Simple foam proa

I'm planing to use FinnFoam XPS insulation board for my pasific proa hulls. It is highly water resistant and I'm confident it won't be a problem.

Because of the size of the boards and the number of them in one package, it would be easy to make the waka
500cm (16.4 feet) long, 20cm (8") wide and 60cm (2 foot) high.
I would still have enough left for an ama
375cm (12.3 feet) x 10cm (4") x 30cm (1 foot).
I was thinking about maybe 200cm (6.6 feet) total width.

The general design should be close to something like this: (More pictures)


I'm trying to keep things as simple and cheap as possible, and I don't like working with fibers and epoxy, so I'm thinkin keeping the foam uncovered. I will attach an aluminum tube or something to the keel to protect it from rocks etc. If I like the boat I might add some epoxy and fiber later.

Questions:
1. Do you think the insulation board will be strong enough or do I need something to stiffen it?
2. Do the dimensions for the hulls and for the total width make sense?
3. Any suggestions?

 

I've recently built a small boat using XPS insulation foam, although I used it as the core for a foam sandwich.

Firstly, the foam is not UV resistant, off cuts of the stuff I used faded very quickly when exposed to sunlight for a few weeks and gave indications that it was starting to breakdown (and I'm in the UK, so the sun isn't that strong!).

Secondly, the foam is very susceptible to impact damage, even when covered with a thin layer of epoxy glass. It's good stuff to work with, but has no intrinsic structural strength on its own, so needs a good epoxy glass, or some other reinforcing fibre, layer on the outside.

If you want some ideas for shaping solid foam and covering with epoxy glass to turn it into structural members, then do a web search for the Rutan home built aircraft construction method, or search by aircraft name, like Cozy. Here are a couple of sites with info on using XPS for aircraft structure that may be of interest, the same techniques translate well into making light weight small boats, I've found: http://www.cozygirrrl.com/menupage.htm (click on the "chapters" menu item on the left), http://cozy.caf.org/index.shtml (a Cozy builders site).

 

Finn foam says it is extruded Polystyrene, so any covering would seem to require the more expensive Epoxy, instead of other cheaper resins.

If you dont like resins, the best bet is to go with marine ply with a craft of that type and size.

You could also use plywood for the crossbeams - you can make very efficient and light members without the need for specialist aluminium tubes if you want to keep things cheap.

The dimensions seem ok, based on an experimental trimaran I built. You might like to consider the Rob Denney style, with the mast bearing, low weight bearing hull being longer and thinner, and the weight carrying hull (always to windward) shorter and fatter.

 

I've been going back and forth between ply and xps, and i've decided to go with xps, because I want to be able to make smooth curves in all directions for the sides. Maybe the next boat will be plywood.

I'm going to have a crab claw sail, and maybe I'll make pockets for the edge spars, a like in Wharram's wingsail. I haven't been able to find any good resources on how big the sail should be, and more importantly, how to make (cut) one. I've found sources that say it "works best cut flat", but I still need to cut the edges so that there is a little roundness to the sail, right? And what about compensating for the bending spars on the sail? Or do I really just cut the sail to the same shape my spars form?

Polytarp Sprit-Boom Sail 101 - Todd Bradshaw. This link has a lot of great information, but not really for a crab claw.

 

If you are talking about compound curves - you will find that foam sheeting is just as difficult to get into two-axis curves as plywood.

I also wonder why you want to go 'crab-claw' for a sail.

It is a very inconvenient shape for trimming, and very inefficient overall. There is lots of discussion about this on the forum.

 

I think the idea is to use the Rutan technique, carve the hull shapes from solid XPS and sand to shape, then cover with epoxy glass. It's a pretty quick way to make nice compound curved items with a lot of structural strength.

XPS carves easily and can be sanded to a fairly smooth shape, after which you apply a thin layer of epoxy/microballoon slurry, squeegeed into the foam surface, followed by glass cloth and more resin. It needs a cloth with good drap qualities, like a crowsfoot weave, and also needs careful planning of the shapes of the pieces of cloth and where to have overlaps, but the results can be very good indeed, as all the homebuilt aircraft built using this method show.

 

That would be a good reason - but early on he said he wasn't going to use epoxy

" I don't like working with fibers and epoxy"

So, if he only uses foam sheets, then compound curves become a problem.

But then he also said
"I'm trying to keep things as simple and cheap as possible"
so maybe the SOR has changed ?

 

The current plan is this: I'll use 50mm foam and use it like stitch'n'glue plywood. The curves are so small, I can easily bend the boards that much. Then I can sand the outer shape to a smooth curve to the other direction. I can use polyurethane glue for the seams, no additional fibers needed at constructing. I'll use some old house paint for uv protection. I'll have a wooden(?) keel running the whole way from front to back, so if I crash the boat somewhere, there is a good chance it will take the hit, not the foam. I'll have some water tight bulkheads in the hull, so the boat will still float quite well if i get a leak. No matter what happens, the foam will float anyway. If I like the boat, get tired of fixing the hulls and get some more money somewhere, i might add the layers of fibers and epoxy anyway.

