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.
The membership association for owners of Dufour Yachts