details in last post - the beams are 1200 mm wide, 50 x 20mm in the centre tapering to 40 x 15 at the ends. The shaped foam weighed about 65g (just over 2 ounces for those from backward countries :slight_smile: - glassed and one side filled/sanded they weigh about 165g.(about 6 oz) I would probably go thinner next time.

Yes, mini40

The hulls are made from a block of solid expanded styrene foam and glassed. I was toying with the idea of removing the foam, but the foam was so light I felt any extra glass strength needed would probably weigh more than the foam. The hull (6oz directional glass) weighs 525g (about 19 oz) and the floats (10 oz cloth) about 385g (~14g). They have solid foam pads in any attachment areas. Loosely, a Water Resist design.

I dont think I will get it in the water fast, as I am in the process of adding hydrofoils to each float (initially each fixed @ 45 degrees to horixontal, but later self adjusting)

Also in the process of making a wing mast (glassed foam with sail track -about 75 x 20 section) in the hope that when it capsizes the mast will have enough bouyancy to stop inversion (and maybe self-right on windy days?)


Well done Jon - and thanks for adding your information. It can only help others to expand their horizons in the multihull direction.

You may also have added info to solve the question - do I leave the foam in, or remove it. With such a small difference in weight (I was surprised) it makes finishing a lot quicker and easier, since there is a stiff backing to remove glass flex when filling. Probably should add that “if” you insist on using carbon fiber, it might be logical to consider removing the foam.

Also, I have found out the Dremel tool at high speed does a wonderful job of removing foam for wooden “hard attach points” to be added. If/when you can - try to document the manner in how you attach to the floats. That seems to be a stumbling area for many. Drilling/screwing into the main hull never seems to be a problem, but for some reason, many get stuck on the arms-to-float connect.

Will you be eliminating foam in the rear of the main hull to allow radio gear? I had thought that being able to remove that part would make sense to get everything below the deck.

Thanks again - good luck with your build.

QUESTION: Is the foam “expanded” or “extruded” ? ie: Is it a bunch of little white pellets molded together? Just curious, as further thought indicates you are going to be pushing 5 lbs. probably ??? (around 2268 grams) total ssailing weight when done.

Thanks to all for their responses! Much appreciated yet I have more questions :slight_smile:

Dick - i have also seen the german website you referenced and am thinking of following his method (foam plug removed after glassing then building it up like a stick-built boat - i especially like the “glass panels” he’s using to cover the topside of the boat - clean and simple.) My two questions are this:

  1. What if i were to take a block of foam and cut out the profile of the beams, then rotate that block and cut out the plan view (while using pins or something to keep the block temporarily together). After that, i’d shape/sand this into a more pleasing shape. The final steps would be glassing it (i’m thinking strips of glass cloth laid up like wrapping a bike handlebar). Would this be strong enough? Light enough? Any comments? Not worth the trouble??

  2. (this one somewhat diverges from the initial post) I plan to do the outrigger hulls in two-part halves. What is the process for joining them together? I’d put bulkheads for structure and alignment-purposes, then just put a bead of epoxy with microballoons around that joining-edge? Would it also be wise (necessary??) to run a 1" strip down the center of the hull overlapping the crack?

quote=eikelben My two questions are this:

  1. What if i were to take a block of foam and cut out the profile of the beams, then rotate that block and cut out the plan view (while using pins or something to keep the block temporarily together). After that, i’d shape/sand this into a more pleasing shape. The final steps would be glassing it (i’m thinking strips of glass cloth laid up like wrapping a bike handlebar). Would this be strong enough? Light enough? Any comments? Not worth the trouble??
    I have photos of some awfully thin, fragile looking beams. Yet - they seem to work. Depending on the final sailing weight, keep in mind the leeward float (and cross beams) must support the center hull and the windward float if/when a gust hike the two hulls out of the water. It certainly would be worth a test layup to see what kinds of weight will cause a failure. Perhaps - just a meandering thought - you could do your shape and make a groove top and bottom of beams. Lay in a series of carbon tows (embed in epoxy) of unidirectional stuff. Then try the wrap method you are thinking about. My initial feeling is the carbon will prevent/reduce tendency of beams to bend and the glass would add structural support to the entire beam to further resist bending. Like in big boats - you build 'em until they break - and most boats are probably overbuilt anyway. I haven’t ever fooled with your construction, so the idea would be new to me as well. Might also check the web, as there are carbon or glass “socks” you can buy that slide over the core and are epoxied and vacuum bagged. They are designed to slide over a mandrel and after cure, removed for a completed tube. Most I recall were large diameter, but maybe you can find something similar that is about 1-2 inches in diameter. This would give you a woven tube as a covering. Just another thought. I can’t recall where I saw them - but maybe do a Google search on braided cloth tubes.

