Page 129 - Introduction to Naval Architecture

116 SEAKEEPING

Bilge keels
Of the passive systems, bilge keels are the most popular and are fitted
to the great majority of ships. They are effectively plates projecting
from the turn of bilge and extending over the middle half to two-thirds
of the ship's length. To avoid damage they do not normally protrude
beyond the ship's side or keel lines, but they need to penetrate the
boundary layer around the hull. They cause a body of water to move
with the ship and create turbulence thus dampening the motion and
causing an increase in period and reduction in amplitude.
Although relatively small in dimension the bilge keels have large
levers about the rolling axis and the forces on them produce a large
moment opposing the rolling. They can produce a reduction in roll
amplitude of more than a third. Their effect is generally enhanced by
ahead speed. They are aligned with the flow of water past the hull in
still water to reduce their drag in that state. When the ship is rolling the
drag will increase and slow the ship a little.

Figure 6.6 Bilge keel