I've logged many hours sailing myself , but this poor guy who was basically dead in the water (anchored) and not knowing if the sailor saw him , had to be scared out of his mind. A 40' sailboat even a 5 mph could probably go half way though a 21' powerboat. If the powerboat was not anchored and underpower it would have been no big problem , simply sailboats right of way.
It was simply a foolish , inconsiderate , unsafe , unnecesary move on the part of the sailboater.
