Prop slip and cavitation.

WWGS

Cadet
Joined
Oct 15, 2020
Messages
6
Bought a new prop for my 1989 85hp Force. It appears my prop is slipping and I have cavitation happening. See the attached photos. The yellow marks on the prop had moved after about 30 minutes of run time. I’d been feeling some slipping or more like grabbing here and there while running it up on plane. I recently had the lower seals replaced, and found that the prop nut hadn’t been torqued. Tightening it to torque made it run much better, but as you can see it’s still slipping. I hope the new prop relieves me of both issues. I’m concerned the cavitation will continue. Anyone had experience that would say otherwise?

thanks for any comments!
 

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Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
13,539
Cavitation is minute air pockets formed when the oxygen is pulled out of the H2O mixture called water. Reduced air pressure, aka vacuum is what does this and in your case I'd say the rough character of your surface is the smoking gun. I have no idea as to how it looked when originally installed and personally don't think the slipping hub has anything to do with it.

I only had cavitation on one prop in 60 years of boating and it was on a new 1971 Chrysler 85 hp with the factory aluminum prop. The leading edge of a slick, painted, aluminum prop, had a hook (reverse curve) right at the leading edge of where the blade extends from the hub. The cavitation erosion, down into the aluminum a good bit, was circular about the size of a quarter dollar and right in the middle of the hook. The remainder of the prop remained slick and no erosion.

Numerous times I have seen responses on here concerning props with nicked blades and the cavitation issue was mentioned with the irregular surface being the source. The other thing might be poor paint and a poor grade of aluminum to cause it to start in the first place, or something like a barnacle, some barnacles attaching themselves due to being idle while immersed in salt water.

Best I can do on this.
 

WWGS

Cadet
Joined
Oct 15, 2020
Messages
6
Cavitation is minute air pockets formed when the oxygen is pulled out of the H2O mixture called water. Reduced air pressure, aka vacuum is what does this and in your case I'd say the rough character of your surface is the smoking gun. I have no idea as to how it looked when originally installed and personally don't think the slipping hub has anything to do with it.

I only had cavitation on one prop in 60 years of boating and it was on a new 1971 Chrysler 85 hp with the factory aluminum prop. The leading edge of a slick, painted, aluminum prop, had a hook (reverse curve) right at the leading edge of where the blade extends from the hub. The cavitation erosion, down into the aluminum a good bit, was circular about the size of a quarter dollar and right in the middle of the hook. The remainder of the prop remained slick and no erosion.

Numerous times I have seen responses on here concerning props with nicked blades and the cavitation issue was mentioned with the irregular surface being the source. The other thing might be poor paint and a poor grade of aluminum to cause it to start in the first place, or something like a barnacle, some barnacles attaching themselves due to being idle while immersed in salt water.

Best I can do on this.
Thank you sir. This is my first boat and is used, so I have no idea how it looked either. I appreciate your insight!
 

WesNewell

Chief Petty Officer
Joined
Jan 3, 2018
Messages
435
the spillage mark just shows the hub is slipping. Don't assume it just slipped that much. It likely slipped many rotations if it was noticeable. You likely need a new hub or prop..
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
23,066
The pitting is from cavitation.----What pitch was the prop on the motor ?-----What pitch is the new prop ?----What RPM is the motor running at ?
 

WWGS

Cadet
Joined
Oct 15, 2020
Messages
6
The pitting is from cavitation.----What pitch was the prop on the motor ?-----What pitch is the new prop ?----What RPM is the motor running at ?
Both the old and new prop are 13inch x 19 pitch. 4000rpm is roughly where I keep it when on plane
 

WWGS

Cadet
Joined
Oct 15, 2020
Messages
6
Both the old and new prop are 13inch x 19 pitch. 4000rpm is roughly where I keep it when on plane
I might add that I had a new aftermarket CMC trim/tilt added because the old Force unit was leaking and no seals were available. Not sure if this could’ve created an issue.
 

WWGS

Cadet
Joined
Oct 15, 2020
Messages
6
the spillage mark just shows the hub is slipping. Don't assume it just slipped that much. It likely slipped many rotations if it was noticeable. You likely need a new hub or prop..
Yes, I'm not assuming it was just that short amount of slip, I already received the new prop, but have yet to install it.
 
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