Air at Sea Level at 20oC (68oF)*

PROPERTY

SI SYSTEM

ENGLISH OR

INCH-POUND

SYSTEM

1.221 kg/m3

0.00237 slug/ft3

Mass density, ρa

11.978 newton/m3

0.07625 lbf/ft3

Weight density, γa

1.50E-5 m2/sec

1.615E-4 ft2/sec

Kinematic viscosity, ν

*

Note that humidity and even heavy rain has relatively

little effect on the engineering properties of air (personal communication with the

National Weather Service, 1996)

4-3

for a ship:

X - Direction parallel with the ships

Longitudinal axis

Y - Direction perpendicular to a vertical plane

through the ship's longitudinal axis

Z - Direction perpendicular to a plane formed by

the "X" and "Y" axes

There are six principal coordinate directions for a ship:

Surge - In the "X"-direction

Sway - In the "Y"-direction

Heave - In the "Z"-direction

Roll - Angular about the "X"-axis

Pitch - Angular about the "Y"-axis

Yaw - Angular about the "Z"-axis

Of primary interest are: (1) forces in the surge and sway directions in the

"X-Y" plane, and (2) moment in the yaw direction about the "Z"-axis. Ship motions occur

about the center of gravity of the ship.

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