SH Cap. 2 — Shiphandling in a Channel

1. Overview

Channel shiphandling demands a fundamentally different mindset from open-water navigation. In a confined waterway, the ship is no longer the dominant object in a large sea; she is a piston moving inside a cylinder, and the water displaced ahead and around her has nowhere to go except through ever-narrowing gaps between hull, bottom, and bank. The three phenomena that dominate this environment — bank effect, squat, and blockage factor — are all expressions of the same Bernoulli physics: as restricted passages force water to accelerate, pressure drops, and the ship is pulled, sunk, or steered against the handler's intent.

The experienced shiphandler responds not by fighting these forces but by exploiting them. Bank suction at the stern can swing a ship around a bend that would otherwise require tug assistance. A fair current striking the quarter assists turns. Speed — the single variable most within the shiphandler's control — directly governs squat: cut speed in half and squat drops to one quarter of its former value.

The chapter also charts the transition from conventional single-rudder/single-screw arrangements to directional propulsion systems such as the Azipod. These omnidirectional drives offer exceptional low-speed maneuverability, but they raise equally important concerns: the illusion that technology can substitute for seamanship, the erosion of traditional bridge resource management, and the loss of shiphandling skills across generations. Ships are ships — the same principles of planning ahead, anticipating forces, and maintaining reserves apply regardless of propulsion type.

2. Bank Effects

2.1 The Two Forces: Cushion and Suction

When a ship moves close to a steep bank, two hydrodynamic forces act simultaneously along her hull. At the bow, water is compressed between the hull and the bank, creating a pressure cushion that pushes the bow away from the bank. At the stern, the same restricted passage accelerates water flow, producing a low-pressure zone — bank suction — that pulls the quarter toward the bank. The net result is a moment that sheers the ship's head toward the center of the channel (or the opposite bank), while the stern creeps toward the near side.

The Bernoulli Principle underpins both effects. A ship proceeding parallel to a bank at any speed will tend to move laterally toward that bank because the narrowed passage between hull and bank accelerates water flow and reduces pressure on that side. A heading slightly away from the bank, or a position near channel center when not passing, is the standard corrective.

2.2 Responding to an Incipient Sheer

When a ship begins to sheer across the channel toward the far bank due to bank forces, the instinctive reaction — reducing engine speed — is wrong. The rudder derives its effectiveness from water flow; reducing revolutions weakens the rudder while suction on the stern remains strong. The correct response is to hold or increase engine speed, allow the head to fall a few degrees off course toward channel center, maintain some rudder toward the near bank, then apply greater rudder angle to check the swing and bring the ship back when she has gained adequate separation from the bank. Only after securing safe distance should speed be reduced.

A practical speed constraint follows directly: a ship in a narrow channel must never proceed at full maneuvering speed, because she would have no engine revolutions in reserve to increase rudder authority in an emergency.

2.3 Using Bank Suction as an Asset

The same force that threatens becomes a tool when planned for. Placing the quarter close enough to a bank causes the stern to swing toward it, amplifying a turn that the ship could not complete unaided at that speed. In the Gaillard Cut of the Panama Canal, turns that are theoretically impossible for many vessels without tug assistance are routinely negotiated using controlled bank suction — the rudder angle indicator often reads amidships through the turn. Bank suction can similarly assist passing another vessel, centering the ship in a channel during reduced visibility, and executing routine maneuvers, provided the shiphandler plans ahead and keeps sufficient speed reserve to come ahead if the sheer grows beyond the desired value.

3. Planning Ahead, Tide, and Current

3.1 Thinking Ahead of the Ship

Excellent shiphandling — as distinguished from merely acceptable shiphandling — is defined by one habit: thinking ahead of the ship so that she reacts to orders, rather than orders reacting to the ship's behavior. This requires understanding which forces are acting, reducing speed in advance of confined sections, and sequencing maneuvers so each sets up the next, much like a chess master planning several moves simultaneously. Understanding ship behavior, reducing speed in good time, using existing forces to assist rudder and engine, and maintaining that mental lead over the ship are the basics of excellent shiphandling.

3.2 Current Effects on Turns

Many rivers cannot be navigated by larger ships without a fair current (flowing in the same direction as the ship). A fair current striking the quarter assists the stern around a bend at a greater rate, while the bow is helped by eddy currents reflected out of the bend together with the lack of current on the point side. In contrast, a ship stemming a head current experiences greater water flow between her hull and the bank, which retards the stern's motion around a turn and sets the bow bodily toward the inner bank.

