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 Three of Four Blades?>		  New Topic  Topic Locked
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jerlim
Master Marine Consultant

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USA
1484 Posts
Initially Posted - 07/30/2018 :  17:57:02  Show Profile
I needed to replace the three blade prop on my 10 year old Mercury 8hp OB, due to a run-in with some debris that I did not see...

The yard could not get a three blade replacement prop for 7 - 10 days, and offered me a 4 blade prop that they could install the next day...so being the impatient sod that I am, I went for the 1 day and four blades.

I've trolled the net but can't find a straight answer as to the difference (to me as the user) between three or four blades. Speed seems the same, mileage is pretty hard to tell, no difference in vibration, but at lower RPMs it does a bit of a whining sound.

Any experiences or thoughts to tell?

Thanks!
Jerry
Edited by - jerlim on 07/30/2018 17:57:35

WesAllen
Navigator

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USA
222 Posts
Response Posted - 07/30/2018 :  19:26:49  Show Profile
Usually it has more thrust. My 9.9 Merc came with a 4 blade.
Wesley Allen
"Breaking Wind"
1982 C-25 SR/TR/SK #2773
Hemlock, MI
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Voyager
Master Marine Consultant

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USA
5231 Posts
Response Posted - 07/30/2018 :  21:14:55  Show Profile
Jerry, I’m sad to hear of your mishap.

Maybe four blade props produce more prop walk?

Perhaps the whining sound is from the bearings that just underwent whatever trauma was sufficient to break your original prop?

You might remove the spark plug and rotate the engine manually with the transmission in gear to see if anything is binding.
Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain — Milford, CT
Edited by - Voyager on 07/30/2018 21:16:55
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Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts
Response Posted - 07/30/2018 :  21:37:04  Show Profile
It's not quite that simple... It's the combination of diameter, pitch, and then the number of blades. The first two might be more significant than the last, since 3-blade props tend to have greater surface area on each blade. The best source for advice is the engine manufacturer (or a knowledgeable dealer, which can be hard to find for small motors).

I discussed this with a Honda manufacturer's rep and another from Yamaha at a boat show--the Honda high-thrust 8 (and 9.9) had 4-blade props and the Yamaha 8 had 3 larger blades. Both had lower pitch than on "ordinary" outboards meant for lightweight boats. I switched from an older Honda 8 with a "standard" prop to high-thrust Honda with with a larger, lower-pitch, 4-blade... The difference in maneuvering the boat was striking. (As I've said in the past, it would stop Passage as if she'd run into a pillow.)

Just about any prop will push a boat... For optimal acceleration, stopping, and economy at the desired speed, a planing inflatable dinghy wants a different prop from a 5,000 lb. sailboat.

BTW, I believe prop-walk increases with pitch more than anything else. It's the tendency of the prop, at lower or no boat speed, to throw water outward (waste) in addition to aftward (thrust).
Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge
Edited by - Stinkpotter on 07/30/2018 21:47:17
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islander
Master Marine Consultant

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USA
3992 Posts
Response Posted - 07/31/2018 :  06:38:06  Show Profile
You dont say what the dimensions are on the new prop compared to the old so hard to say about any differences. Most motors will have three props to choose from if you look up parts for them. Gas mileage is irrelevant with to many variables affecting it. Wind, Waves, Current etc. If your happy with the speed and maneuverability around the dock then your good to go. Since we are pushing @5000lbs with only 8hp most of the differences between the two props like planning, hole shots etc are lost to us.Might matter more to someone putting a 8-10hp on a bass boat. If it matters I found this.
quote:
The most asked question at the boat shows is almost always” what is the difference between
a 3 blade and a 4 blade boat propeller” The difference between 3
blades over 4 blades is that the 3 blade (smaller blade ratio) usually goes
faster and the 4 blade (higher Blade ratio) is better hole shot and better
stern lift as well as better handling and fuel economy because it will keep you
on plane at a lower speed

If you look at a propeller from behind, and visualize a circle that the blade tips would draw, you will see that the blade of a 3 blade prop covers about 50 -55% of the circle. (This is called diameter area ratio, or DAR). You can add thrust within this drive circle by increasing that percent (to say 60 – 65%) by adding a 4th blade. When you look at a four blade prop you will notice that more of this area is covered because of the extra blade. That means more push contact with the water
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


Edited by - islander on 07/31/2018 07:40:25
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jerlim
Master Marine Consultant

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USA
1484 Posts
Response Posted - 07/31/2018 :  09:13:00  Show Profile
As always - the brain trust here is so wonderful ! Thank you all for your thoughts and insights - I appreciate it.
Fair winds,
JL
Jerry
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Voyager
Master Marine Consultant

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USA
5231 Posts
Response Posted - 07/31/2018 :  19:58:58  Show Profile
I think the formula for genius is:
50% personal experience
30% cold hard facts and data
20% purely unadulterated bull sh*+
Mix well and pour over ice.
Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain — Milford, CT
Edited by - Voyager on 07/31/2018 19:59:49
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sethp001
Mainsheet C-25 Tech Editor

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814 Posts
Response Posted - 07/31/2018 :  23:57:04  Show Profile
quote:
Originally posted by Voyager

I think the formula for genius is:
50% personal experience
30% cold hard facts and data
20% purely unadulterated bull sh*+
Mix well and pour over ice.


And all the people said, "Amen."


Seth
"Outlier" 1987 Catalina 25 SR/SK/Traditional Interior #5541
"Zoo" 1977 Morgan Out Island 30
"Nomad" 1980 Prindle 16
"Lost" 1988 Catalina Capri 14.2 (sold - yay!)
"Marine Tex 1" Unknown Origin POS 8' Fiberglass Dinghy
https://whichsailboat.com/2014/07/27/catalina-25-review/
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
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