FWIW, I'd be leery about 400-450 NM range at that speed range except maybe in ideal circumstances, or at slow "trawler" speeds (which might
work fine).
First, the 400 figure would need .8 NMPG all in, i.e., with no reserve... not counting extra reserve fuel on
deck... and I doubt that'll happen on those DDs at that speed range. (Are they 485 HP? Can't remember DD numbers...)
Second, I suspect 10-12 kts would probably be a very inefficient speed range, probably pushing a lot of
water. I'd have guessed comfortable
cruise on plane would maybe be more like 16-18 kts.
Anyway, the waterline length is likely more like 40' -- so maximum theoretical
displacement hull speed would be about 8.47 kts. (Have to refine that, based on true waterline length. Formula is 1.34xSQRT-LWL)
Best fuel economy would likely be somewhere between 6.32 kts and 7.59 kts (1.2x).
Given GPH/engine at a given
RPM, and once you know what
RPM gives what speed, you could figure range at 7.5 kts with a built-in fuel reserve of 10% or whatever.
-Chris