Engine swap pitch question 5.0-5.7

[Chevy13s]

Hey guys, I have a 18' 78 Sunrunner bowrider OMC 800 which I swapped the 305 for a 350 and gained quite a few ponies. The prop says 15x17 on it and its 3 blade alum. I took it out for the first time today and I'm proud to say it ran great but I didn't want to give it more than half throttle because I was already at 5000 rpms and only doing around 30 or so I would guess. I would like it to run around 4200-4600 at WOT instead of half throttle. So what should I do? I heard you can borrow props to see if they are correct? Where can I find these?

Thanks

 

[walleyehed]

Re: Engine swap pitch question 5.0-5.7

Are you using the same out-drive? If you are, something doesn't match-up to me...
It takes "roughly" 17-18HP to gain 1 MPH at this point. There ARE other variables but not any that would show or "cause" a 10MPH gain.
If you were with-in specs with the 15X17/305 combo, with the same outdrive, and a 350, I don't feel the gain will be as much as what your numbers show, and here's why...at 5000RPM with the 15X17 and showing 30MPH something is way wrong...
I figured this with the several ratios.....the slip shows from 35-45%...should be in the 15-18% range for that prop.
One of two, or both is off...either the speedo (GPS or conventional?) and/or the tachometer.
Some insite as to which drive you have may help...2.2 ratio????...the only ratio that would match-up so the tach and speedo would be out of the question.

 

[Chevy13s]

Re: Engine swap pitch question 5.0-5.7

Im sorry, i didnt know you guys do all kinds of math to figure it out. You rock! Speedo was not working at the time, I just guessed, and that was ground speed in a river. I dont know if the combo was matched with the 305 because the motor was rusted solid when I got it. The outdrive is an OMC 800. In my Selec book it says:

Gear Ratios
Upper 21.17
Lower 14.26
Total .67
Overall 1.49

Thanks!

 

[MikDee]

Re: Engine swap pitch question 5.0-5.7

In my opinion, generally a light 18' boat with a 260hp/350 I/O uses at least a 21" pitch prop, or maybe a 23" pitch, but there are variables to consider, such as what's your WOT rpm with the 17" pitch prop, then deduct 200 rpm for every increasing change of pitch, usually designed in inrements of 2", like 19, 21, 23, 25, Till you get to 4600rpm at WOT lightly loaded.