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post #1 of Old 09-14-2009 Thread Starter
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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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