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06-24-2016, 01:20 PM
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#1
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Senior Member
City: Rapid City, SD heading back to the PNW
Join Date: Oct 2014
Posts: 212
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4 blade prop vs 3 blade prop
As I have been searching Yachtworld and other sites for my next dream boat I have seen that some boats have 4 blade props and others uas 3 blade props. What if any are the advantages of a 4 blade over a 3 blade props?
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06-24-2016, 01:38 PM
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#2
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Guru
City: Owings, Md
Vessel Name: Graceland
Vessel Model: Mainship 34 MK1
Join Date: Jan 2016
Posts: 1,381
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4 blades provide greater surface area for a given diameter of prop which generally reduces prop slip. The smaller diameter allows a lower (closer to horizontal) shaft angle as well as reduced draft. 4 blades have higher drag compared to a 2 or 3 blade prop and in some cases can produce a vibration behind full keels at certain speeds.
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06-24-2016, 01:43 PM
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#3
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Guru
City: Owings, Md
Vessel Name: Graceland
Vessel Model: Mainship 34 MK1
Join Date: Jan 2016
Posts: 1,381
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The shape of the hull will dictate the placement of the engine and prop shaft, this configuration tends to drive the selection of the best prop configuration for a given boat.
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06-24-2016, 01:50 PM
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#4
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
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Three blade props are almost always better for trawlers.
The reason to use a four blade is or should be to give the prop sufficient blade area to work propperly w the power of the boat. It's almost always a blade area issue.
If your boat has the wrong gear ratio you may need a 4 blade and there are other unusual circumstances that may dictate the need for 4 blades.
Three blades are almost always more efficient and cheaper.
Basically there is no advantage to 4 blade props unless a more powerful engine has been installed in the boat. Usually it would take considerably more power to get benefit from a 4 blade prop.
__________________
Eric
North Western Washington State USA
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06-24-2016, 02:04 PM
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#5
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Senior Member
City: NC
Vessel Name: The Blue Comet
Vessel Model: Nordic Tugs/ NT32
Join Date: Nov 2010
Posts: 146
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Say it ain't so!!! My trawler has a five bladed prop.
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06-24-2016, 03:17 PM
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#6
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Valued Technical Contributor
City: Litchfield, Ct
Join Date: Aug 2012
Posts: 6,785
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The number of blades on the prop would be about number 99th on my list of things to check when buying a boat.
David
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06-24-2016, 03:48 PM
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#7
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
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Dave,
I think the OP was just curious.
True not usually an issue. Unless the pitch to dia ratio is too far from ideal.
CC,
I tried a 5 blade but it was 1" more dia and was wayyyy too much prop. 2000rpm max. May have worked if it had been 1" less dia. The blades were skinny and long and I was liking that for possible efficiency increase. Still have the prop ... 19".
__________________
Eric
North Western Washington State USA
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06-24-2016, 04:47 PM
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#8
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Guru
City: Salvador - BA
Vessel Name: Rainha Jannota
Vessel Model: Curruira 46
Join Date: Jun 2011
Posts: 667
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Does it matter? That was a technical choice when the boat was built and took in consideration the engine, the draft, the gear, etc.
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06-24-2016, 05:21 PM
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#9
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Technical Guru
City: Wilmington, NC
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 6,194
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One way to sum it up: If a 3b works well, gives good performance numbers, and does not have bad vibes, you are good. If not, you do the same with 4b or 5b and try to dial it in.
Prop selection is half engineering and half black magic.
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06-24-2016, 05:39 PM
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#10
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Senior Member
City: Rapid City, SD heading back to the PNW
Join Date: Oct 2014
Posts: 212
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Thanks for the information, I have noticed that the Bayliners mostly have 4 blades and just about all others have 3. Today I was looking at three different DeFever 44's two were 3 blade and one was a 4 blade. the all had Lehman 135's.
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06-24-2016, 05:47 PM
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#11
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Master and Commander
City: Vallejo CA
Vessel Name: Carquinez Coot
Vessel Model: penultimate Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 12,559
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My four-blade propeller is a good fit for my engine and hull. I'd rely on those professionally knowledgeable to select the proper propeller fit.
