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Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

4 blade prop vs 3 blade

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jerlim
Master Marine Consultant

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USA
1484 Posts

Initially Posted - 06/27/2017 : 09:45:45

So, my 8hp Merc OB needs a new prop. The exact 3 blade replacement is on a long lead back order, but a 4 blade (same pitch) is available. I'm going to make the change.

Any experiences to share? As per the readings I've done, I can expect less vibration and better mileage...anything else?

Thanks,
Jerlim

Jerry

islander
Master Marine Consultant

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USA
3992 Posts

Response Posted - 06/27/2017 : 14:12:04

General rule of thumb is when going from a 3 blades to 4 you should drop the pitch by 1. Adding a blade creates more friction thus reducing RPM. Reduced RPM means less push. It takes more HP to regain the RPM and push. Dropping the pitch by 1 solves this by reducing the drag created by the extra blade. It's better explained here.
https://www.boattest.com/view-news/3276_which-is-better-4-blades-or-3-blades

Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound

Edited by - islander on 06/27/2017 15:11:39

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 06/27/2017 : 18:31:08

Yup, the pitch should be lower. When I bought a new motor for Passage, Yamaha's high-thrust model for sailboats had a larger diameter, lower-pitch three-blade prop, and Honda's had four blades with lower pitch. Both were also 8 hp versions of their 9.9 motors, with higher low-end torque. "Standard" pitches on most outboards are designed for getting boats on plane at higher speeds than a C-25 can ever achieve with any motor. At lower speeds, they tend to generate more turbulence and less true thrust than they're designed for. Lower pitch can also contribute greatly to stopping power using reverse thrust. (As I've said before, my high-thrust Honda 8 stopped Passage as if she had run into a pillow.)

In sum, the prop (and the motor) should be matched to the boat--weight, drag, and expected speed. In my case the high-thrust Honda 8 was a better match than the older standard Honda 8 it replaced, and better than a neighboring C-25's Johnson 9.9 and a friend's older, standard Honda 15 on another C-25. The factory rep at a boat show was right.

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 06/27/2017 19:55:36

jerlim
Master Marine Consultant

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USA
1484 Posts

Response Posted - 07/01/2017 : 12:36:33

Gentlemen - As always, many thanks for your sage advice and insights....best wishes for a safe and happy Independence Day!