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Squat effect

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from Wikipedia

The squat effect is the hydrodynamic phenomenon by which a vessel moving through shallow water creates an area of reduced pressure that causes the ship to increase its draft (alternatively decrease the underkeel clearance of the vessel in marine terms) and thereby be closer to the seabed than would otherwise be expected. This phenomenon is caused by the water flow which accelerates as it passes between the hull and the seabed in confined waters, the increase in water velocity causing a resultant reduction in pressure. Squat effect from a combination of vertical sinkage and a change of trim may cause the vessel to dip towards the stern or towards the bow. This is understood to be a function of the Block coefficient of the vessel concerned, finer lined vessels Cb <0.7 squatting by the stern and vessels with a Cb >0.7 squatting by the head or bow. ^[1]

Squat effect is approximately proportional to the square of the speed of the ship. Thus, by reducing speed by half, the squat effect is reduced by a factor of four.^[2] Squat effect is usually felt more when the depth/draft ratio is less than four^[2] or when sailing close to a bank. It can lead to unexpected groundings and handling difficulties. There are indications of squat which mariners and ship pilots should be aware of such as vibration, poor helm response, shearing off course, change of trim and a change in wash.

Squat effect is included by navigators in under keel clearance calculations.^[3]

Marine incidents

[edit]

It was a cause of the 7 August 1992 grounding of the Queen Elizabeth 2 (QE2) off Cuttyhunk Island, near Martha's Vineyard. The liner's speed at the time was 24 knots (12 m/s) and the draft was 32 feet (9.8 m). The rock upon which the vessel grounded was an uncharted shoal later determined to be 34.5 feet (10.5 m), which should have given her room to spare, were it not for the "squat effect."^[4] U.S. National Transportation Safety Board investigators found that the QE2's officers significantly underestimated the amount the increase in speed would increase the ship's squat. The officers allowed for 2 feet (0.61 m) of squat in their calculations, but the NTSB concluded that squat at that speed and depth would have been between 4.5 and 8 feet (1.4 and 2.4 m).^[5]

Squat is also mentioned as a factor in the collision of the bulk carriers Tecam Sea and Federal Fuji in the port of Sorel, Quebec, in April 2000.^[1]

The third largest cruise ship in the world, MS Oasis of the Seas, used this effect to obtain an extra margin of clearance between the vessel and the Great Belt bridge, Denmark, 1 November 2009, on a voyage from the shipyard in Turku, Finland to Florida, USA.^[6] The new cruise liner passed under the bridge at 20 knots (37 km/h) in the shallow channel, giving the ship extra clearance due to a 30 cm squat.

References

[edit]

^ ^a ^b "Marine Investigation Report M00L0039" (PDF). Transportation Safety Board of Canada. 27 April 2000. Retrieved 9 October 2024.
^ ^a ^b "Navigation and Vessel Inspection Circular" (PDF). Retrieved February 11, 2008.
^ ECDIS Passage Planning and Watchkeeping. Livingston, Scotland: Witherby Publishing Group. 2023. p. 25. ISBN 9781856098168.
^ Marine Surveyors Find Uncharted Rock That May Have Damaged Hull of the QE2, New York Times, 15 August 1992
^ Marine Accident Report—Grounding of the United Kingdom Passenger Vessel RMS QUEEN ELIZABETH 2 Near Cuttyhunk Island, Vineyard Sound, Massachusetts, August 7, 1992 (NTSB/MAR-93/01), pp. 26-30. National Transportation Safety Board: 25 May 1993.
^ Wright, William, "Clearing a Landmark", Captain's Log, Day Three, Royal Caribbean at Oasis of the Seas Archived 2010-06-20 at the Wayback Machine; „Oasis of the Seas“ hat Kurs auf Fehmarn Archived 2009-11-03 at the Wayback Machine, KN-online (31 October 2009) (German)

