If another boat signals intentions to pass you from astern what should you do?

Depending on which side they intend to pass you on, they should give you and audible horn blast. One blast indicates passing on the starboard and tow blasts indicate passing on the port. Remember to return the same audible horn signal to let them know that you heard them and are aware of their intentions. One blast from them gets one blast from you and so on.

The vessel being passed will remain the Stand - on vessel, which means you do not alter your course or speed. They, being the Give-way vessel must alter speed and course to pass.