Page 130 - Introduction to Naval Architecture

SEAKEEPING 117

Passive tanks
These use the movement of water in specially designed tanks to oppose
the rolling motion. The tank is U-shaped and water moves from one
side to the other and then back as the ship inclines first one way and
then the other. Because of the throttling effect of the relatively narrow
lower limb of the U joining the two sides of the tank, the movement of
water can be made to lag behind the ship movements. By adjusting the
throttling, that is by 'tuning' the tank, a lag approaching 90° can be
achieved. Unfortunately the tank can only be tuned for one frequency
of motion. This is chosen to be the ship's natural period of roll as this
is the period at which really large motions can occur. The tank will
stabilize the ship at zero speed but the effect of the tank's free surface
on stability must be allowed for.

Figure 6.7 Stabilizer fin