High water also eases the pilot's task by providing greater underkeel clearance and, when moving with a rising tide, pairing fair current with deeper water simultaneously. Current predictions are unreliable guides; freshets from upriver rain and strong onshore winds can shift the change of current significantly. The professional habit is to watch pilings, buoys, and fixed objects continuously to check actual current against predicted.

4. Types of Rudders and Propulsion Systems

4.1 General Principles

The shiphandler cannot choose the ship's rudder or propulsion system, but must understand its characteristics and work within them. A practiced hand learns quickly how effectively the rudder responds and at what rudder angles performance degrades. Proceeding at moderate speed gives the advantage of being able to increase engine revolutions — and thus water flow past the rudder — to improve steering on demand without creating excessive squat or bank effects.

For diesel ships, large and immediate changes in revolutions are achievable; steam turbine ships increase revolutions more slowly, requiring extra care to keep speed down while maintaining steering control.

4.2 Balanced Spade Rudders

Ships fitted with balanced spade rudders may lose steering effectiveness at large rudder angles. Turbulent flow develops over the rudder surface, destroying its lifting effect — a condition known as rudder stall. A stalled spade rudder leaves the ship tracking her previous course as though no rudder were fitted. If a ship's profile drawings show a spade rudder, use caution with rudder angles beyond 5 to 10 degrees; beyond that range the rudder may stall. One class of large German containerships fitted with such rudders suffered multiple groundings within months of delivery.

4.3 Propeller Design Effects

Propeller direction of rotation governs the direction of transverse thrust when the engine is put astern and influences the diameter of port and starboard turning circles. Propeller diameter affects stopping distance and low-speed steering: a smaller-diameter propeller requires higher revolutions to produce adequate water flow past the rudder at reduced speeds.

4.4 Variable-Pitch Propellers

A variable-pitch propeller offers real advantages: no need to stop and restart the engine for astern, almost infinite speed selection, indefinite astern operation, and no risk of exhausting starting air through repeated maneuvering. However, two significant drawbacks must be managed in channel transit:

• Reducing pitch abruptly — even to zero — severely disrupts water flow past the rudder, degrading steering at the worst possible moment. Pitch must be reduced very gradually.
• The astern mode is less effective than a conventional propeller, so more time astern is needed to stop the ship, compounding the steering problem.

Practical rule: begin slowing a variable-pitch vessel sooner than a conventional ship when approaching a berth, lock, or pilot station; use minimum pitch to maintain steerage at slow speed once headway has been reduced. When the ship is stopped at the berth with the propeller at zero pitch, keep stern lines clear of the water — the shaft continues turning at high RPM and can foul a line in seconds.

5. Directional Propulsion Systems (Azipod)

5.1 Design and Construction

Electric drive pods are fitted externally at the stern of the vessel. Each pod can be rotated 360 degrees, so the direction of thrust changes by rotating the pod rather than by deflecting a separate rudder. The propeller is mounted at the forward end of the pod and can also be reversed in direction and speed. External electric drives receive power from the ship's diesel generators, eliminating the conventional shaft entirely.

Some installations add a fixed centerline pod; other designs place a propeller on both ends of a pod (one pulling, one pushing), or combine a conventional shaft with a pod immediately astern acting as a stern thruster in port. Azipod steering in At Sea mode is analogous to steering an outboard motor: the pod turns to port to move the ship's head to starboard — opposite to conventional wheel intuition but quickly intuitive once the tiller-control concept is internalized.

5.2 Advantages

Omnidirectional drives offer passenger ships and specialized vessels compelling advantages: no shaft reduces noise and frees interior space for revenue cabins; the engine never stops for astern, eliminating the risk of a restart failure during maneuvering; air compressors can be smaller; twin drives in Harbor mode can be split to apply power simultaneously in two different directions. The result is a vessel highly maneuverable at all speeds, capable of operating in marginal ports with fewer tugs and improved schedule reliability.

