In Titanic's day blueprints were not used to construct ships or even boats. The reason is quite simple. A ship must be drawn in three dimensions. Any given point must line up with that same location on the plan, the elevation and the transverse views. Pencil lines are simply too "fat" to be accuate, so the process of "lofting" was developed. A loftsman would take the drawings and enlarge them to full size. This was normally done on a smooth, white floor installed over the building shop -- hence the term "lofting" since it was done in the loft. In the process lines would be "ooched" one way or another in order to achieve the 3-dimensional meetings described above. If the ship was a one-off, that was it. However, if the vessel being lofted was first of a series, the loftsmen would create a "table of offsets" giving the three critical measurements taken from the full size views on the loft floor.
Something else, even parchment or vellum used in conventional drafting changes size with humidity.. This means that it is impossible to take accurate measurements off a scale drawing. This is one more reason you never...repeat, never...take exact measurements off a reduced scale drawing.
As anyone who has done a lick of carpentry knows, "12" is the magic number. It can be divided in half as well as iinto thirds and quarters with simple in-the-head mathematics. Best, there are no pesky fractions or decimals to deal with in these divisions. Until the irrational metric system began to dominate, every bloke in the shop knew the unique math around the number 12. Alas, those were the days! Actually, those are still the days in shops around the world. The number 12 still dominates. (Example: The standard size of plywood is still based around the globe on 4 feet of width and 8 feet of length.)
But, I digress. A table of offsets lists dimensions in feet and inches as well a eighth inches. Plus there are "ticks." A number on the table might read 10-2-2, which would mean "10 feet, 2 inches, and a two eighths." If the same number were written 10-2-2+ it would mean "ten feet, 2 inches, two eighths plus a 'tick' which is 1/16th of an inch. A minus sign at the end would make it "minus 1/16th of an inch.
The reason for "ticks" is human memory. Its easy enough to learn feet and inches. Keeping all fractions in 1/8th inch simplifies that part of what you have to keep in your head. And, you don't memorize anything for a "tick" which is just half an eighth. Believe me, when you have to climb down six feet off a building platform to consult the plans, and then climb back up you want to use a system that makes carrying measurements in your noggin as easy as possible.
While the system sounds crude, it is really quite workable. Wood swells across the grain so planks gain or lose in width sometimes during the working day. Steel gets longer or shorter as it heats and cools. In either case, the measurements made in the morning would not necessarily match precisely those taken at 2:00 p.m. So, there must be a bit of a "fudge factor" in all of this.
-- David G. Brown
