Under sole fuel tank

How does one go about installing a fuel tank under the cockpit sole and then sealing the cover? Are there any particular aspects of this type of installation to be wary of or to pay special attention to? Any reading that would illustrate this project well? Thanks for the help.
Arko

 

Personally, I wouldn't seal any tank of any kind under a sole, you will need to get at it sooner or later and the seal would make this difficult to say the least.

Tanks of all kinds need regular inspection and maintenance. Tanks should be available for such and removable if possible, though this thought escapes a lot of production boat manufactures. Not much worse then a leaking tank and no way short of hacking up a deck, cockpit or sole to get the darned thing out for repair. Engines should be treated the same way, but most aren't.

 

Fabricating the cover as a hatch is the easiest way. You can either have a foam seal or caulk the seam. US law requires inspection plates to access all the hose connections and the fuel sender. Also, tanks are to be fabricated and tested by a USCG approved welding facility and the fuel pickup must have an anti-siphon valve.

 

How is the tank to be mounted? I installed a flange mounted fuel tank under the cockpit and made it accessable by removing the cover which is sealed with a quality foam tape and screwed down. The tank space is vented and there is almost no water leakage. I also think it is folly to permanently seal a fuel tank under the sole. The tank has been removed for inspection and all is well.

 

PAR, Gonzo, and Tom, thanks for your replies. You all at least confirmed that I a little common sense. This will be my first attempt at a powerboat. I don't even know a whole lot about boats so I am trying to learn as I go.
All of you point out that tanks and fittings must be accessible for maintainance and repair. Makes sense and something I had considered as well. I did not want to seal the tank in a permanent manner but something just as Gonzo and Tom mention, a hatch of some sort. Sealed but also accessible. Tom, are you saying that you just epoxy glued a flange around the opening of the cut out for the tank and then sealed with a foam tape? How is that arrangement working? Is it fully waterproof? I really don't know how the tank is supposed to be mounted, if any certain way. My project is not a planing hull, it is a semidisplacement hull so I don't expect to be subjecting neither me nor the tank to a lot of high speed hull pounding but I would assume the tank should be secured somehow. Would it not? If for nothing else, the fittings.
I was looking at one of those plastic tanks from boatersworld and the like.
I am not real sure about this venting thing either. If the tank is sealed in any compartment, do you have to vent the compartment as well as the tank.
Thanks for your help.

 

The mounts on a tank installation should be stout, real robust. Even a semiplaning hull form will take a beating in certain conditions and the tank has to be firmly held down.

The last one I did on a power boat was cylinder shaped, sat in a cradle and I made some straps from 1" wide stainless (16 g I think) sheet metal.

An access hatch is a good idea and I wouldn't be so concerned about total waterproofing in regard to the seal. If you're shipping enough water to breach a dogged down, foam lined hatch you have other more important worries, like the pumps being up to the task of getting all this water out of the bilge.

There is a publication put out by the USCG that will be quite helpful in the requirements for your tank. I'm not sure what it's called, but a quick search on their web site will prove useful.

 

Plastic tanks are really good. For one, they don't corrode. Also, they are a bit flexible, so it is possible to squeeze them in for a tight fit.

 

Is it the "Safety Standards For The Backyard Boatbuilder" ? If so I already had a PDF of that. I just never read it thoroughly. I saved it quite some time ago. It didn't really apply to kayaks so I just saved a copy of it perhaps knowing that eventually I would end up building a power boat. Thanks for jarring my memory.
I can only mount under the sole if I can locate a low profile belly tank. Any thoughts?
Thanks.

 

I would have thought that you would want to have a little room for expansion with these plastic tanks. Ever seen the plastic gas cans sit in the sun for a little while?

 

Yep, It is Safety Standards For Backyard Boatbuilders. It give the requirements but no specifics on how the tanks are to be built or mounted. I completely agree that the mountings should be hefty enough to take punishment even in a displacement boat. I've had sailboats crash off waves with enough force to disloge anything that was not solidly fixed.