5.3 Concerns and Limits

The power and maneuverability of Azipod systems create a dangerous illusion: that traditional seamanship is less important. Experienced masters, mates, and pilots recognize this risk; less experienced officers often do not. Three concerns stand above the rest:

• Terminology fragmentation: At least three major companies use different names for the same operating modes — for the cruise/at-sea mode, "Call Combi" and "Open Sea"; for the maneuvering mode, "Call Azimuth", "Aziman", and "Maneuvering high"; plus a separate joystick/dynamic-positioning control. Standardization is essential for safe inter-company crew transfers and for pilots moving ship to ship.
• Single-person operation: The console layout invites one person to handle engines, thruster, and steering simultaneously, effectively bypassing Bridge Resource Management. Approximately 90% of marine accidents occur in restricted waters; BRM exists precisely to manage those risks.
• Loss of shiphandling skills: Skills lost across a fleet are lost permanently. When a pod motor fails, officers who have never handled a vessel by conventional means will not be able to proceed.

5.4 Three Operating Modes

For practical shiphandling, three modes with minimal configurations cover all situations:

5.5 Configuration #1 — At Sea Mode

Both pods are placed parallel to the centerline and move together in synchronized mode, exactly as rudders are moved on a conventional ship. Steering commands are given identically to a conventional vessel. The pod turns to port to swing the ship's head to starboard — tiller-steering logic. Once internalized, handling at sea in this configuration is as straightforward as steering a small boat with an outboard motor.

5.6 Configuration #2 — Harbor Mode (Bicycle Mode)

The pods are placed at 45 degrees in opposition to each other (V-shape), fixed in position. Speed and direction are controlled by changing the propeller setting — the engine revolutions, ahead or astern — on each unit, not by rotating the pods. The arrangement resembles bicycle handlebars: to turn starboard, advance the port control and retard the starboard. The angle of the pods stabilizes the ship at slow speeds; increasing one pod only holds the stern against wind. Some operators find the high sensitivity of Azipod power makes course-keeping in narrow channels difficult in this configuration; others find it useful for slow-speed open channels and anchorages.

5.7 Configuration #3 — Docking Mode

As the ship approaches the dock, the pilot orders Docking mode. The inboard pod (port pod when docking port-side-to) is set parallel to the centerline for ahead/astern movement. The outboard pod is rotated to the three-o'clock position (90° to centerline) and acts as a powerful stern thruster to move the stern toward or away from the berth. In conjunction with the bow thruster, this arrangement makes the ship completely controllable throughout the final approach. The configuration is intuitive: ahead/astern propulsion is handled by one unit; lateral stern movement is handled by the other.

5.8 Standardized Commands and Bridge Resource Management

Standard commands for Azipod systems are built on three elements: engine commands use percentage of power (0–100%, ahead or astern) in At Sea and Harbor modes; conventional rudder commands in degrees are used at sea via standard wheel; pod configuration in Harbor mode uses clock positions relative to the bow (e.g., port engine at ten o'clock, twenty percent ahead). Percentage power is preferred over conventional bell orders because power through electric drives is fully variable — dead slow/slow/half/full are artifacts of fixed-output machinery.

Standardized commands allow the conning officer to step away from the console and conn from anywhere on the bridge, restoring the traditional bridge organization where pilot, master, helmsman, and officers each play defined roles. BRM is not an anachronism aboard Azipod ships; it is more necessary there, because the power available makes it easier to blunder into an untenable position.

5.9 Computer Control and the One-Ship-Length Rule

Computer-controlled joystick operation is available in all modes. The computer translates joystick inputs into pod rotation, engine speed, and bow thruster orders via preprogrammed algorithms. The fundamental limitation: the system is reactive, not anticipatory. It cannot draw on local knowledge, past experience, or foresight; it detects leeway or set almost instantly, but only after they occur. Skilled shiphandlers plan for expected wind shifts and current changes before they arrive.

Rapid changes at high power settings — half ahead or greater used repeatedly — are the reliable warning sign that a computer-controlled vessel has been put in a position with no reserves. The computer will continue to apply every available tool at maximum effort regardless of consequences. Good seamanship defines a practical threshold: the one-ship-length rule — shift from automatic to manual control when the ship is within one ship length of the dock or a fixed object. Lines have been parted and incidents occurred when the computer attempted to move a ship laterally under automatic control near berths.

6. Effect of Trim on Handling Characteristics

6.1 Trim by the Stern

As a ship's trim by the stern increases, she becomes more directionally stable and her turning circle increases in diameter. The increase in tactical diameter is minor and of no practical significance in most cases. From the shiphandler's perspective, assuming no excessively strong wind on the higher bow, a ship generally steers better as drag (trim by the stern) increases. Once the ship stabilizes in a turn, the pressure couple shifts aft of the center of gravity, reinforcing stability and allowing the rudder to check the swing predictably.

