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Old 08-06-2017, 10:17 AM
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Prop cavitation

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?
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Old 08-06-2017, 10:27 AM
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Sounds like pretty classic cavitation description.
Did the prop "bite" again when you lowered RPM?
Did it do this multiple times?
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Old 08-06-2017, 10:51 AM
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It did bite once I lowered RPMs. It unnerved me enough that I didn't give it a chance to do it again
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Old 08-06-2017, 05:54 PM
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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?
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Old 08-07-2017, 08:26 AM
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"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.
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Old 08-07-2017, 09:45 AM
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Quote:
Originally Posted by alcodiesel View Post
"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.
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Old 08-07-2017, 09:52 AM
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Quote:
Originally Posted by Easy Rider View Post
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.
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Old 08-07-2017, 12:24 PM
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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
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Old 08-07-2017, 01:37 PM
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Quote:
Originally Posted by Dave Neptune View Post
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.
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Old 08-07-2017, 02:18 PM
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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.

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Old 08-07-2017, 09:11 PM
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oldlaxer, the growth here on the Chesapeake has been phenomenal this year. Get the bottom/prop cleaned and report back.
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Old 08-09-2017, 09:00 AM
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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.
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Old 08-09-2017, 09:11 AM
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Please, please confirm a clean prop before messing with the gear.
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Old 08-09-2017, 09:20 AM
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Quote:
Originally Posted by ndutton View Post
Please, please confirm a clean prop before messing with the gear.
Got it. Thanks.
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Old 08-09-2017, 04:59 PM
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Quote:
Originally Posted by Oldlaxer1 View Post
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.
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Old 08-09-2017, 05:07 PM
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John, with cavitation you should have noise emanating from under the boat as the engine revs up. If it is slippage the engine just begins to rev rapidly and no noise under the boat.

Do you feel a "definite" klunk when shifting into forward?

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Old 08-10-2017, 08:44 AM
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Quote:
Originally Posted by Dave Neptune View Post
John, with cavitation you should have noise emanating from under the boat as the engine revs up. If it is slippage the engine just begins to rev rapidly and no noise under the boat.

Do you feel a "definite" klunk when shifting into forward?

Dave Neptune
Yes, I do feel what I think is the klunk. I don't notice any difference when shifting now as opposed to the past 3 years with the boat.
When the engine revs I can't honestly say I hear a change in noise from under the boat but a runaway engine takes more of my focus.
I'm going to check shifter cable ends to be sure and I'll get the prop cleaned and report back.
Thanks for all the input guys!

John
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Old 08-10-2017, 09:24 AM
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I am hitting 0 inches manifold about 300 RPMs lower than usual, so I am diving today - jellyfish please stay away!
Clean running gear is really the first thing to check with the Indigo prop. It is easily sabotaged by just a few barnacles.
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Old 08-29-2017, 09:54 AM
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Once again, you guys were right. Finally rigged up the gopro to have a look at the prop. Hmmm, way past time for a short haul.
Thanks for the help. If weather cooperates she's coming out on Thursday.
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Old 08-29-2017, 12:29 PM
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Yeah...The Indigo doesn't have a chance with that much growth, and that isn't too bad! They are just extremely sensitive I think.
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Old 09-04-2017, 06:50 PM
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Couldn't get the short haul coordinated and had a trip scheduled for Sunday. Heading out, cavitation again. Throttled back and then had a great day on the bay sailing, maybe one of the best. Once we rafted I dove on the bottom and scraped the prop. Coming home today it was her old self. 5.5 kts without working hard. Winds came up and we sailed on home. I love this little motor. We're considering stepping up to a bigger boat but honestly I'm torn because I'm so comfortable with the A4 (and all the help here!)
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Old 09-05-2017, 03:27 PM
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Although cleaning the entire boat is a chore, at least now the sea nettles are mostly gone we can at least swim on the prop for another 4-5 weeks before it is too cold. - The boat is slow with a dirty bottom, and may affect extreme top end performance, at least the engine is usually happy as long as the prop is clean!
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Old 09-05-2017, 11:03 PM
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Quote:
Originally Posted by sastanley View Post
Although cleaning the entire boat is a chore, at least now the sea nettles are mostly gone we can at least swim on the prop for another 4-5 weeks before it is too cold. - The boat is slow with a dirty bottom, and may affect extreme top end performance, at least the engine is usually happy as long as the prop is clean!
Mostly is not the same as all. I cleaned my boat today and a sea nettle ended up pretty much on my face
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Old 09-05-2017, 11:39 PM
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Uh oh, sorry Joe. We did a trip from Solomons, MD to Deltaville, VA (Jackson Creek) over Labor Day and I do not remember seeing a single jellyfish!
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Old 09-06-2017, 07:59 AM
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There were none when I went in, but they showed up later. I put Vaseline on my face if I see them.
The local Western Auto has sun screen + jellyfish lotion. It is supposed to prevent them from stinging. I might have to give it a try - no idea if it works.
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