A vessel not under command

ColRegs, Rule 3 paragraph (f):

The term “vessel not under command” means a vessel which through some exceptional circumstance is unable to manoeuvre as required by these Rules and is therefore unable to keep out of the way of another vessel.

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 Examples of vessels which are likely to be accepted "not under command":

• a vessel which have had breakdown of her engine(s);
• a vessel which have had breakdown of her steering gear;
• a vessel which have lost a propeller;
• a vessel which have lost her rudder;
• a vessel with her anchor down but not holding;
• a vessel riding to anchor chains with anchors unshackled;
• a sailing vessel becalmed;
• a vessel affected in such an exceptional way by weather conditions, that she is unable to keep out of the way of another vessel by alteration of course and/or speed, thus justifying her showing of not under command signals.

With reference to "A Guide to the Collision Avoidance Rules" by A.N. Cockcroft and J.N.F. Lameijer.