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Old 08-30-2013, 12:44 PM   #1
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Performance of 4 blade vs 3 blade

Can anyone tell me what the difference is between 3 blade & 4 blade of the same dia. & pitch or point me to a web site. I have 4 blade & have found a set of 3 that I'am contemplating buying for spares, I'll give more for a set I could leave on the boat rather than "get home spares" that I'ld have to change out again. Thank you for any guidance.

Ron on Northern Lights II
I don't like making plans for the day because the word "premeditated" gets thrown around in the courtroom.
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Old 08-30-2013, 12:52 PM   #2
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There are many variables to props, but an abbreviated explanation is, the 4 bladed prop has more surface area therefore more thrust potential, conversely for a faster boat the added drag of a fourth blade can reduce speed at the high end. You can argue these points (cup, blade configuration etc) but generaliscally that is it.

If the 3 bladed prop is exact same pitch and diameter, you may find it will have a slightly higher WOT rpm in addition not as smooth. If they are real cheap and you have no spares now, that would be a good thing to have, just make sure they are secured very well if kept on the vessel.

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Old 08-30-2013, 01:11 PM   #3
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Generally when going from 3 blades to 4, diameter is multiplied by .94, and pitch by .98. This assumes both the diameter and pitch of the existing propeller is optimum already. When going from 4 to 3 blades 1.06 and 1.02 are the multipliers. So the correct change in size is small.

You said "pair" of props so I will assume they are not behind a keel. Given that, the 3 blade props will be more efficient with perhaps slightly more vibration. If too close to the hull (<15%D) either will cause vibration but at a different frequency and one you may or may not like.

If I heard you wrong and the prop is behind a keel, stick with 4 blades IMHO
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Old 08-30-2013, 01:15 PM   #4
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(Ron- Sent you a PM with the difference)
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Old 08-30-2013, 02:27 PM   #5
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I think propeller loading is the important issue. Trumps # of blades most always. Most people on most boats can't tell the difference between 3or 4 blades.

HP per sq in of blade. I have a 3blade 18" dia but only have 40hp so ideally I'd probably be better loaded w a 2 blade. But I have a wide and blunt trailing edge and the two blade may not be very smooth.

I'm having thoughts about cutting down the LE and TE to reduce the blade area. If you don't have enough pitch more pitch is better. Too many trawlers have 4blade props w very little pitch and are consuming a lot of power just spinning their flatfish blade around. The drag of water flowing over the blades is a lot of drag if you have too much area. Less area and more pitch would be better for many.

Lots of people think diameter is everything in the quest of thrust but not so from my experience.

North Western Washington State USA
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Old 08-30-2013, 03:16 PM   #6
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A propeller with a diameter larger than will fit under my boat was suggested. Can a smaller diameter still be used without hurting my performance significantly? What other adjustments to the propeller should be made to compensate?

If the difference between the optimal and allowable diameter is minimal, say 1 or 2 inches for a medium size pleasure boat, a corresponding increase in propeller pitch will be effective at maintaining reasonable propeller performance. For some situations, it may be suggested to increase blade number, say from 3 to 4, to provide more blade area on the diameter constrained propeller. For some rare cases, particularly on re-power projects, a engine-gear combination is selected with the wrong gear reduction ratio. This can result in a significant propeller selection problem with no real solution other than changing to a different gear reduction ration.

You suggested a 13 diameter x 14 pitch propeller for my boat. Would a smaller diameter propeller with more pitch provide the same performance?

A propeller with smaller diameter and larger pitch might provide the same load on the boats engines but may not have the same efficiency as the suggested propeller. Propeller diameter is chosen to be optimal for the boats particular combination of horsepower, RPM and speed. Deviating significantly from the best diameter may result in slower acceleration, reduced cruise speeds and efficiencies, and more difficulty in slow speed maneuvering.

Can one propeller be best for both maximum boat speed and maximum pulling power, say when pulling a skier or fish nets?

In general, no. If maximum pulling power is desired, the propeller size, particularly the pitch value, is often reduced to allow higher engine RPM (where more power is produced) at lower boat speeds. For maximum top boat speed, the optimum propeller pitch value will typically be higher.

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