When we originally bought the boat, one of the items identified during the survey as needing repair was our AquaDrive bulkhead.
The AquaDrive is a thrust-bearing device for the propeller shaft that is also tolerant of engine to shaft misalignments. It's commonly found on commercial vessels and larger powerboats.
We believe it was installed on our yacht due to the slightly longer unsupported length of propeller shaft compared to a similar stern-cockpit vessel. Our engine is relatively far forward underneath the center cockpit. We've heard of at least one Liberty 458 without an AquaDrive that has had prop alignment problems.
The problem with ours was that the load-bearing bulkhead was made of stainless steel welded to feet which were then glassed to the hull. This arrangement was apparently not strong enough and the bulkhead broke at the weld to these feet. The problem may have been compounded by the weld not being able to "breathe" beneath the fiberglass. The break caused the bulkhead to move slightly when the boat was put in and out of gear.
The original owner had tried to correct the problem by screwing a metal rod through the bulkhead and into a floorboard support. However, this did not prevent movement of the bulkhead, and since the floorboard support is not a structural part of the hull, we felt that relying on this arrangement for a part that absorbs engine thrust might not be the best idea. This conclusion was cemented when one of us stood on the floorboard while the boat was put into gear, and felt the resulting "thunk".
We received a variety of opinions from several mechanics regarding possible solutions. These ranged from "ignore the problem, it's not serious" to "remove the AquaDrive completely, you don't really need it". A call to the AquaDrive's manufacturer indicated that the first opinion was complete garbage, in that the bulkhead absolutely should not move at all. Rumours of alignment problems on other vessels similar to ours led us to believe that in general, keeping the AquaDrive would be in our best interest.
In the middle of the opinion spectrum, there were the options of rewelding the existing bulkhead to new feet or replacing the bulkhead entirely. In neither case did we feel competent to do the work ourselves.
In the end, we chose to go with the solution proposed by KKMI in California. We had them replace the old steel bulkhead with one made from G10, which is a highly compressed fiberglass laminate in epoxy resin. It's very similar to the material used in printed circuit boards, and can withstand fairly high loads. The bulkhead was made larger than the original and was supported forward and aft by gussets or buttresses. It was a good replacement for steel in our system since it bonds well with the existing fiberglass hull, and won't corrode, which is essentially what happened to the welds of the existing steel bulkhead.
Overall, we're very pleased with the workmanship and have had no new problems so far. As well, the prep done by the yard was excellent. We had virtually no cleanup to do ourselves afterwards, and there was no damage to floorboards or walls.
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