Jim Wright says: >Close. How about "minimize *shaft* RPM for a given horsepower output? >That's why we have the reduction gears, so the engine can run nearer its max >horsepower but the shaft RPM can be lower than the engine. Why do we want to minimize shaft RPM?--actually what we want to do is to swing a bigger prop--bigger props are more effecient than smaller props. Thus work boats-may have reduction ratios up to 6 or more to one--3 to 1 is much more common on recreational boats--and thus a 28 to 32 inch wheel can be swung on a boat like the Nordhaven 40. The Engine on the Nordhaven is a Lugger L668 D, which according to the company web site is rated at 105 hp at 2200 RPM continous duty--135 hp high output @ 2500 RPM. I was not able to down load the power curves, but the web site states the engine is fuel effecient at slow speeds. It is a "large" engine for the hp--and should have excellent longivity--the web site estimates 30,000 + hours of service before overhaul. I believe that the transmission is a 3:1 reduction gear. The question poised by Larry Zatlin was ref prop effeciency--calculated to be in the mid 30%, rather than the 50% that might be expected in a passagemaker at this speed. Is this due to too small a pitch, or too great resistance?--If you review the voyage only a minority of the time were the wind and seas behind the boat--I think that this resistance is probably enough, combined with the drag of the stabilizers and a fully loaded boat to account for the apparent loss of effeciency or slip of the prop. If you put the bow of the boat against an immobile object, no matter how many RPM, or HP produced, the effeciency is 0%. If you tried to push a string of barges it might be 10% If the boat was nearly empty, with wind and waves behind, it might well be 60%. The more easily driven the hull, the better effeciency and less the slipage. There is a danger in this "over proping" thread. We have been talking about natural aspirated engines--where a running at low RPM is not particularly harmful, as long as they are kept up to operating temperature. If you attempt to do this with a turbo engine you are much more likely to do dammage.--the engine is now allowed to come up to RPM and spin the turbo adequately to keep carbon deposits from forming. I have very limited experience with turbo engines--65,000 miles in a motorhome where I ran the engine at 85% rated RPM 90% of the time--and my current boat, where I normally run at 80% of rated RPM, but at least once every few hours take the engine to 90% of rated RPM for several minutes. Several mechanics have recommended this mode of operation. If the boat was "over proped" then the 85 to 90% of rated RPM could not be realized. Since the Norhaven 40 is a proven vessel, I suspect that the prop used on this voyage was carefully chosen by the designers. They have had plenty of time to experiment with different props. Many times it takes a number of trials to get the correct prop for a specific boat. There are a few prop shops which have props that can be tried until the proper setup is found. (and as pointed out it will change with load and condition of the boat) Bob Austin (Sailing was much easier! All you had to do was pull some strings and the boat went "thataway".)