InTheWoods

This weekend I had 2 things occur that I never experienced before. At 1st I thought they were unrelated, but now I am not sure...

Saturday - Experienced random & dramatic cavitation while cruising our local lake...
- Running straight (not banking in a turn) in smooth water @ ~35mph
- Normal load, normal trim, normal rpm for that speed
- No known impact of anything: music was off so we didn't hear, feel, or see any impact
- No noticeable vibration. Drive did not kick up
- Almost instant & complete cavitation = RPM climbed VERY quickly as engine raced & we fell off plane as if I had spun the prop off.
- Quickly backed off throttle to keep engine from racing, but I was still in gear & still cavitating for about a minute, maybe 2.
- Throttled to idle & took out of gear. Temp, Oil, & all other gauges looked fine.
- Raised drive, felt prop & skeg = all seamed fine & tight as best I could determine
- Nothing see in water, so I trimmed in & replaned.
- This was at end of day & about a mile from ramp. Boat ran fine & loaded on trailer as usual.
- I have experienced mild cavitation before. It is a 23 degree, non-stepped, non-padded hull. If I am running trimmed out & then bank into a high speed turn, the boat will cavitated / ventilated due to my forgetting to trim in.
- I have no idea what caused such sudden & dramatic cavitation... we were in 12' of fresh water... had I passed over an underwater spring releasing a pocket of air?

Sunday - 1st time ever that my boat engine overheated...
- Back out to the lake for Fathers day
- Put boat in water & within 10 minutes of idling / boarding + 2 minutes of planing, the engine temp buzzer came on & my normal rock steady 160 degree had climbed to almost 200.
- Shut boat down, anchor, let cool, look around, jump in & check for blockage on drive intake. Nothing obvious, so I restarted & watched the temp climb again. Stop, cool, idle back towards ramp... repeat that a few more times, until the boat is back on trailer.
- I didn't want to mess too much on Father's Day, but once home I did run on hose until engine went over 160 / 170, then shut down, remove the thermostat cover. Things where hot, so thermo was wide open as it should be.
- I will solve the overheating issue this week


My ?'s are...

1) Has anyone experienced sudden cavitation like that? If it was an environmental fluke, so be it OR should I be looking at something else with respect to my Engine / Hull / Drive / Prop?

2) Is it possible that the lack of water around my drive & prop that caused the cavitation could have also spun my impeller dry, such that I burnt it up? It was a relatively new impeller, but some say that running it dry can rip it up in 30 seconds.


Boat...
- 94 PowerQuest 270 Laser
- Single engine Mercruiser 7.4 w/ Bravo 1 drive, lower housing & skeg in excellent shape
- 3 blade, SS prop in excellent shape
- No thru hull fittings, No extra transom mountings other than original TrimTabs
- Impeller, impeller housing & thermostat all replaced ~ 50 hrs ago
- All cooling hoses removed & inspected ~ 5 hrs ago
- Engine circulating pump never removed nor serviced

Sorry for the long post.

Thanks, Joe

zz28zz

Maybe you ran over a plastic bag floating in the water. They are almost impossible to see at speed. It might cavitate your prop and definitely would block water intake. Once you stop it could fall off. Just a thought.

I'd definitely check that impeller before running it again.

Trash

First off I have to give you credit for supplying a lot of information. That's the key to getting help on the forums. My first thought was the coupler let go but I don't feel that's the case.

I would echo what zz28zz stated. Clear plastic bags, a piece of carpet etc. could have been just under the surface and caused all that chaos. Once you came off plane it fell off and sank. In the meantime the impeller came apart. I'd check the impeller and clear out the coolant lines of debris.

justinp20012500

Did you spin the hub on the prop? I know they can feel tight, but when placed under a load they can spin.