Driveshaft Stripped

[Scarab47]

I’ve been experiencing a bit of cavitation on the hole shot— enough that it’s been difficult to get skiers up. Never had any trouble hitting top speed though. Read through a lot of posts of people experiencing the same thing with various causes. Checked impeller and it looked clean, had wear ring replaced—still had same issue.

Last weekend it started to get worse and started squealing pretty bad, almost revving up and easing back down (like you let off the clutch in a sports car). Finally there was a thump and the boat completely gave up thrust and had an AWFUL squeal. Got towed in and took to shop.

They pulled it apart and saw that the driveshaft connector had stripped out (pics below). The connecting teeth had completely given out. Has anyone else had this happen? Boats around 200 hours so it’s nowhere near new. But mechanic said this is definitely not typical wear—more poor manufacturing. Looks like to have to replace driveshaft and prop, total about $1000. Heads up to add to the list of causes of cavitation?

 

[tdonoughue]

Is that clearance between the impeller and the wear ring correct? Or is it just sitting back too far? If that were jammed up, could it have contributed to the issue?

 

[Scarab47]

Is that clearance between the impeller and the wear ring correct? Or is it just sitting back too far? If that were jammed up, could it have contributed to the issue?

Possibly, but no evidence so far. The mechanics said everything around the prop was in great shape. Impeller looked great, wear ring was untouched. Totally leave open the chance that something got in there and held up the impeller at one point, but that hasn’t occurred since I’ve owned it (around the last 30 hours) and there was no debris or evidence of anything getting caught up in there.

One thing that couldn’t have helped, was me trying to fight the cavitation to get on a plane in hole shots. RPMs would shoot up to 6000-7000 but I’d still be at 5mph for 10 seconds fighting to get it moving. Ski mode was even worse. The system would basically redline it fighting to get the hole shot until exhaust temperature warning came on a couple times. Driveshaft would be spinning faster than prop as the teeth tried to grip, stripping it worse. Basically it’s equivalent of having a stripped screw but still running your drill on high—making it worse.

 

[Scarab47]

Video of it right after completely lost thrust. God AWFUL sound it made. Can see the prop is still spinning somewhat, but not enough grip between it and driveshaft to generate any meaningful thrust. RPMs basically revved freely like you had a car in neutral.

 

[Rick215HO]

Last weekend it started to get worse and started squealing pretty bad, almost revving up and easing back down (like you let off the clutch in a sports car). Finally there was a thump and the boat completely gave up thrust and had an AWFUL squeal. Got towed in and took to shop.

They pulled it apart and saw that the driveshaft connector had stripped out (pics below). The connecting teeth had completely given out. Has anyone else had this happen? Boats around 200 hours so it’s nowhere near new. But mechanic said this is definitely not typical wear—more poor manufacturing. Looks like to have to replace driveshaft and prop, total about $1000. Heads up to add to the list of causes of cavitation?

I’ve been experiencing a bit of cavitation on the hole shot— enough that it’s been difficult to get skiers up. Never had any trouble hitting top speed though. Read through a lot of posts of people experiencing the same thing with various causes. Checked impeller and it looked clean, had wear ring replaced—still had same issue.

Last weekend it started to get worse and started squealing pretty bad, almost revving up and easing back down (like you let off the clutch in a sports car). Finally there was a thump and the boat completely gave up thrust and had an AWFUL squeal. Got towed in and took to shop.

They pulled it apart and saw that the driveshaft connector had stripped out (pics below). The connecting teeth had completely given out. Has anyone else had this happen? Boats around 200 hours so it’s nowhere near new. But mechanic said this is definitely not typical wear—more poor manufacturing. Looks like to have to replace driveshaft and prop, total about $1000. Heads up to add to the list of causes of cavitation?

View attachment 124673
View attachment 124674

It seems from the picture like there is no grease on the shaft or impeller. or was it cleaned out for the picture? The shaft requires annual or 100 hours inspection and greasing (triple guard grease).

 

[sunbyrned]

This is what I believe has happened to me before. I’ve stated that I think the splines got out of sync somehow and just by pulling the impeller and putting it back in fixed it. Some say that’s not possible. Who knows? Just saying though because if someone experiences cavitation and can’t find the cause, I still suggest pulling the impeller out and put her back in.

 

[Julian]

Someone posted this week that they've had this happen 3 times with their Scarab! I'll find the post and link it.

 

[Julian]

https://jetboaters.net/threads/parts-help-scarab-impeller.27996/#post-460260

 

[Scarab47]

https://jetboaters.net/threads/parts-help-scarab-impeller.27996/#post-460260

Thanks Julian. Good god. Three times in 35h. If that’s not poor design idk what is.

 

[Scarab47]

It seems from the picture like there is no grease on the shaft or impeller. or was it cleaned out for the picture? The shaft requires annual or 100 hours inspection and greasing (triple guard grease).

Good question—I’ll follow up with the mechanics on what they saw there. I took over at 170hrs. Previous owners took really good care of it, but you never know unless it’s under your control.