Converting built in watertank into fueltank?

[SeaHusky]

On my boat the rear end of the bilge has been built as a 90 litre watertank.
I'm not sure if any other boatowners actually have used it for this but probably don't do so today.
The previous owners of my boat have not used it at all.
It is accessible by an inspectionhatch in the front end just in front of the engine compartment



There is a hose pipe going up to the port side and I have yet to discover where it exits.


I think it would be great if I could convert this into a 90 litre diesel tank!
Does anyone know if this is possible or suitable and if there is a good way of doing it?
A rear access hatch would be great but unfortunately the driveshaft is above it.
Any input is appreciated.

[SeaHusky]

Does nobody know if I can fill diesel in a regular polyester tank or if it can be lined or a bladder inserted?

[Godot]

I don't know what diesel would do to the tank. If you just use it as a box to hold a bladder, I imagine that would be ok.

Wish I could be more help.

[Cyric30]

I have to agree with Gogot, without knowing the knowing the type of material the tank is exactly, you dont the reaction the fuel will have with it.  the Option that comes to mind is contact you boats maker, if they are still in business and see if you can get a answer to what type of "plastic" it is, another might be finding somone who worked for the company who might know....the bladder idea might be best, although i dont know anything about them.

[SeaHusky]

Thanks for your replies guys!
This is what I have found out from other people and sources.
Boats from the seventies are made of polyester. Any further then that is just guessing.
Built in watertanks like this often made the water taste of plastic which is why they are seldom used for drinking water.
Without a special coating liquids diffuse through the material. Having saltwater on the outside and freshwater in the tank drives osmosis.
Using this tank for diesel without a special coating would eventually saturate the material with diesel and if not dissolve the plastic it would at least make any attempt at repair jobs to the keel futile. It may also make the boat reek of fuel and render it uninhabitable.
What I can do is cut out the rectangular opening so that I can clean and inspect the tank and if desired put in a standard bladder type water tank of suitable size.
I suppose the real question is if this is a good place to add an additional 100kg of weight?

[CharlieJ]

I dunno about Kg's, but Tehani has a 25 gallon water tank under the cockpit floor, which is 200 pounds. She also has a 17 gallon tank up in the bow, under the Vee berth. Both have bladder tanks by the way. One VERY nice thing about those is there is no vent needed.

[CharlieJ]

Ok, just did the math. About 20 pounds difference.

[SeaHusky]

My boat has 3.0 - 3.4 tones displacement. I'm not sure why they state two figures but it might be empty and estimated cruising load.
The engine is aft, as is the fuel tank, anchors and chains etc.
I was just contemplating how, if I was to have heavy, following seas, it may ride better if I put the weight of the fresh water tanks under the V-berth and/or in the stowage amidships and have an airfilled void to float the rear a little better.