prop size

mayjean123

Seaman Apprentice
Joined
Sep 5, 2004
Messages
41
I have 28 foot 1980 kayot pontoon with a 120 mercruiser inboard engine it has a 3.0 mercruiser out drive what would be good starting point for a prop it does not have a prop right now .it came with a omc stringer out drive but we changed that over to merc,
 

Reel Kahuna

Petty Officer 1st Class
Joined
Apr 3, 2010
Messages
271
Best to run the Toon at WOT to see if it'll reach the maximum RPM. If it doesn't then either decrease or increase the prop pitch. Each inch changes the RPM to about 200 rpm. i.e. Changing from a 19 pitch down to a 17 pitch will change the rpm to 400 rpm.
 

ahicks

Captain
Joined
Sep 16, 2013
Messages
3,957
First prop WAG, maybe 13 or 15". No bigger than 15 for sure. 17" starts getting into stuff used on smaller ski boats.
 

Reel Kahuna

Petty Officer 1st Class
Joined
Apr 3, 2010
Messages
271
I just did a shake-down cruise on my 28' Party Hut re-powered from a Merc 115 2stroke to a Yamaha 115hp 4stroke. Based on my calculation I need to drop the pitch from the 17" at 4800 rpme to an 11" to attain the 6K rpm.
 
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