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Old 08-09-2011, 08:38 PM   #5
Marin
Scraping Paint
 
City: -
Join Date: Oct 2007
Posts: 13,745
RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

As a FWIW data point, we run both our FL120s all the time and run them at 1650 rpm. That gives us a wee bit under 8 knots (we had the props pitched an inch down a couple of years ago, we used to run 8 knots). So far, after 13 years, the engines don't smoke or leave any soot on the transom and each one of them uses less than 1 guart of oil every 100-150 hours which is our oil change interval.

The operators manual for our BW Velvet Drives says the transmission can be freewheeled with no problems at "sailing or trolling " speeds, which is not too helpful of a guideline. But I take it to be "slow." Slower than we go, I think. Also, our shaft logs are cooled/lubed by water feeds from each engine's raw water system so if we have to shut an engine down we have to tie off the shaft. With the prop locked, the boat is no fun to drive at all. The turbulence off the locked prop buffets the rudder behind it something fierce and all this is fed back up the cable-chain steering to the helm (I guess it wouldn't be with hydraulic steering).

And eight knots is painfully slow as it is. To reduce the speed to six or so would be intolerable and certainly not worth (to us) the savings of a few dollars worth of fuel. But then if we could afford it we'd have a boat that could do 30 knots. Actually, we have a boat that does 30 (mph). But it's only 17 feet long and not the sort of thing you'd want to take a three week cruise in with friends.* But it goes the right speed for us :-)
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