Design of wood epoxy trimaran beams

[gonzo]

Have you looked at marinas for old or broken sailboat masts?

 

[Adrian Baker]

As a reference to future readers the 2 pictures attached are, pic 1 the seaclipper 28 beam this is a counter lever beam therefore with no waterstay 140x19 flange top and bottom in Doug fir or similar with 9mm plywood webs each side 250mm deep
Pic 2 is Thomas firth Jone's hummingbird 28 trimaran which did I think , 2 trans Atlantic circuits it's an I beam also counter levered it has a 18mm plywood Web with a 38x38mm wooden flange 1 on each side of the Web and the same on the bottom. Interestingly both beam are the same in material use just arranged differently an I and a box , but the seaclipper box beam I'd say would resist torsional loads better . Wood beams will work fine it's just a compromise in weight and stiffness imo. Both beams should actually weigh the same at 5.7kg per/mtr compared to the aluminium tube of the Buccaneer 28 at 5.4kg per /mtr

 

[oldmulti]

Banjansailor. Sydney to Hobart official weather report was "vague". There are several lows moving toward a high. Result light to moderate winds for the first 12 hours but after that it depends on how long the high sits or if a series of lows push in hard. So it could be an OK start then an uncomfortable result especially for slower/smaller boats. I hope all goes well.

 

[fallguy]

My cat uses Sparcraft S830 mast section for crossbeams. I purchased a full mast back about 2017 before freight went nuts. I had to get the mast cut down into four sections versus shipping it at something around 54 feet long or so. Freight was $500 then. Might be double or worse today.

The notion of finding a boat scrapyard sounds good on paper, but scrapped masts usually end up melted rather quickly I've found. However, a phone call or two might be wise.

The biggest issue with plywood is it can rot; so I recommend a glass wrap. Almost all the unwrapped plywood we see on the plywood boat forum I belong to ends up checking, splitting, and rotting; especially when the plies face up. Wish I could offer you more, but some combination of ply and carbon might be best. That way, the ply is the former and the carbon seals the ply from rot and then lightens the overall.

You can also model the thing and get pretty close to accurate. I modeled the roof structure on my cat. Basically, build a scale model and do some testing to determine E, etc. Might be hard to do on the vertical, though, so a NA might be best.

 

[rxcomposite]

It is a rabbit's hole really.
1. You have to know the righting moment.
2. You have to calculate the force of the riggings. This constitute the initial load on the mast and crossbeams. These forces can sometimes exceed the weight of the boat. Try the Skenes method.
3. You have to calculate the moment of inertia of the tri body. That is the total displacement, the waterplane area of the ama and the main hull and the distances.
4. Calculate the VCG of your boat.

Everything will be downhill afterwards as this is a simple cantelever design and probably built out of marine plywood and screwed lumber for the caps. The sides takes up the shear load and the caps handle the tension/compression.

What is not clear to me is you said "wood and epoxy". Do you plan on gluing the pieces together?

 

[gonzo]

There are plenty of multihulls with wooden beams. However, the successful ones were designed as wood and not aluminum. Wood is fine, but will have a larger section and be heavier than aluminum. Compare a wooden mast with an aluminum one for the same boat design and you will see the difference.

 

[Russell Brown]

Old multi has been super helpful to me when trying to design things like folding systems and figure out scantlings for strip planking, so there must be some designs buried in the pages that would be helpful.
I have zero engineering skills, so I'm often looking at the lighter structures that work for clues to what could work for me. Wood/epoxy is very forgiving and I've seen impossibly light things work fine. Some of them are still going after a very long time too.

Beams with sea stays don't have much load besides compression. The boat I'm building uses sea stays, but has quite high freeboard, so good angles on the stays.

The 25' tri in the photo has reject mast sections as crossbeams and has sea stays. The aft beams are Melges 24 sections (very light) and the fwd ones are Mumm 30 sections. All 4 beams and wires weigh 45 lbs, to give an idea of how light they are. One of the aft sea stays was dragging behind the boat after a very rough sail, so I'm very lucky the boat didn't fold. There must not be much load on beams with sea stays either. The stays and connections have to be bigger than you might think, but the loads on the beams maybe aren't so much.

Obviously, it's best to have an engineer figure it all out, but some of the bigger screw-ups I've seen came from people with formal engineering skills. Sometimes the "educated guess" works as well or better.

