Repurposing a water tank as a diesel tank

[bignick]

My Elan 295 has two water tanks with a combined capacity of about 180 L, which is complete overkill for a weekend yacht. The tanks are located beneath the saloon berths and are connected via a T-piece beneath the cabin sole, with no one way or isolator valves. Consequently, water can flow from one side to the other, resulting in free surface losses and a reduction in stability. For a yacht which can be quite tender and flighty to begin with, this is a somewhat strange choice on Elan’s part. I’m going to remove the T-piece and just retain the tank on the port side. The question is then what to do with the starboard one? Leave in place, remove, or repurpose?

The 80 L diesel tank is located in the aft cockpit locker on the stbd side and the boat has a tendency to drag her transom. I’m wondering about removing it and repurposing the now redundant water tank, to reduce weight and put it where it’s a bit more useful.
I’ll need to remove the water tank and get some welding done on it, to blank the existing outlet and run a new suction line, fit tank sender etc. I may also need to fit an in-tank lift pump, as the tank will then be lower than the engine and filter and would be a PITA to bleed.

Has anyone else done anything similar? Is it more trouble than it’s worth?
Perhaps I should just start by replacing the T-piece with an elbow, so I just fix the free-surface loss issue which niggles me and leave the rest alone?

 

[PaulRainbow]

If done properly, see no reason not to repurpose the water tank. If you fit a 1/4 turn fuel shut off at the tank and another close to the primary filter, you can close the one next to the filter when you do the filter change, should make bleeding simple enough.

 

[NormanS]

I find it astonishingly wrong that the two existing tanks were not fitted with valves. The idea of having twin tanks where there is nothing to prevent water to freely flow to the lee side is crazy.
In theory, I see nothing wrong with converting one of your tanks to fuel.

 

[B27]

I find it astonishingly wrong that the two existing tanks were not fitted with valves. The idea of having twin tanks where there is nothing to prevent water to freely flow to the lee side is crazy.
In theory, I see nothing wrong with converting one of your tanks to fuel.

It would be sensible to have a valve on the outlet of each tank.

Converting the tank to diesel?
1) It's a one way trip I think! Will the next owner want that?
2) It's normal for yacht diesel tanks to draw from the top panel with a standpipe in the tank. That makes it easier to work on any of the plumbing without draining the tank. It's less risk of fuel leaking into the bilge.
3) the water tank may have a large flat bottom which might mean it needs a lot of fuel in it to assure the engine not picking up air at all reasonable angles of heel? You might have to think about pitching motion in a choppy sea and are there adequate baffles?
It doesn't take ever so much air in the fuel line to stop some diesels.
4) you might want to make provision for access into the tank for cleaning? Is that compatible with the need for baffles?
If the weight of 80 litres of diesel in the stern is a real issue, do you really need so much?
Is there anything else which could be moved forwards?

Reducing to 90 litres of water seems a bit dubious for a cruising yacht, it would put me off. It's not many showers.
Getting water can be a pain, and I'd like to be good for a few days personally.

 

[bignick]

The flat tank issue is something I’d have to deal with. A friend has a Dehler 32 which is terrible for drawing air and the engine cutting out; it has done it on two occasions coming back through the North Channel by the Needles in chop, just when you really don’t want it to happen. The first time we put the jib up just before it cut out completely. The second occasion we put the jib up as soon as it misfired and we stopped the engine before it cut out itself, which meant we could restart it when we got into calm water.

I’d wondered about seeing if one end of the tank could effectively be partitioned off into a day tank, to ensure a smooth fuel flow, but that would then require two in tank lift pumps, with even more sensing and complexity.

 

[john_morris_uk]

No comment on the proposal but if anyone has problems getting fuel through for bleeding their engine, I’ve found fitting one of these to solve all problems.

Fuel Primer Pumps For CAV Type And Racor Fuel Filters

I’ve added one to the CAV filter on our main engine and another on the CAV filter on our generator. Bleeding the engine is an absolute doddle. (Fuel for both is via spill pickup pipes from the top of the fuel tank.)

 

[bignick]

I’ve wondered about fitting an inline primer bulb for a while. Maybe that would be the simple solution.

 

[Aja]

I find it astonishingly wrong that the two existing tanks were not fitted with valves. The idea of having twin tanks where there is nothing to prevent water to freely flow to the lee side is crazy.
In theory, I see nothing wrong with converting one of your tanks to fuel.

Astonishingly Marine Project Moody 346 and others have no valve between starboard and port water tanks.

 

[NormanS]

Astonishingly Marine Project Moody 346 and others have no valve between starboard and port water tanks.

If so, I'm surprised. I would have thought that any yacht designer/builder would have some understanding of free surface water, or any other liquid. Even apart from the free surface aspect, one of the advantages of having twin tanks, which can be isolated, is that in the event of a leak etc, there is another tank hopefully with water in it.

 

[Pete7]

I’ve wondered about fitting an inline primer bulb for a while. Maybe that would be the simple solution.

Had a number of diesel Peugeots that had primer bulbs in the fuel line.

 

[B27]

If so, I'm surprised. I would have thought that any yacht designer/builder would have some understanding of free surface water, or any other liquid. Even apart from the free surface aspect, one of the advantages of having twin tanks, which can be isolated, is that in the event of a leak etc, there is another tank hopefully with water in it.

I raced on a boat with twin tanks, where hypothetically, one could have let all the water into the leeward tank before tacking......

 

[bignick]

I raced on a boat with twin tanks, where hypothetically, one could have let all the water into the leeward tank before tacking......

now there’s an idea. I would need to put about 80L in and put a remotely operated seacock between the tanks, so I could control where the water is….

 

[dunedin]

Very easy to fit two stop cocks for the water tanks, just before the pressure pump, so can select whether to run from port or starboard tank. And with only one open water can’t run from side to side. That’s what we have and that’s what I would do.
Unless very certain about all the components I would prefer to keep diesel out of the cabin as even a tiny seepage will make a smell in the cabin.

 

[PaulRainbow]

Water won't normally run from one tank to the other at a sufficient rate to destabilise the boat.

 

[bignick]

Water won't normally run from one tank to the other at a sufficient rate to destabilise the boat.

agreed - infeasible to reduce GM sufficiently to result in a complete loss of stability.
However, potentially equivalent to having an average sized person sitting to leeward after a few minutes on the same tack.

 

[Stemar]

Personally, I'd fit the fuel takeoff and return into the top of the tank, and close off the bottom fitting with a valve so you have an easy way to drain it if necessary. Add a bulb pump between the tank and the filter to simplify priming. On the water side, a T with a tap one side and the takeoff on the other would allow the same thing.

Does each tank have its own filler? If so mark them or, sooner or later, you'll end up with water in the diesel tank or, worse, diesel in the water one, and it WILL happen when you're tired, stressed and hurried.

Unless very certain about all the components I would prefer to keep diesel out of the cabin as even a tiny seepage will make a smell in the cabin.

True, but the stench of diesel will penetrate from the lazarette anyway. Throw a bit of Bio Clean or similar from pet shops in the bilges and it'll soon be gone,

 

[bignick]

Does each tank have its own filler? If so mark them or, sooner or later, you'll end up with water in the diesel tank or, worse, diesel in the water one, and it WILL happen when you're tired, stressed and hurried.

No, there is only a water filler into the port tank. The starboard one fills from the port tank via the link pipe.
thank you for the suggestion of a tank drain. I was thinking of having the bottom exit welded up and fitting a drain instead would save the need to do that.