Help needed with a boat table

Hello;
This is my first post. Since it's boat related, I thought this would be the best place to post. Excuse me if I'm wrong.
I need some advice. I’m building a replacement table top for a friend's boat. The boat has hardware in the deck to accept an aluminum pole, and the tabletop will have hardware on the bottom to attach to the top of the pole, so the table and pole can be stowed away when not in use. The table will be used both inside the cabin and also on the exterior deck. The table will have a trapezoidal shape approximately 30 inches long, with the wide end 22 inches wide, and the narrow end 17 inches wide.

I purchased a nice piece of mahogany, ¾” x 25” x 72, with the intent of cutting two pieces to surface glue and to give me a thickness of one and a half inches. When I cut the board in half, within a few hours one of the pieces cupped in the center almost a quarter of an inch, and then flattened back out over the course of a few days.

Here are my questions:
Is there anything I can do to ensure that no more cupping occurs during glue up or cutting out the final shape? How do I make this stable?
What would be the most effective method of clamping, especially towards the middle of the pieces?
What would be the best finish to use for beauty and durability? We’re more concerned about harsh sunlight than we are about it being waterproof, though waterproof is a desirable quality.

I welcome your input.

Thanks,
David

Glue the two boards concave-to-concave, or convex-to-convex, so as each one attempts to cup it will be balanced by cupping of the other.

To put pressure on the middle of the glue-up, use cauls. Put, say a 3/4"-thick stick along the midline of the glue-up. Put sturdy pieces of wood (cauls) running from edge to edge of the glue-up. Do the same on the other face of the glue-up. Now put clamps on the ends of the cauls to put pressure on that midline stick. You'll be juggling many pieces of wood while you're doing this, so think out how you're going to handle them before you apply glue. You might even try out the clamping scheme without glue.

The traditional finish would be spar varnish. It has UV protection, to better tolerate sunlight. It also looks great on mahogany.

I fully agree with Jamie. I would do the glue up with epoxy, and then epoxify the entire thing, using a heat gun (with organic solvent filters on my respirator) to heat the epoxy up to just before it bubbles for best penetration. Once the glue up has a hot coat and then a coat or two of epoxy at room temperature, then provide UV protection to the epoxy with a good quality varnish.

I would require the buyer to text me pictures of fresh varnish going on at least annually until my warranty expires. Epoxy is not UV stable and must (MUST) be protected if you want it to last.

Thanks for the replies, Jamie and Scott. I have some pieces of oak that are just right for the cauls Also, I had not considered epoxy for the glue-up- I was going to use Titebond III.

I appreciate the responses!

David

I almost always do a dry clamp up before doing the actual glue up. Goes smoother, like Jamie suggested.

This isn't really an answer to your question, but 1 1/2" thick seems too bulky to me. I'd expect it to be 3/4" thick with perhaps another 3/4" layer around the supporting post. Sometimes you see dropleaf boat tables with a thick but narrow center section and wider 3/4" hinged pieces. Also IMO all boat tables should have raised lips around the edges about 3/4" high unless they never leave the dock and that would add to the bulky appearance.

Maybe you could avoid or reduce the problem with a slightly different design.

Thanks, Alan.
The truth is, I really have a lot to learn. After watching the wood cup so badly after crosscutting it, and then watching it straighten out on it's own, I thought that maybe laminating the two pieces together would help eliminate any warping or cupping of the finished project- maybe I should have asked that instead of my original question.

So- using only one layer of 3/4" mahogany, once I get it to the desired dimensions, add a reinforcement piece for the support pole hardware, and finish with epoxy, is there anything else I should/could do to ensure it stays flat?

Thanks,
David

Quote:

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Originally Posted by David Wade

...After watching the wood cup so badly after crosscutting it, and then watching it straighten out on it's own, I thought that maybe laminating the two pieces together would help eliminate any warping or cupping of the finished project- maybe I should have asked that instead of my original question.

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My experience with boats includes a lot of time on salt-water sailboats. Not much of that involved boat construction except for repairs and it ended a while back. Today I wouldn't know where to go to find someone with a boat big enough to have a table. I remember a lot of mahogany. I don't recall warping having been a problem although I don't recall a lot of wide unsupported panels of solid wood either.

My concern was with the appearance, not the construction, and it's not me you have to satisfy, it's the boat owner and yourself. I think laminating the two pieces would work structurally and you already have some good advice about doing that. You could taper the bottom layer to look less bulky and hide the seam. One consideration is that if someone falls into the table or climbs on it, you don't want it to split and the 2 layers would be strong.

As for the design, all this is my opinion. If you Google "boat tables" and look at images, you will find lots of other opinions and you probably should: The raised edges should be vertical on the inside and can be tapered on the outside. They should be tall enough so that the edge of a sliding plate doesn't slip over the top and launch the plate into the air. The corners of the table should have a significant radius to protect flesh from damage and the raised edges can stop short of the corners, making it easier to clean (and build). Those edges will also help stiffen the table top but you won't need that if 2 layers of 3/4". You'd have to be aware of differential expansion between the edges at the ends of the table and the board underneath.

I would have finished it with Varathane (polyurethane) which works but does not refinish well because you can't blend the edges between new and old finish. I hope there are better choices today.

To clarify a detail in Alan's advice (Alan I apologize if I have this wrong) the raised edge must not be cross grained or it will cause problems.

Also, cauls are one way to go. Vacuum bagging may be better and sand bags work well also.

Not to throw water on a nice campfire, but plywood would be more reliable.

Quote:

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Originally Posted by Tom Bender

To clarify a detail in Alan's advice (Alan I apologize if I have this wrong) the raised edge must not be cross grained or it will cause problems.....

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In principle you got it right but there's no point having the raised edge only on the long-grain side. I knew less about things like differential expansion when I was involved with boats and wasn't looking for table details, but I think rails, cross-grain or not, were typically just glued and screwed and varnished solidly in place. Often that was on plywood but I can remember some tables that were not. You could use elongated screw slots. If neither the OP nor the boat owner care, I wouldn't argue for the raised edges. Some boats lead more tame and civilized lives than others.