when the power-driven vessel is towing another boat and has difficulty maneuvering
When it is being overtaken
The sailboat operator is the stand-on vessel so they should keep their course and speed while the PWC is the give-away vessel so it should change course and speed.
The question should be "When is the sailboat the stand-on vessel...? The answer is if the sailboat has an engine and if it is in operation, the sailboat is treated as a power boat in determining stand-on status. The right-of-way would then follow the rules for power boats.
Sailboat
only when under sail alone and is crossing path of powerboat
Only when under sail alone and is crossing the path of the powerboat.
only when under sail alone and is crossing path of powerboat
only when under sail alone and is crossing path of powerboat
only when under sail alone and is crossing path of powerboat
only when under sail alone and is crossing path of powerboat
only when under sail alone and is crossing path of powerboat
only when under sail alone and is crossing path of powerboat