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from Grokipedia

The squat effect, also known as ship squat, is a hydrodynamic phenomenon in naval architecture wherein a vessel experiences vertical sinkage and a reduction in under-keel clearance when moving through shallow waters, primarily due to accelerated water flow beneath the hull that decreases hydrostatic pressure according to Bernoulli's principle.^[1] This effect causes the ship to settle deeper into the water, potentially leading to grounding if not properly managed, and is most pronounced when water depth is less than 1.5 times the vessel's draft.^[2] It arises from the interaction between the ship's hull and the confined water column, where the continuity principle results in increased flow velocity under the keel, exacerbating the pressure drop.^[3] Key factors influencing the magnitude of squat include the vessel's speed, block coefficient (a measure of hull fullness), and the blockage ratio (the proportion of the channel cross-section occupied by the ship).^[2] Squat is proportional to the square of the ship's speed, meaning higher velocities amplify the effect significantly—for instance, doubling speed quadruples the squat—and it tends to be greater in confined channels like rivers or canals due to additional banking suction.^[1] Full-form ships with a block coefficient greater than 0.7 (e.g., tankers) typically experience forward trim (bow sinkage), while fine-form vessels with coefficients below 0.7 (e.g., container ships) trim aft.^[2] Other contributors include the seabed topology, initial trim or heel, and proximity to banks or other vessels, all of which can intensify the hydrodynamic forces.^[1] Predicting and mitigating squat is crucial for maritime safety, as it can reduce maneuverability by 20-30%, increase turning radii, and heighten collision risks with the seabed, particularly in restricted waters.^[3] Empirical formulas are commonly used for estimation, such as the open-water approximation Squat = (C_b × V^2) / 100, where C_b is the block coefficient and V is speed in knots (with a confinement factor of 50 instead of 100 for restricted waters), though advanced hydrodynamic models and software are preferred for precision.^[2] Mariners counteract squat by reducing speed (halving it quarters the effect), adjusting trim, or using real-time monitoring tools like echo sounders to maintain adequate under-keel clearance, ensuring safe passage in shallow drafts.^[1]

Fundamentals

Definition and Overview

The squat effect is a hydrodynamic phenomenon that occurs when a ship moves through shallow water, characterized by an increase in the vessel's draft—known as sinkage—and potentially a change in trim due to reduced hydrostatic pressure beneath the hull.^[1] This effect manifests as the ship settling deeper into the water, distinct from its stationary condition, and is particularly pronounced when the water depth is limited relative to the ship's dimensions.^[3] Sinkage represents the uniform vertical downward movement of the entire hull, reducing the overall under-keel clearance, while trim refers to a rotational adjustment in the ship's longitudinal attitude, which may result in the bow or stern dipping further.^[1]^[3] These changes arise dynamically only when the ship is underway, as opposed to static draft, which is fixed by the vessel's displacement and water density at rest.^[1] From a visual perspective, the squat effect causes the ship to appear "squatted" or lowered in the water, as if the seabed is rising toward the keel, thereby diminishing the available navigation margin and heightening the risk of grounding.^[4] This phenomenon is a critical consideration in restricted waterways, where even modest speeds can amplify the effect.^[3]

Physical Principles

The squat effect arises primarily from hydrodynamic interactions between a moving ship and the surrounding water in restricted depths, governed by fundamental fluid dynamic principles. In shallow water, the ship's hull displaces water, constricting the flow beneath and around it. According to the continuity equation, which ensures conservation of mass in incompressible flow, this constriction accelerates the water velocity in the narrowed channel under the hull compared to the undisturbed flow ahead.^[5] The increased velocity creates a low-pressure zone, as described by Bernoulli's principle, where the sum of pressure energy and kinetic energy remains constant along a streamline. Specifically, the faster-moving water under the hull reduces static pressure, causing the ship to experience a net downward force or sinkage, as the reduced pressure fails to fully support the vessel's weight.^[5]^[6] This pressure reduction is not uniform along the hull, leading to a characteristic distribution that influences both sinkage and trim. At the bow and stern, where the flow converges and diverges, higher pressures develop due to stagnation and deceleration effects, while amidships, the accelerated parallel flow maintains lower pressures. This gradient typically results in greater sinkage amidships, leading to trim by the bow for full-form ships and trim by the stern for fine-form vessels.^[5]^[2] In addition to these inviscid pressure-driven mechanisms, wave-making resistance and viscous effects contribute to squat. Wave-making resistance stems from the energy dissipated in generating surface waves, which in shallow water alters the free surface elevation and amplifies the pressure imbalance beneath the hull, increasing overall sinkage. Viscous effects, including boundary layer friction and turbulence in the constricted flow, further enhance resistance and squat by generating additional drag and eddy formations around the hull.^[5]^[7] The manifestation of squat differs between confined and unconfined waters due to varying flow constraints. In unconfined shallow water, such as open channels, squat is predominantly influenced by wave-making patterns modeled through potential flow theories, resulting in a more symmetric sinkage with moderate trim changes. In confined waters like canals, the proximity of sidewalls intensifies return currents and blockage, leading to greater velocity accelerations and pressure drops, often modeled via one-dimensional theories that emphasize longitudinal flow variations.^[5]^[8]^[6]

