Arriva new fuel tank

I just acquired an Arriva 2001 ski boat long story short it needs a new fuel tank and before I know it I have a full blown project on my hands. I have ripped the flooring out removed the fuel tank and found rot under the tank. Now the hull is fiberglass I don’t believe it is cored with anything but the gas tank coffin is a plywood core. My big question is can this coffin be removed completely? I have a poly tank going in to replace the aluminum tank that was in place but due to the design I want to eliminate the possibility of water being trapped under the coffin by not replacing it and leaving more of a passage for water to run to the bilge area. The original design boxed it off completely and then had a 3/4 tube at the front coffin bulk head and rear on for water to find its way to the bilge area. The design issue is if there was a lot of water it would get trapped behind the tube and never drain which is what I believe caused the water damage over time. Any thoughts?

 

Welcome

More pics would be most helpful. Or a drawing.

I suspect that the coffin supported the fuel tank. If so similar support needs to go back in.

Try replacing the small tubes with much larger cut outs. Big enough to shine a flashlight in, or squeeze a hand in to remove detritus.

Be sure to fully seal the plywood edges before installation.

 

Essentially same answer as Blueknarr.
There has to be a support system for the tank that keeps it in place and distributes the weight of the tank, and doesn't collect water and debris under the tank. Your plastic tank isn't going to rot or corrode but the wood will. So if you replace the wood make sure you seal it on all sides, especially edges with penetrating epoxy. If you aren't comfortable using CPES (it is rather toxic), just thin out regular epoxy and apply several coats to the edges. Another way is to encapsulate the wood in fiberglass. Seal it up well to keep water out.

 

Judging from your picture and colorful description you had an underfloor tank and the tank bearers have rotten. The "coffin" sides are the wood cored floors and stringers and no, you can not remove them, they are structural. The tank bearers were also floors judging by the faint outline in the black area and may or may not have been structural. What you can do is replace the rotten core and reglass. You can use a non rotting core like high density foam or G10 if epoxy encapsulated wood is not good enough for you.

 

I am new to this but my general idea is to add another set of stringers to “box” in the new tank these would be spaced to allow for a 3% expansion of the fuel tank and then the cavity filled in with expanding foam. (Foam being the yellow in the picture and the stringers being the red). The stringers would line up with the transom knee braces and then run to the stringer or bulk head whichever it is called that is at the front of the tank. All would be glassed into the boat with epoxy and fiberglass mat. I am planning on increasing the drain size from a hole to a slot at the front and back of where the tank mounts when I replace these stringers (or bulk heads I’m not sure of the correct terminology sorry) anyone see any flaws in my plan?

 

I’ve had some success with “bagging” the tank support foam in thin plastic bags at each corner, and maybe at the middle if the tank is long. That keeps the foam contained in a smaller area, Leaving bilge drainage open, and it doesn’t stick to everything it touches.
You can also build forms, like you would for concrete, but use old plastic bottles or similar to contain the stuff.
Open top will allow over expansion to be trimmed off after cure.
Looks like the whole floor is coming out? Best to finish exploration/demolition before starting to rebuild, as extensive rot can be a game changer!

 

Floor is all being removed as there was some foam that was saturated with water so while I am in there I want to remove what need to come out now so I am not redoing this again in the future. In hindsight it’s ending up being more of a project than was planned but isn’t that always the way these things go.

 

This should give you the information about tanks and fuel systems you need:
https://www.uscgboating.org/regulations/assets/builders-handbook/FUELSYSTEM1.pdf

 

If you use expanding foam, the odds are you will have to redo it sometime in the future. Expanding foam is notorious for absorbing water. A better way is premade block (or stick) foam. Bag the foam in plastic to keep the water out of it, and then you will never have to redo it. see Boat Building Projects | 1972 Sea Ray 190 Rebuild https://newboatbuilders.com/pages/SeaRay190.html The posted image shows how I did it on my Sea Ray. The foam is polystyrene (not styrofoam) 2lb density insulation foam. Its relatively cheap and available at any hardware or home improvement store. Polystyrene can be damaged by gasoline and strong bilge cleaners, and other petroleum based products, but machine made foams don't absorb water. Expanding foams are usually Polyurethanes which have the advantage of not being affected by gas or oils, but they absorb water.

 

Don't use mat with epoxy, use a woven fabric. I would simply glass some supports for the tank transversly between the stringers, fix the tank solidly in place to them and be done with it, no foam no nothing.