For professional mariners, understanding safe speed raises many questions that are not solely the ship navigator’s concern. One is how the officer of the deck can tell if the ship is squatting. Unfortunately, surface warfare officers (SWOs) are hard-pressed to find a resource that adequately addresses squatting in theory and practice. The primary shiphandling reference used throughout the SWO training pipeline is retired Captain James A. Barber Jr.’s Naval Shiphandler’s Guide, which exclusively focuses on the Arleigh Burke–class guided-missile destroyers without mention of other ship classes, including the littoral combat ships (LCSs), which can operate at high speeds in shallow water.
Moreover, Barber’s primary reference is a 2002 Proceedings article that includes two significant and potentially dangerous misconceptions that require correction.1 The record must be revised with practical tips for a bridge watch team to identify if their ship is squatting.
Recognizing Squatting
As a ship transits, pressure from water being pushed by the bow builds, causing water to flow around and under the hull. This phenomenon is negligible in the open ocean as water flow is virtually unrestricted. In shallow water, where the risk of grounding is a major concern, the flow becomes hampered by bathymetry and the ship’s hull. As “the draft in relation to the available depth of water” increases, the speed of the flow of the water underneath the keel increases, and by Bernoulli’s theorem, creates an area of low pressure underneath the keel (see Figure 1). This low-pressure area beneath the keel effectively increases draft, either uniformly (sinkage) or by the bow riding higher than the stern as if trimmed (squat).2
For officers of the deck and bridge watch teams, six observable squat indicators include:
1. Squatting causes a ship to slow, so the ordered and observed speed will diverge.3 Ships should take caution not to mistake this mismatch between observed and ordered speed as drift when driving into a current.
2. Ship maneuverability will decrease with squat, in particular turning diameter.4
3. The ship’s wake moves back toward amidships and the angle of the wake becomes more perpendicular.5
4. The bow will pitch giving the appearance that the ship is trimmed, but only settles when slowing. Maintaining an increased pitch angle is a key indicator of squat for the Freedom-variant LCS because, as the LCS increases speed, its hull is hydrodynamically lifted on its own bow wave, called being on “plane.”
5. Mud becomes unexpectedly visible around the ship’s hull.6
6. A relatively large transom wave (“rooster tail”) forms.7
Ship Squat Factors
The most influential factor of ship squat is speed through the water. This contradicts the Naval Shiphandler’s Guide, which quotes “speed made good” as a squat factor.8 This could create a potentially dangerous situation, as tidal current can cause a significant mismatch between these two values. The hull shape below the waterline also affects ship squat and is described by a factor known as the block coefficient.
Environmental factors of squat are water depth and width of the waterway in which the ship is operating. Depending on the width and depth of the waterway, flow around a ship can also be restricted and increase squat.
Calculating Squat
The method shown here is derived from Dr. Barrass.9 These values are accurate when the ship is operating in water where the depth-to-draft quotient is between 1.1 and 1.4. The equation also assumes a transit in which flow around the hull is unrestricted.
The block coefficients shown in Figure 2 come from the values used in the navigation, seamanship, and shiphandling trainer (NSST) simulators.10 For the Arleigh Burke–class destroyers, this value differs from the 0.38 value given in the 2002 Proceedings article and referenced in Naval Shiphandler’s Guide.11 Figure 3 shows the equation for squat in open water.12 Using these equations to build a squat table for quick reference in the pilot card will allow all bridge watch teams quick reference to these figures.
The dangers of squat are real. Numerous groundings have occurred because bridge teams have misunderstood squat, perhaps most illustratively the Queen Elizabeth II near Martha’s Vineyard in 2009.13 Squat confusion also reportedly prevented the USS Antietam (CG-54) commanding officer from moving out of shoal water prior to its 2017 grounding. Correcting the available naval literature on squatting is necessary to properly equip bridge watchstanders. The Navy should conduct further research to understand how squat affects ships in common waterways. All Navy SWOs must be competent mariners who understand the forces acting on their ships.
1. Lieutenant Angus N. P Essenhigh, RN, and Commander Michael T. Franken, USN, “Handling the Arleigh Burkes—Part Three.,” U.S. Naval Institute Proceedings 128, no. 6 (June 2002).
2. Russell Sydnor Crenshaw, Naval Shiphandling (Annapolis, MD: Naval Institute Press, 1976).
3. C. B. Barrass and D. R. Derrett, Ship Stability for Masters and Mates 7th ed. (Boston, MA: Butterworth-Heinemann, 2012).
4. Barrass and Derrett, Ship Stability for Masters and Mates, 2013.
5. Crenshaw, Naval Shiphandling, 183; Nathaniel Bowditch, The American Practical Navigator: An Epitome of Navigation (Washington, DC: National Imagery and Mapping Agency, 2002), Section 818.
6. Michael J. Briggs, Marc Vantorre, Klemens Uliczka, and Pierre Debaillon, “Prediction of Squat for Underkeel Clearance,” in Handbook of Coastal and Ocean Engineering (Singapore: World Scientific, 2017), 1029–80.
7. CAPT James Alden Barber Jr., USN (Ret), Naval Shiphandler’s Guide (Annapolis, MD: Naval Institute Press, 2005).
8. Essenhigh and Franken, “Handling the Arleigh Burkes—Part Three;” Barber, Naval Shiphandler’s Guide, 2005.
9. Barrass and Derrett, Ship Stability for Masters and Mates.
10. U.S. Navy, “NSST Special Evolutions Training Instructional Module-Math for the OOD II,” NSS Training Program, 8 February 2017.
11. Essenhigh and Franken, “Handling the Arleigh Burkes—Part Three.”
12. Eric Charles Tupper, Introduction to Naval Architecture 5th ed. (Boston, MA: Butterworth-Heinemann, 2013).
13. Nick Perugini, “Grounding of the Queen Elizabeth 2 (Response),” Hydro International, 26 June 2009.