Influencing Factors

Ship Characteristics

The severity of the squat effect in ships is significantly influenced by the vessel's block coefficient (Cb), which measures the fullness of the hull form relative to a rectangular block of the same length, beam, and draft. A higher Cb, typical of fuller hull forms such as those found in large tankers (often around 0.85), results in greater squat because these designs displace more water and create increased flow blockage under the hull, amplifying the low-pressure zone beneath the vessel.^[2] In contrast, ships with lower Cb values, such as bulk carriers (around 0.75) or finer forms below 0.7, experience less overall sinkage but may exhibit trim changes, with squat predominantly by the stern due to the distribution of buoyancy and pressure forces.^[1]^[9] The length-to-beam ratio (L/B) also plays a key role in determining squat magnitude, with slender vessels (high L/B, often exceeding 7 for modern tankers) generally experiencing reduced squat compared to beamier designs (lower L/B). Slender hulls minimize hydrodynamic blockage and wave-making resistance in shallow water, leading to less pronounced pressure differentials along the hull length.^[10] Beamier ships, by contrast, generate greater under-keel flow acceleration due to their wider cross-section, intensifying the squat effect for equivalent speeds and drafts.^[11] The draft-to-depth ratio (d/h) further modulates squat, where deeper-drafted ships relative to the available water depth (higher d/h) amplify the phenomenon by restricting vertical flow paths and increasing the relative blockage. This intrinsic ship property interacts with external water depth to heighten sinkage risks, particularly when h/d falls below 2.5.^[2]^[12] Specific hull shape features, such as bulbous bows and stern configurations, influence local pressure distributions and thus the trim component of squat. Bulbous bows can alter bow sinkage by modifying inflow patterns, potentially reducing forward trim moments in some designs, while stern shapes—such as those with pronounced transoms or skegs—affect aft pressure, contributing to stern squat in conventional forms.^[13] Service speed and propulsion type contribute variably to squat responses, with faster service speeds (e.g., above 15 knots for merchant vessels) exacerbating the effect nonlinearly, as squat increases approximately with the square of the speed due to heightened flow velocities under the hull. Twin-screw propulsion systems, common in maneuverable vessels, often produce more uniform bodily sinkage compared to single-screw designs, which tend to induce greater stern squat from localized propeller-induced low pressure at the aft end.^[1]^[14]

Waterway Conditions

The squat effect is profoundly influenced by the water depth relative to the ship's draft, denoted as the ratio h/d, where h is the water depth and d is the draft. As h/d decreases below 1.2, squat increases non-linearly due to the acceleration of water flow beneath the hull, leading to reduced pressure and greater sinkage; critical thresholds occur around h/d = 1.1 to 1.3, where even modest speeds can result in under-keel clearances dropping to dangerous levels.^[15] In such shallow conditions, the phenomenon becomes particularly pronounced, with experimental data showing squat magnitudes up to 30% of the draft in limiting scenarios.^[16] Channel width and overall confinement further amplify squat by inducing additional flow acceleration from bank effects. In narrow channels, where the blockage factor (the ratio of the ship's submerged cross-sectional area to the channel's) exceeds 0.1, squat can increase by 20-50% compared to open water, as the restricted flow creates a stronger pressure differential around the hull.^[2] This confinement is especially severe in canals or straits with widths less than 2-3 times the ship's beam, exacerbating the risk of grounding during transit.^[15] Variations in waterway geometry, such as bends, slopes, or irregular depths, introduce asymmetric squat and induce trim changes that complicate navigation. For instance, in curved channels or areas with sloping banks, the uneven flow distribution causes differential sinkage fore and aft, potentially leading to bow or stern trim by several degrees.^[1] Similarly, abrupt depth changes along the path can generate transverse currents, further distorting the squat pattern and increasing lateral drift.^[17] Currents in the waterway modify squat by altering the ship's relative speed through the water. Following currents, which reduce relative velocity, tend to lessen squat magnitude, while opposing currents increase it by effectively raising the speed-squared component of the hydrodynamic forces.^[1] This effect is particularly relevant in tidal channels, where current speeds of 1-2 knots can shift squat by up to 25% depending on direction.^[16] Bottom topography has a subtler influence on the hydrodynamics of squat but plays a key role in associated risks. Soft or deformable seabeds minimally alter the flow patterns compared to hard bottoms, yet they heighten grounding vulnerability once squat reduces under-keel clearance.^[1] Uneven topographies, such as ridges or depressions, can induce additional trim without significantly changing overall sinkage, underscoring the need for precise bathymetric data in confined areas.^[17]

