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Old 04-05-2010, 13:18   #1
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Going from a 2-Blade Folding to a 3-Blade Fixed Prop

I have a 1986 O'day 40 and my folding prop threw a blade. I want to switch to a new prop that will give me more maneuverability, accelleration, and perhaps top speed and I have read that a 3 blade fixed is the way to go (and that the additional drag really won't mean much to me as a cruiser). I know I should go down in pitch, but how far down when switching to a 3 blade fixed? Fat blade, or Campbell sailor? Please advise, there is a local shop full of used props, but I don't know which one to pick. I am guessing, stay with 17" diameter, and go down on the pitch, perhaps to an 11?

Please advise, thanks ahead of time for your advice.

Current prop: 17 diam x 10 pitch.
1 1/4" shaft
LOA 39'7"
LWL 33'6"
displacement is 18,500 (with a shoal keel) drafts 4'11
beam 12'7 1/2 "

engine is a Westerbeke 46, max RPM 3800 (however, with the age, and wear of the engine, 3500 is about as high as go.
Transmission is an HBW, but I have not found the gear ratio yet.
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Old 04-05-2010, 14:09   #2
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Hi pireality,

I have the same engine in 24000lb, full keel Rhodes Reliant. I am turning a 3-bladed, right hand 17 x 13 Michigan wheel, on 1 1/4" shaft. HBW 150 gearbox. I think I have a lot more bottom area than you, and also about 3 1/2" deadwood at the cutless bearing. With a clean bottom I can get 7.2 knots (GPS and Seatalk) at 2300rpm. No vibration, smooth engine, no overheating, 180* @ 70*F, 200* @ 85*F ocean temp.
I think I am going to reduce the pitch by 1" and leave the diameteralone.
The stern walks to port quite powerfully in reverse, and I always try to put the starboard side to the dock. A little kick ahead, followed by a short burst astern, and I can have her 30* off the dock in 20ft of travel.
There are a number of threads on CF which discuss the pitch reduction when going from 2 to 3 blade, and also the pros and cons of being able to attain peak engine revs.
IMHO, if the engine is not smoking, lugging or overheating, why run it at high RPM when lower speed work well.
so many projects--so little time !!
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Old 05-05-2010, 20:00   #3
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2-blade / 3-blade / 4-blade props are a function of power available from the engine versus the physical space available to mount the propeller.
- - With a given amount of power from the engine a 2-blade propeller is the best form for a sailboat as it is possible to "hide" the propeller blades behind the keel when the engine is off. However if the space available for the propeller is not sufficient - i.e., there is the main hull in the way then you need to go with a propeller with shorter blades.
- - The 3-blade propeller for the same "given engine power" will have a smaller blade length. And a 4-blade will have an even smaller blade length. What you get into when going with a shorter blade length and/or more blades is more resistance to water flow (drag) as the blade hang outside the shade of the vertical keel. Additionally as the blades length gets shorter there is less "clear water" for the propeller to bite into that is not affected by the flow of water around the vertical keel.
- - So you first determine how much physical space there is available for a propeller - including blade tip clearance from the bottom of the main hull - and then match the power available from the engine. With small hp engines a 2-blade is probably best because you want to get the blades out away from the shadow of the keel. As the engine power available increases the 2-blade propeller will have to have longer blades and there might not be sufficient room. So you move to the 3-blade and again match the power to blade length. With large engines - say a motor-sailor - a 4-blade may be what is needed due to limited space available.
- - Pitch is used to "fine-tune" the available power to the boat characteristics so that you do not "lug" the engine (black smoke) and cannot achieve max rpms. Or you get max rpm's too quickly/easily. The reason to use pitch to fine tune is that you are limited in pitch range by the propeller manufacturers. They do not want to make too many different pitch "versions" of the same size propeller so generally when the pitch vs size of a propeller overlaps the next size propeller they discontinue making the previous size&pitch. Which boils down to there is a limited number of pitches available for a given size propeller.
- - Best advice I have is research what that make/model/size boat (engine) had originally installed before the folding propeller was installed - the manufacturer's recommended fixed propeller size. Use that as your starting size and find a used propeller place that will lend you the same size prop in different pitches. Try each one to which works best on your particular boat. Then buy that one.
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2 blade, 3 blade, pitch

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