The immersed cross-sectional area of the ship at midships, *A*m , can be

determined from:

(14)

EQUATION:

Values of the midship coefficient, *C*m , are provided in the Ships Characteristics

Database (WATERS TOOLBOX) for DOD ships.

The above methods for determining the transverse current force are

recommended for normal design conditions with moderate current speeds of 1.5 m/s

(2.9 knots or 4.9 ft/sec) or less and in relatively wide channels and harbors (see Seelig

et al., 1992).

If the vessel is moored broadside in currents greater than 1.5 m/s (2.9 knots or

4.9 ft/sec), then scale model laboratory data show that there can be significant vessel

heel/roll, which effectively increases the drag force on the vessel. In some model tests

in shallow water and at high current speeds this effect was so pronounced that the

model ship capsized. Mooring a vessel broadside in a high current should be avoided,

if possible.

Scale physical model tests show that a vessel moored broadside in a restricted

channel has increased current forces. This is because the vessel decreases the

effective flow area of a restricted channel, which causes the current speed and current

force to increase.

For specialized cases where:

(1)

vessels are moored in current of 1.5 m/s

(3 knots or 5 ft/sec) or more, and/or

(2)

for vessels moored in restricted channels

then the designer should contact NFESC.

Recent full-scale measurements with a floating drydock show the transverse current

force equations should also be used to compute the longitudinal drag forces for blocky

vessels.

EXAMPLE: Find the current force on an FFG-7 vessel produced by a current of θc=90

degrees to the ship centerline with a speed of 1.5 m/s (2.9 knots or 4.9 ft/sec) in salt

water for a given ship draft. At the mooring location, the harbor has a cross-sectional

area much larger than the submerged ship longitudinal area, *L*wL * *T *.

SOLUTION: Dimensions and characteristics of this vessel are summarized in the lower

right portion of Figure 4-10. Transverse current drag coefficients predicted using

Equation 11 are shown on this figure as a solid bold line. Physical scale model data

86

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