[Dave1273]

I have been on and around boats most of my life and only recently realized, I have never had to replace a prop. The prop on my latest boat is in need of repair and the dealer said he would take care of it whenever I got it back to him but it would take a week or so to get it back. Since the boat is still new (to me) and I hate the thought of not being able to hit the lake if I get half a chance, I have decided a spare prop would be a good idea but I have no idea what size mine is and there are several options for my engine. Is there any way to tell what size prop I have so the spare will be the same? Also, I wouldn't mind dropping a few RPM's on the top end so I should probably go with a larger pitch right? What other effects will this have on performance?

 

[parallaxbill]

Dave there should be some numbers stamped on your current prop hub. You'll probably have to remove it to see them. If they are there you can trace the numbers back to the motor manufacturer if original or look them up on the internet. I found mine recently just by typing in the series of numbers on Google. I got two hits and one told me it was a Mercruiser prop, diameter and pitch.

If you go longer in pitch you will likely lose 200-300 rpm with each inch of pitch added. But that varies a good bit. What performance changes you may see depends on your hull condition, boat design and general condition of your power plant.

Tell us more details about your boat, hull design and motor.

 

[Dave1273]

The boat is a 17' proline cc with a 90hp Force by Mercury outboard. I'm not sure if I need to drop the RPM's or not but the tach is showing just over 5000 at full throttle and I feel that's a little too high. I'm not really concerned with top speed but I do like the fact that it planes quickly.
Thanks for the reply. I haven't pulled the prop off yetbut as soon as I do, I'll look for the numbers.

 

[Topsail Angler]

It may be different on some outboards, but 5500 is the recommended RPM on my Yamaha 115 4 stroke. 5000 doesn't sound high but check with Mercury.

 

[zooker]

my yamaha runs at 5800 rpms wot. merc should be in that region omc's top out at 5200 rpms.. you may be over propped the moter. 22p or 24p prop is about right for a 90 force.

zooker

 

[Dave1273]

Thanks for the input, Maybe 5000 isn't too high. It's been a while since I had an outboard and I guess I was still in the OMC I/O state of mind. In that case, I'll pull the prop and take a look at the numbers.

 

[Dave B.]

Merc props have a stamping the on the side of the prop hub. If the props been painted in the past, then you may or may not have to scrape some paint off to get to the # (alot less scary than it seems). First # is your hub diameter and the second # after the X is the pitch. There is sometimes a stamping on the the hub where the prop nut is as well (should be on the flange area behind the prop nut washer if an older model).


Mercury website also had a prop chart that you can use too to evaluate your boat size, total load/weight, motor hp rating/model, and etc.. to come up with a good start pitch/diameter wise to put you in the ball park.

if no local dealer or knowledgable dealer, let me know and I'll help research for you.

Normally 15 or lower pitches are for pulling and heavy loads
17 pitches are more for speed and to reach top end/performance of hulls

The theory is,,, the pitch is how far the boat will travel forward on one revolution of the prop... so theoreticly.. a 19P prop should propel the boat 19 inches forward in ideal conditions without slippage and I'm sure other variables...
Diameter plays a part too.. smaller diameter props spin a tad faster than larger diam'd props.. so definately need to get both those #'s if possible from the prop you have now or even look in the owner manual for the motor (if available) for any manufacturer recommendations.