Horn Signals

One short
I am altering course to starboard (my right)
Two short
I am altering course to port (my left)
Three short
My engines are going astern
Four short (pause) one short
I am turning round in the fairway to starboard
Four short (pause) two short
I am turning round in the fairway to port
Five short
You are taking insufficient action to avoid me
Two long, one short
I wish to overtake on your starboard (right) side
Two long, two short
I wish to overtake on your port (left) side
Long, short, long, short
You may overtake me
One very long
I am about to enter the fairway.

BW Bye-Law 12 1965 (part of)

12.(1) Every power-driven vessel navigating on any canal shall be furnished with an efficient whistle.

12.(2) When vessels are in sight of one another the master of a power-driven vessel under way in taking any of the courses hereinafter referred to in this Bye-law shall indicate that course by (the) following signals on such whistle, namely: One short blast to mean "I am altering my course to starboard", two short blasts to mean "I am altering my course to port", three short blasts to mean "my engines are going astern", four short blasts to mean "I am about to turn or to turn round". This signal shall be followed after a short interval by one short blast if turning to starboard or two short blasts turning to port and shall be repeated to any approaching vessel, whereupon such approaching vessel shall take action to avoid collision.