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Old 12-25-2012, 06:10 PM   #1
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Propeller question

I want to replace my 3 blade 26"d, 19"p, LH, 1.75"shaft, propeller with a four blade one. I have read that I should get one with one inch less pitch. T question is will a 24"d, 20"p work?
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Old 12-25-2012, 06:37 PM   #2
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Do yourself a favor and consult the experts. Home // Michigan Wheel Marine
or one of the other folks in the business.
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Old 12-25-2012, 09:13 PM   #3
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Do yourself a favor and consult the experts. Home // Michigan Wheel Marine
or one of the other folks in the business.
Absolutely correct response.
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Old 12-26-2012, 06:17 AM   #4
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Guessing at diameter and pitch is fine for an outboard with a stack of used props to try.

Over about 24 inches props get way too expensive to guesstimate .

One question is why 4 blades?

Is the engine so powerful that more blade area is required?

The more blades the lower the prop efficiency , so why the change?
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Old 12-26-2012, 06:21 AM   #5
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I have always understood that 3 blade was more efficient at your top RPMs but a 4 blade would be at your lower cruise ranges.
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Old 12-26-2012, 06:38 AM   #6
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Do yourself a favor and consult the experts. Home // Michigan Wheel Marine
or one of the other folks in the business.
Yes, good advise. In addition you could read, "The Propeller Handbook": The Complete Reference for Choosing, Installing, and Understanding Boat Propellers.
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Old 12-26-2012, 07:02 AM   #7
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Absolutely correct response.
Agreed. :-)

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Old 12-26-2012, 10:43 AM   #8
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There is a good prop cal at Boardiesel.com
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Old 12-26-2012, 11:18 AM   #9
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http://boatdiesel.com/Members/Member...66&VP=1366:667

You will need to be a member to access the calculator.

Personally, I'd take Anode's advice though...
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Old 12-26-2012, 11:22 AM   #10
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awpptdt,

The question here is blade area not # of blades.

If you need more blade area go to the 4 blade. Most of the time that's all one needs to know. Lots of yachts need 4 blade props because the space for the prop is limited and the prop must absorb lots of power. Trawlers have more latitude as they usually have more room for larger diameter props. For example my own Willy could use a prop w 2" more diameter.

Also it's possible to actually increase efficiency going to a 4 blade if the 3 blade has very wide blade tips. So if your 3 blade prop is starting to look like a ducted fan prop more blades w smaller tips and a higher aspect ratio could/would be more efficient.

So if your 3 blade hasn't got too much blade area for it's blade length stay w 3 blades.

As I recall there is a blade area to disk area ratio # that's used in the industry to numerically show or define the amount of blade area relative to the disk area. The disk area is the area of the circle scribed by the blade tips as they revolve. This ratio is called EAR. The EAR on my prop is .51 or .53 so just over half the disk area is filled w blades. One could not see through a prop w an "EAR" of 100.

So unless you need a 4 blade stay w the three blade.

But don't fret over getting this right as it usually makes very little difference.
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Old 12-26-2012, 11:47 AM   #11
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awpptdt,

my own Willy could use more diameter.
Hahaha fair enough LOL

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Old 12-26-2012, 12:19 PM   #12
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Another propeller question that is somewhat related. Has anyone one compared a 3 blade to a 4 blade prop on a full keel vessel? I had one, and changed to a 4 blade. The prop was tuned and computer balanced, but at higher rpms the 4 blade had a little bit of a buzz type noise. I attributed that to the fact that two blades would be parallel to the keel as they went around. The 3 blade seemed like it was pulling more clean water. Maybe just me. Just wondered if anyone else expierenced something like this.
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Old 12-26-2012, 12:54 PM   #13
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Oh yes I've read about it and talked about it.

I think w wide blades (or very wide blades) the "shadow effect" would be unnoticeable depending on how wide the deadwood (now FG) is. Just another reason to go w a 3 blade prop as I see it. I think we talked about this on the Willard Boat Owners Group and as I recall no one seemed motivated to change anything. There is the efficiency aspect too but it as well seems to be fly stuff.

On your boat Don I suspect the propeller loading could be playing a part in your experience. Could be thin blades acting like tuning forks. Could be a form of propeller "singing" too. Could be the prop shaft bouncing back and forth on the rubber of the cutlass bearing in a state of "resonance". Have you asked a good prop man about it?
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Old 12-26-2012, 01:09 PM   #14
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Oh yes I've read about it and talked about it.

I think w wide blades (or very wide blades) the "shadow effect" would be unnoticeable depending on how wide the deadwood (now FG) is. Just another reason to go w a 3 blade prop as I see it. I think we talked about this on the Willard Boat Owners Group and as I recall no one seemed motivated to change anything. There is the efficiency aspect too but it as well seems to be fly stuff.

