Really useful information , Steve, and others. But suggest not to jump to the elbow solution immediately.
My 34e was holed up in a marina over the worst of Covid, only used once in the season and got very weeded up.
When I took at out at the end of the season, black smoke appeared when I went above half revs and would only go a few knots.
The engineer said it was because it was weeded up and, indeed, it was OK the next spring after being hosed off and lying up over winter.