(Taken from the USCG Navigation Rules (Inland & International)
International & Inland
Rule3
General
(C) The term "Sailing vessel" means any vessel under sail, provided that propelling machinery, if installed, is not be used;
So the answer is, if a Sail boat is using it's engine, regardless if the sails are being used, now has the responsibility to act as a power driven vessel.
When a sailboat is operating with mechanical propulsion, regardless of whether or not a sail is up it is considered a power driven vessel for the purposes of the rules of the road.
A sailboat using engine power
A sailboat with it's engine engaged
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.
You should run your boats power ventilation for 30 seconds at least in order to eliminate fuel vapors in the bilge.
red and green sidelights
When it is not under sail, and under motor power. Then it must follow the powerboat navigation rules. Since you've asked this question, then i recomend taking your state's Boater's Safety Course if it has one. It should be online and it cost about $50. It's expensive but you get a snassy card and you learn lots of things like where to go and where not to go, and things like that.
Maintain speed and course, but stay alert.
Power Boating Canada provides an abundance of information regarding purchasing power boats. The website "power boating" provides links to magazines, videos and blogs among other information on purchasing power boats.
A sailboat must follow navigation rules for powerboats when it is under power (using its engine) rather than sailing. Additionally, when a sailboat is operating in a situation where it is overtaking or crossing paths with a powerboat, it must adhere to the same navigation rules to avoid collisions. It's important for all vessels to be aware of and comply with the relevant maritime rules to ensure safety on the water.
The should comply with IRPCS and be on between sunset and sunrise and in restricted visibility
Sail boats and power boats are the two main types of pleasure craft.