Underwater & architectural LED lighting

How underwater lighting is assessed

An underwater fitting is cooled by the water it sits in. Operated in air, the LEDs reach their thermal limit within minutes and the protection circuit reduces output or shuts the fitting down. A fitting that dims on the hard and operates correctly afloat is therefore not necessarily faulty, and diagnosis is carried out with the vessel in the water — a test in air reproduces a condition that does not occur in service and masks the one that does.

Afloat, the two components that most often fail are the driver and the supply feeding it. Underwater fittings run through an external constant-current driver and are usually fed from a distribution point well forward of the transom, so volt drop across the run, corroded terminations and a driver operating below its input range account for most of what is reported as a failed light. Voltage is measured at the fitting with the circuit under load rather than at the panel, since that is where the difference appears.

Scope of work

Thru-hull & surface-mount installation

Fittings installed and sealed, wiring run and terminated, and the circuit commissioned. New hull apertures are cut with the vessel ashore and scheduled against the yard programme.

Constant-current driver replacement

Driver output measured against the fitting it feeds before it is condemned — a driver working below its input range behaves like a failed light and is not one.

Volt drop across extended runs

Measured at the fitting under load. Lighting circuits losing much more than three percent start showing colour shift and early driver failure rather than simply reading dim.

Bonding, isolation & galvanic protection

Whether a metal fitting is bonded to the vessel or deliberately isolated is decided per vessel and per hull material, then recorded — because getting it wrong in either direction creates corrosion rather than preventing it.

Switching, dimming & colour control

Switched and dimmed circuits, colour-change control and its wiring back to the panel or helm, including the control runs that fail while the lights themselves are fine.

Thermal cutout & fault diagnosis

Fittings that fade, cycle or cut out traced afloat under load, separating thermal protection operating correctly from a genuine electrical fault.

Underwater and architectural lighting control panel with six circuit rockers and indicator lamps, shown with three underwater LED light fittings, one lit cool white

Attendance sequence

Phase	Stage	Actions and deliverables
01	Symptoms by message	Vessel, berth, how many fittings are affected and whether the fault appears immediately or after a few minutes running — the timing separates a thermal response from a supply fault before anyone attends.
02	Berth-side diagnostic	Voltage measured at the fitting under load, driver output checked against its fitting, and bonding continuity or deliberate isolation confirmed against what the vessel is built as.
03	Written plan	Whether the fault is the fitting, its driver or the supply feeding it, what is repaired against replaced, and any hull work that has to wait for the vessel to come ashore.
04	Work, test, document	Circuits repaired or installed, then run afloat long enough to prove they hold output rather than cut back, with readings and photographs issued.

What is measured

Underwater lighting is diagnosed afloat and under load. A fitting measured at rest, or in air, will read correctly and tell you nothing about the circuit that fails once it is drawing current and warming up.

Voltage at the fitting, under load

Taken at the fitting rather than the panel. The whole fault on a long transom run lives in the difference between those two readings.

Driver output against its fitting

Measured while driving the load it is installed to drive, which is the only condition that separates a failing driver from one being starved by its own supply.

Bonding continuity or isolation

Confirmed against what the vessel is actually built as, and recorded. Stray DC current corrodes far faster than galvanic action, so a wrong assumption here is expensive.

Insulation resistance on the run

Measured along the cable run to the fitting, which is where water ingress at a termination shows up before it reaches the light.

Where this work is carried out

Diagnosis and driver, supply and control work at the berth across the North Adriatic. Hull apertures are scheduled with the vessel ashore.

Croatia Pula, Rovinj, Vrsar, Poreč, Umag, Novigrad

Slovenia Izola, Koper, Portorož

Italy Trieste, Muggia

Questions asked most often

The lights worked on the hard and now dim after a few minutes. Have they failed?

Almost certainly not, and it is usually the other way round. Underwater fittings are cooled by the water they sit in, so running them in air takes them to their thermal limit within minutes and the protection circuit reduces output or shuts them off. That is the protection working. Fittings are tested afloat, under load, for long enough to prove they hold output rather than cut back.

Can underwater lights be fitted without hauling out?

Surface-mount fittings and everything electrical — driver, supply, switching and control — are done at the berth. A new thru-hull aperture cannot be cut with the vessel afloat, so that part is scheduled against a lift or an existing yard period, and the electrical installation is arranged around it.

Will underwater lights cause corrosion around the transom?

They can, in either direction, which is why the bonding decision is made per vessel rather than by habit. A metal fitting on a metal hull needs isolation; a fitting that is already electrically isolated can be made worse by bonding it to everything else. What matters more is stray DC current, which corrodes far faster than galvanic action — so the supply and its return are checked as part of the same visit.

One light is dimmer than the others. Is that fitting failing?

Check the supply before the fitting. Underwater lights are usually fed a long way aft from their distribution point, and volt drop across that run — or a corroded termination part way along it — leaves the furthest fitting working below its driver’s input range. It reads as a dim or colour-shifted light and replaces a perfectly good unit if measured at the panel instead of at the fitting.

Underwater LED lighting