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40hp 2str 2cyl 2002 merc

D

DE:1

Member
On a 89 16ft Lowe aluminum mod-v. Has a 10-3/8x 12p on it. Tops out at 4000 Rpms and 26 mph.
What prop do I need to get better performance. Taking suggestions thanks.
 
Rule of thumb is a couple hundred rpm per inch of pitch change up or down for the same prop from the same mfgr. If you change anything, mfgr., material, bed bugs or whatever, it can change that number. If you put on a SS prop that is cupped, it will totally screw up your calculations if you were running a run of the mill alum.......the SS will unload the engine, the cup on the blades trailing edge will load it back up. Buttttt the experience will be exhilarating.

If you are over revving, more pitch. If loading up, less pitch by the numbers I said.

Mark
 
I dropped pitch 2 degrees and gained 1200 Rpms. The fact the engine is
is 40 hp the gain on rpm is significant compared to a large hp motor.
 
Okaye. What did you have for a prop and what do you have? Like I said unless it's apples and apples, it's apples and oranges and the sky's the limit.
 
Had a 10 3/8 x 14p alum. Now turning a 10 1/8 x 12 alum.
Don't know manufacturers. The 14p just couldn't get it over the top.
 
DE:1 said:
Had a 10 3/8 x 14p alum. Now turning a 10 1/8 x 12 alum.
Don't know manufacturers. The 14p just couldn't get it over the top.
Click to expand...

Ok I'll buy your 1200 ( not that it matters to you....grin). That's not how the increase is rpms is usually measured. It is assumed that the boat is in some attitude, usually planing out at WOT and you change very little as far as boat to water relationship.

Not getting on plane, and getting "over the top", aka "on plane, outta-da-hole" where boat speed picks up measurably, means that the boat drag is significantly reduced and the prop can propel the boat properly. Yeah then the rpms could increase significantly.

Mark
 
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