Ship Squat Measurements Using GPS at Charleston Harbor

Global positioning system (GPS) sensors were placed on ships in Charleston Harbor to collect ship motion data. This paper focuses on the vertical motion "squat" that is the downward displacement of a ship while underway which is a function of ship draft, hull configuration and speed, and channel depth, width, and cross-sectional shape. The primary vessels of interest were containerships and bulk carriers whose length exceed 213 m (700 ft). A total of 18 ships were tracked, but only 12 were analyzed because of data collection problems. Environmental measurements consisted of wave, current, and tide levels during the period of the study. Water level information is particularly important for accurate calculation of squat. The large tidal variation (up to 1.8 m) at Charleston Harbor makes an accurate determination of the water surface elevation along the path of the ship very difficult. The procedure for obtaining these measurements required that a 3-person survey crew board the vessel and install 3 GPS antennas near the bow, and the port and starboard bridge wings. Comparisons of measured squat with three empirical predictions are presented and discussed. The evaluation of squat using GPS is a significant improvement over previous techniques. GPS not only provides precise vertical and horizontal position, but also an accurate measurement of ship speed that has been a limitation in previous studies.