gear ratio, slip, RPM??? im confused

B

bigdawggie

Seaman
Joined
Jan 12, 2008
Messages
51
Hello

as you can tell from the subject I am a newbie here and would like to figure out what these terms mean.


I have a 71 kona 18' with a Mercruiser 1500 (which I am yet to find a manual for)

currently it has a 17" aluminum 3 blade prop which is pretty knicked up so I was going to buy a new one but from reading a few posts there is more to it than walking into the store and asking for a 17" prop.

I took the boat out last year and ran at WOT to see what the RPM was at and it reached about 6200. I think the motor is rated at 5500 WOT so I was going to go with a 19" as the new prop.

now I am seeing that gear ratio? & slip? plays a part in the prop but I don't have a clue know what it means or how to figure it out.

your help is appreciated
 
S

stevens

Senior Chief Petty Officer
Joined
May 4, 2005
Messages
799
Re: gear ratio, slip, RPM??? im confused

Two useful links if you want to read up on propeller theory and terms:

http://sites.mercurymarine.com/portal/page?_pageid=126,48572,126_49259&_dad=portal&_schema=PORTAL

and:

http://www.keypart.com/Marine/Propellers/PropellerGuides.aspx

A useful site for calculating slip, etc.:

http://www.go-fast.com/Prop_Slip_Calculator.htm

But you may not need to know all this. Just select the prop that gets you the right WOT RPM. You first need to verify what the proper max RPM should be, then that your tachometer is reading correctly, then what WOT RPM you are getting today. Armed with this, you select a prop accordingly, where every inch of pitch increase will lower your max RPM by about 150-200.
 
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