We are authorized tonnage measurers for UK, Jersey, Guernsey, Gibraltar, Bahamas, BVIs, Cook Islands and many other flags.
Tonnage measurement involves taking the boat's dimensions, and identifying it with a unique marker (such as hull number and/or engine numbers). It is part of the registration process for most reputable flag states.
We measure vessels up to 24m loadline length (which can be up to around 30m in length overall). If you'd like to understand the difference, the US Coastguard has an explanation here.