I've read a lot about the crab claw, and to me it seems like it's quite controversial. Some don't like it and some write texts like this: http://proafile.com/archive/article/rig_options_crab_claw. To me it looks like it would be pretty easy to use on a proa, and for an inexperienced sailor that is important. It also allows low stress rigging, which seems like a good idea for a boat made of foam. Which rigging would you use?

 

Do try a bit of a test, but I suspect you will not enjoy the results. I suspect that any 'stitching' will pull through in stressed places.

Because you are building a two hull craft, your chances of getting a workable, usable craft are greatly enhanced. A monohull needs to have carefully thought out shape and ballast, while multihulls are basically rafts, and are less prone to serious design errors. This is meant to re-assure you into putting a bit more effort into the build, as it has a good chance of working. I built a trimaran with the "this looks about right" method of design, and had a lot of fun - http://www.boatdesign.net/forums/projects-proposals/historic-home-made-tri-41690.html


To keep it simple, and considering it is a Proa, I would opt for a simple bermudan mainsail, perhaps a secondhand small dinghy rig - so that its simply a case of letting the boom rotate 'fore' and 'aft' ( on the leeward side) . Obviously the traditional goose neck where the boom joins the mast will need to be modified to some form of 'snotter', and you will need the stays to be only to windward. I would also be tempted to either bury a half metre of the mast into the hull to stop it falling to leeward, and /or a solid stay(s) extending from the centre beam to a third way of the way up the mast.

It will be a lot of fun.

I would also be glad to give you hints that make fibreglassing less repulsive to you, as a bit of useful re-inforcing will make all the difference to the project.

Hint 1 - you can clean hands and tools of epoxy with Vinegar.

 

I have also read that link before - the figures are all bull&^*t.

The editors note "Other experimentors have been unable to duplicate Marchaj’s test results ." says it all

 

Instead of resins and glass, you might try cloth and paint somewhat like they used to cover wood canoes with. It would add a little something to durability compared to plain foam and plain paint, plus the UV protection.

 

To make a proa like that out of ply would be pretty simple and the glassing and epoxy work requried would be minimal. Have a look at the thread in the boat building section called PeeroSail Build. (spelling?) by Lewisboats. The methods he used to pull the ply boat into shape were very simple, as yours could be.

Alternatively read Gougeons Wooden Boat Building, it can be downloaded from a link on this site for free. There is a section on tortured ply multihull boat building, with some thought and experimentaiton you could get perfectly good shape for a proa using their method and very minimal materials.

The problem in building anything from foam is that the strength has to come from the glass sheathing on the inside and outside surfaces. But the big issue is attachment of rigging and providing enough skin strength provide for the local loads at mast step, shrouds, crossbeams etc. If you dont sheath it in epoxy and glass, it will fall apart as soon as there is any load at all.
I concur with RW, get a simple bermuda rig, there is a tonne of 2nd hand rigging and sails around if you know where too look. If you have experience and want to develop something new then go for it. But if you are new to this stick with the tried and proven, there is enough to learn building a boat let alone trying to get an unproven rig to work.

 

I agree with the above, except there wouldn't be any internal glassing if the hulls were built using the Rutan method, as there'd be no internal surface to glass. Getting strong and stiff attachments would need some care, though.

By way of showing that you can make good small boats using XPS and epoxy glass, my Duck Punt build, here: http://www.boatdesign.net/forums/boat-design/duck-punt-twist-42466.html uses cheap XPS underfloor heating insulation foam covered in glass cloth and epoxy. The mast step was just reinforced with a bit of carbon fibre to get the needed skin stiffness, as was the stem.

 

yes that would work fine.

You may also get some satisfaction using cloth stitched tightly over the foam, say heavy duty calico painters drop sheets with lots of 20lb nylon fishing line, as previously suggested. Heavily painted, it would be waterproof enough.

In the end though, for the slight increase in expense, and a bit of a learning effort, epoxy and fiberglass cloth would create an object with 10 times the usefulness, 20 times the strength and 100 times professional appearance.

 

Maybe I need to get some epoxy then

I found the Gougeon Book, and at quick glance it seems like something to read, though I didn't find the word "tortured" in there. (Edit: The tortured plywood technique is under the name Compounded Plywood Construction on page 307.)

Maybe I'll see if I find any cheap bermuda rigs from here. The figures in my crab claw link seemed too good to be true for me too, but those are not the only reasons why it seems appealing. It maybe more inefficient than a bermuda rig in some situations, but I wouldn't call it unproven. Anyway, I don't need to decide the rigging just yet. I'll see what I can find. How do the traditional proas with a tilting mast prevent it from falling on the smaller hull if the wind suddenly changes directions when running downwind?

The deck is currently flat, because the boat will be easier to build if held on the ground upside down. Kinda like the PeeroSail.

My plans for the main hull so far. This is what it will look like before sanding the edges off. If the waterlevel is 20cm below the deck, I get about 200kg of buoyancy. At 10cm below the deck it's 300kg. Me + my girlfriend = 130kg, so I think it's enough. Any comments on the hull shape?