Thread: Propellers
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Old 02-27-2008, 10:00 PM   #14
Marin
Scraping Paint
 
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Join Date: Oct 2007
Posts: 13,745
RE: Propellers

Mark--

A finer pitch prop will generate less thrust at a given rpm than a coarser pitched prop at the same rpm. So yes, you are correct in that if you reduced your prop pitch you would not be generating as much thrust at any given engine rpm as you do now. Of course nothing is ever this simple, and there are other factors like turbulence, cavitation, waterflow into the prop, etc. that can effect the efficiency of a prop besides just pitch. But looking at pitch alone, a finer pitch will require more rpm to achieve a specific amount of thrust than a coarser pitch.

However---- a finer pitch prop may allow your engine to develop more horsepower than a coarser pitch prop because the engine can turn faster (at least up to its governed speed). This is why many prop-driven aircraft have variable pitch propellers. For takeoff the pitch is set at its finest pitch so the engine can spool up to give maximum horsepower. In cruise the propeller can be set to a coarser pitch so it will move more air at a lower rpm setting for better cruise efficiency.

John---

I'd need to look at the actual plates on the transmissions, but the typical reduction on BW Velvet Drives is (I'm going from memory here so it may not be exactly right) 1.9-something on the port side and 2.1-something on the starboard side. Or it may be the other way round. In any event, the difference is due to the extra gear in the starboard side to counter-rotate its prop. We'll be back up at the boat this weekend and I'll try to remember to look.

Speaking of propellers, here are a couple of photos I took today in the yard where our boat is being worked on. This is not a new boat--- it's 16 years old--- but the owners keep it in immaculate condition. The yard manager told me today that the insides of the fish holds are all polished stainless. The thing under the prop is called a "beaver tail," and its purpose, according to the yard manager, is to keep nets and stuff out of the prop and also act as an anti-cavitation plate, similar in function although perhaps not as effective as a Kort nozzle. In any event, it increases the efficiency of the prop. This boat is apparently worked very hard and very successfully in a variety of fishing modes, from crabbing to longlining to net fishing.

-- Edited by Marin at 00:04, 2008-02-28
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