Making more noise in fog

Nick Tyler takes the guesswork out of broadcasting fog signals

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Rule 35 of the Colregs requires that a vessel in restricted visibility (fog, smoke etc) should sound its horn in specific patterns every two minutes depending on its current status.

And although the rule is only strictly applied to vessels over 12m, it also says that smaller vessels should make some kind of noise every two minutes as well.

My friend and I were discussing this in a local hostelry one cold winter's night, and we both wondered about the practicality of this. He has a 28-footer that he regularly sails single-handed, and my wife and I are new to this sailing lark, and weren't quite sure what to do with one of those aerosol horns beloved of football fans when we found it in the locker of our first boat.

In fog, we wondered, why would a skipper want to stand with a stopwatch in one hand, and an air-horn in the other, while peering into the mist trying to find their way to safety? Surely there has to be a better way?"

My friend wondered if a small processor and relay could be used to automate the horn. You can buy something that does roughly this, but being PBO readers, we'd rather make it ourselves and save our money for more important things, like sails.

I'd spent three years designing electronics in 12V automotive lighting and am also quite familiar with the Microchip family of microprocessors, so rather rashly I told my friend I could knock him up a prototype in a couple of weeks.