Flipping thru a few old engineering drawing books this morning, I ran across some pictures of engineers doing some lofting. This crowd is old enough to know what that looks like. Large, full scale templates, engineers in stocking feet, ship curves, splines, and ducks, etc.. Really brought back some memories. All that stuff is done in a PC at my work these days, using CATIA.
There was a reference in the text to 'waterlines' being parallel to the keel of a ship or airplane. I've seen this term before in airplane drawings from many years ago. Much terminology in airplane engineering has roots in shipbuilding. I would imagine some or much shipbuilding terminology is rooted in architectural engineering as in Drew's post. However, 'waterline' in ships has an obvious meaning in most cases. Even given that, it also is the datum by which all decks and other features needing to be level must use.
I spent the first half of my career in the manufacturing and quality industry, mostly in machined or formed metal products for aerospace or nuclear. During that stint, I heard the term and saw on drawings, 'waterline' many times. Water's characteristic flatness and levelness has been used in engineering for about as long as engineering has been around(first sandcastles probably).
My theory is that term 'watertable' is an identifier of a datum, on a sxs frame, for design and manufacturing. If you study many sxs frames, you see that this feature and the breach face are the only viable ones that establishes a plane on most frames, that can be used for dimensioning, fixturing, etc.. When you are machining things, you need surfaces to hold the part that are meaningful in squareness, flatness, level, perpendicularity, etc.. The 'watertable' of a sxs frame is a datum used in this manner. Where the term came from and by who, is anyone's guess, IMO, but this is mine.
