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Author Topic:   Prop diameter question
MikeG posted 06-29-2003 11:20 PM ET (US)   Profile for MikeG   Send Email to MikeG  
I know everyone usually talks about pitch when discussing WOT rpm range adjustments but I'm wondering what effect diameter has in this equation. I have a 1996 OMC 90hp on a Montauk. My prop is a 14x17 OMC alum. I am only getting 5100rpm at full throttle and 40 mph GPS. I used to have a 88hp with a 13.25 x 17 SS prop. It would move the Montauk to about 42+mph and I saw the rpms go past 5800rpm (rarely ran the boat above 36 mph). Could the prop diameter be affecting the new engine? I would like to try a 13.25 again but I'm not sure if it will help. Any thoughts on the effect of diameter?

Thanks,
Mike

Sal DiMercurio posted 06-29-2003 11:27 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Diameter dosen't effect the rpms like pitch, but you should gain a couple hundred rpms.
By all means get rid of that 14" dia, absolutly "NO" need for all that dia on that small a boat.
I recommend a 13 or 13.25 dia for that engine & boat.
How high up is the engine mounted ?
Sal
MikeG posted 06-29-2003 11:53 PM ET (US)     Profile for MikeG  Send Email to MikeG     
Thanks, I think I'm going to try out a 13.25x17 Michigan Wheel alum. I'm only looking to pick-up 400rpm or so and maybe a few mph. $109 and at worst it will be a decent back-up. The motor came off a Ranger-style bass boat. According to previous owner, he was getting 48mph on that 18ft. boat. The engine is mounted at the same height as the original engine. Cavitation plate is about even with keel. Lower bolts are in the blind holes. Not a huge fan of that design but changing it would require new holes in the transom as it appears the blind holes have different spacing than the other slots on the on the engine's mounting bracket. I do not plan on going back to the SS. I boat in Maine for part of the season and it is largely a rocky bottom. The rocks are not forgiving to $400 props.

Mike

Jamie 20 outrage posted 06-30-2003 12:42 AM ET (US)     Profile for Jamie 20 outrage  Send Email to Jamie 20 outrage     
I understand your wanting an aluminum prop, and any wheel will make that boat fly, but if I were you I would buy a merc prop over a Michigan wheel. I believe Merc props are better in speed, hole shot and fuel economy. But merc props dont stand up to rocks any better.
BKANE posted 06-30-2003 05:41 PM ET (US)     Profile for BKANE  Send Email to BKANE     
Mike G Where in Maine do you go? I live in the midcoast area. Bkane
MikeG posted 06-30-2003 06:16 PM ET (US)     Profile for MikeG  Send Email to MikeG     
BKANE,

My grandfather had a place near the end of Pott's Point, S. Harpswell ME. Right in the middle of Casco Bay. He had a '71 13ft BW. I usually rent a house on Bailey Island overlooking Mackeral Cove. Last year I dinged my new SS prop on rock getting out of the way of a fast moving lobster boat under the Cribstone bridge connecting Orr's and Bailey Island (right near Cook's Restaurant). Chewed up the edges of the prop a bit and maybe bent my prop shaft. I usually head up to Boothbay for a day trip each time I'm in Maine. Very nice harbor and town.

Mike

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