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STANDARDS · ABYC

ABYC H-27: seacocks and through-hull fittings, what surveyors check

UPDATED 6 NOVEMBER 2026 · 4 FINDINGS · 3 SOURCES

What ABYC H-27 asks of seacocks, through-hull fittings and drain plugs, the 500 lb strength test, why a ball valve on a through-hull can fail it, what a surveyor checks and how to cite H-27 in a finding.

AT A GLANCE
ABYC H-27
OFFICIAL TITLE
Seacocks, Thru-Hull Fittings, and Drain Plugs
COVERS
The fittings that make holes in the hull below or near the waterline, and the valves that close them
SURVEYORS CHECK
Seacocks readily accessible and easy to operate, corrosion resistant and galvanically compatible, and strong enough to hold a 500 lb static load for 30 seconds
GET THE TEXT
ABYC members, through the online Standards Library

Every through-hull below the waterline is a hole that can sink the boat. H-27 is the standard for the fitting and the valve that closes it.

What it covers

ABYC lists H-27 as Seacocks, Thru-Hull Fittings, and Drain Plugs. In our words: the fittings through the hull and the valves on them, what they're made of, how strong they are and how they're installed and operated.

What surveyors check against it

Access and operation. Seacocks readily accessible, with handles easy to operate. A seacock behind a drawer or seized open fails its purpose. See seized seacocks.

Materials. Nigel Calder quotes H-27: all materials corrosion resistant in the marine environment and galvanically compatible. Look for pink, dezincified brass and mixed metals. Bonding is a separate argument; see through-hull bonding.

Strength. The assembly must hold a 500 lb static load for 30 seconds. MarineHowTo tested threaded combinations against that figure: a 3/4 inch bronze valve on a matching through-hull failed at 404 lb, and a 1 inch combination at 458 lb. A flanged seacock through-bolted to a backing block is the usual answer.

Citing it in a finding

"Replace the ball valve on the through-hull with a flanged seacock through-bolted to a backing block, ABYC H-27." Name the fitting by its service and location, and say whether it operated.

Findings judged against ABYC H-27

Each page shows how the defect is found, what decides the grade and example findings written up at A, B and C.

UNDERWATER GEAR

Seized seacocks

Why a seacock that won't close is a safety finding, how surveyors check seacocks and through-hulls, the ABYC strength test a ball valve on a through-hull can fail, replacement costs and three example findings graded A, B and C.

How to find and write it up
PLUMBING AND TANKS

Single hose clamps

Where ABYC requires two hose clamps and where one is allowed, why surveyors still call for two below the waterline, what to look for in the clamps and barbs, and three example findings graded A, B and C.

How to find and write it up
UNDERWATER GEAR

Through-hull bonding

Why surveyors disagree on bonding through-hulls, what Nigel Calder and Don Casey advise, when a fitting should be bonded, what a surveyor checks and three example findings graded A, B and C.

How to find and write it up
UNDERWATER GEAR

Leaking shaft seal

How many drips a stuffing box should make, when it needs repacking, what a surveyor checks on the shaft log hose, clamps and dripless seals, and three example findings graded A, B and C.

How to find and write it up

Frequently asked

What is the ABYC seacock strength test?

H-27 asks for the seacock and through-hull assembly to hold a 500 lb static load for 30 seconds without failing.

Is a ball valve on a through-hull a seacock?

Not in the H-27 sense. A ball valve threaded onto a through-hull leaves the threads carrying the load. In MarineHowTo's tests, a 3/4 inch bronze valve on a matching through-hull failed at 404 lb, and a 1 inch combination at 458 lb, both below 500 lb.

What materials does ABYC allow for seacocks?

Nigel Calder quotes H-27 as requiring materials that are corrosion resistant in the marine environment and galvanically compatible. Bronze and suitable composites are common.

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