6.2 Trim by the Head

A ship trimmed by the head is directionally unstable for almost all hull forms. The increase in submerged area forward creates a larger initial positive pressure at the bow during a turn, while reduced submerged area aft weakens the counterbalancing negative pressure at the quarter. The resultant couple remains forward of the center of gravity throughout the turn, causing the ship to continue swinging even after the rudder is brought amidships. To the shiphandler, this appears as a forward shift of the pivot point toward the bow.

A large VLCC with full forward sections experiences similar instability even on an even keel. If that VLCC is additionally trimmed by the head, the instability is magnified. For almost all hull forms, trim by the head in shallow water compounds the problem because underkeel clearance is reduced simultaneously.

6.3 Managing a Directionally Unstable Ship

When a ship with marginal directional stability must be handled in a channel: apply rudder earlier and for longer to initiate a swing; return the rudder to amidships as soon as the swing begins (the rate of turn will continue to increase even with rudder amidships); apply counter-rudder in good time to check the swing before the ship leaves the channel. The rate-of-turn indicator is indispensable in this situation — it makes it possible to limit swing to a known safe maximum. LNG carriers trimmed to even keel for their terminals are safely handled this way by limiting rate of turn to less than $\frac{3}{10}$ degree per second; a rate of $\frac{5}{10}$ degree per second is a safe maximum for an ordinary 36-degree-per-minute turn.

A directionally unstable ship can turn in a very small area — a characteristic that can be exploited if the shiphandler monitors steering closely. A trained helmsman experienced on that particular ship is invaluable; a general helmsman unfamiliar with the ship's tendency to continue swinging can allow a small error to become an unrecoverable sheer.

7. Pivot Point and Turning Characteristics

7.1 The Pivot Point Defined

The pivot point is the apparent instantaneous center of rotation of a ship in a turn. Under normal ahead power it lies approximately one-third of the ship's length from the bow. It is not a fixed structural point; it shifts forward when trimmed by the head and aft when trimmed by the stern. When a ship is making sternway, the pivot point shifts aft, toward the stern.

The practical significance: when deciding when to begin a turn in a channel, it is the pivot point that should reach the turning point at the end of a reach — not the bow, not the bridge. Waiting for the bow to reach the turning point means the turn has been started one-third of a ship's length too late.

7.2 Starting the Turn

Two errors are common: starting too late (requiring excessive rudder and engine revolutions, risking leaving the channel) and starting too soon (requiring the swing to be checked and restarted, which in narrow channels with bank suction may be difficult to achieve once the initial swing is lost). When in doubt about the required rudder angle, use more than seems necessary and reduce as required, rather than using too little and finding the ship will not complete the turn. Ships turn circles, not corners — the theoretical turning radius established during master's trials must be internalized to judge entry point.

7.3 Using Navigation Aids as Turn Indicators

A buoy at a turn can serve as a real-time rate-of-turn indicator by aligning it with a fixed reference on the ship (stay, stanchion, window frame) and observing whether its relative bearing moves toward the bow, holds steady, or opens aft:

• Bearing moving forward: the ship will be closer to the buoy at turn completion; rate of turn is increasing.
• Bearing steady: the ship is turning at a constant rate and will maintain approximately the same distance from the buoy.
• Bearing opening aft: distance from the buoy is increasing; rate of turn is decreasing.

This technique is especially powerful in a strong current, since the buoy reflects the ship's true movement relative to the waterway — the net result of momentum, swing, and current — rather than her movement through the water alone.

8. Meeting and Passing in a Narrow Channel

8.1 Combined-Beam Criterion

If the channel is wide enough, meeting is simply a matter of each vessel keeping to her side. The practical question is what constitutes adequate width. In the 500-foot reaches of the Panama Canal, ships meet routinely when their combined beams total up to 170 feet — a figure established by pilot experience and confirmed by simulator tests. (Panamax class vessels are the exception; they do not meet any vessel in those reaches due to their own handling limitations.) This figure serves as a useful guideline, though meetings do occur in channels narrower than 500 feet under appropriate conditions.

