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Old 01-11-2020, 09:56 PM   #1
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I have been running a 24 x 24 three bladed LH prop on a 98hp Ford in a displacement hull with a cruise speed of 7.5knots at about 1800 to 2000rpm.
I hit a rock today that was not there yesterday!!!! and have bent a blade so am looking at a replacement prop.
Can someone tell me the perceived performance or characteristics between 3, 4 and maybe 5 bladed units.

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Old 01-11-2020, 11:07 PM   #2
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I can't tell you, but if you can find a copy of Dave Gerr's Propeller Book, you can learn oodles, and help yourself.

I got mine from the local library and used it when I repowered.

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Old 01-12-2020, 12:47 AM   #3
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I agree about the book by D. Gerr. Ebay may get you a used copy.

Have you asked the prop shop about repair as they can do some amazing repair work of course depending upon the material the prop is constructed of.

IS the unit you have not suitable or do you simply want to double check that it is a suitable prop? The shop may even be able to add or reduce pitch if wanted.

Generally a four blade is smoother than a three producing less vibration and gives a better grab but the four is less efficient. A four may fit where a three won't as the blade area is higher so the diameter can be reduced somewhat.
The same can be said generally for a five but those are often left unless the other two simply cannot be made to work. Of course cost raises it head so unless there is a good reason for the extra blades it is usually not worth it. u

But get the book as there is lot to learn about them and why one would be chosen over another.
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Old 01-12-2020, 03:26 PM   #4
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Originally Posted by C lectric View Post
I agree about the book by D. Gerr. Ebay may get you a used copy.
I'll second that, and raise you: any book by Dave Gerr is worth buying and spending long hours with.
"It's never too late to have a happy childhood." — Tom Robbins
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Old 01-12-2020, 04:24 PM   #5
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Three blades will be the most efficient and the probably require the largest diameter. Four and five blades, incrementally smaller diameter and smoother as well as less prone to cavitation at speed or slap while cruising.

Third reading Dave Gerr’s propeller handbook. I imagine there is a go to prop guru somewhere nearby who can also advise you if repair is not an option.
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Old 01-12-2020, 07:42 PM   #6
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Gerr "s handbook is a must have. Quite a lot of 2nd hand available.

New from $14
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Old 01-12-2020, 07:59 PM   #7
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Some blades look much like the end of a pointed leaf. Others have wide blade tips. The wide blades usually are wide to deal with lots of power or too much for low aspect ratio blades. The long narrow blades are more efficient as there's not so much tip loss from overspill of water.

Fat blades are generally fat to eat up the hp applied to them. So (all other things being equal) the skinny high aspect blade prop will be more efficient just like and for the same reason airplanes are more efficient w long skinny wings.

Usually a trawler w a 4 blade prop has a problem w too little pitch and too much area. They have too much drag just from water sliding over a surface. Kinda like having a prop w no pitch. It would still take more than a small amount of power to turn the prop w no pitch.

If you had a prop w an infinite amount of pitch as in blades at right angles to the “no pitch” prop you would push no water and the boat wouldn’t move. Just a lot of stirring of the water.

So it’s obvious that a prop somewhere in-between the two extremes is needed. For trawlers don’t use numbers. Like a 16” dia prop should have 8”of pitch. Dosn’t work that way. Too many variables. And if some = good dosn’t apply to number of blades for at trawler. There is a range of pitch/diameter ratios that are somewhat ideal. Your prop man knows or otherwise has such information. For general repair he wouldn’t need to know anything about it. But for changing props or buying an original he does. And if you need such information get another prop man or find an engineer.

PDR’s or (pitch/diameter ratios) that are extreme or very unusual are probably not good. A 26 X 14 prop would probably not load the engine or have very low efficiency due to too much drag from blade area. A 16 X 30 would have very serious tip losses.

Many here seek out a 4 blade prop thinking “that’s what the big boys use”. Like smaller boats have 3 bladed props ... and for various good reasons. What one needs is a suitable amount of blade area for the amount of power. The OP w less than the usual amount of power (98hp) would theoretically want to lean to less blade area. But many other factors are involved. Like reduction gear ratio, propeller blade space (diameter wise), blade aspect ratio and general blade tip design as in skewed or symmetrical blade shape ect ect. This one in the pic is quite skewed and typical of (for a trawler) efficiency and high performance. A prop w elephant ear or symmetrical blades have much more reverse thrust. So many variables.

For the OP IMO a three blade w slightly skewed blades should be a fair guess. I use a symmetrical bladed prop whereas the leading rdge has the same shape and blade area as the trailing edge. Michigan Wheel calls them an MP propeller whereas MP stands for machine pitch. I think it means it has the same pitch everywhere on the blade. It’s a little less efficient than skewed blades but prolly only about 5% (IMO). But in reverse the thrust is perhaps even double.

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Old 01-12-2020, 08:01 PM   #8
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Thinking about the tip loss and aspect ratio thing, is it possible that 4 higher aspect ratio blades could be more efficient than 3 fat blades for the same total blade area?
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Old 01-12-2020, 08:26 PM   #9
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First get your old prop to a good prop shop. They can do amazing things!
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Old 01-12-2020, 08:27 PM   #10
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That’s why I was talking about variables.
Quite simply to answer your question .. yes. But it would require more blade swinging room than almost all trawlers have. So if you have the room go for it. But a high aspect ratio 4 blade may have more vibration than the three blade. But a properly set up and balanced 3 blade will have very little vibration.

Island Cessna,
Yes if the original can be fixed, isn’t too thin or dosn’t show too much copper it may be a good bet for quite a few years.


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