When the radio fails, the fog rolls in, or a vessel needs to warn every ship in sight at once, mariners fall back on a communication system older than radio itself. The International Code of Signals gives every vessel, regardless of the language its crew speaks, a shared way to signal by flag, by sound, and by light. 

This is a working field reference to the three signal types: what the common flags mean, what the whistle patterns say, and how light signals fit in. For the full history and structure of the Code, see the guide on understanding the International Code of Signals.

The Three Types of Maritime Signals Under the ICS

The International Code of Signals (ICS) provides for signaling by several methods, and three matter most in day-to-day practice: flags, sound, and light. Each suits a different situation.

• Flag signals communicate visually over distance in daylight and good visibility conditions, using the 26-letter alphabet flags, 10 numeral pennants, and substitute pennants.
• Sound signals carry through fog and darkness when flags cannot be seen, using the ship's whistle or horn.
• Light signals send letters by Morse code using a signal lamp, working at night and over longer distances than flags.

All three exist so that a critical safety message can be understood even when language, visibility, or equipment would otherwise get in the way.

Signal Flags: Single-Letter and Numeral Meanings

Every alphabet flag in the ICS carries a complete meaning when flown alone, and a bridge officer is expected to recognize the safety-critical ones instantly. A single flag communicates one specific message; flags combined in a hoist spell words or convey coded groups.

Common Single-Letter Flag Signals

The single-letter signals below are the ones every mariner should know on sight, because most concern the safety of the vessel, its crew, or those nearby:

• A (Alfa): "I have a diver down; keep well clear at slow speed."
• B (Bravo): "I am taking in, discharging, or carrying dangerous goods."
• D (Delta): "Keep clear of me; I am maneuvering with difficulty."
• E (Echo): "I am altering my course to starboard."
• F (Foxtrot): "I am disabled; communicate with me."
• G (Golf): "I require a pilot." From a fishing vessel: "I am hauling nets."
• H (Hotel): "I have a pilot on board."
• I (India): "I am altering my course to port."
• J (Juliett): "I am on fire and have dangerous cargo; keep well clear."
• L (Lima): "You should stop your vessel immediately."
• O (Oscar): "Man overboard."
• Q (Quebec): "My vessel is healthy, and I request free pratique." Flown to request health clearance to enter port.
• U (Uniform): "You are running into danger."
• V (Victor): "I require assistance."
• W (Whiskey): "I require medical assistance."

Numeral pennants (0 through 9) carry numbers for positions, times, and coded groups, and substitute pennants let a vessel repeat a flag within a hoist when it has only one set of the code.

Sound Signals: Whistle and Horn Signal Patterns

When visibility drops, or vessels are close enough to hear but need to signal intent, the ship's whistle takes over. Sound signals use two basic elements: a short blast lasting about one second, and a prolonged blast lasting four to six seconds.

In clear visibility, maneuvering and warning signals communicate a vessel's actions: one short blast means "I am altering my course to starboard," two short blasts mean "I am altering my course to port," and three short blasts mean "I am operating astern propulsion." Five or more short and rapid blasts are the danger or doubt signal, used when one vessel doubts another is taking sufficient action to avoid collision.

In restricted visibility, fog signals announce a vessel's presence and status: a power-driven vessel making way sounds one prolonged blast every two minutes, while a vessel underway but stopped sounds two prolonged blasts. These maneuvering and fog sound signals are governed by the collision regulations, and the full set of rules is covered in the guide to COLREGs.

Light Signals and Their Meanings

Light signals extend the ICS into darkness. The primary method is a signal lamp, often called a blinker or Aldis lamp, which transmits the same code letters by flashing them in Morse code. A vessel can send any single-letter signal, or spell out a full message, by light when flags would be invisible.

Sound signals also have a light equivalent for maneuvering. A vessel may supplement its whistle signals with an all-round white light: one flash for "altering to starboard," two for "altering to port," and three for "operating astern propulsion," matching the whistle blasts. This light supplement is useful in close-quarters situations where confirming intent visually adds a margin of safety.

The answering pennant and code flag also play a role at night and by day: a vessel hoists the answering pennant at the dip to show a signal is seen, then close up to show it is understood.

Where to Get Signal Flags and the Official ICS Publication

Knowing the signals is only useful if a vessel actually carries the means to make them. The International Code of Signals is a mandatory backup communication system for commercial vessels, and maintaining clean, visible signal flags is part of a vessel's safety equipment.

A complete International Code of Signals marine flag set includes the 26 alphabet flags, numeral pennants, and substitutes needed to make any signal, built with lock-stitched seams to resist fraying in service. Alongside the flags, the International Code of Signals publication provides the full official reference to every signal and its meaning, the codebook that belongs on every bridge. As an authorized IMO publication distributor, American Nautical Services supplies the authentic publication along with the full range of marine signal flags.

Keep the Signals Ready Before You Need Them

Flag, sound, and light signals are the fallback that works when nothing else does: no battery, no bandwidth, no shared language required. Learning the safety-critical flags, knowing the whistle patterns, and keeping a clean set of flags and a current codebook aboard is basic seamanship that pays off in the moment it matters. Shop signal flags and the ICS publication in the ANS marine signal flags collection, or contact American Nautical Services at +1 (954) 522-3321 or sales@amnautical.com.

Frequently Asked Questions

What does flying a single signal flag mean?

Each alphabet flag in the International Code of Signals has a complete meaning when flown alone. For example, the A (Alfa) flag means "I have a diver down; keep well clear at slow speed," and the O (Oscar) flag means "man overboard." Bridge officers are expected to recognize the safety-critical single-letter signals on sight.

Are sound signals the same as COLREGs signals?

The maneuvering and fog sound signals used at sea are governed by the COLREGs (the Collision Regulations). The International Code of Signals works alongside them as a broader communication system covering flag, sound, and light. In practice, the whistle signals for altering course or navigating in fog come from the COLREGs.

What are the basic maritime sound signals?

The two building blocks are the short blast (about one second) and the prolonged blast (four to six seconds). One short blast means altering course to starboard, two short means altering course to port, three short means operating astern propulsion, and five or more short rapid blasts are the danger signal.

How do light signals work under the ICS?

Light signals use a signal lamp to flash code letters in Morse code, allowing a vessel to send single-letter signals or full messages at night. Maneuvering signals may also be shown with an all-round white light flashing to match the whistle blasts.

Where can I buy a set of international signal flags?

Complete International Code of Signals marine flag sets, including the alphabet flags, numeral pennants, and substitutes, are available from American Nautical Services, along with the official ICS publication and individual signal flags in the marine signal flags collection.