8.2 The Five-Step Meeting Procedure

When combined beam approaches the 170-foot limit, the meeting must be carefully choreographed. The procedure in figure 2-14 proceeds in five stages:

1. Ships approach nearly head-on; when approximately one-and-a-half ship lengths apart, both put their rudders to starboard to move to their own sides.
2. When one ship's bow is abeam the other's bow, the helm is shifted to move the stern to starboard until the ship is parallel to the bank.
3. Rudder is put right again to check the swing. At this moment the ship is susceptible to both bank suction on the starboard quarter and interaction from the passing vessel — both forces promote a port sheer. Sufficient starboard rudder is required to maintain control.
4. Do not increase right rudder further; allow the ship to sag slowly to port so she is again heading away from the bank. The other ship's stern has now cleared the bow; lateral collision is no longer a risk.
5. As the other ship's stern passes clear, mutual suction between the two sterns moves each away from the near bank.

Speed is the governing variable throughout: below full maneuvering speed so that suction is minimized, engine reserve is available to increase rudder authority, and there is room to maneuver if the sheer exceeds the desired value.

8.3 Overtaking

Overtaking in a channel is mechanically routine but speed-sensitive. The overtaking ship must complete the passing maneuver as quickly as safely possible, since the risk is greatest when the overtaking ship's stern is abeam the overtaken ship's bow — suction forces are strongest and the overtaken vessel is most susceptible to becoming unmanageable. The overtaken vessel reduces speed to minimum steerageway to shorten the passing time; while being overtaken, she increases revolutions as needed to maintain steerage. The Rules of the Road place the decision to permit overtaking with the ship being passed, because it is that vessel which is most at risk if the maneuver goes wrong.

9. Conning Stations and Integrated Bridge

The conning station centralizes the readouts that a shiphandler needs in one place: fore/aft and lateral speed (from a complete Doppler presentation), rudder position, heading, wind direction and strength, and vessel position. For large ships or ships with restricted forward visibility, a rate-of-turn indicator is essential — it displays turn rate in tenths of a degree per second and enables the shiphandler to detect the onset of a swing, confirm whether it is increasing or decreasing, and check it before it becomes visible to the eye.

The integrated bridge consolidates these readouts with radar (including ARPA), ECDIS, DGPS and portable piloting units, VHF, AIS, and the helm station itself into a unified workspace. Despite the sophistication of newer equipment, the gyrocompass and fathometer remain the essential tools for channel navigation — direction and depth are the parameters that govern every decision. The fathometer in particular is too often neglected once the pilot is aboard; depth is a primary shiphandling parameter and must be watched routinely, not just checked.

A Doppler log showing both fore/aft speed over the bottom and athwartship movement at bow and stern is necessary to maneuver large ships accurately alongside piers or single-point moorings, where speeds may be fractions of a knot. The single-axis Doppler speed log adequate at sea does not provide sufficient information for close-quarters maneuvering.

10. Blockage Factor, Squat, and Speed in Restricted Channels

10.1 Definitions: Sinkage, Trim, and Squat

Three related phenomena affect a ship's draft in restricted waters:

• Sinkage is the bodily increase in mean draft as the ship moves — an even increase forward and aft.
• Dynamic trim is the rotation about the transverse axis caused by the change in pressure distribution along the hull — resulting in greater draft increase at either the bow or the stern.
• Squat is the combination of sinkage and dynamic trim. In practical terms, the mariner reports that the ship "squats X feet by the head" or "by the stern."

All three components — sinkage, trim, and squat — affect shiphandling and must be monitored. Squat occurs because the ship displaces water equal to her own weight; that water must return astern, flowing primarily under and along the hull. Faster speed equals higher flow velocity, which by the Bernoulli Principle means lower pressure and greater draft increase.

10.2 Blockage Factor and Its Variables

The blockage factor is the ratio of the ship's maximum underwater cross-sectional area (midships section) to the cross-sectional area of the channel. The smaller the available flow area, the higher the water velocity for a given ship speed, and the greater the resultant pressure drop around and under the hull. Six variables govern the magnitude of the effect:

1. Ship's speed through the water.
2. Ratio of ship's draft to water depth.
3. Ratio of ship's cross-sectional area to channel cross-sectional area (blockage factor proper).
4. Block coefficient — a high $C_b$ amplifies all hydrodynamic effects in shallow water.
5. Ship's displacement — greater mass means more water to displace per unit time.
6. Rate and period of acceleration as the ship increases speed.

