Cavitation over 3400 rpm, help with prop selection, mercruiser 260, 24' Sea Ray

[fastang50]

Ran the "new" (nearly thirty year old) boat today. Have a prop question, monitoring speed and RPM I'm pretty much getting solid results until about 3400 and then have cavitation no matter where the lower unit is trimmed. Boat is an '83 24' Sea Ray, pretty heavy. It's wearing a 15x15 prop, 1.5 gears I expect. I'm seeing 20 mph at 3k rpm, builds to 25 by 3400rpm, but anything past that you feel the prop unhook. Very strange feeling, it will literally tach to nearly 5k with no change in speed.
The mercury pop selector site doesn't even address this set of variables! I'm thinking I need to go up at least two sizes on pitch to a 19" or so. We're looking to use this boat for long distance river travel so fuel economy is way more important than acceleration.
Suggestions?
Thanks.

********Updated with a pic several posts down***********

 

[spdracr39]

Re: Cavitation over 3400 rpm, help with prop selection, mercruiser 260, 24' Sea Ray

Are you sure the hub isn't spinning? Draw a line from the nut to the outside of the hub. Run it and see if it stays in line. If not it is spinning and needs repair.

 

[fastang50]

Re: Cavitation over 3400 rpm, help with prop selection, mercruiser 260, 24' Sea Ray

Nope, I'm not sure that isn't what's happening. I didn't even know that was possible. Thanks for the idea.

 

[QC]

Re: Cavitation over 3400 rpm, help with prop selection, mercruiser 260, 24' Sea Ray

Also, what's the condition of the prop, and where is the trim set? Trimmed out they will Ventilate (cavitation is something else. Common misconception) early too.

 

[fastang50]

Re: Cavitation over 3400 rpm, help with prop selection, mercruiser 260, 24' Sea Ray

Prop is in excellent condition, almost looks new. The issue happened regardless of tucking the drive full down or raising the bow. Thanks for the info though, I'll have to read up on "ventilate" vs. "cavitation".

 

[Scott Danforth]

Re: Cavitation over 3400 rpm, help with prop selection, mercruiser 260, 24' Sea Ray

to me it sounds like a spun hub.

like speedracer stated - pull the prop nut. take a paint pen, mark the brass center of the hub in relationship to the prop with a paint line. re-install the prop nut and retainer. take it for a spin. then take a look at the marks. if they moved, the hub is spun.

if the hub is spun, take the prop to a prop shop and get it repaired.

 

[fastang50]

Re: Cavitation over 3400 rpm, help with prop selection, mercruiser 260, 24' Sea Ray

I appreciate the advice everyone. It will be some time before we can test for a spun hub, but I'll check it out. It does make sense to me as I think about how it felt.

 

[fastang50]

Re: Cavitation over 3400 rpm, help with prop selection, mercruiser 260, 24' Sea Ray

The plot thickens. I pulled the prop this evening. Unfortunately I won't have a chance to do the paint marker test for a spun hub for awhile... but looking on the front side of the blades I have what might be cavitation erosion going on. Now if I could just figure out how to post a picture....