5.7EFI / Alpha I Gen II - Coupler or prop hub?

[Strecker25]

Hi, we just went on a 150 mile trip and on the way back I suddenly started to get what I first thought was just ventilating the prop on one motor. The RPMs would increase a bit and if I pulled the throttle back on that motor it would then rev up and push just fine. A little later it happened and my wife went and sat on the aft seat, she said it sounded like something was rattling around in the engine room and the engine was jumping between 2500-3000 rpm. I was able to throttle down and then back up and it was fine for another few miles. It happened again, same fix, then on the last leg of the trip about 10 miles I had no issues.

I can try and pull the prop, but I'd have to do it underwater as the drives don't raise up and out on our boat. Can a coupler "half" fail? I wasnt able to open the hatch to check the smell as we were offshore a bit.

I thought maybe it could also be the hub, these are stainless merc props with the plastic hub

 

[alldodge]

They probably don't come completely out of the water but maybe high enough to do it at a ramp or shallow shore. I change my props at the boat ramp. Drive up until you get close, and have someone ready to stop the boat in the water. After stopping the motor, raise the drive as far up as it will go.

Pivot the boat around to get at the stern and have a white grease pencil handy. Mark the inside of the prop clear across the the center of the nut and across to the other side. Push the boat out, drop the drives and go for another run. Then come back and check the marks, if they moved, its the prop.

In most cases if the engine coupler fails, you going to smell burnt rubber. So if the prop did not move, then you need to pull the boat and the drive to check. In this cases if prop didn't move it may just be the coupler.

 

[Strecker25]

Thanks, I'll do that tomorrow. I actually have the spare hubs so I will probably just replace it tomorrow as well with a good one so we can rule that out. '

I'm not familiar with the construction of the coupler so in your opinion does that sounds like one that is starting to go? In my other research it sounds like usually they go all at once, not a couple times in a trip and then work perfectly. It made a noise like something was rattling around in the bilge.

 

[stonyloam]

A coupler is essentially a rubber disc bonded to a metal plate that is bolted to your engine flywheel. Bonded in the center of the disc is a splined coupler. The driveshaft slides into the coupler and power is transmitted from the engine to the outdrive. The coupler can fail in two ways, the splines can ware to the point where they fail and the coupler spins around the driveshaft, or the rubber bonded to the coupler can fail allowing the rubber to spin around the metal coupler ( usually with a burning rubber smell). Usually a complete failure, once it lets go. When the coupler fails the engine will start to overheat because the impeller in the outdrive is not turning.

 

[Rick Stephens]

I'd agree with your thinking that a toasted coupler wouldn't grab for a while then release and go back to grabbing. Either a hub or a sheared tooth on a gear in the drive could account for the symptoms.

 

[Strecker25]

Update on this, I replaced the hub with a new one and took two cruises with plenty of weight on board, no sign of slipping. The old one definitely had some worn edges on the edge that would account for forward thrust. I'll post a picture of it later for your opinions but I hope that was it. I also re greased the couplers for the hell of it although we've only put 25 hours on this year.