BOAT-COMPARATOR Guide
Signal Flags and the International Code: What a Renter Should Recognise
Alpha for a diver in the water, Oscar for man overboard: a handful of International Code of Signals flags worth recognising when passing other boats.
You don't need to know them all, but a handful of flags from the International Code of Signals come up regularly in leisure boating — recognising them avoids a lot of misunderstandings out on the water.
A code over a century old
The International Code of Signals has existed since the mid-19th century and has been revised several times, most recently by the International Maritime Organization in 2005. It lets ships of different nationalities exchange safety messages without relying on a shared language — flown from a mast, it stays readable at a distance when VHF isn't.
Alpha: a diver is in the water
The Alpha flag (blue and white, swallow-tailed) signals a diver near the boat flying it: passing it means slowing down and giving it a wide berth. Our guide to scuba diving from a rental boat covers when you can bring tanks aboard yourself.
Oscar: man overboard
The Oscar flag (red and yellow rectangle) signals a person overboard and calls on any nearby vessel to assist — a signal rarely seen in leisure boating, but one you need to recognise instantly if it appears on a nearby boat.
Other useful signals to know
Two black balls stacked vertically, or a red-over-red light at night, signal a vessel that cannot manoeuvre freely (engine failure, a trawl being worked): it gets right of way regardless of the usual steering rules. Our guide to navigation lights at night covers the night-time equivalents of these day signals.
What matters in practice
On a standard rental, it comes down to two reflexes: slow down and steer well clear of an Alpha flag spotted at anchor, and alert the coast guard immediately over VHF channel 16 if a distress signal appears on a nearby boat. The rest of the code, more technical, is mostly the concern of professional shipping.
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