 

[Adrian Baker]

Thanks Russell, oldmulti has done amazing job compiling all that info. I've looked though it quite abit and surprisingly ,trimarans with seastay aren't all that common, let alone with wood beams so I haven't really found a good example close to my boat other than the bucc 28 with aluminium tube my feeling is the beam design style of the seaclipper 28 and 34 would work fine although they are optimised for counterlever loads with the top flange in compression and the lower flange in tension . With a seastays it takes all the tension loads with the beam taking the compression load only ,broadly speaking .I'm not quite ready to start building beam so I'll ponder some more on the idea

 

[oldmulti]

Adrian, I am NOT suggesting you follow this design concept even though it was done with the help of the Gougeon brothers as the cross beams were in need of upgrading when this tri was being resold. Adaigo was considered a little heavy so they developed a light weight version, “Cake Walk”, in cooperation with an owner. The tri that was built is 34 x 26 foot that is claimed to be 600 lbs lighter than Adagio. Umm? Then I started to see the lack of a cabin space, low free board, small floats and the structure of the crossbeams. Maybe a 700 lbs boat weight may be possible. Lets look at the cross beams. The cross beam has 3 mm ply across the majority of the beams (doubled at the main hull) with 12 x 19 mm framing about every 150 mm centre lines. This is a very time consuming build. The tri needs a crossbeam repair which the current owner has requested someone to do. No volunteers came forth. The cost of this work would be large. The jpegs of the tri and beams give the idea. Remember, if the numbers are correct "Cake Walk" has a lower righting moment (about 70%) of your proposed trimaran.

Yes, you can go very light in a wooden beam build if you use aircraft quality plywood and exotic build techniques but why? The beam here suits a tri built for specific conditions and require constant inspection and maintenance. Keep it simple, you appear to be going for a reasonable cruiser racer that can be a practical boat. Don't be lured by very light weight just because you may have to disassemble the tri for trailering. Design a folding beam system for that. 25 foot plus floats weigh more than most beams and are pains to move around if they are not controlled during an assembly/disassembly by folding cross beams in some capacity.

 

[Adrian Baker]

Thanks for your input OM, your dead right regarding simplicity thats what I'd like ,I only plan to demount the boat every couple of years and keep it on my mooring the rest of the time,moorings are really cheap here ,$70 per year (I own the tackle ect and rent the spot from the gov.) I live really close to the mooring and plan to disassemble it at the boat ramp where there's heaps of space things are good like that in Tasmania ,
I'd like the beams to be under 50kg ideally so two people can lift them, the floats are 110kg I've finished them, I use a 2 wheeled trolley under the centre to move them around which I'm able to do easily, I'll do the same with the mainhull but with 4 wheels I haven't weighed it but I think about 250kg +- I haven't glassed the outside yet ,but I can still lift one end just, it's very narrow at 830mm wide and 1200 headroom with one bunk . It's really hard to get a good photo but I tried

 

[oldmulti]

Sydney Hobart official forecast from the Bureau of Meteorology less than 24 hours out.

• BOM said there is "uncertainty" to the Sydney to Hobart forecast on Boxing Day
• Rain, wind speed and thunderstorms could create dangerous conditions to the race

The start could be in sunshine or thunderstorms. The most dangerous time will be half way down the course when crossing Bass Strait where storms and heavy weather are anticipated. Big boats do the trip 630 mile trip in 36 hours, small boats take up to 10 days if the weather is bad. There are two 70 year old ladies doing the trip in a 33 footer in the double handed version. Makes me feel old.

 

[Adrian Baker]

After some more thought, I think this design has promise,not as light as I'd like but no one wants broken beams. The idea and ply/wood configuration is borrowed from Richard Wood's beams . The heavier flanges are to take the daggerboard loads

 

[Russell Brown]

Your tri looks great, Adrian. How long is it? How much do you think it will weigh, rig height, beam, etc?
I'm more a fan of a plywood box with big, high grade cleats in the corners to take the loads and a plywood bulkhead every couple of feet to keep it from "going out of square".. That being said, my proa beams have had solid lumber top and bottom surfaces, but they had some loads to deal with without sea stays.

 

[Adrian Baker]

Russell ,the tri is 29'6" long ,22' beam weight I'm hoping sub. 1ton the floats are 110kg there 29ft long ,the beams I was hoping to get in at 50kg . I've planed on a max displacement of 1.5 ton,mast height 36'. Sail area main and jib 440 sq ft .I also do like the beam you described Its the other contender see the attached picture.
Blue bell, yes I'll be rounding the corners and 2x4 are actually 3 1/2 x1 1/2 , not spruce but ,around spruce density but. No spruce in Australia

 

[Adrian Baker]

I made 2 samples it's always good to get a feel for it .
The first one is 170x170mm o.d. ply box with 38x38mm in each corner
The second pic is 200mm high 158mm wide o.d
with 140 x38 flanges .9mm ply webs
These sizes are a bit random, but in the ball park .,from looking at other beams used on other similar boats