by GreenLake » Mon Apr 05, 2010 1:25 pm
Wayne,
for the weight, you need to simply run an estimate. I know that one can build a Mirror, which is 11' x 5', but a whole boat, not just benches, so that it comest to about 100lbs. So you would need perhaps 50% of that.
A 1/4" plywood panel, is estimated at 25lbs, for a half box of 1' x 12' with 1' of vertical, you'd need 1.5 panels. So you are approaching 40 lbs.
Because of the way I visualize that your seats taper, I'd expect that you save enough wood to roughly offset the weight of the epoxy coat, fillets, and glass tape for the corners. Your existing seats are unlikely to be lighter than that, but perhaps not much heavier (in their dry state).
If you have 10% of the volume of 1" balsa filled with water, that's 1-2cubic ft of water, given your seat geometry....
So, if your diagnosis is correct, that you have waterlogged benches, and if my estimates are realistic, I would expect a weight savings from their replacement.
Stiffness: duplicating the existing structure should ensure that the stiffness remains the same. These plywood constructions are plenty stiff, and the corner helps.
Putting it together: since your benches are longer than a plywood panel, you need a joint. I would start with making the four panels, by joining them with a scarf joint or lap joint. Then you can cut out the shapes (I assume that they have curved edges) and dry fit them. At about that stage I would epoxy coat the panels on both sides.
Now comes the question how to assemble them into a seat bench. I would be afraid that, despite best efforts, a pre-assembled seat might be harder to fit. Here's a suggestion for an in-hull assembly method for your consideration:
First, I would epoxy a few small wooden blocks along the side of the hull and the bottom. These would serve as supports/stops when fitting the panels. They would be fully epoxy coated.
Then, I would epoxy something like a 1'x1' triangular cross section along the inner upper edge of the vertical panel. That way, you can fit the vertical panel first, then lay the seat on top. (you can epoxy one or two "stops" underneath the seat surface, so your vertical panel has something to rest against if it should flex, or you can use nails that you remove before the epoxy has fully hardened.)
Finally, you would fit the seat. It should rest securely on the wood blocks on the hull and on the 1x1 for the vertical while you glass tape the connection to the hull. After the epoxy joint to the vertical is cured enough, you can sand the seat edge to perfection, and re-coat with tape and epoxy.
Even though, with this method you assemble in the hull, you can have everything fully epoxy coated. And by using the stops you can provide some pressure/weight to firmly hold the panels into place.
The one downside of the method is that the work of sanding the edge has to take place in the boat.
With a bit of forethought, you might even be able to have the epoxy coat on the panels still be in the "green" stage when you glue them together, so that you get the strongest bond.
Also, this approach could be used to have the vertical panel slightly curved - rather than having the seat edge a straight line, it could be a constant offset from the gunwale, for example, before it starts tapering. A slight bend in the vertical panel would make it stiffer in the other direction.
OK, that was a bit detailed. I hope this can help you clarify your thoughts on the matter.
Oh, and whatever you do, fill the void with blocks of foam. That way you will have positive flotation even if your seats don't make an air tight seal with the hull... Yes, the foam weighs something, but from a safety point of view it's not optional.
~ green ~ lake ~ ~