prop question

D

duckjunkie

Cadet
Joined
May 31, 2010
Messages
7
i'm posting this under the tohatsu thread even though my engine is a 2003 mercury 40 lightning xr. i know this engine is actually a tohatsu made for mercury, black with mercury decals. i have ordered tohatsu parts (rectifier, starter, switch, and relay) and all fit and use tohatsu wiring codes. my mercury manual shows a top engine speed of 5500 while the 2003 tohatsu manual shows a top engine speed of 5700. the engine came with a 12.5 pitch aluminum prop from the factory and i am currently turning it at 5500 wot. i don't believe this prop has any cup built in it. i am wanting to go to a stainless prop but not sure if i should go with a 12 pitch or a 13 pitch. the boat jumps up on plane now like a bullet. i know the stainless props i am looking at have cup built in, and i know cup is like adding pitch. any thoughts and help would be greatly appreciated. thanks for your time, duckjunkie
 
pvanv

pvanv

Admiral
Joined
Apr 20, 2008
Messages
6,579
Re: prop question

This question is probably better posted in the prop forums. However, here are the rules of thumb: Aluminum flexes more than stainless. So going to stainless, you can usually reduce pitch slightly. Same for adding cup -- reduce pitch slightly for similar WOT RPM. So try the 12. Remember that you must actually test the prop to get real numbers. Every boat reacts a little differently. Prop selection is always a bit science, a bit of an art, and a bit of a crap shoot.
 
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