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Old 08-22-2020, 02:10 PM   #1
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4 blade prop

Has anybody tried a 4 blade prop ? Any advanrage ? Any increased speed ? RPM change ? I'm running a Ford Lehman 120 in a 30 ft. Island Gypsy. 1700RPM gives about 7 knots.
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Old 08-22-2020, 02:34 PM   #2
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4 blade prop

A 4 blade prop will be somewhat smother, but it will be a different size ,
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Old 08-22-2020, 03:16 PM   #3
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No advantage other than a hair smoother ... so small that you may even have trouble measuring it w electronics.
And unless you have a prop w smaller dia than usual you’ll have too much blade area and less efficiency.

You know your max diameter possible? The dia and pitch of your present 3 blade? Engine WOT rpm and rated rpm (it’s 2500(l’ll give you that))?
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Old 08-22-2020, 03:56 PM   #4
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You probably won't make 8 knots. Hull speed is the controlling factor. You want to go faster, get a lot more horsepower or a longer boat.
A 3 blade is more efficient than a 4 blade.
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Old 08-22-2020, 05:54 PM   #5
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What size prop is currently installed?
Can you reach 2500 rpm?

Typically you would drop an inch or two of pitch for a 4 blade. Diameter is limited due to distance from hull. I doubt you would see a noticeable difference.
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Old 08-22-2020, 06:19 PM   #6
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Given the same hydrofoil design, the fewer the blades the more efficient; the more blades the smoother. The fewer blades will need to be greater diameter to absorb the same horsepower. In applications with excessive power, fewer blades often result in an unacceptably large diameter, hence more blades.

If I were looking for more efficiency or less vibration I’d be looking for a more advanced design hydrofoil. Prop design has come a long way in the past few decades, particularly since the widespread use of advanced modeling.
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Old 08-22-2020, 07:46 PM   #7
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PPandE,
Most all props that you’d find on a trawler have about the same aspect ratio. This means that blade area is dependent on dia and the number of blades. Since the number of blades can only be changed if you are right in-between needing a 3 blade or a four blade. And then with either a 3 blade or a 4 blade you will have the least efficient prop for your boat. They make props for a given diameter in 3 and 4 blade configuration. So for the most part you don’t really have any choice. If you have a boat whereas you can increase prop dia 20 to 30% you then have a choice to run a 4 blade. But only if you can significantly change your dia If can maintain a reasonable aspect ratio.

Most trawlers have dia limitations that dictate a three blade prop. Bigger boats like 50’ and up frequently find the best prop. But smaller boats are better off w a three blade prop.

Interestingly my boat has prop clearance that would allow a 4 blade. But Willard (the builder) Put 3 blade props on all the 30’ boats. I tried a 5 blade w high aspect ratio blades but still had too much blade area. Out of the barn I had 2000rpm WOT engine speed after significant de-pitching.
The three blade I bought is a low efficiency prop compared to a more typical prop. I’s an MP Michigan 18” three blade probably w a bit more blade area than the typical prop. And there is no change in pitch from close to the hub to the outer edge. Also (and finally) the Profile shape is symmetrical and a big advantage to that is relatively high reverse thrust. It’s hight and day to me. The MP (machine pitch) Michigan propeller is less efficient but it’s a very small loss.

First two pics are of my Michigan MP prop.
Third pic is the five blade I tried.
Fourth pic is of a big motorsailer with high aspect ratio blades.
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Old 08-22-2020, 08:25 PM   #8
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My understanding is that in addition to being smoother and being able to transfer more HP into the water for the same diameter, there was also less slippage. Dropping the boat into gear had more traction (so to speak) immediately than a three blade. It was also my understanding that for towing purposes, more blades and surface area for the same diameter was preferred.

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Old 08-22-2020, 09:43 PM   #9
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Ted I would think more “traction” when popping into gear would be a measure of how over propped one was. Less instant surge should indicate may be one is propped correctly.

Not related .....
Something I’ve heard or most likely read was that boats Like trawlers w a “fat” and abrupt keel end like seen in my two lower photographs above would send turbulence aft right into the vertical blades simultaneously causing a blip in thrust that would/could generate some vibration. Three blade props should fair better in this as they would react much like a 3 cyl engine rather than a twin or two cyl engine.
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Old 08-23-2020, 01:18 AM   #10
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Quote:
Originally Posted by SoWhat View Post

Typically you would drop an inch or two of pitch for a 4 blade. Diameter is limited due to distance from hull. I doubt you would see a noticeable difference.
We have 4 blades on this, had 3 blade on the closely related predecessor. I thought they looked small, the above explains it, though I still think it`s a tad underpropped.
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Old 08-23-2020, 05:18 AM   #11
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A three blade propeller will always be marginally more efficient than a four as there's one less blade to disrupt water flow. A two blade would be even more efficient and a single the most efficient in this respect however the obvious mounting and vibration issues make them impractical in most cases. Speed is dictated by waterline, SL ratio, hull form, available horsepower and the pitch and diameter of the propeller not the number of blades. Primarily going from a three blade to a four blade is to compensate for a restriction in diameter caused by insufficient aperture area, second would be to address vibration issues.
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Old 08-23-2020, 07:53 AM   #12
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Quote:
Originally Posted by Nomad Willy View Post
Ted I would think more “traction” when popping into gear would be a measure of how over propped one was. Less instant surge should indicate may be one is propped correctly.

