What size boat must be registered, and how do registration rules vary by location?

There is no worldwide minimum boat size for registration: the rules depend on the country, sometimes the state, the boat's propulsion and whether it is used privately or commercially. Check the requirements for your intended flag and the waters where you will keep the boat, because national registration, local boat licensing and permission to operate are separate matters.

Denmark: the tonnage threshold

For Danish pleasure craft, compulsory ship registration generally applies above 20 gross tonnage, following changes introduced in 2018. Most family sailing yachts and small motorboats fall below that threshold.

Gross tonnage, or GT, measures enclosed volume, not the boat's weight. A yacht weighing 20 tonnes does not necessarily measure 20 GT. Use the vessel's tonnage documentation, rather than its displacement or an estimate based on length.

Older Danish guidance describes voluntary registration for boats between 5 and 20 GT and excludes boats under 5 GT from the Register of Shipping. Treat this as historical guidance when buying a boat. Confirm its existing registration status and the current procedure with the Danish Maritime Authority, especially if it is near the threshold.

Check outstanding mortgages before paying. Registered mortgage rights can survive a sale. For smaller boats, relevant security may appear in Personbogen, but a clear search alone does not establish that every possible debt has been disclosed.

UK: coastal and inland registration

A privately owned pleasure boat used in UK coastal waters generally does not need national registration simply because it exceeds a particular length. Inland navigation authorities can, however, require a licence or registration for their waterways, even for small craft.

• Part III, the Small Ships Register: a simplified nationality register for eligible pleasure vessels under 24 metres. Owner eligibility also matters.
• Part I: the fuller register, commonly used to record ownership and registered mortgages.
• Inland waterways: check the relevant authority's requirements for licensing, insurance and any Boat Safety Scheme certificate.

For foreign cruising, arrange suitable nationality documentation before departure. Small Ships Register registration does not provide the same ownership and mortgage record as Part I. Neither replaces evidence of VAT or customs status.

USA: motors often trigger registration

In the USA, state rules usually matter first. Most states require registration or numbering for motor-powered recreational boats, including small tenders fitted with an outboard or electric trolling motor. Requirements for unpowered sailing boats, dinghies and kayaks vary. Some states apply length thresholds.