that is the diference between plywood and fiberglass?

I am intrested that is the diference in weight beetwen marine ply and fiberglas (the same strenght)?

 

You will get a lot of answers
A plywood hull can be half the weight of a traditional grp hull.
You have to look at e-modulus and density.
For small light boats the local stiffness is often the dimensioning factor.

 

The Gougeon Brothers present a very convincing case that the question is not as simple as that.
According to their testing an equal weight of fiberglass composite is initially stronger, but after being exposed to repetative stresses becomes considerably weaker, as compared to a wood/epoxy construciton.
Of course ply/epoxy is considerably stiffer for a given weight as compared to an uncored glass/resin layup.

 

I think plywood over stringers can be almost as light as a typical cored sandwich.

 

Plywood can flex further for a longer time and distance than GRP can with out breaking.

But wood rots,

For the "ultimate" in strong & light , look to small aircraft .

FAST FRED

 

You have a tough time beating plywood. It's an incredibly good building material. The last time I looked at the Gougeon brother's book (which details thebe's experiment) It didn't look like the second moment of area was the same for both test peices. obviously, testing in bending, this will give silly answers.

Fibreglass (and Caron) do have some redeeming features over plywood though...
1) It's very easy to mould.
2) It's very easy to repair.

Most boat hulls can be made of ply, stiffeners usually need to be stripwood, and that's where the weight starts to go up. Keep the stiffening light and you've got a very stong, light structure.

One of the biggest problems with wood is that it rots as Fast Fred said. so, as soon as you've laminated the hull and sanded it smooth, put a coat of thinned (75% dope to 25% thinner) cellulose dope (available from model shops in small tins(is rather expensive)) on, then sand the surface back and put on a coat of full thickness dope. Ideally you'd like to start at 50% thinned and work up to 100% dope over 4 coats, then do another coat at 100% dope. That would be expensive. Once you've done that leave it to dry fully for a while (few days), then start finishing the surface with glass/epoxy until it's beutifully smooth. The inside of the hull should have the same doping treatment.

The cost of the boat will go up, but the life of the boat will go up too. It won't necessarily stop rot, but it will certainly slow it down. The primary advantage is that the plywood shouldn't be as keen to delaminate, as the water will have a very tough time getting to the glue joints.

Tim B.

 

Strenght is a word that has no meaning unless you specify some parameters. For example, do you want abrasion, puncture or bending resistance. Do you need a smooth panel or can you have nerfs? A solid wood (not plywood) lapstrake is one of the lightest possible boat structures.

 

All building materials have good points and bad. Wood and wood products (plywood, etc.) are the building material of choice around the world, hands down. The reasons are complicated, tied to economics, utility, availability, renewablility, familiarly, commons skills, commonly available tools, etc. This combined with the physical properties of plywood, make it very difficult to beat as a construction material, in pound for pound comparisons with other materials.

Tim, we've long since moved past dope for sealing out moisture from the cellular structure of wood. If you're going to use epoxy anyway, there is no need for the additional pre-treatment of dope. Epoxy (more correctly, penetrating epoxy) will easy out perform dope in this regard.

I disagree again, that one of the biggest problems with wooden construction isn't rot. It's routine maintenance. It's not done any more and this is what promotes rot. When I was a boy, I watched my friend's father next door, wipe down the morning dew on the little bits of bright work he had on his power cruiser, with a chamois. He was performing routine maintenance, so he wouldn't have to freshen up his varnish as often as others and it looked better longer. Gone are the days when an owners, first job when picking his yacht up at the mooring or slip was to go below, sponge and bucket in hand, and empty the bilge. Rot needs the conditions to be just right and you can defeat this with a little method and maintenance. Yes, rot will get in sooner or later, but good maintenance schedules and proper care will prevent major issues from becoming a surprise needing attention.

We live in a world where we lock our cars, houses and boats tight as drums. We use the car and house often enough to keep it in good order, but the boat gets the shaft an has to suffer considerable neglect, before we even show up at the dock, let alone do any real maintenance. This wasn't always the case and isn't the case with me and my lot of vessels, but for the extreme vast majority of pleasure craft in the world, a sorry hand has been dealt it, unfortunately. Wooden craft can tolerate the least amount of time in this neglected state and as a result has an undeserved reputation for decay.

 

I am a metal freak. However, that said, nothing touches cold moulded ply, strong and light. Would prefer this over any fibreglass boat...

 

I quite parshal to plywood/epoxy myself and if you use a lightwieght cloth over to keep the abrasion away, coat the entire boat ( inside and out) 3 coats and painted and you take the rot factor out until you break the surface of the epoxy woood connection. Another factiod is Ester's never stop hardening where epoxy reaches a certain cure point and thats it, it does not cure any more.