We are all familiar with those evocative seaside sounds. As the fog rolls in thick a familiar foghorn sounds it's bass warning like a bullfrog croaking in the night. Mariners attuned to the sounds hear a lighthouse bearing on some blanketed rocky point or hear the identifying signal of a nearby boat. Each sounding of a whistle and ringing of a bell has a direct purpose at sea. Many recreational boaters don't realize it is their responsibility to also carry some sound signaling device. Whistles and bells are a requirement for informing other vessels of your maneuvering intentions and for navigating in fog. Both international and US inland navigation rules require every boat to have an efficient sound signalling device onboard at all times. In general, the larger the vessel the bigger the sound. For larger vessels the size of the whistle and bell depends upon the overall length. Annex III of the COLREGS (international Collision Regulations) specify the decibel levels and frequencies of whistles, more commonly called "horns." In general, the larger the vessel the deeper the sound of the whistle or "horn." A bell is also required on any vessel over 12 meters [40 ft] in US Inland waters (20 meters [66 ft] for International Rules). Bell requirements are likewise specified in Annex III. "The diameter of the mouth of the bell shall be not less than 300 mm [~12 inches] for vessels of more than 20 meters in length, and shall be not less than 200 mm [~8 inches] for vessels of 12 to 20 meters in length. The mass of the striker shall be not less than 3 percent of the mass of the bell. The striker shall be capable of manual operation." Any vessel that is in excess of 100 meters [328 ft] must also have an accompanying gong. Honestly, who knew fog horns and bells were so exacting?
In truth, repeating sound signals every minute or two in fog can be a bit tedious, especially after an hour. Rather than bringing along a "bell boy," most boats sailing outside the fairweather window choose to mount an automated sound signalling device. A marine loudhailer is one common solution to comply with requirements. They have the sound signals programmed into them and broadcast through a speaker. At the press of a button, a loudhailer repeats the required sounds at the prescribed interval. Simply mount it forward on the bow or mast to let other boats on a possible collision course know you're there. The hailer's electronic control box looks very similar to a VHF radio, and is usually mounted by the helm. Some high end marine VHF radios also double as a loudhailer. With this setup sounding in fog is as easy as one touch of a button. A bell and whistle capable of manual operation are still required in addition to the automated loudhailer. Sufficient backups include a simple handheld air horn and brass bell for small boats. A permanently mounted horn is the preferred signalling device whenever sounding maneuvering signals or attracting attention. Inland rules require passing agreements be signalled with whistles (horns) anytime vessels are within 1/2 mile. You are required to have whistles readily on hand for these maneuvering signals. Examples of passing signals required by COLREGS are the danger signal of 5 or more short rapid blasts when two or more boats are nearing collision or are in doubt as to the safety of the other boat's intended course, or a prolonged blast when rounding an obscured bend. You can also use the whistle to attract attention, similar to using your car horn. For instance, alert a fairway idler messing with their fenders they're not the only boat in town, or arouse boats perpetually blocking the channel while drifting and chatting over the gunnels. Listen for these sounds next time you're around the pier.
Mounting whistles is a fairly straight forward procedure (pun intended). The horn should be mounted on deck, preferably aloft well above ear level for crew safety such as on a cabin-top. The horn should be directed "with it's maximum intensity pointed straight ahead." The sound should be clear of structural obstructions on the boat. While placement of a bell is not specified directly, it should obviously be mounted on the exterior of the boat and in a location where it can best be heard in all directions.