You are in your 13m powerboat in a relatively narrow channel transiting between some islands at night, visibility is good and all in all it has been a quiet watch when you hear the following sound signal from behind you.
2 prolonged blasts followed by 2 short blasts
How long is a short blast?
How long is a prolonged blast?
What does the sound signal indicate?
What is the response if you agree?
What is the response if you disagree?
What are the rules you are going to apply when deciding your response?
The vessel behind you has clearly seen what is assumes is your stern light
What is the arc of coverage of your stern light?
What is the minimum distance it can be seen from you?