Boat Hull Number (HIN / CIN) Explained + Decoder
Every boat built for the UK or EU market from 1998 onwards carries a 14-character Craft Identification Number, and every US-built boat from 1984 onwards a 12-character Hull Identification Number. It is the closest thing a boat has to a registration plate — and the first thing to check when you buy.
Try it: decode a hull number
This is the same decoding SoundHull's app does when a boat is added. It reads the format; it cannot tell you who owns the boat, because the UK keeps no such register.
What the characters mean
A UK/EU number such as GB-ABC00042-A3-24 breaks down like this:
| Part | Example | Meaning |
|---|---|---|
| Country | GB | Where the builder is registered (two letters). GB, FR, DE, PL are common on boats sold here. |
| Builder code | ABC | The manufacturer's identification code, three characters, issued to the builder. The same code appears on every boat that yard built. |
| Serial | 00042 | Five characters chosen by the builder — usually the hull number within a model. |
| Month of build | A | A letter from A (January) to L (December). |
| Year of build | 3 | The last digit of the year the hull was laid up. |
| Model year | 24 | Two digits. A hull laid up in late 2023 is often sold as a 2024 model, so the two years can differ by one. |
A US number is the same without the country, for example ABC12345D404: builder ABC, serial 12345, built in April (D) of a year ending in 4, 2004 model year.
Where to find it
- The transom, starboard side, upper corner. Moulded into the gelcoat or on a riveted plate, within a few inches of the top edge. On some boats it is on the hull side near the stern.
- A second, hidden copy. Builders are required to put a duplicate somewhere inside the boat that is not obvious — under a locker lid, inside a cockpit locker, behind a panel. Finding both and seeing that they match is a strong check against a swapped transom plate.
- The builder's plate (the CE plate on EU/UK boats) in the cockpit or by the companionway repeats the builder and often the number.
- The paperwork. The declaration of conformity, the original invoice and any survey should all carry it.
What to do with it when buying
- Ask the seller for the number before you travel and decode it. A number that does not fit the format on a boat built after 1998 needs an explanation.
- At the viewing, read it off the transom yourself and photograph it. Then find the hidden copy.
- Compare all three — transom, hidden copy, paperwork. One digit different is enough to stop and ask.
- Check that the builder code belongs to the yard the seller says built the boat. Builder codes are public; the boat's own manual and the builder's website are the quickest check.
- Keep the number with your records. If the boat is ever stolen, it is the identifier the police and insurers work from.
Boats without one
Boats built before the requirement, home-built boats and some imports have no number, or a number in a format of their own. That is not a fault. It does mean the identity check falls back to the builder's plate, the engine number, the sail number and the paperwork chain, and that it is worth having the surveyor note the identifying marks they can find.
SoundHull records the hull number as the boat's permanent identity: when a boat is added, the number is decoded exactly as above, an unusual format is accepted only after the person confirms it is what is moulded on the transom, and the same number cannot be added twice by two different owners without both being told. The record then follows the boat when it sells.
Questions people ask
Is the hull number the same as the registration number?
No. The hull identification number is given by the builder and stays with the hull for life. Registration numbers (Part 1 or the Small Ships Register) are allocated when an owner registers the boat and can change.
Can I look up who owns a boat from its hull number?
Not in the UK — there is no public ownership register for leisure boats. The number lets you check the builder, the build date and that the paperwork matches the hull; ownership is shown by the bill of sale.
The number on the transom does not match the paperwork. What now?
Stop and ask. It can be an honest typing error on an old invoice, but a transom plate that differs from the hidden copy or the builder's records is exactly what a stolen or re-plated boat looks like. Do not pay a deposit until it is explained in writing.
The record that goes with this.
SoundHull guides the inspection, fingerprints every photograph and signs the report. Not a survey — the record you keep so the survey goes well.