Horizontal pressures on the sides of a ship cancel
forces acting in opposite directions (fig. 12-17). The
vertical pressure may be regarded as a single
force--the force of buoyancy acting vertically upward
through the CENTER OF BUOYANCY (B).
KB - FEET
Figure 12-19. Curve of center of buoyancy above base.
Figure 12-17. Relationship of the forces of buoyancy and
To read KB when the draft is known, start at the
proper value of the draft on the scale at the left
When a ship is at rest in calm water, the forces of
(fig. 12-19) and proceed horizontally to the curve.
buoyancy (B) and gravity (G) are equal and lie in the
Then drop vertically downward to the baseline (KB).
same vertical line, as shown in figure 12-17. The center
of buoyancy, being the geometric center of the ship's
Thus, if our ship were floating at a mean draft of
underwater body, lies on the centerline and usually
19 feet, the KB found from the chart would be
near the midship section, and its vertical height is
approximately 11 feet.
usually a little more than half the draft. As the draft
INCREASES, B rises with respect to the keel.
Figure 12-18 shows how different drafts will create
different values of the HEIGHT OF THE CENTER OF
A ship may be disturbed from rest by conditions
BUOYANCY FROM THE KEEL (KB). A series of
which tend to make it heel over to an angle. These
values for KB (the center of buoyancy from the keel) is
conditions include such things as wave action, wind
obtained and these values are plotted on a curve to
pressures, turning forces when the rudder is put over,
show KB versus draft. Figure 12-19 shows an example
recoil of gunfire, impact of a collision or enemy hit,
of a KB curve for a warship.
shifting of weights on board, and addition of off-center
weights. These conditions exert heeling moments on
the ship that may be temporary or continuous.
When a disturbing force exerts an inclining
moment on a ship, there is a change in the shape of the
ship's underwater body. The underwater volume is
24 FOOT WATERLINE
relocated, its bulk being shifted in the direction of the
20 FOOT WATERLINE
16 FOOT WATERLINE
heel. This condition causes the center of buoyancy (B)
to leave the ship's centerline and shift in the direction
of the heel. (The center of buoyancy moves to the
geometric center of the new underwater body.) As a
result, the lines of action of the forces of buoyancy and
gravity separate and in doing so exert a MOMENT on
the ship. This moment tends to restore the ship to an
Figure 12-18. Successive centers of buoyancy (B) for different