Should I stick with plywood or go for foam core?

I would much appreciate some advice about what materials I should use for my first boat project.

I want to build a small trimaran like e.g. the W17 but I am very much in doubt if I should use epoxy and plywood or epoxy and foam core or someting else? I live in Denmark where the climate cold and wet and I prefer the kind of boat I am used to which is gelcoat coated fibre glass that can be left alone without worring about standing water in cockpit and such. I plan on having the boat on a permanent mooring during the summer.

Can I make plywood totally waterproof or should I use foam core or something else?

Since this will be the first boat I build I would prefer to build it exactly as specified wich would be using plywood but I just worry that the boat will rot.

 

First of all you should build in a material that you are comfortable to handle. If that is wood, than building in foam core at least means a learning curve. Some details just are different.

Second, what you are describing is a polyester boat. Keep in mind that even polyester boats suffer from water ingres, algae growth, and just getting dirty. A one off even suffers more, as most likely you will have no gelcoat, but PU paint. So the least you can do is design such that everything is draining water.

A well built wood/epoxy boat should not suffer much from water damage. Same for a foam cored boat. It is the "well built" thing that is the most important factor. Doing things the wrong way, means it can go catastrophically wrong.

My PERSONAL choice would be for foam core, but from your point of view things might be different. Building methods could be drastically different.

 

Deagle,

I would like to know which is better plywood or foam core. But, as much as I have read so far, I am still not sure.

Some plywood is actually a natural foam core - balsa, or cork. And some ply uses polystyrene foam core.

So, from what I do know about the materials. It seems that the purpose of 'core' material is not 'strength,' but support. The thicker the foam, balsa, cork, or wood core, the stiffer the under material is. The skin then multiplies the strength of the underlying material - most of the strength of the skin comes from it being a stronger material and in tension over the surface.

Having said that, I think I marine grade birch or okoume plywoods are about the best out there.

OK, what I wrote was general, you will need some of the experts to explain what it all really means.

wayne

 

Plywood is a wonderful material, but it eventually rots and there is little that can be done to totally prevent that. If the boat is kept out of the water most of the time it's a great material. But if the boat is larger and is kept in the water all the time you would be better off using a synthetic core material like Airex, just because you won't have the rot issue.

And yes, you can totally encapsulate the wood and it might not rot, but I doubt that it can be done perfectly enough to totally prevent it.

 

The Gougen Bros 30+ year old catamaran might disagree with you - and dozens of other old plywood/epoxy boats around the world.

There are plenty of problems with foam, mostly caused by problems in construction true. Its far from a total problem solver, as well as being a lot more expensive to build.

Check out
http://www.yachtsurvey.com/core_materials.htm

 

If you can get past his terrible writing style, you can learn something.

But, he not only meanders without purpose, he meanders with a lost purpose.

 

That was a lot of great input. Thanks a lot all of you! I can't say that I am convinced of either wood or foam but here are the areas where either fibreglass and epoxy coated plywood or foam has the upper hand as I understand it after all that reading

Price: Plywood
Ease of build: Plywood (this is a biggie for me as a first time builder: I can build it exactly the way the designer had in mind if I use plywood)
Ease of maintenance: Foam Core (provided your build quality is good and this is not easily achieved for a first timer)
Weight: Maybe foam but probably not a big difference on a 17 foot boat
Stiffness: Foam but probably marginal too
Toughness on impact: Plywood

After writing that list I think I am leaning towards plywood

Ole

 

Deagle, you can build a plywood boat that will last for years if you do it right in the beginning.

The general idea is to isolate the wood from water, particularly fresh water. You can do a creditable job of making that happen with the WEST (wood epoxy saturation technique) and subsequent paint. It is not an absolute but it can and does yield a structure that will resist rot, soaking, and weight gain for longer than you are likely to keep the boat. It is not hard to do and adds only a moderate amount of initial cost.

 

the shape of your boat is a consideration. Foam is versátil , plywood wont conform to many shapes. I would save more money, study resin infusion then go with foam.

 

I guess the other thing is, do you plan to go on and build lots of boats ?

If you are just doing one or a few, then the simple, non-time sink method of plywood / epoxy will get you there with less learning curve.

If you plan to make a career of boat building, then its worth learning foam and infusion eventually , as that is far more suited to commercial production techniques.

If this is your first boat, perhaps starting with plywood is the way to go.

