14 in dia vs 15 in dia

paulray

Cadet
Joined
Jun 20, 2008
Messages
9
'89 Starcraft 22ft American Express Dry Wt 2750 lbs. AQ170C/270 power/io. Came with a 15x15L prop. I had to replace it after some rock climbing with a 14x15L. With the new prop it was running 25mph GPS at 4000 RPM. These are supposed to run 25 mph @ 4200-5000 but I could never get this performance out of the 15x15. Well, more rock climbing (tough area) and now had to use a 14x12 for the weekend. 17mph @ 4200 rpm. So shopping again, should I go 14in or 15in dia (think I'm staying with the 15 pitch)? What difference would/could 1in in diameter make?
Thanks
 

wire2

Lieutenant Commander
Joined
Jun 25, 2007
Messages
1,584
Re: 14 in dia vs 15 in dia

Very little. The larger diameter will have slightly less slip for a given pitch.
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,771
Re: 14 in dia vs 15 in dia

4200 - 5000 RPM is a pretty broad rpm spread which will mean speed will also be in pretty broad range depending on the boat, engine condition, load, etc. I also question a 14x12 pitch and turning only 4200 rpm????? That engine should be screaming unless the 4200 RPM is not wide open throttle in which case all of your numbers are meaningless. Prop testing needs to be done at wide open throttle. Are you sure the tach is accurate?
 
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