One of my strategies for going to wonderful, but isolated and remote, cruising destinations is to gather what I call “gurus” for consults in areas where my expertise is sorely lacking or areas where my efforts have brought no joy. I am willing to pay for the time these gurus spend helping me sort out an issue. They do not get consulted often, maybe once very couple of years, but when I have needed their help, it has proved invaluable.
I sent this problem to a mechanic/engineer (and general boat maintenance/repair expert) and he gave me permission to share his response, copied below. I thought might be of general interest to the Forum readers in part, as he may be correct, but also as an example of thinking and trouble-shooting problems that are complex and elusive.
My best, Dick Stevenson, s/v Alchemy
Hi Dick, I wonder if anyone has checked the travel of the shift linkage / shift lever? I have had several engines act in a similar manner ( minus the Aquadrive ). They tended to “buck” fore and aft as the clutch engaged and slipped/disengaged rapidly, primarily during acceleration. The problem would ease as the boat speed came up and the engine load lessened or if the cone clutch seated itself more firmly . Additionally, if the helmsman had separate shift/throttle levers a forceful shove on the shift lever would usually solve the immediate problem by squeezing that last bit of travel out of the cable. I have pushed the lever with my thumb while the engine was bucking and had it instantly correct itself. The longer term fix of this problem is to ensure proper shift lever travel for full engagement. And hope that the slippage hasn’t worn the clutch too badly ( probably hasn’t). In this instance the term “ easing into forward from neutral” makes my antennae twitch a little. The torsional movement athwartships would be more of a fore and aft motion , I suspect, if there were no Aquadrive doing it’s job in a perfect way of preventing thrust transmission. Since it is so robust and firmly attached to the hull the only motion available to the engine is to twist, as the torque of rotation loads and unloads in concert with the clutch slipping. Also, it sounds like they’ve replaced everything but the toilet seat in an effort to make this go away. Not to mention that when the problem “fixes” itself it stays fixed even when RPM is reduced. My guess is, that is because the the clutch has seated and is held in place by the spiral shaft of the cone. I’d be happy to take a call but feel free to forward my comments and let the owner try the simple process of pushing the shift lever while the engine is bucking.