  2. (this one somewhat diverges from the initial post) I plan to do the outrigger hulls in two-part halves. What is the process for joining them together? I’d put bulkheads for structure and alignment-purposes, then just put a bead of epoxy with microballoons around that joining-edge? Would it also be wise (necessary??) to run a 1" strip down the center of the hull overlapping the crack?
    [i]Once you have the bulkheads installed in their location for beam attachments in the floats, you can add a light, thin balsa (or even foam) strip inside the one half, allowing it to stick out about 1/4 - 3/8 inch. Allow to cure on one half, then apply thick epoxy and slide into the other half hull. It will act like a locating pin/strip. I concur that a strip of glass to cover the exterior seam and faired into the general hull or float surface would add to the strength in that area. My old full size cat, had a lip that hung down inside on each vertical half of the hull. They epoxied this seam, filled with putty and then gel-coated it. I was always worried if the dagger board hit bottom or submerged log, it would slice back like a knife and I would have two half hulls. :scared: Fortunately it didn’t happen. I had a heck of a time getting the prototype 1 Meter bulkheads into place in a vertical split, but after some time fitting and shaping they came out OK.

[/i][COLOR=Black]In the photos, (#1) you can see I used “locating tabs” but in the end (and future) I would use full length strips (foam or balsa) along top and bottom of hull seams - (at least where they are easy to glue in) to provide a surface for the thick epoxy to join both sides.

The second photo is Clyde Jones of Ohio assembling his “NIGHTMARE” design F-48/Mini40 and you can see how relatively small in cross section his crossbeams are - they are carbon of course.

If you do go the “wrap” idea, please let us know how it went, and what kind of strength you found.

Dick, as always, you have been a huge help. I have no facilities here to do vacuum bagging but the idea is interesting – someone should try this out! :stuck_out_tongue: I will heed your advice on the full length “tabs” for aligning the hull halves.

As i begin construction, i’m running into the issue of overall size… you see i am working in our apartment and my wife isn’t too happy about that to begin with :mad: and my hot wire cutter only has a 4" reach. No i know i could get a car battery charger and some wire and with a little know-how make myself a larger one, but frankly i don’t have the time, space or extra money to spend on that project just to complete this project. I’m not going to go as cheap as is possible on this because i do want a respectable final boat, but i also need to balance the costs out (don’t we all??) That being said, if i were to go with 2" of foam between each shadow versus 4" i’d end up with a 24" +/- boat. Some simple math would lend a 36" boat if 3" foam was used (however i’m not sure this is readily available - are you aware??) Is a 24" LOA boat going to be too finicky in the water to even sail? i keep reading the larger ones are so much easier to sail? Would 36" LOA solve this? Would increasing the overall beam solve the problem? Actually, what is it specifically that makes them more difficult to sail? Is it simply the scale of the boat in comparison to small waves, gusts of wind, etc? (an inability to resist them in the way a larger boat would?)

Sorry for all the questions/rambling… One day this will come together! :smiley:

Jon - your post made me curious, so I just weighed my F-48 float for the WATER RESIST design - float only, and no glass.

At 1.2 meter (48 inches) in length it weighed 147.4 grams (5.2.oz). If I leave foam inside, I only need one layer of 4 oz. cloth (at most) to cover exterior. I will need to add some hard attachment points and reinforce areas of stress, but I feel it will be much less than your 385 gram. (14 oz) weight you mention. Now that I have given some thought, leaving foam in and a light cover versus removing foam and needing a heavier glass cover.

See photo attached for float shape and relative size for comparison purposes.


I used expanded styrene foam for hull and floats (medium density with beads slighly larger than 1/8"). I did write down the weights as I went, but unfortunately on the hull and float themselves which recently were sanded off!

I was going to remove the foam from the floats, so I used a heavy (10 oz) cloth. My memory had the shaped float foam about 135g, with glassed weight 385g - a large increase. The foam hull again from memory was about 250-270g.

If you get a good sanded finish on the foam, and use 4oz glass, you should be significantly lighter than my floats. I glassed my floats in port/starboard halves - one half at a time. The hull I glassed top/bottom halves. Both with a slight overlap of cloth. I used “peel ply” over the glass cloth to maintain a good finish and reduce later filling/sanding.

To get to the minimum weight, you have to make moulds.


Excellent and thanks - clears up a lot and no longer confused. :cool:

Tonite I think I will run some tests and some numbers to verify your/my thoughts. The extruded foam sands so nicely, so if time is spent on a final shaping with the extruded foam (household insulation foam up here) I might even be able to get away with 2 oz. or lighter weight cloth. I have some .75 oz. - but that is so fine, it may not provide the protection to bumps and knocks that I am looking for.

