Ship Roll Stability
Question:
I recently was on a cruise ship which was fifty feet from
waterline to upper deck and drew only 20 feet of water. What
keeps it right side up?
Answer:
Here is the deal on keeping ships upright. Ships are built with
as much heavy stuff low in the hull as practical but most of the
weight of most ships is above the waterline. The thing that keeps
them right side up is the same thing that makes rocking chairs
possible. See if you can find an old fashioned rocking chair
around the house and conduct this little experiment. If you can
not find one, just play along using you imagination and
experience.
Everything, all solid objects anyway including ships and
rocking chairs, have what is called a center of gravity. That is
the point where gravity seems to exert its influence on the
object. To locate the center of gravity of a rocking chair, get
down on the floor, sitting or on hands and knees will do, and
face the side of the unoccupied, unrocking chair. You will notice
that the rockers each contact the floor at a single point. The
center of gravity of the chair must lie directly above that
point. If it were off one way or the other, gravity pulling the
chair down and the floor pushing the chair up would tip the chair
forward or back.
Now
reach out and rock the chair slightly. Notice what happens to the
point of contact between the rockers and the floor. It moves back
when the chair rocks back and forward when the chair rocks
forward. The rockers are shaped so that when the chair tips back,
moving its center of gravity backwards, the point of support,
where the rocker touches the floor, moves back even farther.
Because the center of support always stays outside the center of
gravity, the down pull of gravity and the up lift of the floor
combine to tend to straighten the chair up whenever it is
disturbed from its resting state.
Like a rocking chair, a ship has a point of support called its
center of bouyancy. When you place a steel ship in the water it
will sink. Sink that is until the weight of the water displaced
by the ship equals the weight of the ship and everything in it.
The geometric center of that displaced volume is the ship's
center of bouyancy. That is the point where the bouyant force of
the water seems to act on the ship. It is similar to the point
where the rocker rests on the floor in the rocking chair
example.
Now if you, or some higher power, reach out and rock the ship
as we did the rocking chair, the shape of the displaced volume
will change to conform to the shape of the ship's now tipped
hull. The hull is designed so that the center of bouyancy moves
father than the center of gravity when tipping takes place. The
combination of the upward force of bouyancy at the center of
bouyancy and the downward force of gravity at the center of
gravity tends to tip the ship back upright again.