The ColRegs are clear on this issue. If a sound signal is detected forward of the beam and it can not be determined if risk of collision exists, they shall reduce speed and stop if necessary until it has been determined if a risk of collision exists.
That's what the rule says, but it doesn't make it safe. Particularly, if you stop and lose rudder authority. Now you're a sitting duck. I always slow to a point that I think I'll be able to evade, given the visibility, but maintain control of the ship.
Recreational sailors who chose not to sound their sound signals because they feel their skill alone with a RADAR is sufficient to prevent a collision is truly frightening because I would trust few pleasure craft RADARs to adequately detect a small fibreglass or wood boat and I would trust even fewer pleasure craft RADAR operators to effectively use their RADARs or even be reasonably familiar with rule 19.
I'm not advocating breaking the rule. I sound the signal. But I have a hard time agreeing that it makes much difference. It does make one feel like they are helping. The concept was invented before radar and AIS existed. If the operator isn't good enough to navigate with a (modern) digital radar, there is no way I can conceive of them doing so by hearing a fog signal.
In my former career I dealt with some collisions in fog, not pretty. I worked on one particular vessel a good sized comercial vessel (over 200 ft) that had collided with about a 50-60 ft steel fishing vessel killing all on board. Both vessels were equipped with RADAR, sound signals were not used. One had at least 3 persons on the bridge. I wasnt on board at the time, but many of my former shipmates were and it never sat right with them.
Fair enough, but there is no way to know the use of a fog horn would have prevent it.
There are some rules that dont really matter that much with pleasure craft, but when it comes to sound signals in the fog, people die because the rule is ignored.
Again, I'm not advocating breaking the rule, I just find it hard to believe that a fog signal is going to prevent many accidents. Here's a scenario I found myself in a couple of years back. We came through Quick's Hole, into Vineyard Sound and fog unexpectedly rolled over us. There are lobster pots and lots of traffic in the sound, so we slow way down. As it was unexpected, I had not raised my radar reflector (my mistake, although, not required in the ColRegs I might add). I'm monitoring radar and note a large ship gaining on us quickly from our six. I can hear it in the distance. No doubt, it's one of the high speed catamaran ferries heading to the island. I'm fairly close to shore, so find it odd they are too. I can't really evade by turning toward shore, but turning toward the center of the sound is what I expect them to do. I blow the daylights out of the horn. I even sounded 5 blasts, as I was getting spooked. They are making over 20 knots, which is slowed down for them, but not adjusting course. There is no bloody way they could possibly hear the horn on that deck. I don't recall hearing their horn, but I doubt you could hear it over their engine noise if they did. They're louder than the horn. When they get within a half mile or so, they turn out toward the middle of the sound and I eventually hear them roar past.
In all likelihood, they saw us on radar the whole time. Nevertheless, I find it impossible to believe my horn mattered at all.