Hull identification
How to Find the HIN on a Boat (and What to Do If It's Missing)
By The VesselBook Team · Published July 28, 2026 · 6 min read
On almost every boat, the Hull Identification Number is on the outside of the transom, in the upper starboard corner — the back right of the boat as you sit in it facing forward — within a couple of inches of the top edge. It is 12 characters, at least a quarter of an inch tall, and either moulded into the fibreglass or on a permanently fixed plate.
If you are standing at the dock with a phone in your hand, start there. The rest of this article covers the cases where that does not work: the boat has no usable transom, the number has weathered away, there is a second number that disagrees with the first, or the seller cannot explain any of it.
Step 1: the transom
Look at the back of the boat from the outside, on the starboard side, close to the top. Common obstructions:
- Outboard brackets, swim platforms, boarding ladders and transom doors. Look above, below and behind them. On boats with a large platform, you may have to get in the water or lie on the platform to see the hull face.
- Paint, wax build-up, growth or a name decal applied straight over the number. Wipe the area with a rag; the characters are moulded, so they can usually be felt with a fingertip even when they cannot be read.
- A boot stripe or antifouling line that has crept upwards during past yard work.
If the characters are faint, hold a phone light at a low angle across the surface rather than straight at it. Raised or recessed characters cast a shadow and become readable. A pencil rubbing on paper works on a moulded number when a photograph will not.
Step 2: the hidden duplicate
US boats built since 1972 carry a second, unexposed copy of the same number, precisely so the hull can be identified when the transom number is gone. Builders put it where it will not be seen in normal use:
- inside a cockpit locker or the anchor well,
- on a transverse bulkhead or stringer, visible only with a hatch or floor panel up,
- under a deck fitting, a cleat backing plate, or the fuel filler,
- inside the engine bay, on the hull side or under the engine mounts,
- under the helm console or behind a removable trim panel.
The duplicate should be character for character identical to the transom number. It is worth finding on any boat you are about to buy, and worth photographing on a boat you already own — if the transom is ever damaged, that hidden number is what proves which hull you have.
Step 3: match it to the paperwork
Read the number off the hull, then read it off the documents — do not do it the other way round, and do not read the paperwork first and then "confirm" it on the hull. People see what they expect to see, and 0/O, 1/I, 5/S, 2/Z and 8/B are the pairs that catch everyone.
Check it against:
- the state registration certificate or title,
- the US Coast Guard documentation, for documented vessels,
- the insurance policy or quote,
- the bill of sale and any survey report,
- the builder's certificate, if the boat still has one.
All of them should carry the same 12 characters. A single mistyped character on a registration is common and fixable, but it has to be fixed — an insurer or a state agency will eventually notice, usually at the least convenient moment.
Boats where the transom rule does not apply
- No transom, or a transom that cannot carry a number. The number goes on the starboard side of the hull, aft, near the stern.
- Personal watercraft. Usually on the hull just above the rub rail on the starboard side, at the stern, and often small. There is normally a second number inside the engine compartment.
- Inflatables and RIBs. Typically on a plate fixed to the transom, or on a stiffener inside the boat. On soft-transom inflatables, look inside on the hull fabric or on the plywood floor structure.
- Sailboats. Same transom rule, but sugar-scoop sterns and wind vanes hide it. Check inside the lazarette and on the hull side under the aft bunk for the duplicate.
- Home-built and pre-1972 boats. These may legitimately have no HIN. States issue a number in that situation; the state paperwork is then the identity of record, and the state's issued number is what a surveyor or insurer will want to see.
If the number is missing, unreadable, or looks altered
Take this seriously, in this order:
- Look for the duplicate before concluding the boat has no number.
- Ask the seller for the paperwork, and see whether the number on it can be found anywhere on the hull.
- Look at the surface itself. Filler over the number, a plate riveted on top of moulded characters, a ground-flat patch of gelcoat, or characters in a different font from the surrounding moulding are all reasons to stop.
- Contact the state boating agency where the boat is registered. They deal with missing and damaged HINs routinely and can tell you what a replacement requires.
Removing, altering or obscuring a HIN is illegal in the US, and a hull whose identity cannot be established is a hull whose history cannot be established either. No price makes that a good trade.
Not every missing number is a warning sign, though: a pre-1972 or home-built hull may never have had one, and states assign numbers in exactly that situation. What to do if your boat has no HIN covers how that works and why you must never apply a number yourself.
Photograph it while you are there
Two photographs, every time: one close enough to read every character, and one wide enough to show where on the boat that number is. On a boat you own, keep them somewhere you will still have them in ten years. On a boat you are considering, they are the first thing worth having, because everything else you learn is filed under that number.
Look the number up
Once you can read the number, you can check what is publicly on file for that hull. The free HIN Lookup shows the hull's identity, whether ownership has been verified, records the owner has made public, and whether the hull appears in federal vessel data. No account is needed. Nothing on file is not a warning sign in itself — most hulls have no public record yet — but it does mean the seller's account of the boat is all you have.
If you are still choosing a boat, browse listings on VesselBook, where a seller can attach the documented history to the hull rather than describing it in a paragraph. And before you commit to anything, work through what to check before buying a used boat.