When we had our Nordic Tug 32 it had electric horns. One of our top of the line ICOM VHF radios was hooked up to a hailer so that it could sound the proper signals in reduced visibility. I never felt that the hailer signals were loud enough and in fact, they were a lot less loud than the horn itself. Loud is very important as you want other traffic to hear your signals in spite of their engine noise. I searched for a fix and found a horn controller that was inexpensive and easy to install. It made the signals automatically using the electric horn and was a far superior solution. The brand I got is no longer manufactured but there is one on the market
http://www.fogmate.com/ I would not be without a horn controller. On our NT37 we had a Kahlenberg dual trumpet air horn system with a controller on it. Now, that was loud and heaven help you if you forgot to close the overhead hatch in the pilothouse before engaging the horn controller!
I am not sure whether the Fogmate will work with small air horns (no air storage, horn sounds when air compressor runs) but can see no reason why it won't. It absolutely will not work with big air horns where the horn button activates a solenoid in the air line between a storage tank and the horns.
I consider the horn controller to be an essential safety device along with an autopilot. Think about how much of your brain you use when operating in fog to make the signals properly and to steer a course. That leaves little brain capacity (speaking strictly for myself!) for a proper watch of one's surroundings due to the concentration required. Yet, those 2 tasks can easily be outsourced to the horn controller and the autopilot. Now, you have a lot more capacity and less distraction with which to focus on your course, surrounding, other traffic etc.
Those of you who do not know what fog is, won't have the same needs. However, one year I taught a radar course at a Nordic Tug Rendezvous in mid July and had that year alone run more than 300NM on LI Sound in fog with 1/4NM visibility or less by the time I taught the course! it would be a lot worse boating in Maine.