Hard Spot

[hilbert]

I am replacing the cylindrical fuel tank from under the port deck to a rectangular tank in the starboard lazaret. I have built a frame that will secure the tank. The frame will be positioned between the main bulkhead and a piece of 3/4” plywood, that will be parallel to the bulkhead. If the plywood were rectangular, it would be approximately 14” wide by 20” high. However, it is shaped to conform to the inside of the hull, of which a 32” arc is in direct contact.

The tank has a capacity of about 11 gallons and would be around 100 lbs when full. Can I fiberglass the plywood directly to the hull? Would this create a hard spot? If I need to put something between the plywood and hull, what material (product) would I use?

Thanks,

Trofast, Cape Dory 28

[sailingdog]

Ideally, you should have about a 1/2" trapezoidal foam piece separating the wood from the hull. The trapezoidal shape will allow you to easily tab the plywood in place using fiberglass cloth and epoxy resin and create a nice solid connection without forming a hard spot that will cause the fiberglass in the hull from hinging and fatiguing.

Ideally, the foam should be closed cell airex foam or something similar.

[deniseO30]

Epoxy putty you make by mixing epoxy and micro fibers, make it the consistency of drywall compound. If it needs to be structural, use cloth after the first app of putty is firm but not cured. then apply more to fair the seam. It will be stronger then the hull or plywood when it's all cured. If you want it less permanent use polyester (bondo) it may hold for a few months or years. I don't think the hard spot in the hull will mater.

[tager]

The hard spot in the hull will be noticeable. Foam is the preferred solution. I prefer bolt-on bulkheads as they minimize rot, but to each their own.

[hilbert]

My thanks for the advise form everyone that has replied.

I am confused as to where I can obtain a 1/2" trapezoidal foam piece of airex type material. I only require 32", but it must bend 90 degrees in that short distance. I could order something online, but I don't know how easily it cuts (to make it trapezoidal) or whether it would conform to the sharp bend (and no one sells such small quantities).
Would a piece of foam ˝” pipe insulation from Home Depot work?

To make room for the fuel tank, I tore out a battery box that was constructed from plywood. This plywood construct was epoxied and fiber-glassed directly to the hull. Maybe I should just do the same as I don’t know where or what to buy for a foam barrier.

[mitiempo]

Any kind of foam will work. It's purpose is only to keep the plywood from the hull until it is glassed in. Some builders (Morris) space the bulkhead and after one side is glassed remove the spacer and glass the other side. The goal is to create an epoxy bond that takes the load evenly without any hard spot against the hull. Tim Lackey at Northern Yacht Restoration | Tim Lackey:* One Man, One Boat at a Time uses pink foam that is available at Home Depot and other building suppliers. I used Ethafoam, a softer foam. You could use plastic spacers and remove them after glassing one side if you wish. Epoxy doesn't stick to plastic.
The trapezoidal shape helps when glassing and can be cut in foam with a razor knife. Or you could not cut the foam and create a shaped fillet with thickened epoxy but it will add to the cost a bit. Use epoxy and not polyester as it bonds much better to cured fiberglass (the hull).

[sailingdog]

Any good closed-cell foam will work. Do NOT use an open cell foam, as it can cause problems by wicking the epoxy away from the fiberglass and starve the laminate of resin.