Reply
 
Thread Tools Search this Thread Display Modes
 
Old 12-25-2012, 05:10 PM   #1
Senior Member
 
awpptdt's Avatar
 
City: Wilmington, N.C.
Vessel Name: Donnchaidh
Vessel Model: Marine Trader 40' sedan
Join Date: Nov 2010
Posts: 167
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?
awpptdt is offline   Reply With Quote
Old 12-25-2012, 05:37 PM   #2
Guru
 
Anode's Avatar
 
City: Missourah
Vessel Name: M/V Scout
Vessel Model: Sundowner Tug 30'
Join Date: Jul 2010
Posts: 645
Do yourself a favor and consult the experts. Home // Michigan Wheel Marine
or one of the other folks in the business.
__________________
Chip

Deliveries & Yacht Services
www.captainchip.com
Anode is offline   Reply With Quote
Old 12-25-2012, 08:13 PM   #3
Guru
 
Moonstruck's Avatar
 
City: Hailing Port: Charleston, SC
Vessel Name: Moonstruck
Vessel Model: Sabre 42 Hardtop Express
Join Date: Nov 2008
Posts: 8,276
Quote:
Originally Posted by Anode View Post
Do yourself a favor and consult the experts. Home // Michigan Wheel Marine
or one of the other folks in the business.
Absolutely correct response.
__________________
Don on Moonstruck
Sabre 42 Hardtop Express
When cruising life is simpler, but on a grander scale (author unknown)
https://moonstruckblog.wordpress.com/
Moonstruck is offline   Reply With Quote
Old 12-26-2012, 05:17 AM   #4
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 22,553
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?
FF is offline   Reply With Quote
Old 12-26-2012, 05:21 AM   #5
Guru
 
psneeld's Avatar
 
City: Ft Pierce
Vessel Name: Sold
Vessel Model: Was an Albin/PSN 40
Join Date: Oct 2011
Posts: 28,153
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.
psneeld is online now   Reply With Quote
Old 12-26-2012, 05:38 AM   #6
TF Site Team
 
Larry M's Avatar
 
City: Jacksonville
Join Date: Nov 2008
Posts: 11,683
Quote:
Originally Posted by Anode View Post
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.
Larry M is online now   Reply With Quote
Old 12-26-2012, 06:02 AM   #7
Guru
 
Hendo78's Avatar
 
City: Perth
Join Date: Nov 2012
Posts: 3,314
Quote:
Originally Posted by Moonstruck View Post

Absolutely correct response.
Agreed. :-)

iPad Forum Runner
__________________
Builder and former owner of MV SOLSTICE
Hendo78 is offline   Reply With Quote
Old 12-26-2012, 09:43 AM   #8
Guru
 
motion30's Avatar
 
Join Date: Jan 2010
Posts: 1,181
There is a good prop cal at Boardiesel.com
motion30 is offline   Reply With Quote
Old 12-26-2012, 10:18 AM   #9
Guru
 
City: Hotel, CA
Join Date: Sep 2011
Posts: 8,323
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...
__________________
Craig

It's easier to fool people than to convince them that they've been fooled - Mark Twain
CPseudonym is offline   Reply With Quote
Old 12-26-2012, 10:22 AM   #10
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
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.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 12-26-2012, 10:47 AM   #11
Guru
 
Hendo78's Avatar
 
City: Perth
Join Date: Nov 2012
Posts: 3,314
Quote:
Originally Posted by manyboats View Post
awpptdt,

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

iPad Forum Runner
__________________
Builder and former owner of MV SOLSTICE
Hendo78 is offline   Reply With Quote
Old 12-26-2012, 11:19 AM   #12
Guru
 
Moonstruck's Avatar
 
City: Hailing Port: Charleston, SC
Vessel Name: Moonstruck
Vessel Model: Sabre 42 Hardtop Express
Join Date: Nov 2008
Posts: 8,276
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.
__________________
Don on Moonstruck
Sabre 42 Hardtop Express
When cruising life is simpler, but on a grander scale (author unknown)
https://moonstruckblog.wordpress.com/
Moonstruck is offline   Reply With Quote
Old 12-26-2012, 11:54 AM   #13
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
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

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 12-26-2012, 12:09 PM   #14
Guru
 
Moonstruck's Avatar
 
City: Hailing Port: Charleston, SC
Vessel Name: Moonstruck
Vessel Model: Sabre 42 Hardtop Express
Join Date: Nov 2008
Posts: 8,276
Quote:
Originally Posted by manyboats View Post
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.
__________________
Don on Moonstruck
Sabre 42 Hardtop Express
When cruising life is simpler, but on a grander scale (author unknown)
https://moonstruckblog.wordpress.com/
Moonstruck is offline   Reply With Quote
Old 12-26-2012, 12:26 PM   #15
Guru
 
Brooksie's Avatar
 
City: Cape Cod, MA
Vessel Name: Island Seeker
Vessel Model: Willard 36 Sedan
Join Date: Oct 2007
Posts: 1,306
Quote:
Originally Posted by Moonstruck View Post
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.
Brooksie is offline   Reply With Quote
Old 12-26-2012, 12:28 PM   #16
Guru
 
Join Date: Aug 2011
Posts: 577
Quote:
Originally Posted by psneeld View Post
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.
__________________
TIME well wasted
1984 34' Mainship III
Arkan'tsaw
twiisted71 is offline   Reply With Quote
Old 12-26-2012, 12:35 PM   #17
Guru
 
Join Date: Aug 2011
Posts: 577
Quote:
Originally Posted by Moonstruck View Post
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.
__________________
TIME well wasted
1984 34' Mainship III
Arkan'tsaw
twiisted71 is offline   Reply With Quote
Old 12-26-2012, 12:39 PM   #18
Guru
 
Moonstruck's Avatar
 
City: Hailing Port: Charleston, SC
Vessel Name: Moonstruck
Vessel Model: Sabre 42 Hardtop Express
Join Date: Nov 2008
Posts: 8,276
Quote:
Originally Posted by twiisted71 View Post
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.
__________________
Don on Moonstruck
Sabre 42 Hardtop Express
When cruising life is simpler, but on a grander scale (author unknown)
https://moonstruckblog.wordpress.com/
Moonstruck is offline   Reply With Quote
Old 12-26-2012, 02:31 PM   #19
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
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.
Attached Thumbnails
STH71269 copy.JPG  
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 12-26-2012, 03:49 PM   #20
Senior Member
 
awpptdt's Avatar
 
City: Wilmington, N.C.
Vessel Name: Donnchaidh
Vessel Model: Marine Trader 40' sedan
Join Date: Nov 2010
Posts: 167
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.
awpptdt is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


» Trawler Port Captains
Port Captains are TF volunteers who can serve as local guides or assist with local arrangements and information. Search below to locate Port Captains near your destination. To learn more about this program read here: TF Port Captain Program





All times are GMT -5. The time now is 06:56 AM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012