Join Date: Feb 2012
Location: New Orleans Louisiana
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Re: Diesel Keel Tank Replacement
The ballast value of the tank is going to be pretty minimal. No designer worth discussing is going to rely on the ballast value of a consumables tank for anything. It MIGHT make the ride a bit more comfortable, but it could just as easily give the boat a snap roll when filled. Frankly given the location the difference is going to be roughly the same as one person sitting on the centerline versus moving 3' towards the high side. There just isn't enough lever arm to matter.
As far as what do do with the space... I would need to see it. I could imagine batteries would be fine if you could build a box for them, or bulk long term storage (canned food, booze, rice&beans). Or perhaps just give yourself a deeper bilge. It really depends on the arrangement.
Whatever you do, do not turn it into an integral tank. This does work with metal boats, but there are numerous reports of diesel eating away at the resin in fiberglass tanks, let alone fiberglass hulls. Water is almost as bad, think blisters on the inside of the hull. Not to mention the issues of dealing with an integral tank can be massive (how do you install a drain in it?).
Really I think you are over thinking this, your existing fuel storage can get you a good ways across the Atlantic, I wouldn't worry about it. Figure you will get about 6kn at 1gallon/hr fuel burn. Likely much better than this, but being conservative. My Beneteau 381 is more like .5gallon/hr @6.5, and my old Irwin 54 was just under 1gall/hr at 7.5kn. I doubt you will get worse fuel economy than the Irwin, and will almost defiantly fall in between the two.
Trying to eek out the last but of fuel range just isn't worth it.