Is there an official way to determine the length of a boat?

For instance, I am captain of a wooden bateaux that the builder
claimed was 23' long. Measured from the very rear of the sternpost to
the very forward of the stem, it is 23' 9". Measured at the height of
the gunwale and including the stem and stern posts, the boat is 23'
0'. Exclusive of the stem and stern posts, the boat is 22' 6". The
typical 'fullen laden' water line is 21', while the length along the
bottom (it is flat bottom, so has no keel), is 19' 6".

So, my questions a (a) when telling someone the length of the boat,
which number do I use, and (b) has there been any standard on this
over time?

Thanks much.

- David
David Manthey

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