ERDC/CHL CHETN-IX-14
March 2004
Charleston Harbor Ship Motion
Data Collection and Squat Analysis
by Michael J. Briggs, Stephen T. Maynord,
Charles R. Nickles, and Terry N. Waller
field data collection of ship motion data using Global Positioning System (GPS) sensors in
Charleston, SC, Harbor during April 1999. Comparisons of measured squat with several empirical
predictions are presented and discussed.
Laboratory (CHL) conducted a field study at Charleston Harbor between April 16, 1999 and
April 25, 1999. Prototype data of environmental forces (waves, currents, and water levels) and ship
motions during inbound/outbound transits of the deep-draft entrance channel and interior channels
were collected. The purpose of this field study was to provide data for (a) validating physical model
measurements, (b) enhancing the ship simulator model, and (c) improving USACE design guidance
for deep-draft navigation entrance channels. The study was funded by the "Design of Deep-Draft
Coastal Navigation Entrance Channels" and the "Depth/Width Requirements for Inland Navigation
Channels" work units of the Navigation Systems Research Program.
Charleston was selected for the study because the entrance channel is relatively long and exposed to
the wave environment for much of its length. The two-way entrance channel (Figure 1) is 21 km
(70,000 ft) long by 244 m (800 ft) wide, and 12.8 m (42 ft) deep (mllw). The data from this study
were later used in an idealized physical model of the Charleston Harbor entrance channel. Ship squat
is the downward displacement of a ship while underway that is a function of ship draft, hull
configuration, and speed, and channel depth, width, and cross-sectional shape. The results of
analysis of the ship motions to determine ship squat is presented and compared to several empirical
equations.
PROJECT PLANNING: Prior to the start of the survey, CHL personnel met with the Charleston
Branch pilots and U.S. Army Engineer District, Charleston, personnel to discuss the project and
request assistance. Without the cooperation of the pilots, the project would have been logistically
impossible. The pilots agreed to allow the survey crews to ride on the pilot boat and assisted in the
selection of vessels and obtaining required permissions.
The USACE operating procedure was to board/disembark with the harbor pilot. On outbound transits
the equipment had to be taken down before the end of the channel so that the survey crew could get
off the ship with the pilot. On inbound transits the ship was generally in the channel before all the
GPS were collecting data. The inbound tracks typically were longer than the outbound tracks.
VESSELS: The primary vessels of interest were container ships and bulk carriers whose length
exceeded 213 m (700 ft). A total of 18 ships were tracked. Their origin (outbound) or destinations
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