On your boat Don I suspect the propeller loading could be playing a part in your experience. Could be thin blades acting like tuning forks. Could be a form of propeller "singing" too. Could be the prop shaft bouncing back and forth on the rubber of the cutlass bearing in a state of "resonance". Have you asked a good prop man about it?
Eric, it is not on my present boat, but was on a Mainship I owned in the past. The discussion here reminded me of it. The Sabre is twin screws with no keel. It runs smoothly with no noticeable vibration. The Mainship at higher rpms did have the buzz. Seems to me that I would prefer an odd number of blades with a full keel arrangement.
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Old 12-26-2012, 01:26 PM   #15
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Another propeller question that is somewhat related. Has anyone one compared a 3 blade to a 4 blade prop on a full keel vessel? I had one, and changed to a 4 blade. The prop was tuned and computer balanced, but at higher rpms the 4 blade had a little bit of a buzz type noise. I attributed that to the fact that two blades would be parallel to the keel as they went around. The 3 blade seemed like it was pulling more clean water. Maybe just me. Just wondered if anyone else expierenced something like this.
I have not made this comparison, however on the "downeast" boat forum of which I am a member (all single screw, full keels) there is complete consensus that 4 blade props are needed behind full keels and many trials to back it up. That said however, these are all high HP high speed boats which may be on the verge of cavitation due to lack of blade area with a normal 3 blade prop so naturally most have had good results w/ 4 blades.
I have often wanted to try a 3 blade on my downeast boat which is quite low powered, has a borderline small shaft diameter, and does suffer from some vibration which seems mostly to come up through the rudder. I am also of the opinion that an odd number of blades would, all other things equal, yield less vibration behind a deadwood.
My Willard 36 has the origimal 5 blade "shrimper style" prop which was in the original Garden specs. It is smooth as silk if not as efficient as a 3 blade would be.
Propellers are too expensive to fool with and the 4 to 3 size recommendation I got from Michigan/Federal some years ago made no promise that it would be either smoother or more efficent so I never did it.
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Old 12-26-2012, 01:28 PM   #16
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I have always understood that 3 blade was more efficient at your top RPMs but a 4 blade would be at your lower cruise ranges.
'tis true. The higher the speed the fewer blades preferred. Fewer leading edges that have to "cut" through. Way back in the day of early OBs you could get a 3 blade "power" prop or a two blade "speed" prop on the same engine. Now-a-days manufacturers have had to dumb down the selection and give a prop that allows the engine to reach max revs. That has become a sticking point with surveyors and others who buy into it. This is especially true for planing hulls. On a displacement hull or any hull that is used at displacement speeds I prefer a slightly larger than "spec'd" engine and I'm going to set it up to give the speed I want at lower (but still reasonable RPMs), it can do this as it has more rpm "flexibility" due to producing more power at a lower range.

On our commercial trawlers a four blade was preferred for more "bite" when pulling trawls or pushing nets. This was true on the low speed boats, if the boat was expected to plane, to and from working grounds, they usually got a 3 blade esp. with gassers. But there were some who had 350HP 6 cyl Caterpillar powered 27' skiffs. They had the oomph to swing a 4 blade, and needed it to put all that power to use. It was an amazing sight to see a Lafitte skiff with skimmer nets sticking 16' in the air "loafing" down the pass at 35 kts! This was unheard of until the mid-late '80's when the lightweight high HP diesels began to hit the market. Before that you had a 6-71 DD, or a turbo/aftercooled 3208 Cat (bad rep as they didn't hold up putting out over 210 HP under the demands of a commercial shrimper who wanted the grunt/economy of a diesel AND the speed of a BB chevy). So while it has become regurgitated gospel that a boat HAS to reach max RPM fully loaded, remember that is so the 4 weekend/year Capt. Bligh can have a warranty. My boat is a 1983, I have no delusions about taking her out and holding her on the pins to head for the Dry Tortugas for the weekend. Common sense will play a big role in the longevity of just about anything. If you run your boat hard and it has marginal HP for the performance you want stick with the 3 blade, your engine will thank you. If you have an engine that the market has demanded the HP be doubled or more from its original design/ratings and you intend to use at its new rating, do not be alarmed when it goes KaBlooey with relatively few hours compared to what "they used to get"! The 250 HP 4BT Cummins is a prime example, as is the 375-425 3208 Cat. Just because it can be done doesn't make it a good proposition. A 40 yr old cypress Lugger with its original 4-71 DD was nothing rare. A planing skiff with a hot 3208 more than 3 yrs old without major repairs was an anomaly.

What I'm trying to say is that on low speed boats, while important its not as critical to have the "perfect" prop. I would much rather my trawler speed boat be at its Tq peak or PDC at the top of my cruising speeds. I.E. 1600 RPM at 8.5-9 kts. I don't care if it will turn 2800 with that prop/gearing configuration, its not where I operate my boat.

Let the flaming begin.....its my boat and my opinion based on my experience, not some industry concocted blanket rule.
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Old 12-26-2012, 01:35 PM   #17
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Eric, it is not on my present boat, but was on a Mainship I owned in the past. The discussion here reminded me of it. The Sabre is twin screws with no keel. It runs smoothly with no noticeable vibration. The Mainship at higher rpms did have the buzz. Seems to me that I would prefer an odd number of blades with a full keel arrangement.
Don, another very likely reason to consider is the keel induced cavitation at high speeds causing the buzz/vibration. More blades means a higher frequency that they hit that turbulent stream behind the keel. I've noticed on newer full keeled vessels designed to operate at higher speeds that the trailing edges of the keels are shaped to give a clean stream off the keel, instead of the 90* squared off keels usually found on slower rigs.
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Old 12-26-2012, 01:39 PM   #18
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Don, another very likely reason to consider is the keel induced cavitation at high speeds causing the buzz/vibration. More blades means a higher frequency that they hit that turbulent stream behind the keel. I've noticed on newer full keeled vessels designed to operate at higher speeds that the trailing edges of the keels are shaped to give a clean stream off the keel, instead of the 90* squared off keels usually found on slower rigs.
Thanks, that makes sense.
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Old 12-26-2012, 03:31 PM   #19
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Here's my ridiculously "dirty" keel trailing edge. I briefly tried a Michigan Star five blade w high aspect ratio (skinny) blades and a fairly high frequency vibration resulted. So sometimes the variables will cause the "rules" to be not true.
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Old 12-26-2012, 04:49 PM   #20
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Propeller question

My propeller turns fairly slow as it is driven by a 2.65 transmision with the engine turning 2,400 rpm. The result is at speed there is a definate chop, chop, chop vibration. The prop has been reworked and I have been told a 4 blde propeller while not as efficient will make it smother.
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