It is not in "The Rules"! Why not?
Perhaps because it is not a rule?

"Frank Boettcher" wrote

"Joe" wrote:
Blue over white is displayed on the Boliver - Galveston car ferries &
most ferries. As normal pass on 1 whistle unless other arangements are
agreeded upon.

The mine sweeper is in chapmans, the ferry is not. Guess I should
write it in.