Rich posted "....8.1 knots at 3.9 GPH and 5 GPH at 8.6 knots....Assuming a speed increase is proportional to hp (and fuel consumption) cubed: taking the ratio of speeds 8.1/8.6 and cubing would suggest a fuel savings of about 0.83 (savings of about 17%)... .83 x 5 GPH is 4.2 calculated instead of his 3.9 actual... quite close considering unknown factors such as currents, winds, and sea conditions.... J&M posted: "...."weight" (as weighed on a lift and DISPLACEMENT (the weight of the water displaced) can be very different-that is where hull shape becomes a factor...." Both aspects of that statement are incorrect. (Archimedes discovered the relationship.) A 40,000 lb boat always displaces 40,000 lbs of water...while afloat....regardless of hull shape, regardless of whether fresh or salt water....But this displacement is different than a calculated "tonnage" for USCG documentation purposes. ------------------------------------------------------------------------ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now.

Robin Brueckner wrote: > ...But this displacement is different than a > calculated "tonnage" for USCG documentation purposes. > > FWIW, "tonnage" has nothing whatsoever to do with weight (or displacement). It's a measure of volume, figured differently on gross or net basis. A "ton" in tonnage is 100 cubic feet. It matters not weather those 100 cubic feet are filled with air, water, or lead, it's still a ton. Completely unrelated to the other meaning of "ton" having to do with weights (where, even there, how much a ton weighs depends on which ton you're talking about -- short, long, metric, etc.). Tonnage is properly measured, not calculated. But it's hard to measure the volume of an irregular shape such as a boat, and so you are permitted, for certain purposes, to "estimate" tonnage using a formula. -Sean http://OurOdyssey.BlogSpot.com