Fifty gallons of
diesel weighs in at about 358 lbs and I can guaranty you that you don't want a bladder that weighs in at that much rolling around on your fore-deck and it will roll in any sea, I can guaranty you of that. Moreover, the warnings on most bladder
tanks stipulate that the tabs are not sufficient to secure the tank when filled, which should be installed in a secure enclosure free of chafe.
Your Morris is fitted with a 75 gallon tank of which at least 70 gallons is usable. The typical
engine for that yacht was a 56 hp
Yanmar the burns roughly a gallon an hour and should give you about 6 knots on your water-line. You might want to carry a couple of jerry jugs of spare
fuel, which can be lashed to 2x6's fastened vertically between two of your stanchions on each side of the yacht just aft of amidships (and they'll be protected to some extent by the bulworks) but beyond that you really shouldn't need much more
fuel than that for the trip--and surely not 50 gallons. After all, it is only 1500+/- miles.
Before we bought our
current boat, we had occassion to inspect an
Oceanis 411 in Palm Beach that had once been fitted with a 50 gallon bladder tank in the cock-pit for an intended trip to the
Bahamas. The
family got into the
Gulf Stream where the thing managed to break it's lashings and get thrown around enough that it tore out several of the stanchions and made it impossible to free the lee sheet. They eventually made it back to the coast and into Jupiter Inlet where the wife quit the
boat, vowing never-to-return--hence the
sale offering. We passed.
FWIW...