Prediction Methods

Empirical Formulas

Empirical formulas provide practical tools for mariners and engineers to estimate ship squat quickly using basic ship and waterway parameters, derived from extensive model tests and field observations. These methods prioritize simplicity for onboard calculations, focusing on key variables like speed, hull form, and water depth. The Barrass formula, developed from analyses of merchant vessel data, estimates maximum squat in open waters as

S = \frac{C_b V^2}{100}

, where

S

is squat in meters,

C_b

is the block coefficient (dimensionless, typically 0.7–0.85 for full-form ships), and

V

is speed in knots. For confined waters, a factor of 2 is applied:

S = \frac{2 C_b V^2}{100}

. This relation accounts for hydrodynamic sinkage in relatively shallow conditions (h/d ≈ 1.1–1.4) and is applicable for speeds below critical Froude numbers. Variants incorporate blockage factor

S_b = \frac{A_s}{A_c}

, such as

S = 2.08 C_b^{2/3} V^2 S_b^{2/3} / 30

.^[18]^[2] The Huuska method (also referred to as Huuska/Guliev), offers a relation for squat and associated trim, incorporating the ship's length-to-beam ratio (L/B) and a speed-squared term adjusted by depth Froude number. The core equation is

S = 2.4 \nabla Fn_h^2 K_s

, where

\nabla

is displacement volume in cubic meters,

Fn_h = V / \sqrt{g h}

Fnh=V/gh

is the depth Froude number (with g ≈ 9.81 m/s²), and $K_s$ is a channel width correction factor (often near 1 for open waters, increasing in confinement). Developed from systematic model experiments on tankers and bulk carriers, it predicts trim by bow or stern based on L/B > 6 or < 6, respectively, aiding in dynamic stability assessments. The constant 2.4 has units of m^{-2} for dimensional consistency.^[19] The ICORELS criteria, established by the International Commission for the Reception of Large Ships, emphasize operational limits to control squat and maintain safe underkeel clearance (UKC). Guidelines recommend limiting depth Froude number $Fn_h \leq 0.7$ to ensure UKC remains at least 10–20% of draft for large vessels in approach channels. This stems from full-scale trials of VLCCs and is integrated into port design standards to prevent grounding risks at higher speeds. A related empirical speed limit is approximately $V \approx 5 \sqrt{UKC}$

m/s (about 10 knots for UKC=1 m), adjusted for vessel type.^[20] These formulas perform best at speeds up to 10–12 knots in waters with h/d ratios of 1.1–1.5, but accuracy diminishes in extremely shallow (h/d < 1.1) or highly confined waterways (blockage > 0.25), where wave-making and bank effects dominate; validation studies against model tests show errors within 20% for typical cases but recommend complementary numerical approaches for extremes.^[19]

Numerical and Experimental Approaches

Computational Fluid Dynamics (CFD) simulations provide a powerful numerical approach for predicting ship squat by solving the three-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations to model viscous flow, pressure distributions, and free-surface effects around the hull in restricted waters.^[21] These methods capture nonlinear phenomena such as wave-making and boundary layer interactions that influence sinkage and trim, using turbulence models like k-ε and volume-of-fluid techniques for the air-water interface. Commercial software such as Star-CCM+ or open-source tools like OpenFOAM are commonly employed, with dynamic fluid-body interaction modules allowing the hull to heave and pitch freely during simulations. Validation against experimental data for container ships, such as the Duisburg Test Case, shows CFD predictions of squat within 10% error for Froude numbers up to 0.515 and water depths of 1.2 times draft.^[21] For instance, in a 2010 study on inland cargo vessels, CFD simulations predicted maximum squat values of approximately 0.28 meters at full scale for speeds of 10 knots in shallow channels, aligning closely with measured resistance increases.^[22] Potential flow theory offers a faster, inviscid approximation for initial squat estimates, relying on slender-body theory or panel methods to solve Laplace's equation for the velocity potential around the hull. These approaches, assuming irrotational flow, use source distributions or boundary element methods to compute wave resistance and induced sinkage, particularly effective for preliminary assessments in open or dredged channels. Seminal developments include Tuck's 1966 linear theory, extended to nonlinear variants for better accuracy in confined geometries. Tools like ShallowFlow implement linear-2D and nonlinear-1D methods for varying canal widths, while HullWave applies double-body linearization and GL Rankine uses Rankine-source panels. A 2016 comparative study validated these against 1:75-scale KVLCC2 tanker tests in rectangular canals with width-to-beam ratios from 1.05 to 9.05, finding double-body methods consistently accurate across geometries with squat predictions within 15% of measurements at subcritical speeds.^[23] Slender-body approximations, refined in subsequent works, provide squat estimates scaling with the square of the depth Froude number, suitable for high-speed displacement vessels.^[7] Model tank experiments remain essential for validating numerical predictions and studying squat under controlled conditions, using scaled physical models in towing tanks to measure sinkage, trim, and resistance while preserving the Froude number for dynamic similarity. Scaling laws ensure that gravitational effects dominate, with model speeds adjusted as

