From Handbook of the Nautical Rules of the Road by Llana & Wisneskey
Vessels using only their sails for propulsion are included, even though they may be fitted with an engine. Operation of the engine to generate electricity or to heat water, for example, does not make the sailing vessel a power-driven vessel, so long as the propeller (or paddle wheel) is not engaged. Rule 18 tells us what the responsibilities of sailing vessels are with respect to other types of vessels, and Rule 12 does the same with respect to other sailing vessels.
Chris Llana is a former Coast Guard officer with a B.S. in naval architecture and marine engineering and advanced degrees in marine affairs (MMA) and law (JD). During his tenure as a civilian at Coast Guard Headquarters, he drafted the annexes to the Inland Navigation Rules and wrote other regulations implementing both International and Inland Navigation Rules. Subsequent to that, he worked for Comsat Corporation on policy issues concerning the International Maritime Satellite Organization. He currently writes novels and maintains a web site on the U.S. transition to the ATSC digital TV standard.
George Wisneskey is a graduate of the Coast Guard Academy and holds a master's degree in education from the George Washington University. As chief of the Coast Guard's Rules of the Road Branch before his retirement in 1982, he oversaw the drafting of the Inland Navigational Rules Act of 1980. He is currently an active player in the Neuse River Foundation from his home base on North Carolina's coast.