Cavitation (maybe?) questions

[bobsyeruncle]

Multi faceted post...a few issues at hand
255 open WE 2019.. 2x300
I am confused on a situation.. A month or so ago, out with some friends on the lake and everything was working perfect (45hrs on each engine). Backed into shore a little too close and sucked up some rocks and debris. It seemed to clear all up, but definitely some rocks went through. After that, all was well, boat working perfect still. Decided it was time to leave that end of the lake and head home. About 15 min. into cruising down the lake at about 5000rpm (perfectly smooth engines) something happened.. Port side (red rpm indicator) fell off like it was cavitating.. happened instantly. Engine shut down (safety?) due to high rpm? Fired back up and ran perfect (both sides)..until you hit 4500 rpm...then port side would fall off again (rpms falling behind sb side), other side stayed strong, but of course, couldn't go much higher as the port side would fall way off. I presumed we picked up something in the lake along the way...Limped back (4 hours) at 4000rpm smoothly with zero other issues.
Once the boat was out of the water, immediately spotted two rocks jammed on the exit side of the impellers on SB side...(realized later this isn't the side that was causing issues?)
The boat was due for 50hr service (notifications popped up on way back to launch) , so left the rocks in there for dealer to see and do full inspection on drives/impellers, etc when it went in for service. ( a few days later)
Got the boat back, and finally decided to get back out on the water yesterday... Fired up the boat, and immediately the service engines warnings came up. Boat just came back from service a month ago...warnings weren't cleared. Kind of frustrating as nearest dealer is 4hrs away..but, whatever, can deal with the warnings.
Got away from the dock, and hit 4500rpm....exact same problem. Port side cavitating/falling off (rpms falling way behind sb side)

Limped to the nearest beach and let the kids play for the day...
Got home and climbed underneath with the flashlight... Starboard side had a stick up in the intake side of impeller ..but unsure if that could really cause an issue or it would just be spit out immediately as we could have picked that up while loading the boat? Or...would it be stuck there causing issue? (Even though this side wasn't causing any problems) What I did notice was maybe a fair bit of damage to impellers? Maybe this isn't much? Hoping you guys can help shed some light... Could the rpm (perceived cavitation) issue not be related to the drives at all? What else could cause this happening mid cruise in the middle of the lake?

Here is a photo of the port side (no stick) and sb side with the stick... are these impellers damaged too much to work effectively? If so, shouldn't the dealer have caught this upon inspection?

Also a presumption...but the right and left buttons for starting should correspond to the side of the boat the engine is on? So much more going on here than my old 470 with a Hamilton..lol

Thanks all.

 

[Dean P]

You are correct, the buttons should represent port and starboard or left and right. The wear rings look pretty grooved up. I think the rocks really did some damage and YES your dealer should have pointed this out. Did they take it for a test run before you picked it up?

 

[bobsyeruncle]

Thanks, I was wondering if those sleeves should have those "rings"... Apparently they did not take it out...or run it (or they would have noticed the engine service prompts) I couldn't tell if the rings were grooves for the impeller blades or not..
Do you think that stick that was in there would have spit out or could it be from the original issue? It just seemed very odd that the issue arose after cruising for about 15min down the lake..and happened instantaneously...I want to say with a bang..but, I don't there was a "bang"..just an engine that raced briefly...and now won't go over 4500 without cavitating.
After the rocks, was fully expecting to replace impellers and whatever there was there for wear. They gave us the rocks back and said we were good to go...not at all impressed with them..but that is another issue..haha More trying to figure out what the issue is and if we need to go back to the dealer.
Sounds like we will be visiting a new dealer (still 4 hours away) very soon.

 

[Dean P]

I too have had a plastic bottle cap and then a piece of wood wedged in there. Both did not do any damage. Rocks are much harder and will easily carve a groove. Your impellers (from the pics) don't look that bad. I would replace the rings first and see how it performs. And, while the impellers are in hand, inspect/try and smooth them out the best you can. Good luck!

 

[Scarab255openid2019]

Bobsyeruncle what was the fix for this? I had the exact same thing happen to me. Would love to hear what happend. Thanks!

 

[gRobi]

Take off the impellers yourself and give them to someone to be fixed or do it yourself. Not big of a deal really. Wearrings looking like this will cause cavitation while heavily accelerating, could be changed when everything is open. But your problem is located at the front sides of your impeller. (In my opinion) grinding it to be even not causing cavitation did the trick for me after a similar situation.