Wednesday, July 11, 2007

Dry-fitting the Aquadrive

Completed the dry-fit of the Aquadrive thrust bearing. Why do this now? Well, access. Everything is open in the stern. As things get built up back there access will be more difficult. Reminder that the Aquadrive is a device which separates the thrust of the propeller from the engine. A typical straight shaft installation pushes on the engine to move the boat forward. With an Aquadrive the thrust is transferred directly to the hull. The primary advantages are:
  • less noise. Since the engine is not bearing the thrust, softer engine mounts can be used, dampening noise and vibration.
  • alignment is less critical. With a straight shaft installation, the 300lb+ engine needs to be precisely aligned with the prop shaft otherwise premature failure of the cutless bearing (in the stern tube, supporting the shaft) may result. The Aquadrive uses CV joints between the thrust bearing and the engine. The CV joints are flexible and can allow up to an 8 degree deviation between engine and the thrust bearing.
However, the alignment of the thrust bearing with the final drive is critical for the same reasons mentioned above. Compared to a retrofit of a finished boat, things are easier here as nothing is yet permanent and everything in the driveline can be tweaked if necessary.
There were a few gotchas of course.
The instructions suggest installing the bearing to a 3/4" plate. This could be made of plywood, steel, etc. But it is emphasized that it be strong and the attachment to the hull should be very robust as it will be bearing the thrust of the prop.
Well, I anticipated an aquadrive during the design of the aft floors and planned to use one of the floor supports as the plate for the bearing.
To beef it up, I made it about two inches thick (two 3/4" pieces laminated with glass in between).
Turns out this extra thickness made it impossible to get the bearing in there with the rubber bushings and necesary shims to get the right angle.
In the end I had to route out 3/4" in the forward side to provide the necessary wiggle room(see picture).
In the end the shaft and coupling match the thrust bearing spot-on.
As it is still even a pain to get in there I may simply make the install permanent.
This simply means rounding over some sharp edges, smoothing out some surfaces and applying
a little paint.
Then I will wrap it up in plastic in place until we get to the engine installation (a ways away yet).
More later...

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