Prop cavitation

[Oldlaxer1]

We were out yesterday and motoring into a pretty stiff headwind and current. I upped the rpm's (Indigo prop, no tachometer) and after a minute or so the engine reved fully up as if I had accidentally shifted into neutral (I hadn't). I'm due for a short haul for bottom and prop cleaning so I'm thinking this could have been caused by barnacles on the prop and it suddenly cavitating. Does this sound feasible or should I be looking at tranny slippage as the culprit?

[roadnsky]

Sounds like pretty classic cavitation description.
Did the prop "bite" again when you lowered RPM?
Did it do this multiple times?

[Oldlaxer1]

It did bite once I lowered RPMs. It unnerved me enough that I didn't give it a chance to do it again

[Easy Rider]

was it rough enough that the prop may have come closer to the surface or perhaps the underwater flow changed the dynamics. Just a thought. When you slowed down and the prop "bit" was the boat bouncing around less?

[alcodiesel]

"It did bite once I lowered RPMs."

When my gearbox was slipping out of forward it would not re-engage by lowering RPMs. I had to manually shove it back into forward. Not a difficult adjustment, by the way.

I am voting for cavitation, even though props on sailboats seem to be pretty far below the waterline.

[joe_db]

"It did bite once I lowered RPMs."

When my gearbox was slipping out of forward it would not re-engage by lowering RPMs. I had to manually shove it back into forward. Not a difficult adjustment, by the way.

I am voting for cavitation, even though props on sailboats seem to be pretty far below the waterline.

Submarines suffer cavitation. This is when the pressure on the low pressure side of the blades gets low enough for the water to flash to steam. Ventilation, a common issue with outboards, is when air from the surface gets sucked into the prop.

[Oldlaxer1]

was it rough enough that the prop may have come closer to the surface or perhaps the underwater flow changed the dynamics. Just a thought. When you slowed down and the prop "bit" was the boat bouncing around less?

Not terribly rough at that point so I don't think that caused it. On the Tartan 30 the prop exits through the trailing edge of the fin keel so it is almost in the middle of the boat.

[Dave Neptune]

Is your shifter fully in the detent locking the forward clutches? How was the "power of the prop" up to the cavitation/clutch slippage?

Just another thought.

Dave Neptune

[Oldlaxer1]

Is your shifter fully in the detent locking the forward clutches? How was the "power of the prop" up to the cavitation/clutch slippage?

Just another thought.

Dave Neptune

I'm almost positive it was fully engaged. I told my wife when we went out earlier in the week that the bottom and prop probably needed cleaning because the boat felt much slower at cruising speed. I don't have a speedo so it is just how it felt. Last bottom and prop cleaning was about a year ago so I think we're due. I had just never experienced that before. First thought was we lost the prop!
Thanks everyone.

[Dave Neptune]

A year is a very long time so it was probably cavitation. Get her cleaned, take her out and give it a bunch of throttle to see if the prop and trans hold.

Dave Neptune

[sastanley]

oldlaxer, the growth here on the Chesapeake has been phenomenal this year. Get the bottom/prop cleaned and report back.

[Oldlaxer1]

I got down to the boat last night and fired it up while tied up in the slip. I could run it just below cruise without issues. If I pushed it up just a bit I had 3 occurrences of the cavitation/slippage. Totally calm water. I'm beginning to believe it could be slippage rather than cavitation. I'm pulling out the manual and reading up on adjusting one notch. Before doing so I'll be checking the shifter cable to be sure it is not slipping at either end.
It will probably be next week before I can get back to the boat. I'm scheduling the short haul for the week of August 21 when we're back in town.

[ndutton]

Please, please confirm a clean prop before messing with the gear.

[Oldlaxer1]

Please, please confirm a clean prop before messing with the gear.

Got it. Thanks.

[roadnsky]

I got down to the boat last night and fired it up while tied up in the slip. I could run it just below cruise without issues. If I pushed it up just a bit I had 3 occurrences of the cavitation/slippage. Totally calm water. I'm beginning to believe it could be slippage rather than cavitation.

Note that you are way more likely to have a cavitation episode while throttling up at the dock.
The reason is that you are not moving THRU the water giving the prop "clean" water to keep biting into.
Basically the prop is sitting in place and cutting a "hole" in the water.
Sort of like spinning your wheels in a mud hole.