Almost nobody will ever measure your boat, even roughly; but almost every marina will charge you according to its length. That "length" is determined by one of two methods: what you put on the marina registration
card when you sign in; or what your yacht registration
says. The latter is simply copied from the builder's certificate, issued when the boat is built, and as you have already discovered, this "length" may have little relationship with reality. In general, I think, builders overestimate the length of the boat, because bigger boats command higher prices.
Since you're the builder
, I think you can simply state a length on your application for registration. I wouldn't worry too much about accuracy, given that nobody else does.
If you're going to keep your boat in a marina very much, I'd say you should estimate your boat's length conservatively. Definitely keep it under 50 feet, because that's a common point for price
changes; and if you're ever going to be in Europe
, maybe keep it under 15 meters (48.75 feet) -- though at some point you have your conscience to deal with.
The one problem with underestimating the length is that once you register the boat, you're stuck with that length. This may have significant impact when it comes to selling the boat, as price
is definitely correlated with hull
length. So if you underestimate your boat's length now, you might eventually lose all the money
you save at marinas!
When you try to go through the Panama
or Suez Canals, you don't have to worry about what you put on the registration -- the boat gets measured from scratch, and what they find determines what you pay.
Hope this helps.