Comes from the days of steam, when it was important to know the carrying capacity of coal. Hence depth is related to coal capacity, length is measured from the fore part of the rudder stock to the stem, so only coincidentally related to WL length on some boats, nowhere close on most. At least that is how it is here in Canada and since the US and Canadian rules were created back in the mists (clouds of coal smoke) of time, they have similar origins.
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Keith
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