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 Catalina/Capri 25/250 Sailor's Forums
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 O/B prop		  New Topic  Topic Locked
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jerlim
Master Marine Consultant

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USA
1484 Posts
Initially Posted - 04/06/2010 :  12:06:48  Show Profile
So...the yard is suggesting that I change the factory prop on our 8hp Mercury, 4 stroke O/B (we've had it 4 years and it performs like a charm). Their advice is to upgrade and install a prop designed for sail boats...they do not mean the folding props or adjustable pitch props for inboard motors...they are talking about O/B props...I can't find any info on-line about this, I've asked them for further details...(being the suspicious type, I suspect, this is about revenue enhancement)...any thoughts ? ?
Jerry
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Bill Holcomb
Admiral

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USA
769 Posts
Response Posted - 04/06/2010 :  12:31:35  Show Profile
Hi Jerry,
Professional propeller shops often have "cupped" propellers for small outboard engines. Each blade on the prop has an extra bend in it to help with slow engine speed power (like around the marina). I bought one of these props for my original engine - a 7.5 Honda. That engine has since been replaced and I - like you - now have an 8 hp Merc. Frankly, I'm not planning on changing the Merc's propeller.
Bill Holcomb - C25 Snickerdoodle #4839
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Dave5041
Former Mainsheet Editor

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USA
3758 Posts
Response Posted - 04/06/2010 :  12:46:28  Show Profile
Maybe you'll get up on a plane faster, your skiers will appreciate that.
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Nautiduck
Master Marine Consultant

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USA
3704 Posts
Response Posted - 04/06/2010 :  12:55:35  Show Profile
If your current OB "works like a charm" I'd use the money for something else!
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islander
Master Marine Consultant

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USA
4024 Posts
Response Posted - 04/06/2010 :  13:47:05  Show Profile
Sounds like Prop Wash to me.
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jerlim
Master Marine Consultant

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USA
1484 Posts
Response Posted - 04/06/2010 :  13:53:00  Show Profile
Bill - thanks much for the particulars...I agree w/ all on every point !
Edited by - jerlim on 04/06/2010 13:53:52
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JimB517
Past Commodore

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USA
3285 Posts
Response Posted - 04/06/2010 :  14:50:20  Show Profile  Visit JimB517's Homepage
A "high thrust" prop creates more push at low speed but at the cost of higher RPM while running. Do you want to be cruising at 6 knots and 5000 RPM? I think not. The regular prop delivers good enough performance at low RPM and plenty of thrust for maneuvering around docks. We rarely run our motors above 1/4 throttle.

Stick to the stock 6 or 7 inch pitch, 3 blade, prop.

These high thrust props are best for large power boats who run an outboard as a "kicker" for trolling at 2 or 3 knots with the main engine turned off.
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delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts
Response Posted - 04/06/2010 :  15:05:11  Show Profile  Visit delliottg's Homepage
What they're probably talking about is a lower pitch propeller which would give it more low end torque (good for sailboats), as opposed to top end speed (not very necessary for sailboats, they're only going to go so fast no matter how fast you spin the prop).

When I bought my Tohatsu, I talked myself into a lower pitched prop figuring I'd swap it out after giving the factory prop a try. Since I can move along at 4.5 knots at 1400 RPM, I haven't seen any reason to swap it in. Now just consider it a spare.
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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts
Response Posted - 04/06/2010 :  15:48:07  Show Profile
I replaced a 199x Honda 8/standard prop with a 2003 Honda 8 "Power Thrust" with a larger diameter, 4-blade, lower pitch prop. The new one would accelerate faster and stop the C-25 as if it had run into a pillow, but a 5-knot cruise was at a higher RPM, probably costing some gas mileage. Keep the dough.
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.