Page 131 - Introduction to Naval Architecture

118 SEAKEEPING

Active fins
This is the most common of the active systems. One or more pairs of
stabilizing fins are fitted. They are caused to move by an actuating
system in response to signals based on a gyroscopic measurement of roll
motions. They are relatively small although projecting out further than
the bilge keels. The whole fin may move or one part may be fixed and
the after section move. A flap on the trailing edge may be used to
enhance the lift force generated. The fins may permanendy protrude
from the bilge or may, at the expense of some complication, be
retractable, Figure 6.7.
The lift force on the fin is proportional to the square of the ship's
speed. At low speed they will have litde effect although the control
system can adjust the amplitude of the fin movement to take account of
speed, using larger fin angles at low speed.

Active tank
This is similar in principle to the passive tank system but the movement
of water is controlled by pumps or by the air pressure above the water
surface. The tanks either side of the ship may be connected by a lower
limb or two separate tanks can be used. Figure 6.8 shows a system in
which the air pressure above the water on the two sides is controlled to
'tune* the system. The air duct contains valves operated by a roll
sensing device. The system can be tuned for more than one frequency.
As with the passive system it can stabilize at zero ship speed. It does not
require any projections outside the hull.
The capacity of the stabilization system is usually quoted in terms of
the steady heel angle it can produce with the ship underway in still
water. This is then checked during trials. It is possible to use modern
theories to specify performance in waves but this would be difficult to
check contractually.

SUMMARY

It has been shown that a ship's motions in irregular ocean waves can be
synthesized from its motions in regular waves. Roll, pitch and heave
responses in regular waves have been evaluated and the effects of
added mass and damping discussed. The energy spectrum has been
shown to be a powerful tool in the study of motions as it was in the study
of waves. Factors limiting a ship's seakeeping capabilities, including the
degradation of human performance, have been discussed and it has
been seen how they can be combined to give an overall assessment of
the probability that a ship will be able to undertake its intended