How many people can actually sleep aboard?

Far fewer than the number in the advertisement. Ask for the overnight berth count specifically, because that is a different figure from the one most operators lead with.

A vessel commonly carries three different numbers. The day passenger limit, the offshore limit which is often lower, and the overnight berths which are usually much lower again. A boat advertised for 30 people that sleeps 6 is quoting the day limit and the berth count, and both are true.

Where the numbers come from

The Survey: see glossary">Certificate of Survey belongs to the vessel and sets the maximum people aboard, the operating area, and often a minimum crew. It is not marketing. If the certificate says twelve, twelve is the hard ceiling regardless of how many bunks you can find. Safety and commercial vessel rules explains the certificates.

What to ask instead of capacity

• How many double cabins, and how many singles or bunks.
• How many heads, and whether any cabin has its own.
• Whether any berth is a saloon convertible, which means someone sleeps in the walkway.
• Whether any berth is a forepeak, which is the worst berth at anchor in a swell.
• The maximum overnight number on the certificate, not the day number.

Do the honest maths

A four-cabin catamaran sleeps eight in doubles, and that is comfortable. The same boat sleeps ten or twelve if you use the saloon and the forepeaks, and by night three that is a different holiday. For a week aboard, count couples and cabins and stop there.

Catamarans generally give more usable cabin space and separation than a monohull of the same length, and they do not heel, which matters if anyone is unsure about sleeping at sea. Choosing a vessel compares layouts.

If the group is large, two smaller boats sailing together often works better than one boat at its limit. Families and groups covers that, and the operator directory shows which fleets have enough hulls to make it possible.