Any help or advice most welcome on our annoyingly intermittent propulsion problem.

On Shimshal we have a 75hp Yanmar J4HTE which runs beautifully smoothly and slips in and out of gear with no trouble at all. For at least 99% of the time the prop draft spins smoothly when motoring with no hint of judder or vibration. We have an Aquadrive CVB15 which is a constant velocity joint to diminish vibration and take care of minor engine misalignment. That drives a 3 bladed folding prop through a Sleipner Stern tube and cutlass bearing. There is some wear in the cutlass bearing with about 1.5mm of movement in the vertical plane.

Here is the problem. Very occasionally when slipping from neutral into slow ahead a thudding can be heard. As revs are slightly increased a vibration develops which is most prominent between the Aquadrive and gearbox. So long as revs are kept low that vibration will continue but when increased to 1500 rpm it disappears completely and does not return when revs are dropped back. This only occurs occasionally and most of the time it is completely smooth at all revs.

Last winter we changed the prop thinking it was the cause but to no effect. I now think it is the Aquadrive that’s at fault but there is nothing on the internet to point to Aquadrive failure. I have recorded a 1minute video of the vibration happening that can be viewed on this link: https://vimeo.com/291519307

Advice most welcome.
edited by simoncurrin on 9/24/2018

That's a nasty wiggle as you increase the revs! I would suspect cause to be related to the worn cutlass bearing (1.5mm seems like a lot to me - no expert) and possibly exacerbated by some soft engine mounts (when were they last replaced)?

We had a similar sort of vibration some time ago - but it was during motoring in a following sea as we crested a wave. Everyone thought it was the prop, but in the end was traced to poor engine alignment. I gather your Aquadrive takes care of that - but presumably if it's well out of whack, alignment might also be an issue...

Bill Balme