Prop Cavitation Problem

[Buck_Hunter04]

I bought a new prop for my Mariner 25HP last year and I got the right one. (According to the Stamped #'s) Problem now is when I begin to get moving my engine revs up and I cavitate. I have to slowly build up speed and then take off. I did put 2 12v marine batteries in the back, about 1/3 from the back. However, I didn't have this problem with the old prop. The old prop was dinged up and ground down. Probably way out of balance. Only thing I can think of is too much weight in the back throwing off my center of gravity... And the boat is a 14 Northwoods Troller. Thanks for any answers anyone can give me.

[Finns]

Is it a "Brand New" prop or new to you prop? The reason I ask is if it is a used prop (new to you) it sounds like maybe a 'spun hub'. Brand New prop, I have no idea.

[Big-Al]

I was asked to troubleshoot a like problem on the same motor on a pontoon boat. The symptoms that were described to me were of a spun prop. I looked at the prop and it did not appear that the prop was spun. I took if for a drive and I have one very brief time (under a second) that it revved a little bit. Other than that it was fine. Is it possible for a prop to slip and then be fine or was this situation likely cavitation? Don't intend to hijack the thread Buck Hunter, just solicit more discussion.

[BartmanMN]

Hmm, I have been getting a lot of cavitation on corners lately so I decided to order a new 4 blade prop. I'll need to look at the old one to see if it has signs of a spun hub.

[FshBloodRiverMud]

What are the signs of spun hub?

I have had similar things happening in my boat on occasion...Even though i'm probably going to replace my leaker this weekend with a new boat, it would be nice to know if i have signs of a spun hub for resale.

[leech~~]

To check for a spun hub.

I'd scratch or use a marker on the splined insert inline with a scratch on the propeller housing (include the rubber too). Take her out and bring her back in and see if the scratches still line up.

[Buck_Hunter04]

It was a brand new prop. in the Mercury box and the numbers were right. Might not mean anything. as long as I get up to roughly 10mph I'm fine, but it didn't do that with the old prop. Just can't gun it anymore. Confused?!

[leech~~]

Were the pitch numbers of your old and new prop the same?

[jmg]

Probably doesn't have anything to do with your situation, but a no. of yrs. ago bought a 1975 boat w/ 1975 40hp. Merc on it. Prev. owner had gotten different prop for it to get up on plane quicker, but then sold the boat. Well, when I would gun it to go, it would just sit there spinning like the prop was in air. If I eased way up on the throttle, it would eventually get going.

I found out there was a spacer (thick washer) that was supposed to be removed for that prop. After I got rid of the spacer, it worked great and never had the problem again. Ironically, now I had to replace a prop on an Evinrude, due to a spun prop, after doing the test someone else(Leech) described. Good luck with your problem.

[Kyhl]

I was asked to troubleshoot a like problem on the same motor on a pontoon boat. The symptoms that were described to me were of a spun prop. I looked at the prop and it did not appear that the prop was spun. I took if for a drive and I have one very brief time (under a second) that it revved a little bit. Other than that it was fine. Is it possible for a prop to slip and then be fine or was this situation likely cavitation? Don't intend to hijack the thread Buck Hunter, just solicit more discussion.

Yes, props can slip like that. It depends on what they were designed to do. Adding more rake or cupping (which adds rake) can fix this but at the cost of being less effecient because it creates more resistance as the prop spins.

Props with less rake will be more prone to this if the motor is set high enough. I've had props that would briefly lose grip in the corners or accelerating off cruise to WOT.

So they make props with different designs that fit your desired use. That's my experience anyway.

I still haven't decided which way I want to go with my current setup. Started with little rake. Moved to a lot of rake. Now little rake with extra cupping. I may go back to the original with little rake and accept that it loses grip at times.

[Buck_Hunter04]

Thanks for the input. Anything I can do to avoid spending more money on a new prop again if not necessary.

[Kyhl]

Thanks for the input. Anything I can do to avoid spending more money on a new prop again if not necessary.

Soderblooms.

[dfv87]

I spun the hub in my motor 2 summers ago, and replaced the prop. if the hub is worn in a new prop you may not be getting a clean fit and thus the slipping. The cavitation could be the new props added speed/power. I would try moving those batteries and maybe a gas tank forward to even out your weight. this might be 2 separate issues...JMHO.

[Tom7227]

Consider sending the old prop to Soderbloom's for repair and keep it for a spare. A fraction of the cost of a replacement and you will be very pleased with the result.