Fuel Tank Support

[riverboathank]

I've built 2 diesel fuel tanks (P.S.&S.B.), composite construction to fit the contour of the hull & chine, bottom dimensions of 6 1/2'x 2'(100 U.S. gal.). I would like to install continous support and am thinking of using 2 part foam. Access to area is only 1/2"-1" around perimeter of tank, and not having used pour-in-place foam before, is this feasible? The gap between tank bottom and hull is also 1/2"-1"(varies). Are there any other options?
Thank You, Seasonal regards, Hank.

[On Vacation]

This is feasible. One question, does the area of location have a possibility for bilge water? I dry fit the conture by first sticking in some small strips in place with some small mixes of foam holding the strips down if curved area, with some small strips pushing on it, running the distance of the area that the tank will sit on, then grinding down uniform, checking with a piece the the size of the bottom of the tank.

At that point, you can either stick it in place with some small amounts on the bottom, or some small beads of 5200, and then foam the sides in place, with small mixes, keeping in mind the foam will move the tank if you use too much mix to start. I use just enough to stick the tanks in place and ten go back and finish the job.

Sometimes I will mock up another tank with just luan, since the weight and handling of the size of the tank in the finish mode, is usually heavier than the mockup,since you don't get a second chance with active foam, and fit the bottom section.

Try to keep that stuff off your hands using long stir sticks for mixing, lots of them avaliable before mixing the foam, to work it in the voids, upon kicking time, but not cured yet..

[ 12-29-2003, 08:02 AM: Message edited by: Oyster ]

[riverboathank]

Thanks Oyster. The area is sealed off by 2 bulkheads and a longitudinal stringer so bilge water should not be a concern. the bottom of the tank is contoured, so I would have to use the tank itself as a mold for the initial strips. Is the foam material compressive enough to absorb some irregularities? Is it possible to inject foam through a small gap? I intend to wrap the tank bottom & sides in poly so that I won't get any adhesion and make for easier removal if necessary, and hold the tank down with top perimeter cleats.
Thank you,
Hank

[On Vacation]

The size of your mass, even with resin or plastic coats, and the hull form, this is not a workable situation with active foam , and a lot of cured foam surrounding the 100 gallon tank. In my experience with the 2 part, I have bent harderened steel straight claw hammer handles pulling old metal tanks out from hulls when they leak. Do a test. This tank will need to be held in place, securely before the foam goes in place on the sides. I also use some small sticks stuck around the sides, in the initial placement before foaming in place.

Do not over foam at the start of this job.The dry fit is very critcal, or you will hate yourself. If this tank raises up, with foam around some of the bottom corner, with the size of this tank, moving it back in place, or getting it out will be almost impossible.

[ 12-29-2003, 08:35 AM: Message edited by: Oyster ]

[On Vacation]

The two part foam. in a large mass, will and has blown out hull sections. If the hull does not give, you may fracture the tank itself.

[Tom Lathrop]

Hank,

I ain't going to argue with Oyster too much since he has done more of this than I have. However, I think that mounting the tank with flanges molded into the sides and resting on solid stringers may be a better solution to your problem. Some added, FUEL RESISTANT, foam material fitted under the tank could help prevent drumming of the tank when slamming into waves.

This assumes that you built the tank strong enough to stand on its own and don't need to depend on the hull for support of the tank bottom. You did do this, didn't you? All auto tanks are mounted this way with flanges.

[Ian McColgin]

Oyster's remarks are very well taken. Two part foam when going off can be very hot and very expansive. Small batches are where it's at.

When I've had large complex spaces to foam`I use dixi cups - one for part a, one for part b and labled and kept seperate! - as my measuring devise. This works out to batches that are a bit under 1 cup mixed fluid.

One place I part company from many - I pour the parts into a largish (1 qt or bigger) paper bucket and give it a blast with the paint mixer on the end of a drill - the really big heavy all metal type. About 5 seconds of this is a complete mix. I bring the mixer up still twirling so's the mix is tossed off the mixer onto the sides of the bucket.

Then pour.

At this point scrap sticks may be helpful to pull most of the glop out of the bucket, but often you're pouring down a wee hole (like 6"D) and there's not room to spoon it out. When all that's going to pour easily is out, put the bucket on one side on a shelf such that the lip is over the void. You'll end up with a collection of bucket tounges.

G'luck

[On Vacation]

You got it guys. Great points. Well taken.

[riverboathank]

As usual, your responses Oyster, Tom & Ian provoke more thought and changes. The idea of trying to get foam into a 1/2" to 1" gap around a 2 1/2'x6 1/2' tank sounds a bit ...ah, what's the word? The tank itself is solidly constructed, but I would need some cushioning support spaced around the bottom. Fuel resistant foam?, hopefully that aspect won't be tested, what is it? Strips of it spaced every 1' or so would do the job?
Excuse my ocassional delay in responding, I've taken a month or so off work to push boat progress, and between running from shop to cyber. and finding the phone line tied up, well..
Thanks, Regards, Hank