Also, since you live in a cold climate, a trimaran is a fairly wet, cold way to go.

 

I think it would be quite difficult to get a composite panel to be versatile enough to be left on a mooring for season after season. It would be fine if you wanted to spend a lot of money building a boat that you planned to mollycoddle, but it is fussy to try and work out all the reinforcements you would want in the takes-care-of-herself version. The hull shell would not be lighter than a ply panel hull of equal durability and you might have trouble finding core that is thin enough for the application. Composite panels in the interior might be put to better use. If you seriously want to build one that can live on a mooring, I'd use Meranti marine BS1088 ply. I'd coat the panels with a very hard epoxy resin, let it kick off for a day, and then drape with 10oz woven cloth that has a very fine weave wetted out with ordinary laminating epoxy, Or perhaps two drops of 6 oz deck glass that is even flatter and easier to completely fill. (You can actually get the glass off the boat without marring the plywood if you do this.) I'd run a drape of 4 oz stuff on the inside on the hulls as well. This can be squeegeed over that same, hard, neat epoxy seal with bubble filled laminating epoxy. Wrap inner over edges and lap an inch, then fit hardwood cap piece over all edges. Hull to deck joint is cap piece to cap piece, which is then wrapped in glass tape. Boats get up to all kinds of mischief on a mooring. Better would be to plan on 12 days in and 2 days out for the summer.

The W-17 looks like a fine little boat that wouldn't find industrial strength hulls objectionable. Cutwaters should have at least 40 oz of glass built up one way or another for mooring duty. Chafe happens. What does the designer list for bare hull weight? And does the designer have any ideas as far as rigging a mooring bridle? Not all boats take to a mooring. Some cannot be made to sit quietly no matter what you do.

 

There is no benefit to use foam in this manner on this size boat. If you do not seal your core well from moisture intrusion, be it plywood or foam, the structure will fall apart eventually. the core material must be protected from moisture intrusion no matter what you use. Foam will eventually rot, mold and decompose it if gets saturated with water, just as much as wood.

You still have to seal the foam from the moisture as much as plywood, there is no benefit in terms of durability to use foam core over plywood. Durably comes from construction method and the quality of build. Both have to be well sealed and well maintained to preclude moisture intrusion into either core material.

That said, plywood is usually far less expensive to build, easier to work with and can weigh less that foam cored fiberglass. Plywood contributes to the primary structure, while foam has very little structural properties. Because of this you will need much more fiberglass on the outside to make it the equivalent strength, adding costs and extra work, as well as weight.

Unless you are planning on using costly carbon fabrics and vacuum bagged or epoxy infusion construction process, the much simpler plywood and fiberglass will weight much less and cost much less.

For example, in a similar size hull, in sea kayaks, production hand laid up fiberglass over foam sandwich hull, vacuum bagged and hand finished, for 17 ft single sea kayak, will weight about 45 to 60 lbs (depending on the accessories, hatches, seat style, etc) as compared to an equivalent hand built marine ply stitch and glue with a single layer of fiberglass and epoxy (hand laid up, no vac bagging), and several coats of clear finish for UV protection, will weight in at about 30 to 35 lbs, similarly equipped.

The only way you will get lighter than about 30 lbs for a 17 ft sea kayak hull is to go to exotic and costly materials and construction materials. Unless of course you go to skin on frame, using no fiberglass, it will weight about 25 lbs. But this method of construction has certain limitations on hull shape and it limits interior volume because of the frame, and durability can be a problem where moisture gets trapped between the skin and the wood frame.

For what you are building, I would go conventional stitch and glue fiberglass over plywood. It will be much more forgiving, a more common way to go, it will take far less tools and special tooling, cost less, weigh less, and come together much faster than using foam core. There is no benefit to using foam core on such a small amateur built project. It would be a mistake to use anything except plywood and fiberglass.

 

Check out the foam on the NZ Americas Cup Boat after impact.

They are super lightweight, but a plywood wouldnt have suffered that much damage.

 

The seeming preference for plywood over proper (read PVC) sandwich construction on the forum continues to puzzle me, you have a clean, seamless construction that is virtually free of framing in some cases, opposed to something that is all joins and framework, it is just a no-brainer, except the foam sandwich one-off is more difficult to execute.

 

The problem with foam is cost. Plywood makes a great boat at much lower cost