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Thanks, Larry, for the great summary. The typical lack of room to swing a
maximum efficiency diameter prop is one reason that trawlers with smaller
engines (relative to boat displacement) are more efficient than those with
bigger engines. Bigger engines need to swing a bigger prop to approach the
roughly 50-60% propeller efficiency that should be feasible (but is rarely
achieved) in our boats, and the room is usually not there to swing such a
prop.
When I bought Pooh (46' and 33,000 lbs), he had a 75 hp Westerbeke engine
and was turning a 20" diameter prop. After some
engine/transmission/propeller analysis using Dave Gerr's "Propeller
Handbook", I concluded that to get best efficiency from the 80hp Jphn Deere
I planned to install, I needed to swing a 28", or at least a 26" diameter
3-blade prop, using a 2.8 transmission ratio. How many boats swing a prop
this size with only 80hp? More typical would be a 26" prop for 135hp.
As I was aiming for trans-Atlantic range from the 460 gals of diesel tankage
that could be crammed-in comfortably, efficiency was a very high priority
for the project. I spent considerable time and effort to increase the prop
diameter space; by lengthening the shaft, building a new strut, and even
removing about 2" vertically from the V of the hull over the propeller area.
The operation was a success, with Pooh achieving 5 nmpg at 7.5 kts.
Mark Richter, m/v Winnie the Pooh, Ortona, FL on the Okeechobee Waterway
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