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GLOSSARY · HULL AND STRUCTURE

What is a transom on a boat? What it carries and how a surveyor checks the core

UPDATED 10 NOVEMBER 2026 · 3 SOURCES

The transom is the aft face of a boat's stern. What it carries, why cored transoms get wet, how a surveyor sounds one, and where the HIN sits on it.

DEFINITION
Transom

The flat, aft-facing surface across the stern of a boat, which on many powerboats carries the outboard or sterndrive and, by US rule, the hull identification number.

The US boat rules define the transom as the surface at the stern of a boat, projecting or facing aft. On an outboard or sterndrive boat it's a structural part: the engine hangs on it or bolts through it, and it takes the thrust.

Many transoms are cored with plywood. Every fastening through it, from engine brackets to transducers, ladders and drain fittings, is a path for water into the wood. Soundings lists loose, badly fitted or badly bedded hardware as the way in.

What a surveyor checks

Sounding the transom in a grid. A wet core gives the deep "thup" Soundings describes, and the hammer doesn't bounce back as strongly. A moisture meter on dry gelcoat to map it. Dirty cracks and weeping round fittings. With the engine loaded or levered, any flex. On a sterndrive boat, the bellows and the transom from inside.

The HIN on a US boat is on the starboard side of the transom, near the top, so the surveyor reads it here too.

In the report

"Transom, starboard of the drive, area about 400 by 300 mm dull on sounding with high readings. No flex under load." See wet transom core.

Frequently asked

How do you check a transom for a wet core?

Sound it with a hammer in a grid, then follow up with a moisture meter. Soundings notes a wet core gives a deep hollow sound and the hammer doesn't bounce back as strongly.

Where is the HIN on the transom?

On the starboard outboard side, within two inches of the top of the transom, gunwale or hull-deck joint, whichever is lowest, under 33 CFR 181.29.

Findings to write up

HULL AND STRUCTURE

Wet transom core

How water gets into a cored transom, how a surveyor finds it with a hammer and a moisture meter, what the sounds and cracks mean, what repair involves and three example findings graded A, B and C.

How to find and write it up
UNDERWATER GEAR

Sterndrive bellows

Why sterndrive bellows sink boats at the dock, how to inspect all three, how often to replace them, what to check on the drive at the same time and three example findings graded A, B and C.

How to find and write it up

Related terms

Glossary
SURVEY TERMS

HIN

The 12-character number moulded or fixed on a boat's transom that identifies its builder, serial number and build and model year, like a car's VIN.

HULL AND STRUCTURE

Delamination

Layers of a fibreglass laminate coming apart, or a cored panel's skins separating from the core, which costs the panel its stiffness and strength.

UNDERWATER GEAR

Sterndrive

A drive leg bolted to the outside of the transom and driven by an inboard engine through it, steering by turning the whole leg.

HULL AND STRUCTURE

Bulkhead

A wall across or along the inside of a boat, bonded to the hull and deck, that stiffens the structure and often carries rig or engine loads.

HULL AND STRUCTURE

Cored hull

A hull or deck built as a sandwich: two skins of fibreglass with a light core of balsa, foam or plywood between them.

HULL AND STRUCTURE

Gelcoat

The thin, hard, coloured resin layer on the outside of a fibreglass boat, which gives the shiny finish and keeps water off the laminate behind it.

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