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Newbie prop question 13" vs 14"

Hi, sorry to bother you with such a basic question which probably is googleable, but i cannot find the right keywords.
I have a small hardtop with a 1979 Mariner/Mercury 70hp 3-cyl. The previous owner was a very big person and was using the boat for water skiing, so he changed to a lower pitch propeller - a Solas aluminium, 13" pitch, 11.1" dia.
The "original"/previously installed was a 14" pitch 10.25" dia, with a Mercury part no so i'm guessing its the one that came installed with the engine.
I'm not using the boat for water skiing, nor am i particularly heavy. Should I install the original prop? Is there even a big difference between them since the lower pitch prop "compensates" with a bigger diameter?

Thanks
Johan
 
Ayuh,.... You should run whatever prop that allows the motor to run the rated rpms at Wot, when loaded with yer average load,.........
 
The key for getting the correct prop is to make sure your engine can just get in to its rated WOT (Wide Open Throttle) range. Check what that is in your owners manual. It is probably in the 5000 to 5500 rpm range. You want to have the prop that will consistently get you in that range when the throttle is all the way forward.

The reason for that is that it is the optimum rotation for keeping your engine running at it's best efficiency without putting too much strain on the engine or drive train.

Furthermore, make sure that the anti-cavitation plate on the lower part of the engine is mounted so that it is even with the lowest point on the keel. When on plane, that plate should appear as if it is just skimming over top of the water stream coming off the bottom of the hull.
 
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