What sound means a vessel is underway in fog?

Answer 1

There are several sound signals for a vessel underway in restricted visbility.

• A power drivel vessel underway, making way- 1 PROLONGED Blast (4-6 seconds duration) Not to exceed every two minutes.
• A power driven vessel underway NOT making way-2 PROLONGED every 2 minutes, not exceed two minutes.
• A vessel not under command, a vessel restricted in maneuverability, a vessel towing, a vessel engaged in fishing, a sailing vessel & a vessel constrained by her draft (International only) 1 PROLONGED blast followed by two short blast (duration about 1 second per short blast) every two minutes, not to exceed two minutes.
• A MANNED vessel being towed shall sound 1 PROLONGED followed by 3 SHORT blasts every two minute, not to exceed two minutes.

See the link provided for the International and Inland Navigational Rules of the Road. Rule 35.
for power driven vessels over 12meters it's one long blast every 2 minutes