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Maritime flags and their meaning

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What is the issue?
Code Flags are commonly displayed on commercial vessels during regattas and races or used as bunting at shoreside events. But the principal use of code flags is communication between vessels.
Commercial vessels use code flags and shapes alongside sound signals Experience when moving, anchored or alongside terminals to communicate their intentions.

Why address this?
Although it may seem an archaic means of communication, it is important for mariners to understand the meaning of some of the more common flag signals used within shared waterways. Particularly the 'One-flag' signals used in case of danger or breakdowns.

How to address this?
The below signals and code flags are part of an internationally recognised system which can be used by any vessel to communicate with those around it. One-flag signals are urgent or common signals that are frequently displayed, and it is important to understand their meaning when flown individually.


The red-and-white flag and 'Alpha' or 'Alfa' flag both mean 'I have a diver
down; keep well clear at slow speed'

Image: Michael Harpur

The diver down flag, or scuba flag, is used on the water to indicate that there is a diver below. Two styles of the flag are in use. Internationally, the code flag alfa/alpha, which is white and blue, is used to signal that the vessel has a diver down and other vessels should keep well clear at slow speed. In European waters vessels accompanying divers are required to display the Code Flag ‘A’ when divers are in the water. The North America convention is the red with a white stripe from the upper left corner to the lower right corner. However, it should be noted that a Red flag with the diagonal white stripe is also an internationally recognised flag and it is more commonly used on surface marker buoys.

Dive vessel showing the Red flag
Image: Eric T Gunther via CC BY-SA 3.0

When you see a vessel or surface marker displaying either of these flags give it a wide berth and pass at an appropriate speed. Divers may travel some distance from the vessel so you should also keep a good watch on the water around you. Look for rising bubbles and surface markers which can indicate a diver is close to the surface.

D: I am manoeuvring with difficulty; keep clear.
Image: Michael Harpur

E: Altering Course to Starboard
Image: Michael Harpur

I: Altering course to Port
Image: Michael Harpur

P: About to sail
Image: Michael Harpur

S: Engines going astern
Image: Michael Harpur

M: I am stopped and making no way through the water
Image: Michael Harpur

V: I require assistance
Image: Michael Harpur


The complete set of the International Signal Flags and illustrated below along with their international meanings.

            A: Alpha – diver down; keep clear
            B: Bravo – carrying dangerous cargo
            C: Charlie – yes
            D: Delta – keep clear
            E: Echo – altering course to starboard
            F: Foxtrot – I am disabled
            G: Golf – I want a pilot
            H: Hotel – a pilot on board
            I: India – I am altering course to port
            J: Juliet – vessel on fire keep clear
            K: Kilo – I want to communicate with you
            L: Lima – stop your vessel instantly
            M: Mike -my vessel is stopped
            N: November – no
            O: Oscar – Man overboard
            P: Papa – the vessel is about to sail
            Q: Quebec – I request free pratique
            R: Romeo – reverse course
            S: Sierra – engines are going astern
            T: Tango – keep clear
            U: Uniform – you are heading into danger
            V: Victor – require assistance
            W: Whiskey – require medical assistance
            X: X-ray – stop your intention
            Y: Yankee – am dragging anchor
            Z: Zulu – I require a tug


Although officially retired, storm warning flags are still internationally recognised code flags. Small boat stations, marinas and coastguards still use flags to alert boaters of impending weather alerts such as small craft advisory warnings, as well as gale, storm and hurricane warnings.

Small Craft Advisory Pennant
Image: Michael Harpur

“Small Craft Advisory”: Issued for sustained winds or frequent gusts of 20 to 33 knots and/or seas of seven feet or greater expected in a particular zone. A single red pennant can be hoisted for such an event.

Gale Warning Pennants
Image: Michael Harpur

“Gale Warning”: Issued for sustained winds or frequent gusts of 34 to 47 knots. Two red pennants can be hoisted for a gale warning.

Tropical Storm Warning Flag
Image: Michael Harpur

“Tropical Storm Warning”: Issued for the threat of sustained winds or frequent gusts of 34 to 63 knots associated with a tropical storm. A square red flag with an embedded black square can be hoisted for an imminent tropical storm.

Hurricane Warning Flag
Image: Michael Harpur

“Hurricane Warning”: Issued for sustained winds or frequent gusts exceeding 63 knots. Two square red flags with embedded black squares can be hoisted for a hurricane warning.

Flying hurricane warning flags
Image: CC0

With thanks to:
Michael Harpur
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