Design of wood epoxy trimaran beams

Hello,
Here's my input . I have a Newick 30 that is 26' wide and demountable, about 2500 lbs. The plans show main hull attachment similar to cakewalk, but waterstays were added after the initial build. I plan on building new arms at a higher point off the water and similar to these for a good low tech, cheaper and easy to build.
Tim

 

Ive had a good look at your plans pictures now, Tim those beams are so light with only 4mm ply and small corner cleats no wonder waterstays were added, but it obviously work just fine now but thanks for posting them up

 

Will you make your beams sealed or open at the ends? I remember the Piver tri's having the beams open, but with epoxy, I guess sealed is better if you can make sure that water never gets in.
On the little Humdinger tri, one could lean over the edge when driving hard and sight through the beams to insure the sea stay tuning was good.

Bulkheads every couple of feet are necessary. I guess limber holes in the lower corners would be a good plan and maybe a small port somewhere just to be sure there's no water. I use plastic deck fill fittings (for filling water or fuel). These are big enough to see through and fish a rag taped to a stick through, but have a good o-ring and seal well. The spigot part (where the hose goes) can be cut off.

 

Some good ideas Russell. I made up another section , larger at similar weight

 

I like the rectangular cleats better. Without uni fiber on the outside, those cleats are what take the loads. The quality and size of the cleats will help determine the ultimate strength.

 

In an old AYRS publication, I saw mention of sprung plank beams for multihulls. Cut two identical plywood planks, tapered if you want to. Install a wooden slat with a right-angled triangular section on each edge of one of them so that the right angle is on the outside of the plank, and the hypotenuse of the triangle on the inside. Lay the plank with the triangular edges down and right down the middle, install a vertical shear web out of plywood. (Would a plywood shear web be stronger if it were cut so that the wood fibres were angled at + and - 45 degrees instead of 0 degrees and 90 degrees?) That shear web needs to be just high enough that when you put the second plank on top and bend them both, they make a 90 degree corner on each side (or whatever other angle you gave that triangular slat) and you can glue the planks together along their edges. A section through the beam has a lenticular shape.

The people discussing this way of making beams liked them, but did not mention any scantling rules or equations for engineering. They were building small boats, and perhaps those were cheap enough that they could afford applying an experimental approach.

I would check whether the wood's modulus is similar enough to glass that it makes sense to let them share the load, or whether glass is only good to keep a slightly thicker layer of epoxy on the wood to protect it better against water. I find that epoxy without glass eventually cracks. Once I noticed in time, once not. I read that WEST System epoxy supposedly has a maximum elongation (is it 3%?) that is less than the maximum movement of wood as it takes on and sheds water vapour that still goes through the epoxy, and that is what makes it crack. In my experience, a thin layer of glass helps, though if I ever use epoxy on plywood on a structure that is supposed to last, I would look for a less brittle resin.

Then again, there are vastly more experienced people than me here who may correct me.

 

Is Bell Bay Aluminium a possibility Andrew?

 

At Bell Bay they only make Aluminium they don't sell it, that done through retailers like carpal. Most of it is probably exported .but good thinking
I'd prefer to go wood epoxy composite, I've got most of the materials on hand ,just got to sort a design

 

Contact Kurt Hughes at multihulldesigns.com he has some trimaran designs of similar size with demountable wood epoxy beams. In my experience he will sell just the parts of the drawings you need on request i.e. beams only and can consult on how to adapt them to your design.

My advice to you in regards to demountable beams is cantilever type non waterstayed beams are easier to setup. Waterstays are a nuisance to get the tension right as I noticed with my Tremolino, with a non waterstay beam you put it in position and attach it, that's it.

 

Adrian and Russ,
I don't have any pics of my boat together, that hasn't happened since the early 1990's. Here's some of how the boat is now. I can't get to the boat now because of deep snow, but I estimate the water stays to be around 9mm. I did talk to Dick Newick once regarding my boat and he said that making the vertical plywood 5mm 0r 6mm would have made the waterstays obsolete. The boat was pretty floppy with the current set up and the low beams, I'm hoping that raising the beams will help the stiffness. Have you made a decision which way you're going to build yet?
Tim

 

An update on progress:
I brought 2 sections yesterday both over 6.2mtr there anodised as well but have quite a few 4mm rivet holes and couple of bolt holes as It was once a mast . The section is 185x 135mm and 3.3mm thick and weighs 4.5kg per mtr ,at total of $150 it was to good to ignore . So I'm looking at tidying them up and using them as beams ,maybe painting them as well since the anodised looks a little shabby .Regarding strength in way of comparison the bucc 28 uses 140 mm tube with a 4.77 mm wall thickness and weighing 5.5kg per mtr,mine are lighter weight and thinner in the wall thickness but a larger sectional size ,so hopefully they'll be ok.

 

Hi Adrian,
I've been reading your thread here it is very informative,
do you know what alloy are the masts? Do you plan to do something with the bolt and rivet holes

thanks

 

Hi Ivan, no I don't know what alloy it is but the mast was built by a professional so I'd assume it a suitable alloy/temper
The majority of the rivet holes are from a now missing external sail track I either put a rivet in them or I'll use them to fit another track or saddles for the lashing of the trampoline. The bolt holes I'm thinking I could could put a soft pad eye in and pretend it was meant to be there,the mainsheet track will cover some too.
After cleaning the sections up yesterday I weighted them ,the finished weight of each beam should be 28kg at a length of 6240mm ,maximum beam of the boat is 6660mm and length of 9100mm .At 28kg per beam I'll be able to handle them fine with help from my wife ,

 

hello Adrian,

sounds great, wish you good luck and looking forward to updates.
Have you decided about the design characteristics for the attachment of the crossbeams to the amas?