V_m = V_p \sqrt{\lambda}

Vm=Vpλ

, where $\lambda$ is the scale factor, to replicate prototype wave patterns and squat magnitudes. Facilities like the DST or BSHC conduct tests with free-heaving and trimming models, often appending rudders and propellers, across speeds corresponding to Froude numbers of 0.2 to 0.6 and under-keel clearances of 10-20% of draft. For the DTC container ship at 1:53.3 scale in a 10-meter-wide tank, experiments revealed maximum bow squat of 0.035 meters (model scale) at 1.027 m/s, increasing nonlinearly with speed and blockage ratio.^[24] Similar tests on the KVLCC2 tanker in asymmetric channels confirmed squat amplification near banks, with measured values up to 0.02 meters at Fr=0.142 and h/T=1.2, highlighting viscous scale effects that require corrections for full-scale extrapolation.^[24] Hybrid methods integrate CFD or potential flow results with empirical corrections to enhance real-time squat prediction, particularly for non-standard configurations as recommended in PIANC guidelines for channel design. These approaches leverage high-fidelity simulations to calibrate adjustments for factors like multi-ship interactions or irregular bathymetries, then apply simplified models for operational use. For example, combining RANS simulations with blockage-based corrections from PIANC Working Group 49 yields probabilistic under-keel clearance estimates, validated against field data from rivers like the Elbe.^[25] A 2022 study on modern container ships used CFD-generated datasets to derive hybrid formulae via regression, achieving squat predictions with mean errors below 8% for speeds up to 20 knots in shallow water, outperforming pure analytical methods in wavy conditions.^[26] Recent advancements as of 2025 include AI-enhanced hybrid models for probabilistic real-time predictions.^[27] Compared to empirical formulas, these numerical and experimental approaches excel in handling complex scenarios such as non-prismatic channels, wave disturbances, or interacting vessels, where traditional methods falter due to idealized assumptions. Post-2020 validations, including CFD benchmarks for the KRISO Container Ship, demonstrate up to 20% improved accuracy for unconventional hulls, enabling safer navigation planning without extensive physical testing.^[22]

Operational Impacts

Effects on Maneuverability

The squat effect impairs a ship's turning ability primarily through increased hydrodynamic resistance and reduced forward speed in shallow water, resulting in a larger turning radius and overall maneuvering diameter. For instance, experimental analyses of minesweepers demonstrate that at speeds around 17.5 knots, the turning radius measures 195 meters in deep water, but in shallow conditions, a 20-30% speed reduction enlarges this radius significantly due to diminished rudder effectiveness and heightened inertia.^[3] This degradation occurs as squat induces eddy currents beneath the keel, which disrupt flow over the rudder and reduce directional control during turns.^[3] Squat also influences ship stability during maneuvers through bodily sinkage and trim changes. These alterations are exacerbated in shallow water, where the confined flow amplifies hydrodynamic forces acting on the hull.^[28] The effects of squat on maneuverability are highly speed-dependent, with maximum squat occurring near the critical speed defined by the depth Froude number

Fr_h = \frac{V}{\sqrt{gh}} \approx 1

Frh=gh

V≈1, where $V$ is the ship's speed, $g$ is gravitational acceleration, and $h$ is water depth. At this transcritical regime, the interaction between the ship's hull and the shallow-water wave system intensifies.^[29] Studies indicate that such conditions can increase the maneuvering diameter by 20-30% compared to deep-water operations, underscoring the need for speed management to maintain controllability.^[30] Trim changes induced by squat further compromise propulsion and handling, particularly through stern squat, which increases propeller immersion by drawing the stern deeper into the water. This heightened immersion alters inflow to the propeller, potentially affecting thrust efficiency.^[31] The consequent loss of propulsive power amplifies the overall reduction in maneuverability, as the ship struggles to maintain speed or respond to helm inputs during transit.^[14] In confined waters, squat diminishes directional stability by amplifying yaw moments from asymmetric flow around the hull, where uneven pressure distributions on the bow and stern promote unintended deviations from course. This effect is pronounced near channel banks, where the squat-induced acceleration of water under the vessel creates lateral forces that exacerbate yaw, requiring constant corrective rudder action to sustain straight-line tracking.^[32] Such instability can extend stopping distances and complicate precise navigation in restricted areas.^[32]

