On Sun, 06 Feb 2005 14:49:55 +0000, Chris Newport
wrote:
In reality 20 knots is going to be too uncomfortable and most
longer trips will be at somewhere between 8 and 10 knots, giving
typically between 1 and 2 mpg. Semidisplacement boats can be very
economomic at low speeds.

Having the power available to go faster in calm conditions is
nice to have, but comes a price in terms of the fuel consumption
of a large engine not being optimised for slow running and the need
for regular blasts at full power to blow out the sooting.

==========================================

Exactly right on both points which is why we ended up with a GB49
trawler instead of a Hatt 53 motor yacht. My experience on offshore
runs has been that anything over 12 kts gets uncomfortable when the
seas are more than 3 feet or so, which if common. All of the motor
yachts and sportfish in that size range have turbo engines which
demand some full power running time to keep them operational.
Naturally aspirated 671s however will run almost forever at 1200 RPM
and regularly do so in commercial generator service.