Most of us assume that every ship we encounter crossing an ocean will be listening for calls on VHF channel 16.
Reality is somewhat different.
The GMDSS (Global Maritime Distress and Safety System) regulation wording is;
"Every ship while at sea shall maintain, when practicable, a continuous listening watch on VHF channel 16."
In simple words, ships are not required to listen on channel 16 VHF, but they should do so if they can.
In practise most do, most of the time. But there is no guarantee they will hear your call at the moment in time you feel the need to talk to them.
To further complicate matters, if you transmit a voice call, they may not immediately realise it is them you are calling.
So what is the solution to this conundrum?
Obtain the MMSI for the vessel you wish to call from your AIS, and make an INDIVIDUAL DSC call on your VHF radio using that MMSI. Set the traffic channel to channel 06.
All ships are required to maintain a DSC watch on channel 70 (DSC calling) and this will sound an alarm on the ship's VHF radio. The bonus is, they will have your MMSI when you call them, which they will see on their AIS overlay on the ships primary radar.
They immediately know who is calling and perhaps really important in the circumstances, WHERE you are relative to the ship.
Learn how to transmit an individual call on your VHF radio while you are at the dock!
Nothing above applies to calling a ship if you are in distress.
If in distress, on your VHF radio, press and hold down the red distress button for 5 seconds. The radio will automatically change to channel 16.
Then transmit a voice distress call on channel 16, and listen for a response.