Mercruiser 5.7 EFI slipping/over revving

[khe]

I was out on Lake Michigan this afternoon and the waves were about 1' and my boat was running fine. The waves built to 2-3', then 3-4' so I headed back. My engine over revved and it acted like it was slipping. At 3200 RPM I was only running about 10 MPH. It typically cruises at 24MPH at that speed. When I was a mile from the harbor, the symptoms seemed to go away and the boat was picking up speed. Not wanting to risk anything, I slowed it down and made it back to the ramp.

When I got home, I pulled the prop thinking the rubber hub was sheared but I can't see anything wrong. In fact, I can't see any rubber in he hub. The prop is a Quicksilver Vengance stainless steel propeller.

My next thought was the engine coupler but I always thought when those went bad, there was no power transfer to the lower unit - they either worked or they didn't.

I inspected the bilge and did not see rubber shavings from the coupler. What should I check next? There is a propeller shop near me and I was thinking about having them look at the prop to make sure nothing is wrong.

The boat only has this issue when the lake is rough. When it is calm, there doesn't seem to be any problem.

 

[harringtondav]

..assuming you have an Alpha 2 with dog clutch (post your drive info) your out drive is solid, and I think you've ID'd the usual suspects. I'm thinking you took enough air for your engine to over rev and when you plunked your prop back in the torque spike caused rubber to slip. If your prop has a removable hub, replace it. Else have your prop shop press in a new hub. I know several good prop shops, and none have the ability to test breakaway torque on a hub of your size prop.

Take a known good prop back out and see if you are fixed. If you still have slippage, suspect your coupler. It's like an auto/truck clutch. Once it slips, it's toast. If you have a Bravo, ...someone else can coach you about cone clutch failure.

 

[khe]

It's an Alpha one, gen 2 drive. Boat is a 1997 Sea Ray 240 Sundancer.

I don't have another prop that I can install - maybe it is time to buy an aluminum prop to keep as a spare. The Vengance stainless steel prop doesn't appear to have any rubber in it. I'll post a picture when I download it from my phone to the computer.

 

[thumpar]

Mark the prop at the prop shaft and shaft itself. Take it out and try it. If the marks don't line up after you have a spun hub.

 

[khe]

Mark the prop at the prop shaft and shaft itself. Take it out and try it. If the marks don't line up after you have a spun hub.

I plan to do just that. Thanks. I just need some rough water to make it act up. It's possible, that it is slipping slightly in calm seas and I don't notice it.

 

[harringtondav]

Once a hub slips it looses torque capacity. You should be able to mark and test in smooth water. Try several hard acceleration hole shots, check your WOT speed and tach. Then check the marks.

 

[khe]

Update: I discovered I don't have a press in rubber hub - I have the Flo Torq hub system that uses a large plastic sleeve. I plan to take the sleeve out and inspect it this evening. If it looks OK, I'll mark it and put it back together and test it the next time I go out.

 

[thumpar]

You’ll be able to tell by looking at those hubs if it spun or nit. I doubt it spun because of the way those are made I don’t think you would make it back if it did.

 

[harringtondav]

Your prop may have a Merc Flo-Torq removable hub. The newer ones have break away tabs that are the torque fuse in a prop strike or over torque. If you have one of these it's simple to remove the hub sleeve for inspection. Replacement hub sleeves aren't too expensive. https://www.youtube.com/watch?v=UA1yshDrMME

 

[khe]

My prop is the Flo-Torq but I have the six tab washer and a slightly different splined component. It must be an earlier system. I removed the plastic piece and could not see any damage or evidence of slippage. This is the first time I have seen that type of system - it looks like a huge improvement over the press in rubber hubs. As thumper stated, I highly doubt this spun. I marked all components with a paint marker, applied spline grease to the splines and installed the prop. The next time, I'm out, I'll put it through some hard accelerations and see what happens.

When the issue happened yesterday, there was no burning rubber smell and there was not any rubber shavings in the bilge upon inspection when I got it home. The engine temperature was 168-170 degrees which is normal and it did not spike up. I doubt the coupler rubber is sheared as everything I've read indicates the coupler either functions or it doesn't. Possibly cavitation could have been the issue but not sure what would have caused that and why the symptom went away after a couple of hours?

Do the splines in the coupler or the drive yoke ever strip out? Seems rare that would happen and again - if that were the case, it seems like there wouldn't be any power transfer from the engine to the outdrive.

 

[harringtondav]

Do the splines in the coupler or the drive yoke ever strip out? Seems rare that would happen and again - if that were the case, it seems like there wouldn't be any power transfer from the engine to the outdrive.

Coupler splines fail, normally from lack of grease. That's why regular out drive pulls are recommended so you can regrease the coupler yoke shaft splinrd. There is also a grease zerk on the coupler hub...if you can find it behind your engine. You may have to bump the starter a few times to orient the zerk.

The coupler splines are in a sleeve that's bonded to the coupler rubber hub which is also bonded to the coupler body. Slippage can occur at either of these bonds, but generally the spline sleeve breaks loose. And as you've read, once the coupler slips it doesn't transfer much, if any torque.

Your water test will narrow down the suspects. FYI...and likely not your case, I once caught some floating grassy vegetation in deep water. Same symptoms as you. I thought my prop hub slipped. I learned to stop, give the boat a solid shot of reverse to clear the trash, and carry on just fine. Cavitation or aeration slippage ends once you power down, get a grab, and take back off.

 

[khe]

I pull my drive yearly to check the engine alignment and grease the splines. I looked for a grease zerk on the coupler a few years ago and could not locate it - my engine bay is so cramped and I was so contorted in there, I almost wondered how I was going to get out. I need to take a look again for the zerk.

Hopefully, it was just some weeds on the prop that fell off during the trip back. Usually, when there are weeds on the prop, it causes a vibration.

Hopefully, it is all fine when I lake test it next. I really like that Flo-Torq hub design. No more need to have a shop R&R a hub when the rubber fails. It looks much more durable than the old style rubber hubs.

 

[khe]

I ran the boat a couple of days ago - several WOT hole shots and did not have any issues. The water was calm so I was unable to duplicate the conditions I experienced a week ago. I'm thinking I had a small amount of weeds on the prop that were not enough to cause a vibration but enough to cause cavitation. That's the only explanation that makes sense.