1st post, looking for advice

[earle]

Fair enough. Is 1/2" plywood sturdy enough to carry the weight of an average size guy? Not trying to be a wise guy, I just have a hard time envisioning it being heavy enough to carry a load spread over 6'. Would certainly simplify things tho.

[SkunkBoat]

You are going at it from the top? How many squares?(The deck is made of squares of plywood). Pull off the carpeting if you haven't already an you can see the squares.
if you have the option of working from below.
you can cut off either the top skin or bottom skin. NOT BOTH!

Top means you have to fair it and paint and do pretty work. But its easy to access.
Bottom means you can do sloppy work but its a hard angle.

Bolt in a peice of angle aluminum across the bottom to reinforce...easy...

[earle]

I"ve gone at it from the bottom. Cut out the lower skin and removed all the plywood squares. Just finished grinding the excess epoxy off today. Gone as far as I can until Tuesday, when the stores open again. Pick-up the fibreglass etc and start stage 2. Never thot about a piece of angle, but makes sense. Thanks

[smokeonthewater]

You are building a bridge, not a concrete slab... The 1/2" ply sheet is only the deck of the bridge... The cross braces on edge are the framework.

If you have any doubt, build a braced test plank...

[earle]

Originally Posted by smokeonthewater

You are building a bridge, not a concrete slab... The 1/2" ply sheet is only the deck of the bridge... The cross braces on edge are the framework.

If you have any doubt, build a braced test plank...

Never thot of it that way, but makes sense. Going to make a mock up tomorrow and see. Thanks

[smokeonthewater]

Keep in mind that in the boat it will be supported on almost all edges and that the glass will add a lot of strength.... I'd estimate that the finished product will be about twice as rigid as the test plank.

[scook]

Like Smoke says, it's a bridge. Any bridge, or simpler to visualize, a beam relies on tension and compression. When you stand on (load) a beam, the downward force tries to bend the beam down. That makes the bottom want to stretch and the top want to compress (get shorter). Fiberglass is is both hard (resists being compressed) and the fibers are strong (resist stretching).

Like he says, when you glass the bottom, particularly the support stringers, the reinforcing in the glass will add LOTS of strength. You do also want to adhere the plywood well to the fiberglass deck above, so that when the load makes it want to compress, it can't buckle (to get shorter) instead of staying in place and resisting compression.

[earle]

Originally Posted by smokeonthewater

Keep in mind that in the boat it will be supported on almost all edges and that the glass will add a lot of strength.... I'd estimate that the finished product will be about twice as rigid as the test plank.

Good stuff. Built a "bridge" last night and it will be plenty strong enough. Just a couple of final questions.
The plywood I removed was dozens of 6"x6" pieces. Is there a reason for this, or can I replace them with 2 or 3 larger sections? I know plywood has a tendency to bow, but wouldn't adding support stringers stop that?
Also, should the supports be attached directly to the plywood and the whole thing glassed over, or glass the plywood first, then add the stingers and another layer of glass?
Sorry for the questions, but working on my back under the deck is a PITA, and I only want to do it once.
Thanks again for all the comments and advice.