The squat effect poses significant grounding risks in shallow waters by reducing under-keel clearance (UKC) to potentially zero if not accurately predicted, particularly in channels where water depth is marginal relative to the vessel's draft.^[33] Critical UKC thresholds, such as a minimum of 10% of the ship's draft in static conditions, are recommended by PIANC to account for dynamic effects like squat, ensuring a safety margin against seabed contact during transit.^[34] Unpredicted squat can lead to the vessel's hull touching bottom, especially at higher speeds where the phenomenon intensifies, exacerbating the hazard in areas with uneven bathymetry or sediment accumulation.^[35] Squat risks are further compounded by interactions with tidal variations and variable loading conditions, which dynamically alter the vessel's draft and available UKC. During ebb tides or periods of falling water levels, the effective water depth decreases, amplifying squat's impact and potentially pushing UKC below safe limits even if initial calculations appeared adequate.^[36] Similarly, changes in ballast or cargo loading can increase draft unexpectedly, reducing the margin for squat-induced sinkage and heightening grounding potential during maneuvers.^[37] These factors necessitate continuous monitoring to prevent cumulative reductions in clearance that could result in loss of propulsion or structural damage. Regulatory frameworks from organizations like the International Maritime Organization (IMO) and the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) address squat hazards through guidelines for shallow water navigation, including mandatory speed restrictions to limit the effect's magnitude. IMO Resolution MSC.137(76) on Standards for Ship Manoeuvrability emphasizes maintaining safe stopping distances and turning abilities in deep water. IALA Guideline 1078 on the Use of Aids to Navigation in Fairway Design incorporates squat predictions into channel planning, recommending aids like buoys and lights to enhance situational awareness in restricted waters. These standards promote proactive speed management, typically capping transit velocities to below critical thresholds where squat exceeds 20-30% of draft in confined areas. In voyage planning and risk assessment, squat is integrated into tools like the Electronic Chart Display and Information System (ECDIS) for real-time UKC monitoring, allowing masters to evaluate dynamic clearance against planned routes. ECDIS safety settings, such as safety depth contours adjusted for squat and draft, trigger alarms when projected UKC falls below predefined minima, facilitating informed decisions on passage adjustments.^[36] This approach, aligned with IMO voyage planning guidelines, uses empirical squat models to simulate risks, ensuring compliance with port-specific UKC policies and reducing the likelihood of navigational errors.^[38] Multi-ship interactions, such as those during overtaking or passing maneuvers, can amplify squat effects through hydrodynamic coupling, increasing collision risks in narrow channels. When vessels pass in shallow water, the induced pressure fields cause mutual sinkage and yaw, potentially steering ships toward each other or the banks. This interaction demands heightened vigilance, as the combined effects degrade directional control and UKC, particularly in high-traffic areas where precise coordination is essential to avoid allisions.^[39]

Mitigation and Management

Speed and Route Adjustments

To minimize the squat effect during transit through shallow waters, captains and pilots implement speed reduction protocols that limit vessel velocity to 70-80% of the critical speed, where critical speed is defined as the point at which hydrodynamic interactions intensify, approximately

\sqrt{gh}

with $g$ as gravitational acceleration and $h$ as water depth.^[40] In particularly restricted conditions, such as when the water depth to draft ratio ( $h/d$ ) falls below 1.2, speeds are often capped at less than 6 knots to prevent excessive sinkage, as squat magnitude scales with the square of velocity, allowing a 20-30% speed reduction to halve the effect.^[41] These adjustments are dynamically enforced using onboard systems that integrate real-time data to maintain under-keel clearance (UKC) margins, ensuring safe passage without compromising maneuverability. Route optimization forms a core operational tactic, prioritizing deeper or wider channels to increase the $h/d$ ratio and thereby diminish squat-induced risks. Pilots select paths that avoid sharp bends or narrow constrictions at higher speeds, where bank effects can amplify hydrodynamic pressures; for instance, transiting central channel axes in broader fairways reduces lateral interactions compared to edge navigation.^[40] This planning extends to voyage appraisal, where alternative routes are evaluated using electronic chart display and information systems (ECDIS) to favor segments with greater effective depth, minimizing overall squat exposure during the entire leg. Tidal window planning is essential for maximizing UKC margins, with transits scheduled during peak high-water periods to offset squat and static draft. Systems like the Dynamic Under Keel Clearance (DUKC®) tool calculate optimal windows by forecasting tidal heights, currents, and vessel-specific squat, allowing deeper drafts or extended transit times; for example, ports such as Melbourne employ this to widen safe windows by up to 20% under favorable conditions.^[41] Captains coordinate with port authorities to align arrival times precisely, ensuring a net UKC of at least 10% of draft, adjusted for predicted squat.^[40] Real-time monitoring enables dynamic adjustments, utilizing echo sounders for instantaneous depth profiling, GPS for positional accuracy, and integrated prediction applications to track squat evolution. Real-time kinematic (RTK) GPS systems, for instance, compute UKC by combining antenna altitudes with bathymetric data, directly accounting for squat without separate modeling, as demonstrated in trials within the Torres Strait where measurements validated adjustments to maintain clearances above 0.5 meters.^[42] Portable pilot units running DUKC® software provide alerts for speed tweaks, such as increasing from 6 to 8 knots when UKC permits, ensuring proactive responses to variations in water levels or vessel trim.^[41] Crew training emphasizes bridge resource management (BRM) to foster squat awareness, as mandated by the STCW Convention through IMO Model Course 1.22, which incorporates squat effects into teamwork exercises on passage planning and hazard mitigation.^[43] Officers practice allocating resources for monitoring shallow-water transits, including communication protocols for speed and route changes, aligning with STCW Table A-II/1 requirements for effective resource prioritization and situational awareness to prevent grounding incidents. This training ensures collaborative decision-making, with pilots briefing teams on squat risks per voyage-specific contingencies.

