Final answer:
The proper channel to call for help on a marine VHF radio is Channel 16, which is the International Distress, Safety, and Calling frequency.
Step-by-step explanation:
In order to call for help on a marine VHF radio, the proper channel to use is Channel 16. This channel is designated as the International Distress, Safety, and Calling frequency for vessels at sea. It is monitored 24/7 by the U.S. Coast Guard and other maritime authorities around the world.
When making a distress call, it is important to use the proper emergency procedure. The distress message should start with the word "Mayday" repeated three times, followed by the name of the vessel in distress and its call sign, the nature of the emergency, and the vessel's position.
For example, a distress call might sound like this: "Mayday, Mayday, Mayday. This is the sailing vessel ABC123, calling from coordinates 39.1234° N, 75.5678° W. We are taking on water and in danger of sinking. We have four crew members on board and are in immediate need of assistance."