Diesel Leak

[Howler]

Our diesel tank has several ports: engine supply / return, generator supply / return (capped and unused), and fuel gauge sensor. Each is mounted on top of the fuel tank with a screwed on metal plate. The supply / return ports are mounted on a surface of the tank that’s lower than the fuel level when the tank is pretty full and / or the boat heels. Diesel is escaping between the metal plates and the plastic tank.

In order to improve the seal, I’m thinking of:

1. Cutting a gasket
2. Applying diesel-proof caulk between the two surfaces
3. Both

I’m thinking the gasket would be best in case the ports need to be opened. For servicing.

Any recommendations would be appreciated.

[donradcliffe]

In many boats, the fix is don't fill the tank.

[sailingharry]

Plastic tanks are usually polyethylene. Nothing sticks to it. Polyethylene mixing tubs are great for mixing epoxy, because once cured you can pop out the hardened epoxy and reuse the tub. The point is that you will be best served with a gasket, and not an adhesive.

[Howler]

Originally Posted by donradcliffe

In many boats, the fix is don't fill the tank.

Of course, but when the boat heels, the fuel moves there even if only partially full.

[Howler]

Originally Posted by sailingharry

Plastic tanks are usually polyethylene. Nothing sticks to it. Polyethylene mixing tubs are great for mixing epoxy, because once cured you can pop out the hardened epoxy and reuse the tub. The point is that you will be best served with a gasket, and not an adhesive.

Makes sense. Thanks!

[jamhass]

Sealant does not need to adhere well to affect a seal. A non-hardening seal, such as Permatex black, will work fine. The sealant acts to fill minor imperfections. The cover and gasket keeps all in place. One issue is to use a sealant that does not attack the plastic, but that is unlikely.

[MicHughV]

I tried countless different type of sealants on a Beneteau plastic tank..but none really worked....the threaded hole in plastic tanks is just not suitable for a tight fastener.
Finally tried some 5200 in both the threaded holes and around the perimeter...

The tanks in a Beneteau are relatively flat and thin....so when heeled over, fuel is right up against the metal plate.

So far, 5200 seems to be doing the trick.

[MicHughV]

I should add, I also used a different threaded bolt to attach the plate to the tank, with a slightly larger diameter to give it a better bite in the plastic.

[hlev00]

Similar to McHughV I had a leak from one of the self tapping screws that had stripped out. I went up a size and that has held for about six years. Just don't over tighten. After first putting the larger screw in I had a little leak, then I just tightened up a bit more and that fixed it. I have one of these rings to install someday which should provide a proper seal on the plastic tank.
https://kus-usa.com/product/fls-u/

[sailingharry]

Originally Posted by jamhass

Sealant does not need to adhere well to affect a seal. A non-hardening seal, such as Permatex black, will work fine. The sealant acts to fill minor imperfections. The cover and gasket keeps all in place. One issue is to use a sealant that does not attack the plastic, but that is unlikely.

Indeed, I agree. I have used permatex rubber gasket dressing in other applications. It never dries. Might work well here.
https://a.co/d/e3xxJpT

[Howler]

Update… I opened the ports (removed the metal plates to which the feed and return fittings are welded). The gaskets were in terrible shape… cracked and split. There is no question that this was the (or at least a) source of the leak since the top of the tank near these ports would be wet with fuel when the problem was first noticed.

I made new gaskets and reinstalled the port plates. I went out for a sail to make sure the fuel moved around. On return, the area around the ports was bone dry.

Success? Not so fast. There is still a small amount of fuel getting to the bilge. I keep looking, examining the fuel hoses, engine fittings, as well as the fill and vent hose. Everything checked out. There’s no fuel in the engine room.

So is the fuel left over from the previously leaking port gaskets, and just taking its time to work its way to the bilge?

Or it possible the tank is leaking elsewhere than at the ports? It’s one of those black plastic (polyethylene?) tanks that comes with newer Beneteaus. The boat is five years old and there was no issue with diesel leaks the first years. Do these tanks ever just spring a leak?

Any further troubleshooting recommendations are also welcomed! Thanks.

Thanks.