Accomodating Cross Grain Expansion

[Bill Adamsen]

Rebuilding a Dyer 9 frostbiting sailboat for racing ... so trying to keep this "build" consistent with what the manufacturer has done for 50+ years while also following general woodworking best practices.

The issue I'm faced with is installation of the rear seat. It's a piece of Honduran Mahogany approximately 42" wide X 3/4" thick X 11-15/16" deep. In the original install the mfg glued the seat into the boat with a heavy glass gusset underneath. It is also nailed (from the bottom up) into the transom with copper nails. Many "rebuilds" install (epoxy) cleats to the hull sides and then glue/screw the seat to those cleats. Either of those approaches - to my mind - creates the potential for pretty significant cross grain stresses on the Mahogany. I have such a beautiful piece of wood (photo) going in, that I hate to think of installing it in such a way that it will split, warp, and break the bond just due to natural forces. Conversely, not afixing it rigidly to the boat will allow flexing that will reduce its racing competitiveness. Plywood is not class legal. What to do?

Considered solutions ...
1) Install the way the factory did - just use lots of low amine epoxy to seal the seat before install
2) Cleats leaving the front free to move with a slide mechanism gripping the cleat

Love to hear thoughts and other "outside the box" ideas. Anyone have a "low amine" favorite epoxy for coating?

Debated putting this in the boat building section, but this area of the forum gets more traffic.

[Dave Richards]

This sounds like a cool project.

I think encapsulating the seat in epoxy is a good idea but I would still assume there'll be some cross-grain expansion. I would keep the mounting fairly simple, though. Elongate the forward holes in the mounting cleats and use round head screws with flat washers. If the cleats are wide enough, you could recess the screws and flat washers below the bottom surface so they don't show.

As for low amine epoxy, I've used Raka epoxy for years and never had any problems with amine blush.

[Keith Pleas]

I don't have an answer, but...don't you get exactly the same force (acting vertically) on the wood in the transom?

What's the issue with amine? Are you worried about the amine blush? UV exposure? I would use WS 105/207 on that seat, which will also give you some seat of the pants slip resistance. Heck, I just coated my slippy iPhone 6 cover while doing a small piece of wood last night:
306829

The kitchen counter was a great place to work - good lighting, nice height, space for my wine glass. Oh, did I mention my wife is out of town at a conference? ;)

[Bill Adamsen]

I had heard the iPhone 6 was slippery. Do you coat the front, back or both? (thanks for first laugh of the day).

The transom is less than half the height of the seat depth, and I've never seen it fail. Part of that could be that the back is fully glued - hence - at least the back is completely waterproof. Or it could be that the frp skin on the transom flexes in that direction. I'm not worried about the transom.

WS 105/207 could work. I don't have 207 but have lots of 105. I have never had good luck with using epoxy as an undercoating with varnish. I'm sure that's an "operator error." I typically use many coats of varnish, and I'm pleased with the result. Not so pleased with having to redo the work every few years. Plus, in this application I want epoxy as the coating so that I could - if I choose that approach - epoxy to the hull. That was the concern about Amine. I didn't want a layer on the coating that might inhibit bonding when bonding to the hull.

I am willing to try again, and what you've recommended, or Raka, are the approach I'll take.

[Dave Richards]

FWIW, amine blush can be washed off with soap and water if it comes to that.

[Keith Pleas]

I had heard the iPhone 6 was slippery. Do you coat the front, back or both? (thanks for first laugh of the day). ...The transom is less than half the height of the seat depth, and I've never seen it fail. Part of that could be that the back is fully glued - hence - at least the back is completely waterproof. Or it could be that the frp skin on the transom flexes in that direction.
I coated the bumper which was slippery - it's much better now!

I have a new idea - pre-break the seat into slats that are attached to the cleats on the hull side. Unless that's a class violation too? Same principle as expansion joints.

[Andrew Hughes]

Mahogany is just about the best choice when it come to stability.Im not a boat guy but I have some jigs I made in mahogany years ago one is a perfect 45 I use for shoot miters.Its still holding its shape and I will be using it next week.I also have some walnut that pretty stable but mahogany is the king!

[Wade Lippman]

If it will never be in a dry heated building I don't see a problem. The seat isn't very big; the magic number for trouble is 18". The epoxy encapsulation should inhibit movement.
I wouldn't worry about it.

I build a tight fitting drawer the wrong way because it looked better; if I had a problem I would fix it. 8 years now; never needed to be fixed. might be wrong, but didn't matter; I would bet yours is also.

[Bill Adamsen]

Thanks folks for the responses. I picked up some coating epoxy (WS207) and plan to "moisture proof" before installing. Cleats versus the glass underneath. You have increased my confidence this approach will work.

[Jim Matthews]

I'm uppa heya near the Newport boatyards.

The longterm concerns refitting sailboats and yachts
tend to stick with traditional fixtures - those that have proven reliable.

If you're really concerned about movement, make a hexacell core seat.
That way you'll save weight and have a racing dinghy.

[Keith Pleas]

If you're really concerned about movement, make a hexacell core seat. That way you'll save weight and have a racing dinghy. If plywood isn't class legal then I'd bet aircraft panels aren't going to be either!