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Old 08-08-2011, 09:50 PM   #2
Marin
Scraping Paint
 
City: -
Join Date: Oct 2007
Posts: 13,745
RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

The drag from the unpowered prop will be considerable. Not as much if it freewheels but some transmissions won't tolerate that. Also some boats need cooling water fed to the shaft log from the engine's raw water system, which it won't be if the engine isn't running. This is the case with our boat. So the shaft needs to be locked off which means the locked-off prop will be producing even more drag. (It's a popular myth that a locked prop produces less drag than a freewheeling prop but MIT showed in a series of tank tests quite awhile ago that this is just a myth.)

The founder of the Grand Banks owners forum ran some pretty comprehensive tests a number of years ago with his own boat, a twin-engine Alaskan 45, and in the end found that running on one with the unpowered prop freewheeling and locked did not save enough fuel to care about, and at some power settings didn't save any fuel at all or even used more. His conclusion was that unless one was doing some really long-distance cruising, in which case the best solution is to remove the unpowered prop altogether, running on one with a boat like a GB is not productive, particularly in light of the fact that fuel is probably the smallest factor in the cost of owning a boat, even at today's prices.

Running either a single or twin with less power definitely does save fuel. The only limiting factor is engine temperature. You don't want to back off the power to the point where the engine is running too cool. How detrimental this is will be dependent on the engine type-- some apparently tolerate it better than others. But even a 100 rpm reduction in power will reduce the fuel consumption by a not-insignificant amount if you're willing to go slower (we aren't :-) )
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