Change Pitch or Stay the Same

[stratos92]

I have a question regarding whether to change props or not. I have a 17 1/2 foot fish/ski with a '94 Mercury 150 Xri on the back. The boat will run approx 55mph at 5200-5300 rpms with a SS 25p 4-blade prop. The rpm range for the motor is 5000-5600. Should I leave it running as is or drop down to a 23p prop? Suggestions?

[Art Bernard]

Your fine where your at according to the info I've researched (been going thru the same decision this past week). Removing a pitch will give you approx 500 more rpm and adding a pitch will decrese approx 500rpm. If your looking to tweek your top speed you could remove a bit of cup to your blades which is worth about 1/2 a degree of pitch or 250rpm approx, or you could add cup which would take you the other way. With only 600 rpm to play with, I'd leave it alone if she's running fine and doing the job (why take a chance on killin your engine). What you could do is look at how you're boat is loaded weight wise and make adjustments there to effect rpm such as reducing weight when pulling skiers, etc... The info I've givin you is based on what I've learned from other members of this board and several conversations I've had with the techs at Marine Wheels here in Panama City FL. as well as my own research. Hope this helps.

Art

[Robbabob]

Rule of thumb: 1" pitch change = 200 rpm
Pitch down = rpm up
Pitch up = rpm down

ONLY CONSIDER A PITCH CHANGE WHEN YOU KNOW IT IS NECESSARY.

Pitch is a reading in inches, not degrees. It is the theoretical distance the prop will screw into the water during one revolution.

Stratos, has your rpm's always been this low or has there been a change we should know about? Review the sticky at the top of this prop forum and give a little more information.

Good luck!

[stratos92]

The motor has always been at that RPM (52-5300). This motor was recently put on the boat. The motor that came on the boat when I bought it had a blown powerhead. This motor and prop came off a little bit bigger bass boat. The boat is a 92 Stratos F/S (17' 9" long/91" beam/1370 lbs hull wt) and the motor is a 94 Mercury Xri 150hp. The prop on it now is a stainless Renegade Bass 13.5 x 25p. Again, the boat tops out at about 5200 RPMs with the wife and I and fishing gear loaded with the max RPM range being 5000-5600. My concern is not really performance based, it is more a concern of running the motor WOT at the lower end of the RPM range. I just don't want to do any damage to the motor. The boat is primarily not run at WOT, but at about 4500 RPMs/40-45 mph. Any advice would be greatly appreciated.

[stratos92]

Also, could I go with an aluminum prop or should I stay with SS if I change? If I go to aluminum, would I still drop to a 23 to gain the same amount of RPMs? Sorry for all the questions but I am most definitely a newbie boat owner!

[Robbabob]

Stratos,

I see you being fine... if you like the current performance, (i.e., hole-shot and top end where you feel good). You may not always run with the family in the boat, so the rpm will be higher without them. You are within motor range loaded and (probably) when near empty. No need to change a good thing, IMHO.

Going to a 23P would be too much when you run empty and, if not watched, could send you over rpm max. Do a test run by yourself with low gas and a near empty load... I'm thinking 5500 rpm will be reached. If it stays below 5400, you could do a 24P and see increased performance. Still, no reason to change a good thing.

Ennnnnjoyyyyyy!

[Robbabob]

Also, could I go with an aluminum prop or should I stay with SS if I change? If I go to aluminum, would I still drop to a 23 to gain the same amount of RPMs? Sorry for all the questions but I am most definitely a newbie boat owner!

You will likely exceed rpm with an AL 25P, never mind the 23P AL. It has to do with reduced efficiency (flex of the blades and usually less cupping with the AL). This is all rule of thumb as some (very few) AL props can perform nearly as well with most SS props. I try to keep my answers simple (KISS) because rule of thumb is easiest to understand for a newbie.

Many questions are fine... it only gets frustrating when the same question is repeated trying to use different words.

I know you'll do fine...

[stratos92]

Thank you very much for your help and suggestions. I really appreciate it! I will do the test runs your suggested and make changes only if necessary. Thanks again.