Marine light failures: what the rules require and what actually breaks

A failed light is one of the more embarrassing detentions, because it is cheap, visible, and entirely preventable. It is also one of the more common, because the fault is usually not where people look first.

What the rules actually say

The International Regulations for Preventing Collisions at Sea set out, for each light, a required colour, a horizontal arc and a minimum visibility range that scales with the length of the vessel. A masthead light shows forward over a wide arc; sidelights show red to port and green to starboard over a narrower one; a sternlight covers the remainder.

Two points are routinely missed:

• The arc is a construction requirement, not a suggestion. A fitting mounted where a mast or a container stack cuts into its arc is non-compliant even though the lamp works perfectly.
• Vessels above a certain size must carry a second, independent light for the required positions. An inspector will ask to see it energised, not just present in a locker.

Ranges are specified in nautical miles and depend on the length of the vessel, so a fitting adequate on one ship is not automatically adequate on a larger one. Check the specification against your own vessel rather than assuming a like-for-like swap is compliant.

What actually fails

In practice, roughly in order:

• Corrosion at the terminal block. Salt gets into the housing through a perished gasket, and the connection goes high-resistance long before it goes open-circuit. The light appears dim rather than dead, which is why it survives a casual check.
• The gasket itself. Rubber hardens with UV and heat cycling. Once it stops sealing, everything downstream of it is on borrowed time.
• The lamp holder, not the lamp. Contacts lose spring tension and arc. Replacing the bulb fixes it for a fortnight.
• The supply cable at the gland. Flexing plus vibration plus salt, at the one point where the insulation is already compromised by being clamped.
• The panel and its alarm circuit. A light monitoring panel that no longer detects a failed lamp is itself a defect, and it is one that hides other defects.

Notice that only one of those is solved by carrying spare bulbs.

Before an inspection

• Energise every required light, including the secondary set, and look at them from outside the vessel rather than from the panel indicator.
• Open one or two housings and check the gasket and terminals rather than trusting that a lit lamp means a healthy fitting.
• Confirm the light monitoring panel actually alarms when a lamp is removed. Test it; do not assume it.
• Check that nothing added to the deck since the last inspection now obstructs an arc.

Replacing a fitting

Match the specified visibility range and arc for your vessel's length, not simply the physical shape of the old fitting. Housings from different manufacturers can share a footprint while being certified for different ranges.

Where the original fitting is out of production, an equivalent from the same era is usually easier to fit than a modern one, because the mounting and gland arrangement match what is already on the deck.

If you are trying to match a fitting, send us a photograph of it, or see what is currently listed.