Foam around polyethylene fuel tank

[Wade Major]

I am in process of rebuilding a 17.5 foot, 1982 fiberglass Double Eagle. I have the floor, tank and about 500 pounds of wet foam. I have three questions that maybe someone can answer. I plan to install a 35 gal Moeller polythylene belly tank. What is the current thinking on foam around plastic tanks? I have read lots about the problems with foamed in aluminum.

Secondly, this boat do not have any bilge drainage. The four stringers and cross members were all glassed in place without any limber holes but they are all in good shape as there were completely enclased in glass. Should I put this boat back together the way it was or leave the foam out of the middle section where the tank will be, the rear section and install a drain in the low part of the transom?

Third, I want to install an in-floor fish hold in the compartment aft of the tank. I can easy build a box to fit and glass it but I am struggling with how to fit a cover that is easly removed (hopefully to drop fish in) but is reasonably water tight. Does anyone have any ideas or plans?

Thanks to everyone for all the great posts. I have used lots of the information already on the floor removal.

Wade Major:)

[Ike]

Don't. Foam won't adhere to polyethylene and plus that the first time you put fuel in the tank the tank will expand and break the foam. Polyethylene tanks also permeate a very small (really small) amount of fuel vapor. They need to breath. There needs to be space around the tank. So in short no one foams polyethylene tanks.

I would leave the foam out of the tank compartment. Install a small drain.

I don't know about the fish tank. I have seen pre made tanks at boat shows so you should be able to find one you can just drop into place and hook it up.

[Wade Major]

Thanks Ike,

It makes sense not to foam in the tank if it will not stick. How much expansion room is needed? I had assumed that the expansion would be minimal since the tank will be vented to the outside of the hull.

Thanks,

Wade Major

[Ike]

I don't know exactly maybe a 1/2 inch or more. It depends on the size and the thickness. Moeller can tell you that http://www.moellermp.com/

[Wade Major]

Thanks for the link to Moeller Marine. Their site does contain the actual installation instructions which clearly state "not to foam in a plastic tank" and to leave 3% expansion room around the tank.

[Ike]

Good to hear that. I will have to remember the 3%/