The Boaters Hand book gives the following for the blast sounds and the meaning of each signal.
One short blast means: I am turning to starboard (right)
Two short blasts means: I am turning to port (left)
Three short blasts means: I am stopping or going into reverse
Four short and then one short means: I am turning right round to starboard (right)
Four short and then two short means: I am turning right round to port (left)
One long blast and two short blasts means: I cannot manoeuvre
One long continuous blast of ten seconds means Warning I am here