As an example, in the NTXBG, a 1/144 scale Richelieu (close to 6' long, in the 35-45 lb range) is well overspeed with 2 380's (aka Speed 400s) and a 3.2:1 gearbox. The actual true speed is about 4.9 km/h (which is 49 scale knots under our rules), and has to be dialed back (at 6v) to be within her legal, scale limit.

I am thinking a 280 with a 5:1 gearbox (Tower sells a 5:1 enclosed gearbox for well under $10) would probably be adequate for the ship you are building (many of our much smaller ships get by with a single one of those, and, in fact a 39" Liberty ship, when given more voltage, sill throw out a rooster-tail).

Probably either would be fine, though you will not need as much gearing as in tanks. And some of this depends on the prop size and pitch (think of that as additional gearing).

Cheers,

Wreno