There is no such thing as a "long blast", it is called a "prolonged" blast lasting between 4-6 seconds.
-If you agree with the arrangement you would sound the same signal, then take the necessary steps to effect a safe passing. If there is any doubt though the danger signal of five short needs to be sounded.
ETA: Rules of the Road, Rule 34-international
A.) One Short blast means "I am altering my course to Starboard."
Two Short blasts mean "I am altering my course to Port."
Rule 34-Inland
A.)One short blast means "I intend to leave you on my port side."
Two short blasts means " I intend to leave you on my starboard side."
Be alert and observant.Scan 360 degrees and act accordingly. ALWAYS look before you leap.
Assuming you are referring to a fog horn or a warning signal, generally find where the other vessel is and change course accordingly
You are operating a motorboat. You hear one prolonged blast from the horn of another vessel. What should you do?
A sailing vessel operator should sound a blast on the horn when approaching a PWC head on. However, it is up to the operator of the motorized craft, in this case the PWC, to take evasive action.
B
The Vessel is turning to Starboard .
Stay out of the way! One prolonged blast is the whistle signal for a ship leaving a berth or in a blind bend in a channel.
there is a dangerous situation
One prolonged blast from the horn of another vessel indicates reduced visibility (and the presence of that vehicle). The reduced visibility may be due to geography, such as a river with significant bends, or weather features, such as fog.
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.
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.
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.