Prop pitch

[Sandman1775]

Hello everyone. My first post/curiosity/question. I’m new to boating and just bought our first boat a few weeks ago. 2004 aquasport osprey 205 22ft long, Johnson 175hp. I did some research and found the WOT rpm’s should be 4500-5500. I’m not getting more than about 4400, and my top speed at WOT is about 26-28mph. I couldn’t find anything that said what my stock prop was. The prop that’s on it now is a SS14.75x17. Thinking I might try a 19 pitch, for a little more top speed. And I realize I will sacrifice a little acceleration, or hole shot. Correct me if I’m wrong. Anyone with a similar size boat/motor have any advise? Thank you. Semper Fi.
Jacob.

[Capt. Bob]

Start here.

https://classicaquasport.com/smf/index.php?topic=6417.0

[boatnamesue]

Couple rules of thumb and advice.....
- - - -
For every inch of pitch  =  change of 150-200 rpms
Increasing pitch decreases rpms...vice versa
Ideally want to reach top end of rpm spec at WOT
- - - -
Do not use speed as a measure of engine performance....too many variables effect speed
Rpms are the "window" for observing engine performance
Check the engine AV plate is no more than 1.5" above or below bottom of boat (ideally they are even)
Prop testing should be performed in calm conditions and with a light load

If doing the above w/ur 17" prop and still not turning higher rpms...then you dont have a prop issue, you have an engine issue.  Your off by 1k rpms, so theoretically you'd need to drop down to a 11" prop to reach top end rpm spec....and 175hp don't run w/11's. 

[wingtime]

You have something going on besides your prop as mentioned you are at least 1000 rpms low.  17" pitch is probably about right for that hull engine combo.  You want to be on the upper end of the WOT RPMS even a tad above it with a light load.  I'd start with engine height.  Where is the anti cavitation plate riding when on a full plane with the engine trimmed properly?  You should be able to see it riding on top of the water.  If you cant see it or there is lots of spray going over it the motor is mounted too low.  Also be sure your tach is reading correctly.  If the engine height is correct then you may have a cylinder that is not firing. 

[Sandman1775]

Thanks for the help. I’ve been thinking the same thing. Going to check things out this weekend hopefully.