At present, I am thinking we will leave the foam in place for floats, but will have hybrid main hulls for future builds - leaving interior space mid to aft for radio gear. It certainly will speed up the building process, except a new “master plug” is needed for each new build if someone else comes along. We are too disinterested in multihulls up here in the US to go through the bother of building molds for production builds. You can’t earn back your tooling expenses or time.

I’ve been following this post and the various ideas people have suggested in terms of building techniques very closely. Initially i was getting the impression that solid (i.e. glass over foam) hulls would be too heavy - but this conversation seems to suggest otherwise. Tonite i began cutting out some 4" sections for my hulls. In the end it will be 48" in length - when finished shaping them i’ll post my weights as well.

I have had to rethink my position as I read Jon’s post.

In the past, I knew that factually, a foam PLUS epoxy/glass hull just had to weigh more than a hollow epoxy/glass only hull. Thus I was a proponent for hollow hulls with the foam removed and hadn’t considered the point that Jon brought forward - which I completely overlooked - that being the hollow hull would have been built with enough plies to add stiffness and strength - and with each layer of glass … and resin, weight was increasing until it may easily have weighed more than the foam and only a single, lightweight skin of epoxy/glass.

Add in that with today’s weighing confirmed at 5 oz. for the float (sans glass of course) and another European builder posting a “projected” all-up sailing weight of 2.5 kg. (5 1/2 lbs.) I realized that the added work of removing foam, might well be off set by only having to use a single layer of glass if foam was left inside. Thus, if I could manage 2 floats at 6 oz. each, plus a main hull at 10 oz. or so, I would have a lot of weight left to spread over glass, rig, sail and radio gear and still try to hit the 5 lb. or less target.

With this comparison in mind along with an actual weight to back-up my musings, I must admit I am less skeptical about leaving foam inside - and I will now go down that path to see a final weight. While it still may be less weight to build in carbon - when cost is added in for materials, time and labor - it may prove to be very economical and fast to build with glass over foam. I certainly will look at it from a new perspective… all thanks to Jon ! :sly:

Of course, I also understand there are a lot of areas where light weight can be squandered (too much resin, masts of aluminum or wood, and metal fittings - not to mention big and bulky sail winch, battery packs and rudder servos). Filling and fairing with a couple coats of paint will all add up - but then, it’s time to get serious, and consider vacuum bagging and molds for all parts like dagger-board, board trunk, rudder, and even the radio board.

The idea certainly calls for more research and study by building and testing and weighing. In the end, the quest for light weight must also be tempered by a design that sails well - and more importantly tacks well to eliminate lost time on the course trying to get the bows over to the new tack. Add in skipper experience and decent knowledge and use of tactics and perhaps any excess weight is really taken out of the equation. I certainly can attest that shaping foam goes faster and easier than gluing up strips of balsa ! :lol:

I still think the main hull needs to be hollow FG just for ease of use of internals. Now the floats are another matter. Some of the internal foam weight can be carved out leaving maybe 1/2" of foam for FG support. It will be faster easier build, but will require carving an additional foam float.

If the foam plug is removed to make a FG hull, at least when you are done you still have the foam molds to make another if you choose.

I’ve got my foam glued and rough cut. Starting to cut my templates so making some progress on mine.


Since a few of us are just starting new builds i was wondering if it would be helpful to each start a new thread, a build log, to update with progress and pictures. That way everyone can follow along and see how the other fellow builders are coming along. How does that idea sound?

Great idea. May I suggest ground rules and a first post “sticky” perhaps?

Often build logs get hammered and off topic. I know, I am one and had one.

Perhaps we could agree that the log is build info only - questions and commentary on a separate thread with reference to the “post number”. That way, the builder is free to post the build info - as well as any confirmation or clarification to questions asked outside. Would keep the thread on topic, and easy to follow. Some build logs meander and finding one specific post is very difficult when filled with questions. As example, if the log says “vacuum bagging was used” a discussion with pros and cons of how/why bagging is good/bad should not be part of the log, unless the builder actually has a reason to include - or add.

Just an idea - and while it sounds complicated, it would keep the log/blog clean.

Is this a logical choice for the BLOG feature now available?

That’s exactly what i was thinking! :slight_smile: I was just hoping someone wiser than i could establish some ground-rules to follow. Just let us know how to do it and i’ll follow suit.

I’ve PM’d Chad (Administrator) to see if one sets up a BLOG, can it be read only, or is posting comments an options that can’t be disabled. Will advise his response, unless he answers here.

I have read this thread with much interest.
Has anyone got approximate weights of the hulls,beams, etc, using any different construction methods?