Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

xx

happen to fall exactly on even powers or roots,

such as square or cube roots. In the case of

Formula 24-4, the data show that chain weight

does not vary as an even square or cube, but

as diameter in inches to the 1.858 power. This

can be rewritten a number of ways:

Not only is the 1.858 exponent easier to

enter into a calculator, it’s easier to write by

hand or to type as well.

Try it yourself. Take your calculator and

punch in the number for chain diameter.

Then hit the exponentiation key (usually the

X

y

key). Now enter the exponent, and finally

press the equal key. That’s it; there’s your

answer. For example, if you raise a chain

diameter of 0.25 inch to the 1.858 power, the

answer will be 0.076, while a chain diameter

of 0.5 inch raised to the 1.858 power will

equal 0.275.

THE ENGLISH SYSTEM

OF MEASURE AND THE

METRIC SYSTEM

Throughout this book I refer to Englishunits

and to metricunits. These are the terms most

often used in informal conversation in the

United States and are easily understood.

Though some fault the term English units, it

is correct usage and refers specifically to the

English engineering system of unitsor to the

somewhat different English gravitational

system of units, which employs the unit of

mass known as the slug(see sidebar page 183),

and so is internally more consistent than the

English engineering system. Since much of

the world has switched to the metric system,

these “English units” are also less specifically

called United States customary unitsor U.S.

customary units. Whatever they are called,

this is the foot/pound/second units system. It

is similar to the British imperial system of

unitsbut is not the same. For instance, impe-

rial units use imperial gallons, not U.S. gal-

lons, as well as units of weight such as the

hundredweightand the stone, which are not

found in U.S. customary units. Accordingly, it

is incorrect to refer to the English units in this

book as imperial measure or imperial units.

All liquid volumes in this book are based on

U.S. gallons.

The metric system refers to one of

two systems of units—the cgsand the mks

systems. The cgs system is largely employed

by chemists and physicists. Its base units are

the centimeter, gram, and second. Most people

and most engineers use the SI units system

or mks system (Système International

d’Unitésor International System of Units).

Its base units are meters, kilograms, and

seconds. Where the term “metric units” or

“metric” is used in this book, it refers to the

SI (mks) system. There are two exceptions in

my usage:

• The fundamental unit for temperature
  in the SI system is degrees Kelvin. This
  is virtually never employed in everyday
  work (though it is critical for some
  engineering calculations). Accordingly, I
  have used degrees centigrade (C°, also
  called degrees Celsius) for metric tem-
  perature unless otherwise noted.


• Also, the basic unit for angle in the SI
  system is the radian, not the more com-
  mon degree (°). I have used angles in
  degrees throughout.

Author’s Notes

(chain dia. in.) is the same as

(chai

1.858

( nn dia. in.) (chain dia. in.)

is

(^858) 1,000
× )
tthesameas
(chain dia. in.)×1,000(chain dia.iin.)^858