Up above this post, experienced skippers have posted very good advice. Thanks. Reading those posts stimulated my thinking on a subject that fascinated me.
What follows are just a few observations and opinions to add to the mix.
Sailing in fog on San Francisco Bay or off the California coast is common (in the summer months the fog is very thick and blankets the coast and the bay.
Sailing in fog is an experience in using your senses (and good sense).
My own observation was that so many of the boats I crewed on did not have anything more than a "can of air" horn. Those usually get held for imminent collision warnings by most boaters I met as they don't want to use up all of their air (usually they have just one can on the boat).
I have seen huge commercial vessels emerge from under the Golden Gate Bridge in dense fog, with sailboats crossing their path (apparently unaware of the oncoming traffic). Risky.
The old style of "lung powered" trumpet style fog horn may not seem as "modern" as the "can of air" horn, but it is dependable as long as you have a breath. And it can be used for hours if needed. Same with ship's bells. These things may seem like decorative items for a traditional styled boat, but I would want them on the newest yachts. They don't require batteries.
I think a good thing to do is have "all EARS on deck" if possible (or more than one person and more than just the helmsman) as more EARS on watch is a good idea. Must keep the chatter down (no talking) so the crew is attentive to the sounds around the boat. and stop any clanging of deck gear. Good idea to post someone up forward away from the sounds of the cockpit. While I like pilot house boats, I think a sailing yacht would be prudent to have crew outside of the pilot house during fog, the better to hear what is out there.
Unfortunately, fog has a way of making sounds and determining sound direction somewhat tricky.
There is nothing more attention getting than hearing the sound of crashing surf on rocks that are out of sight due to the fog, especially if one does NOT have a radar and is using dead reckoning to navigate! Scary.
Well, maybe ONE thing is that scary: the sound of a huge motor vessel (commercial freighter etc.) somewhere in the fog and getting louder and closer! Really scary!
For those who have not sailed in dense fog, it is an experience that is hard to match. Even sailing at night is not the same as there is usually visibility at night of lights and breakers even if only by starlight.
Moving forward through heavy fog with zero visibility is incredible, as it seems as if the whole world has shrunk down to the few of deck you can see below your feet.
This may seem like overkill to sailors who sail on lakes or protected waters or never in fog, but on a my future cruising vessel I will definitely have Radar, AIS, active radar transponder, passive radar reflector, manual fog horn, GPS, accurate position awareness, and an ALERT crew on deck with all EARS on watch!
Finally, this is not a time to have crew members listening to their iPods, chatting, or watching a video down below.
Oh...one more thing. There was a question about clipping on (the crew clipping on to jacklines). Would it be safer to be attached to the boat or not? My opinion: It would be MUCH safer to be WITH the boat in the fog, rather than overboard as in MOB . And there is a greater chance of someone going overboard than there is of getting hit by another boat. Imagine trying to find a MOB in the fog!
Safe Sailing Folks!