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Old 11-08-2016, 09:37 AM   #26
Nomad Willy
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City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
Peter,
To get the right amount of "slip" most w trawlers choose a prop w about .75 X the dia. Like 24" dia X 18" pitch. Too much dia and too little pitch like 24X12 means there is too much blade area and too much surface area friction drag. On the other hand too much pitch like 19X21 would have too much water at the blade tips spilling over to the other side loosing pressure differential that would otherwise create thrust. And a 4 blade prop should only be used if a 3 blade w an appropriate pitch/dia ratio lacks blade area. The 4 blade is usually used on a trawler if there isn't enough room for a 3 blade w the right blade area.

Re FD my boat has 40hp, 8 tons, 27.5' WLL, 3000rpm rated power, 6.15 knots cruise at 50% load w 2300rpm. A BW gear w 2.57-1 and an 18X13 prop. The prop was 18X14 but couldn't get close to rated rpm. Took out 1" pitch and am still about 25-50rpm short and have called that good enough. At the next haulout I'll cut the leading edge of the prop back perhaps 1/16" to 1/8" and hope for 25-50 rpm above rated.

I once had a wild/crazy idea of cutting the power in half of an overpowered trawler by removing half of the pistons, valves, ect ect as a poor man's way to cut the power to weight ratio in half. A GB36 twin may run at 50-60% load and 6.5 to 7 knot cruise. Just a guess but the sometimes overheating #6 cyl may forever run cool depending on what cylinders were choosen to be closed off. Never heard of it being done and mention it just for laughs.

These are my opinions and there are some here that have different opinions but I think most (close to all .. or all) engine manufacturers would agree. There is lots posted in the past on engine and prop choices so go surfing.
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Eric

North Western Washington State USA
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