10.3 Speed Governs Squat

Speed through the water is the variable most within the shiphandler's control, and it governs squat geometrically: squat varies as the square of the speed. Double speed, quadruple squat. Halve speed, reduce squat to one quarter. This relationship makes speed management the primary tool for maintaining underkeel clearance in shallow channels. Note that the relevant speed is speed through the water, not speed over the ground — a ship stemming a strong current experiences greater flow under the hull than her ground speed suggests.

The Barrass formula consistently overestimates squat, providing a margin of safety. For specific channels and ship types, the Tuck/Huuska and Eryuzlu/Hausser models (documented in PIANC) may give more accurate predictions; waterway-specific calculators can be built from survey-grade DGPS measurements.

10.4 Squat by the Head or Stern

Whether squat occurs by the head or the stern depends on hull form. A commonly accepted rule of thumb: a ship with a large block coefficient ($C_b > 0.75$) — typical of fully loaded tankers and bulk carriers with full forward sections — tends to squat by the head. Container ships with $C_b < 0.7$ typically squat by the stern. The curve of submerged cross-sectional areas for the specific vessel provides a more reliable predictor: if the curve's maximum lies forward of midships, the ship will likely trim by the head under squat.

10.5 Practical Speed Limits and Acceleration

The Panama Canal Commission squat studies confirmed that 6 knots is a practical speed limit for ships in channels where underkeel clearance is 5 feet or less, accounting for speed, acceleration, and rolling effects combined. This rule of thumb applies in the absence of specific local knowledge to the contrary. Pilots with local knowledge may safely operate at higher speeds on mud bottoms or in well-surveyed channels, and more conservatively on rocky bottoms.

Acceleration is a critical additional factor. Initial squat during acceleration from rest can be approximately double the squat calculated by constant-speed formulas. A ship ordered directly to half ahead from stopped may strike bottom while accelerating, even though sufficient clearance would exist once that speed is reached and stabilized. The correct technique is to accelerate in increments: dead slow, slow, then half ahead, monitoring underkeel clearance at each step.

10.6 Handling Effects of High Blockage

As the blockage factor increases, the ship behaves like a piston in a cylinder and progressive resistance limits her speed: a ship making 16 knots at 80 revolutions in open water may achieve only 9 or 10 knots with the same revolutions in a shallow, narrow channel. When this limit is reached, the wake becomes short and steep with breaking crests spreading at a greater angle — the ship is said to be "pulling a lot of water." Steering also degrades: when underkeel clearance is less than 0.5 of the ship's draft, the ship becomes more directionally stable and thus harder to turn. Greater consideration must be given to starting turns earlier and applying more rudder.

11. Confined-Channel Effects: The Gaillard Cut

The Gaillard Cut in the Panama Canal is the reference environment for confined-channel shiphandling. All the effects described in this chapter converge in one waterway: steep banks producing suction, narrow cross-sections creating high blockage factors, bends that theoretically cannot be negotiated without tugs but are routinely made using controlled bank suction, and varying channel geometry that changes blockage factor — and thus squat — along the transit.

Panama Canal Commission studies using survey-grade DGPS equipment on actual ships produced landmark findings that extend beyond the Canal to all narrow and shallow channels:

• Speed through the water is the most critical factor for underkeel clearance.
• Rolling in turns is at least as important as squat for wide-beam ships with low GM. A 106-foot beam Panamax ship increases draft by approximately 11 inches per degree of roll.
• Changing channel cross-section symmetry — as the canal transitions between reaches — causes abrupt changes in underkeel clearance. The draft of a 612-foot-long by 95-foot-beam bulk carrier changed by as much as 2 feet as the channel shape changed, independent of speed change.
• Squat generally increases 50% as two ships pass in the channel, and can increase by 60–100% depending on speed and separation distance. A 900-foot containership at 11 knots in a 700-foot channel immediately increased draft by more than 7 feet as it passed another vessel at approximately 150 feet separation.

For the meeting/passing case: a bulk carrier squatting 4 feet meeting a ship squatting 3 feet will experience a cumulative squat of approximately 7 feet as the ships pass. This interaction is rapid and drastic — a ship proceeding at high speed in a narrow channel could ground immediately at the moment of passing.

The implication for the overtaking case: by conservative extrapolation — and until dedicated measurements are available — assume the draft increase when overtaking equals or exceeds that when meeting, and lasts longer due to the slower closing speed between ships.