Bruce, are you using the special antifouling for aluminum? I used that on our old sail drive, it actually seemed to be better than my bottom paint. We had two serious problems with ours, but it was an old design, an OMC sail drive.. There was a giant O-ring shaped gasket that kept the water on the outside of the boat. That deteriorated from age and the only replacement we could find was in Seattle. The second problem was not the fault of the unit. A lift operator did not follow my sling marks exactly and lifted the boat with the sling over the sail drive. It barely cleared the water when I saw it but by then the damage was done, it broke loose the mountings and started pushing the drive up into the hull. That took the better part of a winter to have repaired.