Nature doesn't go in for 10s, except for the number of fingers on an ape's paw, so he won't strain his brain trying to figure out a complicated concept like 12!
- From Saga of a Wayward Sailor by Tristan Jones
Dozens of Reasons to use Feet-Inches-Eighths
As you may have heard, most of the civilized world now conducts its business in Metric, a system based on powers of ten. Aside from realms of economy (our money is metric) and science, the USofA is, by and large, one of the last holdouts of the Imperial system, based on multiples of twelve.
Unlike ol' Tristan, I'm a fan of Metric for many tasks, particularly where logorithms or scaling are involved. But, unless one obtains sheet materials fully based on Metric dimensions, Imperial gets my vote.
TriloBoats are based on even fractions of sheet materials standardized on imperial dimensions. Not only are these the only ones available to me, they happen to work out very well for a wide range of boating needs.
Trying to force Metric onto such sheets yields non-mnemonic numbers of fantastic length. For instance, a 4ft x 8ft sheet is 1219mm x 2438mm... and that's not an exact fit.
Ten factors (divides evenly) by the following numbers: 1, 2, 5 and 10.
Twelve factors by the following: 1, 2, 3, 4, 6 and 12. This series usefully implies 8 and 9, as well.
The difference is immense, giving 'zounds more ways to multiply or divide. For example, a four foot (48 inch), width of plywood can be divided into even 1/2s, 1/3rds, 1/4ths, 1/6ths, 1/8ths or 1/12ths without mental strain.
The tricky part comes in trying to add or subtract fractional values. Before they must be converted to a 'common denominator', and the answer likely 'reduced' to 'mixed fractions'. Error prone process, this. Complicated by notational conventions that run numbers together, or use 'tick marks' for feet and inches, which get mixed in to the number, or lost in the confusion.
Fortunately, boatbuilders came up with a system called Feet-Inches-Eighths (I'll call it FIE, from here on out).
In FIE, a number, such as five feet, three inches and seven eighths (5'7 3/8'', in tick notation) comes out as 5-3-7. Simple!
If you're the finnicky sort, you can adjust up or down by one or two following signs (+ or -)... the first one signifies plus or minus 1/16th of an inch, and the second use 1/32nd of an inch. I've only ever rarely used even the first in TriloBoats.
Addition involves carrying:
9-12-9 (Our initial, unadjusted answer... 9ft - 12in - 9 eighths)
9-13-1 (Take eight eigths and SHAZAM... they become an inch in the middle column)
10-1-1 (Take twelve inches, and SHAZAM... they become a foot in the left column :Answer!).
Subtraction involves borrowing:
4 -0-0 (Don't like the looks of this... let's borrow feet...)
3-12-0 (Still don't like it... lets borrow an inch.)
3-11-8 (There... still the same number we started with, but now we can get to work!)
- 2 -3 -4
1 -8-4 (Answer: 1ft - 8in - 4eighths)
[Thanks to astute reader, ROB, for correcting a sloppy mistake on my part!]
Multiplication and division, should you ever need them, are little different (I won't go into them, here... just remember that they are essentially repeated addition and subtraction, respectively).
It takes a little practice to wrap one's head around FIE, but the payoff is considerable. No common denominators (they're all eighths). The notation is straightforward.
If you are truly lazy (as I mostly am), consider skipping feet, and work entirely in Inches-Eighths. That's one column less borrowing and carrying.
So... we don't have to use FIE for building TriloBoats, but I sure recommend it. The hours or less it will take you to become proficient in this simple system will quickly pay for themselves...
With the very first big mistake we avoid.
For a full treatment of FIE, and a generally great book on boatbuilding, see Go Build Your Own Boat, by Harold 'Dynamite' Payson.