On modern merchant ships, a mast no longer carries sails — it serves as a vertical support structure for essential navigation and communication equipment.
What a Modern Mast Supports
- Navigation lights — masthead lights, required under COLREGs to be visible from specified distances and angles, critical for other vessels identifying your vessel’s type and orientation at night
- Radar scanner — often mounted at mast height for optimal detection range
- Antennas — VHF, GPS, and other communication/navigation antennas
- Signal halyards — for flying flags and signals
Foremast and Mainmast
Larger vessels may have multiple masts — a foremast (forward) and mainmast (typically near the bridge or midships) — though many modern commercial vessels use a simplified single mast structure integrated with the bridge structure.
Masthead Light Requirements
COLREGs specifies precise requirements for masthead light visibility, arc of visibility, and (on larger vessels) the requirement for a second masthead light aft of and higher than the forward one — these details directly help other vessels judge your vessel’s size, type, and aspect at night.
Why Mast Height Matters Practically
Mast height affects a vessel’s air draft (total height above waterline) — a critical figure for passing safely under bridges and other overhead obstructions, and one of the figures checked carefully during passage planning through restricted routes.
Exam Relevance
Navigation light requirements and their relationship to mast positioning are core COLREGs topics in deck officer competency exams.