Hull Design and Monitoring Tools

Hull form optimizations play a crucial role in mitigating the squat effect by minimizing hydrodynamic interactions in shallow water. Ships with lower block coefficients, typically below 0.7, experience reduced overall squat magnitude compared to fuller forms with higher coefficients, as finer hull shapes cause less obstruction to water flow beneath the vessel.^[1] These designs promote even bodily sinkage rather than pronounced bow or stern trim, enhancing under-keel clearance during transit.^[44] Model basin experiments have validated such optimizations, demonstrating how refined hull geometries can lower sinkage by altering pressure distributions around the hull.^[16] Onboard tools for squat prediction and management include software applications that integrate empirical methods, such as those developed by Barrass, to estimate sinkage in real time based on vessel parameters like draft, speed, and blockage ratio.^[44] These calculators, often embedded in navigation systems like UKC Manager, allow bridge officers to input static and dynamic data for immediate under-keel clearance assessments.^[45] Voyage data recorders (VDRs) further support post-voyage analysis by logging motion, speed, and environmental data, enabling retrospective evaluation of squat occurrences to refine future operations.^[46] Sensor technologies enhance real-time monitoring of squat and under-keel clearance (UKC). Forward-looking sonars, such as the Vigilant system, provide high-resolution 2D/3D seabed mapping up to 600 meters ahead, with automated alarms triggered when UKC falls below predefined thresholds.^[47] Dynamic draft gauges, integrated into systems like those outlined in IHO S-129 standards, use sensors including tide gauges and acoustic Doppler current profilers to compute real-time vessel sinkage and issue alerts for low UKC zones during port maneuvers; as of 2024, IHO S-129 Edition 2.0.0 has been endorsed to further support dynamic UKC management.^[48]^[49] Emerging innovations incorporate artificial intelligence for predictive squat management, leveraging neural networks trained on hydrodynamic data to forecast sinkage in shallow waters. Post-2023 research highlights artificial neural networks (ANNs) for simulating maneuvering responses, including squat, with improved accuracy from limited datasets in real-time scenarios.^[50] These AI-based systems integrate computational fluid dynamics outputs to anticipate squat variations, supporting autonomous navigation adjustments.

