The type of through-hull fitting used for boat underwater lights depends on the hull material of the boat. Clamp-in fittings are typically used in composite hulls, while weld-in fittings are the preferred option for metal hulls.
Different manufacturers provide through-hull fittings made from different combinations of materials that may include low carbon or mild steel, 316 SS, 5000/6000/7000 series aluminium, brass, bronze, sapphire windows, high impact plastics, and more.
We prefer wrought metals that provide the greatest strength and maximum material density, that is the clamp-in THSL designs provide two mechanical barriers and four seals between the ocean and vessel interior. A weld-in version in development provides three barriers with six seals.
Clamp-in through-hull fittings work very well in composites hulls. Clamp-in through-hull lights with metal housings require an insulating plastic sleeve for installation in a dissimilar metal yacht hull. Of the engineering plastics, UHMW-PE (Ultra high molecular weight polyethylene) is among the best. Known as a tough, wear-resistant material, it combines outstanding impact strength, excellent chemical resistance, broad temperature range, and is readily adhered to by such marine adhesives, such as 3M 5200. Insulated lights must still be connected to the ship’s bonding system.
A weld-in through-hull fitting may be required by Class (and standard good practice) for larger vessels. The designer of a weld-in through-hull light fitting may choose to move the window seal surface away from the weld-heat-affected zone, minimizing potential heat distortion and possible damage from weld splatter. After the module is welded in, the yard crew must install the window, window seal, and window retaining ring. The light assembly is attached inboard of that.
Secondary containment may be required in some larger vessels. An interior cofferdam provides a recessed hull section behind the light. One can also provide for installation through interior tanks intended to later hold fuel, potable water, or other liquids. Where a lamp driver is used to power a HID (High Intensity Discharge) or LED (Light Emitting Diode), a remote light head, connected by a cable, can be installed with a small recessed fitting. The cable may then be passed through a standing pipe that passes through and out of the holding tank to a point above the waterline, providing additional design safety.
A weld-in fitting may be subject to thermal stress caused by a sharp temperature drop in the metal after heat is removed. The stress can create distortion warping seal surfaces, and promote premature cracking of the weld. ‘Stress’ reduces these effects. A portable, non-heat stress-relieving method using subharmonic vibration energy has been developed by Bonal Technologies®, called Metal-Lax® that some shipyards have found useful.
Should an owner have an electrical problem with a through-hull light, perhaps as simple as changing a bulb, he would want to perform maintenance on the fixture without having to haul the yacht. A through-hull light that may be opened from the interior of the vessel, with an outer window providing a fixed barrier to the sea, allows for continued vessel operation even if an interior component need be returned to the factory. This maintenance feature also provides a convenient upgrade path to a new color option, or to take advantage of new and brighter LEDs.
There is much to consider when choosing an underwater light for your boat. The correct fitting material and design will help ensure a safe, long-lasting installation.
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