The length of keel is a variable and hypothetical length used in calculating the ship's tonnage, and is not of help in scaling drawings.
The molded beam is the maximum span (at the 'dead flat') across the outside of the frames.
The extreme beam is the same distance plus twice the outer planking thickness.
The depth in hold is usually the distance from the underside of the lower deck beam at the centerline to the upper side of the limber strake. In contemporary drawings the latter is indicated with a hatched line.
Be aware that the drawings in any Anatomy of the Ship are not necessarily accurate: for instance N. Roger Cole points out that in the volume on Alert, the rake of the stern post is at a different angle on each of several drawings!
If you can obtain National Maritime Museum drawings of Beagle, you will be
further ahead in accuracy, and just use the 'Anatomy' book as a secondary
source for a model that you will be investing a considerable amount of time in.
( I assume that you wish to build Beagle of 1820, Darwin's exploration ship.
There seems to be only plans of ships of the same class in the NMM listings,
but you should inquire as to what they have.)
{David Antscher}
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