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Dave and Anke
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Saturday, February 8, 2014

Dozens of Reasons to Use Feet-Inches-Eighths

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. 

And worse:

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:

+  4-9-2
    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.


  1. Very well said! This is coming from a Frenchman who had metric rammed down his throat: I don't like metric. I particulierely dislike going to the wood place when i want plywood in 8' x4', and the bloke there sniffily say, oh you mean 1219 x 2438....I feel like smacking them, this is England you know. The only time where I think its easier than imperial is for very small measurements such as below 10mm, and in tenths of mm. For everything else, feet and inches is what I use. The boat building measurements you talk about are great to use. They are plenty accurate enough. I've no patience for for those people who go on endlessly about accuracy of boat building Down to the mm! Yep, grumpy old' man speaking!!

  2. Hi Joel,

    Thanks for sharing your perspective! I had an inkling that 4ft x8ft is an international plywood standard, but had never heard whether feet-inches-eighths had made it off our shores.

    RE Accuracy - All well and good, but it is one of those manias that keep good boats from completion. Modern, gap-filling adhesives have taken the last smidgeon of practical advantage from over-precision.

    Git 'er done got 'er did!

    Dave Z

  3. Hi Dave,

    I'm from metric land so what you're doing here doesn't come naturally to me. In your subtraction example you began with 4-0-0 and after you'd borrowed a foot you ended up with 3-11-0. Shouldn't you have had 3-12-0?



    1. Hi Rob,

      Right you are! I'll go back and correct that. I started the example, borrowing both columns in one step (3-11-8)... when I thought I'd break it into two for exemplary purposes, I screwed up.

      Not the good example I meant to set! 8)

      Thanks for pointing it out!

      Dave Z

  4. I think you confuse arbitrary units with ratios... I am totally with you as regards building using materials out of modern manufacturing processes with minimal effort and waste...but at the end of the day a base 10 math is far more convenient for technical work. Of course, technical work is what put us in the current somewhat dicey global situation, so maybe I will slink back to lurkdom...

    1. Hi SG,

      I think we agree. As I say, I'm a metric FAN, for most things. Engineers actually use decimal fractions of an inch (to further complicate things!).

      Part of the back story is the concept of radix (aka, base). I won't go into it, but essentially, the arbitrary part is the answer to the question "what is the value of 10". In 'base 12', we'd have two, extra digits, twelve would be expressed '10', and we could still factor with a lot of options.

      For we end-users, the units are arbitrary, the radix is a given (base 10), and FIE is merely practical (when dealing with standards built on one set of arbitrary units).

      You may be right about technical work (in all its many manifestations) leaving us on our limb. For when the bough breaks, though, I vote for picking up tools, materials and techniques!

      Dave Z

    2. Sometime ago, that is to say, back in the day, an author in WER (Whole Earth Review) wrote a piece on this very matter, coming much to the same conclusion... and about boat-building, too. I'm glad to see this issue come up again.

    3. Sometime ago, that is to say, back in the day, an author in WER (Whole Earth Review) wrote a piece on this very matter, coming much to the same conclusion... and about boat-building, too. I'm glad to see this issue come up again.

  5. Posted on behalf of TOM:

    As a Drywall man, we work in inches and 1/8's all the time, no feet, just inches,and have for decades.

    The reason being is that you are constantly communicating with the cut man. A lot of the times on a noisy construction site. If the cutter had to listen so well as to have to hear 1/8's 1/4's 1/2's and play with feet, its just too much.

    So, all numbers are inches and 1/8's run off all at once, like 1234 would mean 123 and a half. And it would be called to you as, one, two, three, four, not one hundred and twenty three and four eighths.

    It was fun seeing it broke down in a way that can be written !

    Most carpenters thought we were idiots and just didn't know how to read tape measures when they heard us telling each other numbers.


    1. Hi Tom,

      I like that system. We'll give it a try, and add in an 'answer-back' (number repeated by person receiving) to double check.

      Any jobsite can establish a system that works for them, so long as all are agreed and consistency is encouraged. And the less words or symbols involved, the better!