Repurposing a Water Tank For Fuel

Still searching for the right boat in todays very tight market. Went to look at an Albin 36 today. The bones seem fine and the engine has no obvious issues despite its' 6500 hours.

It does have the usual old trawler issues including the teak decks and fuel tanks. One of the tanks is toast and the other is questionable. While checking out the steering linkage I noticed the two large 110 gal. water tanks in the stern. The thought struck me: Why not repurpose one of those tanks for fuel? 110 gals of diesel would be plenty for my needs and I could cut the two old tanks out of the engine compartment and add a huge amount of storage space while greatly increasing engine accessibility. Has anyone out there done something like this? Is there any reason I shouldn't do it?

Thanks in advance for the schooling I am about to receive.

It all depends on your chosen cruising ground and boat trim.
I had a single engine centre cockpit Birchwood 33 with a fuel tank in the engine room and 2 water tanks.
Refuelling was always a hassle with cans, only able to refuel on one side etc so I removed the tank, made it larger, with a cleaning hatch and a fill pipe on both sides and I was able to refill direct from a 45 gall drum which made life much easier.
The galvanised fuel tanks were shot so I replaced them with s/steel, again with cleaning hatch, and a levelling pipe.
The whole point was to keep the balance and trim of the boat correct whatever the fill state of the tanks.
110 gallons of water at the stern I would think would make the boat tend to 'squat' when under way.
This is just my opinion, others may differ.

The builder placed those two water tanks in the stern of the boat, so it would stand to reason that they took boat balance and loading into account when they did so. Weather one tank is filled with diesel instead of water should have no discernable effect on boat trim.

Are you sure water tank material would withstand fuel? Better to check first.
If you use your water tank for fuel where would you store water? I personally run out of water before running out of fuel and I only have 500L max capacity.

L

There are two water tanks in the stern of the boat. One on each side. I would leave one tank for water and repurpose the other for fuel.

The water tanks are stainless steel; as are the two fuel tanks in the engine compartment. I was surprised when the seller told me the fuel tanks were leaking. I know these old Taiwan trawlers are notorious for having iron fuel tanks that rust out and leak, but the ones in this boat are both stainless.

You may check why/where they are leaking. If it is a hose or a fitting this could be an easy fix.

L

Could be the welds on the stainless causing rusting.

Lou_tribal said:

You may check why/where they are leaking. If it is a hose or a fitting this could be an easy fix.

L

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Yes, definitely check this possibility out, as Lou suggests. Stainless fuel tanks would be rare in a boat of that age, and should not have rusted tops as so often happened with leaking teak decks and mild steel tanks. It makes one wonder if those tanks have already been replaced - or they are not stainless. If they are, the leaks may well be fixable.

However, even if the tanks are cactus all is not lost. When I bought my boat, the immediate previous owner had already taken the action of emptying and blocking off the original mild steel tanks, and left them in situ because of the cost of removal. They were huge - (about 1000L each) - far larger than needed for almost all likely uses other than ocean crossings. I tended to regard them as positive buoyancy flotation tanks, in the case of an engine-room inundation.

However, there were two 400L stainless water tanks in the lazaret, which he had converted for diesel, and they were interconnected, so self-levelling. For water he had just put in a large water bladder - he was a frugal character, but had spent quite a lot already repairing water damaged timber and decks, so I understood where he was coming from.

I had two 250L food grade plastic water tanks made and I installed them in between the fuel tanks - also interconnected and self-levelling, and the whole set-up worked well.

The other unexpected benefit was that far from messing up the boat's trim, it actually made it more level, with the combined weight in the broadest most buoyant part of the hull, and the rearward weight worked well in avoiding pounding into a head sea, and broaching. The boat just loved travelling down wind, and could surf, 'literally' on large waves with no hint of broaching, and a light helm. That arrangement was still working well when I sold it 16 years later.

How are you going to correct trim as fuel gets used? Or would you consume water from the oppoite tank at roughly the same rate?

Removing ~100 gal from the extremety of the boat with no counterbalance would likey give several degrees of list and it sounds as if there isnt a sister tank to alternately draw from.

GoneDiving said:

How are you going to correct trim as fuel gets used? Or would you consume water from the oppoite tank at roughly the same rate?

Removing ~100 gal from the extremety of the boat with no counterbalance would likey give several degrees of list and it sounds as if there isnt a sister tank to alternately draw from.

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Which, if he has to do this, and the current fuel tanks can't be repaired, is why it would be better if he does what I described done on my boat, above, and use both water tanks for fuel, and put in two new plastic tanks for water...

Peter B said:

Which, if he has to do this, and the current fuel tanks can't be repaired, is why it would be better if he does what I described done on my boat, above, and use both water tanks for fuel, and put in two new plastic tanks for water...

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Thanks for all the replies guys.

I have never owned a boat with twin fuel tanks before. I have read bits and pieces here on the forum about people interconnecting the tanks or drawing from first one and then the other, but keeping them separated in case one should become contaminated. I can certainly see where having one large tank full while emptying the other would cause an imbalance of weight; especially with the tanks being so far outboard. What is the usual practice with single engine users - do you interconnect the tanks so they are always at the same level, or do you go below and switch from one tank to the other every few running hours to keep the levels consistent?

I agree with Peter; I wonder if the original tanks have already been replaced with newer SS ones. If that is the case they should theoretically be repairable. The question would be how practically possible that would actually be. How do you get at a tank to find and fix a leak in such a confined space. Again; I don't have any experience with this. Is this sort of repair actually done?

Complicating the problem is the fact that the current owner has already found someone who will remove the current tanks by cutting them up in place and removing the pieces. This person has already "demonstrated" his ability to do this by cutting a piece out of one of the tanks.

Thanks again for all the replies. It is starting to scare me how much I don't know about owning this kind of a boat.

Good on you for doing your diligence! Just one more potentially relevant observation to complement what has already been said; we have a CHB 41, a bit longer but similar boat. We have two 200g fuel tanks midship, and two 200g water tanks in the very stern. If I were to fully drain the fuel tanks and fully fill the water tanks we would be quite stern heavy; I certainly wouldn't want to make a passage trimmed like that.

I like the idea of new smaller plastic water tanks. I've got a full boat re-plumb on my list anyway; the idea of having new tanks as well sounds great.

Wondering if all the tanks are the same vintage ? Something to ponder. If 1 or 2 leak now.....

R. Bush said:

What is the usual practice with single engine users - do you interconnect the tanks so they are always at the same level, or do you go below and switch from one tank to the other every few running hours to keep the levels consistent?

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Ideally both. In calm seas it's easy to just draw from both tanks at the same time, automatically keeping the boat trimmed. However, in rougher seas or if the vessel does pick up a list, you want to be able to draw from one side to correct this and prevent even more fuel flowing to the lower tank, making the problem worse.

Don't forget that many diesel engines return much more fuel than they use. It's good to also be able to choose where this fuel is going too. On a full days motor I can literally empty one 2500ltr tank into another.