Marina Board Exam: As defined in the Rules, which vessel would qualify as a "vessel not under command" among the listed?

Marina Board Exam

As defined in the Rules, which vessel would qualify as a "vessel not under command" among the listed? 

A. a vessel having an accident in her machinery forcing her to reduce her speed drastically 
B. a vessel affected by severe weather conditions 
C. a vessel fishing with trolling lines 
D. a vessel with her anchor down but not holding


Answer:  D.  a vessel with her anchor down but not holding