Historical Context and Incidents

Evolution of Knowledge

The squat effect, observed as early as the late 19th century through anecdotal reports of vessels unexpectedly grounding in shallow rivers due to hydrodynamic forces, began gaining systematic attention in the early 20th century via model experiments. These initial observations highlighted how ships in restricted waters experienced bodily sinkage and trim changes, often attributed to pressure variations beneath the hull, though quantitative predictions remained elusive until mid-century advancements.^[8] Formalization accelerated in the 1950s and 1960s with theoretical and experimental work, including early studies on shallow-water resistance by Thews and Landweber (1935), which laid groundwork for understanding sinkage components. Pivotal model tests conducted at the Netherlands Ship Model Basin (now MARIN) in the 1960s, notably by Graff, Kracht, and Weinblum (1964), provided empirical data on sinkage and trim for full-form ships in confined channels, influencing subsequent slender-body theories like Tuck's 1966 analysis. The 1970s saw influential contributions such as Huuska's 1979 study on trim predictions, which correlated squat with block coefficients and water depth ratios to forecast bow or stern immersion. By the 1980s, empirical approaches dominated, with Barrass's 1979 and 1981 formulas deriving practical squat estimates from over 300 model and full-scale measurements, emphasizing speed-squared dependencies for operational use.^[51]^[52] Regulatory frameworks evolved in the 1990s through the PIANC Working Group 30 report (1997), which standardized squat criteria by compiling 11 empirical formulas and guidelines for underkeel clearance in approach channels, promoting consistent design for safe navigation. Post-2000, international bodies like the IMO integrated squat considerations into broader underkeel clearance policies. Recent advances in the 2010s leveraged computational fluid dynamics (CFD) for validation, with studies like Mucha et al. (2016) demonstrating URANS models' accuracy in predicting squat and resistance within 5-10% of experimental data for container ships in varying depths.^[53] In the 2020s, research has increasingly addressed climate-impacted waterways, where fluctuating depths from droughts and sedimentation exacerbate squat risks, necessitating adaptive dredging; for instance, studies highlight up to 20% navigation capacity losses in low-water scenarios like the Rhine, prompting revised empirical adjustments. Post-2020 papers on varying depth effects, such as Elsherbiny et al. (2022), use CFD to model non-uniform bathymetry, revealing amplified trim in irregular channels that traditional formulas underestimate by 15-25%. Further CFD studies, such as Terziev et al. (2022), addressed scale effects in squat predictions. As of 2025, PIANC continues refining channel design guidelines amid climate-driven low-water challenges in rivers like the Rhine, with recent droughts causing 15-25% capacity reductions. These developments underscore a shift toward integrated, climate-resilient prediction models for enhanced safety.^[54]^[55]^[56]^[53]

Notable Marine Events

One notable incident involving the squat effect occurred on August 7, 1992, when the RMS Queen Elizabeth 2 (QE2), a 963-foot passenger liner, grounded twice on a rocky shoal off Cuttyhunk Island near Martha's Vineyard, Massachusetts, while en route from New York to Southampton. The vessel was traveling at approximately 19.5 knots in waters with depths around 37 feet, exceeding its static draft of 32.5 feet due to unaccounted hydrodynamic forces. Investigations determined that squat reduced the under-keel clearance by an estimated 1.5 to 2 feet, with some analyses suggesting up to 8 feet under the prevailing conditions of speed and shallow water, leading to hull damage costing over $20 million in repairs and lost revenue.^[57]^[58] In a more recent example of squat amplification in confined channels, the container ship Ever Given ran aground in the Suez Canal on March 23, 2021, blocking the vital waterway for six days and disrupting global trade. The 1,312-foot vessel, with a draft of about 50 feet, was navigating a narrow, dredged section at around 13 knots when strong winds, combined with bank effects and squat, caused it to veer and embed across the canal. Post-incident analysis highlighted how the confined geometry intensified squat, reducing effective under-keel clearance by 1-2 meters and contributing to the loss of steering control, though wind was the primary trigger; the event underscored vulnerabilities in high-traffic Asian approaches to ports like those in the Middle East.^[59]^[60] Another case illustrating squat during high-speed transits in dredged channels involved the very large ore carrier Stellar Banner, which grounded near the port of Tubarão, Brazil, on February 24, 2020, after deviating from its planned route. The 1,115-foot vessel, loaded with iron ore and drawing 22.5 meters statically, proceeded at approximately 5 knots in a channel with variable depths around 24 meters, where investigators calculated a dynamic squat increasing the effective draft to 23.5 meters. This unpredicted increase, exacerbated by shallow water and proximity to banks, led to the initial touching of the seabed, subsequent flooding, and the ship's eventual scuttling; the incident highlighted risks in port approaches similar to those in Asian dredged channels.^[61]^[62] In European waters, the ro-ro freight ferry Seatruck Performance experienced a grounding on May 8, 2019, while transiting the Greenore Channel in Carlingford Lough, Ireland, after departing Warrenpoint, in shallow, confined conditions. Operating at speeds up to 10 knots in depths as low as 8 meters against a draft of 6.2 meters, the crew failed to adequately account for squat in under-keel clearance calculations, resulting in grounding on a mud bank, from which it refloated after developing a list. The UK Marine Accident Investigation Branch report emphasized how the effect amplified in restricted waterways, akin to riverine environments like the Rhine, where barge operations in the 1990s faced similar amplification risks leading to multiple groundings, though specific Rhine cases were often compounded by low water levels.^[63] These incidents collectively demonstrate that unaccounted squat magnitudes of 0.5-2 meters can critically erode safety margins, prompting updates to international guidelines such as those from the International Maritime Organization on passage planning and real-time monitoring tools to predict and mitigate hydrodynamic effects in shallow or confined areas.^[57]^[59]

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