UFC 4-159-03
3 October 2005
The immersed cross-sectional area of the ship at midships, Am , can be
determined from:
Am = Cm * B * T
(14)
EQUATION:
Values of the midship coefficient, Cm , 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, LwL * 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
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