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  #1  
Old 09-14-2009
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Converting water tank to diesel tank - fittings?

On my boat I have 1 poly diesel tank (250l) and 3 x 250 poly water tanks. Since I've got a functioning watermaker I believe the the two for'ard water tanks totalling 500l are sufficient and would like to convert the aft water tank to hold diesel. As I'll be heading out through the Canal this year I'd like to extend my boat's range under engine without having to resort to stowing canisters on deck or elsewhere on the boat.

The tanks are at the same level within a foot of each other, underneath the bunks in the aft port (diesel) and starboard (water) cabins. the material is the same for both tanks, but the water is blue and the diesel black.

I'd like to replace the water filler cap with a diesel one, including the hoses, so that I can fill from either side of the boat. I'd also like to combine both tanks so that (a) they both drain at the same time and (b) they don't potentially cause balance/trim problems when heeled over by draining from high to low tanks.

At first I though I could just connect the tanks at their lowest points with a relatively small diameter hose that has a cutoff valve. I could leave this open to allow the tanks to equalize except for times when the boat will be heeled over for extended periods. My thought was that the small diameter would prevent fuel rushing from port to starboard and vice-versa in dangerous amounts in rolling conditions but still allow both tanks to drain equally. The advantage to this solution would be that I wouldn't have to change the fuel pickups and tubing currently installed for the genset and diesel; just connect the two tanks (and removing all freshwater-related tubing and replacing the fueling caps & tubes).

The other method that comes to mind is not connecting the tanks but fitting each with identical fuel pickups, connecting those pickup lines in a T-valve and letting gravity and vacuum pressure ensure that both tanks get drained at the same time. This solution is more complicated technically, but would have some advantages - I could separate the two tanks and put fuel filters inline between tank and T-valve and could (theoretically) replace one tank's filter while the engine is running and the T-Valve set to take fuel from the "good" tank instead of from both.

The 250l is woefully inadequate considering my Yanmar 4JH4-HTE's fuel consumption of 15l/hour at max cruising speed. I really don't motor too much (6 months cruising up and down the Caribbean this year and only filled up once and topped off another time prior to storage) but want that security of being able to turn on the "iron wind" generator and avoid storm systems.

I'd be very interested in hearing from those who have done something similar or who have better alternatives (or who can tell me why my method won't work in practice).
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Old 09-14-2009
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No can do! The plastics used to make fuel tanks is not the same stuff they use to make water tanks. You must use cross link polymer plastics for use as a fuel storage tank.
I'm quite sure your insurance company will also have something to say about your plan.
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Old 09-14-2009
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What about simply adding a fuel bladder? You could even put the fuel bladder inside the water tank?

I will be doing something similar and am going with a bladder.

You will probably have to pull the fill hose to something appropriate for diesel. I also would leave them independent since it gives you more flexibility and safety. I would think your biggest concern would be getting bad fuel versus off-balance.

Just my thoughts.

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Old 09-14-2009
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Off balance is not an issue.
Can you now sail with water tank empty and fuel tank full? Or vice versa ?
It is like two people sitting on one cockpit bench or the other one.
It does not matter for your size boat.
I would keep them separated and use a T valve to select one or another.
The material of the tank is an issue. Bladder in the tank may be a good idea.
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Old 09-15-2009
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I agree with Tomaz. I would not connect the tanks. One nice feature of two seperate tanks is the knowledge that when one runs empty you know precisely what you have left.

If you choose to keep them seperate be aware that the fuel return line on your Yanmar has to be switched from tank to tank at the same time as switching the supply or you will end up over-filling one tank at the same time as emptying the other.

Also running both tanks down at the same time means two places to gather condensation while the tanks are less than full. Your return fuel will be warm and this promotes condensation. You should try to avoid this in both tanks.

The other (perhaps off topic) is your fuel consumption. Maybe you're propped differently or maybe you motor really fast (full throttle?) because I get about 5.5 knots at under 2000 RPM (same engine) and my consumption is consistently around 5 litres an hour. 15l/h would scare me.
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Old 09-15-2009
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I will check into which material is used for the water tank - luckily it is in a position where it can easily be replaced if necessary.
Because the fuel pickups & return tubing is complex I wanted to keep it simple and just use one tank, but after reading the replies I think I will put in 2 separate systems. I think I'll go the route of a new tank versus a bladder, though - I have this image of the bladder chafing and me having 250l of diesel in the bilge.

Omatako - the fuel consumption of 15l/hour is at max cruising speed (i.e. what I'd use to get out of the way of a big depression) and is the published figure by Tohatsu for 2800RPM. I get good mileage at 2000rpm as well and fuel comsumption is in line with 5l - even better when motorsailing with the Gori prop set to "overdrive"; but I would like to be able to have the option of running more than 24hours at "flank speed" - naturally in the hopes that I'll never actually have to do it. As mentioned before, I don't motor much and most of my fuel is consumed running my genset, but for the big pacific distances I would prefer to have reserves.
(addendum) Yanmar 4Jh$ fuel consumption
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Last edited by Zanshin; 09-15-2009 at 05:40 AM. Reason: Added URL
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