Not necessarily. Available power at idle will determine speed at idle. So a boat with idle RPM as a higher percentage of WOT RPM will idle faster relative to its top speed. That's why some big sport fishes do 7 kts at idle. My own boat does just over 4 kts at idle and I'm not overpropped at all.

The "traction" Ted is talking about is related to blade area and slip. A prop with more blade area will accelerate the boat faster when placed in gear at a stop, as it'll slip less when first engaged with the load of a stopped boat.

That's why ski boats often run 4 blades, as they're willing to sacrifice a little top speed (due to needing less pitch to make up for the extra blade drag) in exchange for being able to take off faster to get a skiier up.
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Old 08-23-2020, 08:38 AM   #13
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I have used both on my 36 foot Albin, 120 F.L. Don't notice any real difference, the four blade may be a little slower at 1700 rpm.

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Old 09-22-2020, 08:56 PM   #14
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Need New Props

Great topic! So what’s would be the consensus, of you all. Needing to replace props on my 40’ new to me Tolly Sundeck. Props have been reworked to many times, 1/2” to small in diameter and blades are thin and sharp as knives.
Currently 3 blade 20”x 17”, they are now 19 1/2” diameter and we can’t get a vibration out of one that was badly damaged prior to us. And yes shafts are new and were measured for tolerances.
Was supposed to come with 3 blade 20”x 18”, I was thinking going with new 4 blade 20”X 18”. Current props are quite noisy in the aft bed when under way and was thinking 4 blade may quiet things down a db.
Not looking for WOT performance, hopefully low end 8 knot performance.

For the little time we have had it out fuel burn is good at 8.5 running 16-1700rpm.

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Old 09-22-2020, 09:00 PM   #15
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What rpm do those engines hit at WOT with the current props? I'd want to see at least 4200, up to 4400 would be fine.
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Old 09-22-2020, 09:19 PM   #16
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If you don’t care about thrust performance the 4 blade will give more prop tip to hull clearance. That should make it quieter and perhaps a very small reduction in vibration. But it’s likely there will be less prop noise. Just an opinion.

And of course a little more fuel burn and less speed. More rpm at same cruise.
All these differences will be quite to very small except the noise.
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Old 09-22-2020, 09:55 PM   #17
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You will likely have to give up some pitch for the drag of the extra blade. No free lunch.
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Old 09-23-2020, 06:44 AM   #18
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Sorry I don't fully understand everything about the full function of pitch and diameter.

My goals:
Less Vibration
Less Prop Noise
Fuel Economy

Not worried about top speed or hole shot, we cruise at a comfortable 8.5 knots and do appreciate alittle more speed 10-12 to get out of dodge.
Not sure if we will hamper the hull speed playing in this range and making fuel burn worse.

To be honest we have never pushed the engines at wide open throttle to max rpm $$ and don't get a thrill at those speeds. If need be to properly size props, will get it done next trip out.

Unless I'm missing something in the definition of thrust performance, or understand the purpose of pitch.

Really interested in more knowledge in props to tune our boat from low speeds to mid range.
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Old 09-23-2020, 07:29 AM   #19
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It matters to know what RPM the engines turn at WOT. With carb-ed gassers, being overpropped may save a little fuel at very low RPM, but will waste fuel everywhere else by getting the carbs into power enrichment too early. On top of that, they're under heavier load at any given RPM if overpropped, which makes it easier to cook exhaust valves and such if you ever do push the throttles up a bit.

A 4 blade should definitely help the noise and vibration issue. Maybe something in the 19x18 size range, but hard to say for sure on sizing without knowing what it does with the current props.

Also, do you know what gear ratio your transmissions are? I'm going to guess around 2:1 based on the factory prop size, but it would be good to confirm.
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Old 09-23-2020, 08:03 AM   #20
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It matters to know what RPM the engines turn at WOT. With carb-ed gassers, being overpropped may save a little fuel at very low RPM, but will waste fuel everywhere else by getting the carbs into power enrichment too early. On top of that, they're under heavier load at any given RPM if overpropped, which makes it easier to cook exhaust valves and such if you ever do push the throttles up a bit.

A 4 blade should definitely help the noise and vibration issue. Maybe something in the 19x18 size range, but hard to say for sure on sizing without knowing what it does with the current props.

Also, do you know what gear ratio your transmissions are? I'm going to guess around 2:1 based on the factory prop size, but it would be good to confirm.

Thanks RS, ok you touched on a few things there that I didn't think of. I guess WOT is in order to get this right as far as diameter and then we can play with pitch if we don't nail it the first try.

Does pitch play a bigger role in response or hole shot? Or rpm? My thoughts if I can keep my cruising speed in the 8's and lower rpm means saving money?
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