Surveyor’s Notebook: Using the Proper Seacock

A fine example of a dangerous installation

According to Boat US, boats sinking at docks account for a disproportionate number of insurance claims. Frankly, I’m not surprised. Even a modest boat often has six or seven through-hull fittings below the waterline. Should one of these fail, the inrush of water will swiftly sink the boat.

All below-waterline through-hulls should be protected by a seacock. A proper seacock can go from fully open to fully closed with a 90-degree turn of its lever-type handle. Normally the handle is in line with the hose when it’s open and at a right angle to the hose when closed. You should only use seacocks made from suitable materials, which means bronze or Marelon.

ABYC guidelines also state, and I quote, “A seacock shall be securely mounted so that the assembly will withstand a 500 pound (227 Kg) static force applied for 30 seconds to the inboard end of the assembly, without the assembly failing to stop the ingress of water.” The seacock in the picture at left is not a seacock at all, but merely a plumbing gate valve that probably came from a hardware store. There is no backing flange, the valve is simply attached to the end of the through-hull, so you can’t tell at a glance if it is open or closed, and it would surely not stand up to the ABYC load test.

There is also only a single clamp holding the hose to the fitting. All hoses below the waterline should be double clamped. Otherwise, the only thing standing between your boat and the ocean is a single screw and one thin strip of metal.

Lifelong boat addict and marine surveyor Mark Corke can be reached at

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