prop size

jc44

Cadet
Joined
Mar 11, 2012
Messages
22
just bought a 97 crest 111 pontoon with 27" tubes.has a spare prop on now ,dont know size yet will look later,motor is a 1976 75hp johnson, now does 13mph at 4200 rpm,any ideas where to start for right prop. thanks.
 

crb478

Lieutenant Junior Grade
Joined
Dec 6, 2006
Messages
1,036
Re: prop size

See what the pitch of the prop you have now is. You should be turning 5000-5500 rpm and the higher the better. I suspect that you need around a 13 pitch prop or maybe even an 11. I am a fan of four bladed props on pontoons, I believe they run smoother in the turbulent water behind the pontoons, but that they also give you better control in reverse.
 

MH Hawker

Vice Admiral
Joined
Jul 13, 2011
Messages
5,516
Re: prop size

I would think with a 75 hp you snould be running in the 20 mph range.
 

jc44

Cadet
Joined
Mar 11, 2012
Messages
22
Re: prop size

thanks for info, boat being delivered in a few minutes,will post results after checking prop.
 

jc44

Cadet
Joined
Mar 11, 2012
Messages
22
Re: prop size

the original prop is not on the motor as it was damaged,it isa 13x17,the previous owner said ran decent.wondering if i should try a lower pitch around a 11 to 15?
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: prop size

A 17 seems like a lot of prop.Your present wot rpm and gps speed will help determine the best prop.
 

crb478

Lieutenant Junior Grade
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Dec 6, 2006
Messages
1,036
Re: prop size

I would drop to al least a 13 but probably an 11 pitched 4 blade would be about right
 

jc44

Cadet
Joined
Mar 11, 2012
Messages
22
Re: prop size

pulled prop off motor,had to go by serial number,no size stamped,took it to shop guy said it was 133/4 by 15. ran boat boat in the river where it was calm,most i could get was 16mph at 4100 at wot. how much should i go down on pitch,11,12,13????. any suggestions would be appreciated. what about a 4 blade?. thanks.
 

crb478

Lieutenant Junior Grade
Joined
Dec 6, 2006
Messages
1,036
Re: prop size

If you are only getting 4100 rpm with a 15 pitch you may need to go to a 9. I believe that motor should be turning 5000-5500.
 

lakegeorge

Senior Chief Petty Officer
Joined
Mar 19, 2002
Messages
660
Re: prop size

pulled prop off motor,had to go by serial number,no size stamped,took it to shop guy said it was 133/4 by 15. ran boat boat in the river where it was calm,most i could get was 16mph at 4100 at wot. how much should i go down on pitch,11,12,13????. any suggestions would be appreciated. what about a 4 blade?. thanks.

I would go to the 11 pitch.
 

jc44

Cadet
Joined
Mar 11, 2012
Messages
22
Re: prop size

thanks will do. sorry i just noticed i had repeated my-self in an earlier post.
 
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