What is a Thru Hull Fitting?

A hole in the hull may conjure up visions of flooding or sinking, but look at any boat’s hull and you’ll see at least one hole, which is likely a thru-hull fitting.

What is a through hull fitting

Water inlets and exits to cool inboard and sterndrive motors; drains for sinks and live wells; transducer mounts; exhaust exits; bilge outlet and transom drain plugs all require a hole though the boat’s hull for them to function.

Obviously a hole in the hull is a very serious matter, so to ensure these holes remain watertight and don’t compromise the hull, a thru-hull fitting is used.

Thru-Hull Fitting Definition

A thru hull (or through hull) fitting is defined by the American Boat and Yacht Council (ABYC) as a device that allows the passage of liquids or gases through a hull.

Boat thru hull fittings

Essentially, a through hull fitting is a device that’s secured to and creates an opening through the hull, to which a pipe or duct can be attached, allowing the passage of water or gas into or out from the boat.

Above and Below Waterline Thru Hull Fittings

All through hull fittings fall into one of two categories: Above the waterline and below the waterline.

Above waterline thru hull fitting

Above the waterline thru-hull fittings may include genset exhaust, head discharge, livewell drain, bilge outlet, or sink drains.

Below the waterline thru-hull fittings may include engine coolant intake, engine exhaust, livewell inlet, A/C inlet, or drain plugs.


A fault with any through-hull fitting poses the risk of water entering the hull, with below the waterline thru-hulls presenting a greater risk because they’re permanently submerged. Because of the potential hazards associated with thru-hull openings, certain standards have been established to minimize their risks and maximize their dependability.

Flow Rite thru hull fitting

Flow-Rite thru hull fitting black

Standards for Through Hull Fittings

ANSI/UL “Standard for Marine Through-Hull Fittings and Sea-Valves” 1121.8.1 states that:

     The component parts of a through hull fitting must be made of galvanically compatible materials and have sufficient resistance to corrosion and strength to withstand both intended and abnormal use.


In addition, ABYC “Seacocks, Thru-hull Fittings and Drain Plugs” Standard H-27.5.1 states that:

     Any through hull fittings situated below the maximum heeled waterline must be equipped with a seacock to stop water from entering the hull if the fitting were to fail.


While H-27.5.3 adds for fittings above the waterline:

     Through hull fittings that are not equipped with a seacock must use reinforced piping or hose that resists collapse or kinking.

What are through-hull fittings

These standards require thru hull fittings to be suitably strong and corrosion resistant, include a valve (seacock) that can be closed if the through hull malfunctions for fittings below the waterline, or incorporate reinforced pipe/hose for through hull fittings above the waterline.


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