I built my tank of fiberglass with integral mounting flanges that were screwed to ledgers in the tankage space. In addition, I fixed nylon straps under them as a belt and suspenders measure. The covers screwed down on ledgers surrounding the tank with the foam sealing tape on the ledger. I don't see why you should make it easy for water to enter the bilge even though there may be manual and/or automatic bilge pumps. My tank is under the cockpit which is self draining and that affects my wish to seal the cover. I like the plastic tanks better than metal ones, especially aluminum ones that corrode easily.

Don't forget that if the tank space is open to the boat interior or bilge, it must be positively ventilated.

I don't think that there is any need to provide expansion space around the tank since the tank must have expansion air space inside to take care of that. Those portable tanks expand only when sealed.

 

I don't think it should be easy for water to get into the bilge. As I said, if water is coming through a flange with a foam seal, that's too much water or bad design or construction or something that needs addressing, preferably before the conversation with the SeaTow skipper about the options of getting positive buoyancy back in the boat . . .

Tom, I've often wondered about 'glass tanks, but wasn't willing to climb out on the limb. Did you do anything special, or just a standard layup (epoxy) with cloth/mat? Form fitting tanks with a lot less cost then the custom ones, got to be a catch . . .

 

There's a special resin to use in fuel tanks. I can't remember the name, but most resin suppliers should know of some. It's easier to design the tank into a new build, but it should be easy enough to build one to fit an existing compartment.

As for aluminum fuel tanks, they will last for years if given the proper air space around them. The aluminum requires the air to form the proper coating which minimizes the corrosion. The problem with most boat builders is they bury a tank down low in the bilge, then pour polyurethane foam around it. This allows no air circulation and mixed with water, which even closed cell foam will absorb after a while, creates the perfect condition for a rotted fuel tank. Then watch the repair bill soar when we have to cut half the boat apart to replace it.

Keeps me in business

 

PAR,

First, there is very little water that makes it past the foam seal. So little that I am not ever concerned about it. There is an automatic bilge pump but it has never cut on unless I am just testing it. After several rains there may be one or two teacups. It's just not worth it to make the seal perfectsince I go in to check on things every year or two..

The resin I used is called isothalic and is recommended for fuel tanks. I made the tank from a sheet of fiberglass cut and formed into the designed shape with X baffles inside. Corners were taped and the whole thing laminated with several layers of cloth and roving. The inside was heavily coated with resin to prevent any wicking of the glass fibers. It was pressured tested over a perid of two days by fitting a tire valve in the gauge flange before use.

The capacity is 25 gallons. I chose to build my own tank to allow best use of the available space. I included a water tank of 30 gallons in the same space in a tank built of epoxy and plywood. We do not use this water tank and use a smaller 8.5 gallon tank under the sink that works fine and can be taken out to fill or dump after use. This was an advantage a few times when water had to be carried to the boat. We have a manual pump to conserve water and the thought of pumping that big tank dry after a cruise is the main reason it has never been used since my wife insists on fresh water for each use. Considering this, if I were building again, I would opt for a commercial plastic belly tank for fuel.

Regardless of the reason, the stack of old corroded aluminum fuel tanks to be found in many boatyards will keep one off my boats. If I insisted on metal, it would be stainless.

 

Gentlemen I have the very interesting discussion about fuel tanks. I have a 1970 19ft Mako this year it gave up its transom so I have to redo it. Since that has to happen I would also like to put the gas tank below the deck. The 1970 never had them under the floor. From your discussions I think you said I should moumt the tank on the stringers and support it with stainless straps. I should not be mounted directly on the hull. Am I gettting the right Idea? also I have some Ideas on how to do the transom but I would like some more opinions. First In taking the old transom apart it apears It was replaced once before. The plywood (2,3/4" sheets are from side to side but appear to start at the engine mounting surface (20" from the very bottom of the hull) to just below the deck floor. Plus there are a lot of globs of resin plastered in places and what appears to be disbonds in the lay up. My question is how far shouln the wood go? Anyone thanks Ken

 

Is this an outboard powered boat? Plastic tanks can be mounted without air gaps. The bottom needs to be supported. Plywood works well.