__________________
Kar-KEEN-ez Koot
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06-24-2016, 06:53 PM
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#12
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Guru
City: Longboat Key, FL
Vessel Name: Bucky
Vessel Model: Krogen Manatee 36 North Sea
Join Date: Oct 2009
Posts: 5,196
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In at least one case I know of, a cat owner had Michigan Wheel make a 4 blade unit to replace the original 3 blade that came standard on PDQ 34 catamarans with twin 75 or 100 HP Yanmars. According to the owners on the PDQ forum, the efficiency and vibration was improved markedly, especially the "singing" the three blade units were known for. At least one boat claimed to gain two knots on top end but all seemed ecstatic over the sound levels...some claimed 50 percent improvement. PDQ was known for their design prowess, but this was apparently something they missed. Rare case, but sometimes it works out.
__________________
Larry
"When life gets hard, eat marshmallows”.
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06-24-2016, 06:56 PM
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#13
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Master and Commander
City: Vallejo CA
Vessel Name: Carquinez Coot
Vessel Model: penultimate Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 12,559
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My four-blade propeller sung. After two attempts, the propeller guy fixed it.
__________________
Kar-KEEN-ez Koot
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06-24-2016, 07:00 PM
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#14
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Guru
City: Ft Pierce
Vessel Name: Sold
Vessel Model: Was an Albin/PSN 40
Join Date: Oct 2011
Posts: 28,149
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Not necessarily a simple answer....but simplistically.....
A 3 bladed will be slightly more efficient if your boat design will allow it's diameter...if not a 4 bladed might be better.
Plenty of similar boars that run one or the other and no one can tell which one is really better.
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06-25-2016, 05:49 AM
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#15
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Guru
City: Fort Myers, FL... Summers in the Great Lakes
Vessel Name: Slow Hand
Vessel Model: Cherubini Independence 45
Join Date: Dec 2010
Posts: 12,834
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Quote:
Originally Posted by markpierce
My four-blade propeller sung. After two attempts, the propeller guy fixed it.
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Singing is a function of the leading edge of one or all of the blades being tapered too much, common by novice prop reconditioners. The leading edge needs to be slightly blunt. It pays to use a seasoned professional when it comes to prop work.
Ted
__________________
Blog: mvslowhand.com
I'm tired of fast moves, I've got a slow groove, on my mind.....
I want to spend some time, Not come and go in a heated rush.....
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06-25-2016, 10:13 AM
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#16
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Technical Guru
City: Wilmington, NC
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 6,194
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Singing on my 3b tells me my prop is nice and clean. No sing means I need to jump in and clean it.
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06-26-2016, 06:31 PM
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#17
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Guru
City: UMR MM283
Vessel Name: Northern Lights II
Vessel Model: Bayliner 3870
Join Date: Jan 2013
Posts: 1,357
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I just had my 3 blade set of spares put on a couple of weeks ago, replacing the 4 blade set. Both sets are 20"x19", I gained 2 to 3 tenths of mph at 1000 to 1100 rpm. That is about the only difference I've seen. I'am going to take the 4 blade to a prop scan shop and reinstall at next years haul out.
__________________
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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06-26-2016, 09:17 PM
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#18
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
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RC,
What was your WOT rpm w both props? Does your 3 blade have very high blade area per blade? That's the only thing that would make them comparable IMO. Also IMO you should put the prop on that has the most rpm if it's less than 100 rpm over rated WOT rpm.
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Eric
North Western Washington State USA
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06-26-2016, 09:50 PM
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#19
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
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Five blade prop
Colony Cove,
Here's my 5 blade. Probably dos'nt look much like yours. the blades are very high aspect ratio .. long and narrow. I think it's a Michigan Star wheel.
__________________
Eric
North Western Washington State USA
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06-26-2016, 10:45 PM
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#20
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Technical Guru
City: Wilmington, NC
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 6,194
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Cool prop!! I've seen those on real old chris craft cruisers, others from back in the 50's?? I'd love to try one with modern power. Just have no idea how that skinny blade 5er would do..
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