PARALLELOGRAM OF FORCES

265

determining her speed of turning. Think of a round flat tub floating in
water and of a deal plank kept floating on its edge by means of a keel.
It is obviously much easier to turn the tub about a vertical axis than the

Fig. 4 —Rudder Angle 20°

Fig 5 —Rudder Angle 40C

plank. Ships are designed on lines between these extremes, and a
short, flat-bottomed, beamy ship turns quicker on her rudder than a
long, deep narrow ship.

S

r" \

v----V

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f--*-,;

+ I

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Fig. 6.
The Turning Centre of a ship when under way is situated about
one-third her length abaft the stem, and it is necessary to keep this in
mind when manoeuvring in narrow waters as the stern of the ship
sweeps round the circumference of a larger circle than the stem.
Figure 6 indicates the relative diameters of the circles, assuming for the