Re: Rule 23a question
There is also a Canadian modification
In the waters of the Great Lakes Basin, a power-driven vessel
when underway may, instead of the second masthead light and
sternlight prescribed in paragraph (a), carry, in the position of
the second masthead light, a single all-round white light or two
such lights placed not over 800 millimetres apart horizontally,
one on either side of the keel and so arranged that one or the other
or both shall be visible from any angle of approach and for
the same minimum range as the masthead lights.
Where Canadians and Americans share waters there is often congruence between Inland Rules and Canadian modifications.
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As I sail, I praise God, and care not. (Luke Foxe)