In another attempt to source to performance problem, I motored off my mooring and removed the transmission cable from the transmission. I was concerned that perhaps the transmission was not engaging fully in forward. Alas, it made no difference and I still only managed 3 Kts. So, either the problem exists somewhere else in the engine or it's, as the mechanic at Gartside said, there's too much growth on the bottom.

It will be interesting to see if I can do better than 3 Kts under sail. That will be the next test.