NOMOGRAPHS OF CVN 68 MOTION IN SHALLOW WATER
Shallow water motion transfer functions where developed for the CVN 68 class ship to aid
in predicting the ship's underkeel clearance for a variety of different entrance channels. The
motion transfer functions were validated by model tests conducted at the US Army Corps of
Engineers Waterways Experiment Station.
The nomographs of CVN 68 motion in shallow water were developed by combining the
shallow water motion transfer functions with a variety of wave and ship operating conditions.
The waves used in calculating ship motions were developed to simulate the shallow water
environments. The modal wave periods ranged from 6 to 14 seconds and the significant wave
height ranged from 1 to 10 feet. The wave energy was spread using the idealized JONSWAP
spectrum which is consistent with fetch limited conditions generally found in shallow water, and
the energy was spread +/- 90o to simulate shortcrested seas.
The ship operating conditions used for these nomographs were the following. The ship
speeds were 6, 10, and 14 knots which covers most transit conditions for CVN 68 class ships.
The ship-to-wave heading on the nomographs are head, bow, beam, quartering and following
seas. Defining these headings, head seas are directly at the bow of the ship, bow seas are 45o off
the bow, beam seas are directly at the beam or side of the ship, quartering seas are 45o off the
stern, and following seas are directly off the stem.
The vertical motion and velocity at the bow and stern of the ship is calculated for each of
these conditions. The extreme motion expected in 100 transits is then calculated from the vertical
motion and velocity using a statistical formula generated by Ochi (1973). The largest resulting
vertical motion is then used in the nomographs.
Ochi, M.K., "On Prediction of Extreme Values," Journal of Ship Research, Vol. 17 (1973).