Ship Handling & Manoeuvring

Put the wheel hard over from a steady course and the ship traces a roughly circular path with three named landmarks on it. Advance is how far she has travelled in her original direction by the time the heading has swung 90°. Transfer is the sideways displacement over that same swing. Tactical diameter is the transfer once she has turned a full 180° — the distance between the original track and the reciprocal one. A fourth figure, final diameter, is the steady circle she settles onto once the initial transients have died away, and it is usually a little smaller than the tactical diameter. All four come from sea trials and sit on the pilot card, and all four change with loading, trim and speed, so a ballast-passage figure is not a laden-arrival figure.

The reason the turning circle has the shape it does is the pivot point — the point on the centreline that appears, for a moment, to not be moving sideways at all. Forward of it the hull is being pushed bodily into the turn; aft of it, the flow past the stern and the rudder's side force are doing the steering. Going ahead that balance sits roughly a third of the ship's length back from the bow. Put her astern and the flow over the hull reverses, the rudder's authority drops away, and the pivot point slides aft — often close to the stern, sometimes beyond it. That single fact explains why a ship that answers her wheel crisply going ahead can feel unresponsive and unpredictable going astern: the bow, now well forward of the pivot, describes a much wider arc than it did on the way in.

An engine order of full astern does not stop a ship the way brakes stop a car. There is a delay while the engine actually reverses — longer on a large slow-speed diesel than a manual candidate might expect — and once astern thrust does arrive it is fighting the ship's momentum, not simply cancelling it. The distance covered from the order being given to the ship coming to rest is her headreach, and it is one of the few manoeuvring figures with a genuinely simple physical basis: kinetic energy goes as the square of speed, and the astern propeller is doing roughly constant work against it, so headreach scales with speed squared rather than speed itself.

That squared relationship is the whole reason "slow down early" beats "stop late" — a modest reduction in approach speed buys a disproportionate reduction in the distance needed to stop. It also means the reverse is true: a small speed increase over what the pilot card was worked from eats far more stopping distance than it looks like it should. A crash stop is also rarely a straight line. A right-handed propeller driven astern generates transverse thrust that walks the stern to port, so the ship curves away from her original track as she slows — useful to know if the reason for the crash stop was something dead ahead, because "stopped" and "stopped on the original track" are not the same promise.

Run a ship into shallow water and the flow that has to pass under and around the hull is forced through a smaller gap. By continuity that flow speeds up, and by Bernoulli's principle the pressure under the hull drops as it does — so the ship sinks bodily and usually trims as well, typically by the head on a full-form vessel and sometimes by the stern on a fine one. That combined sinkage-plus-trim is squat, and because it grows with the square of speed it is cheap to control and expensive to ignore: a small reduction in speed buys a large reduction in squat, for very little time lost on passage.

"Confined" here means a channel narrow or shallow enough relative to the ship's beam and draught that the water displaced by the hull cannot escape sideways as easily as it can in open water — a river, a dredged channel, a fairway between banks. The block coefficient matters because a full-form hull (high C_b) shoves more water aside for a given length than a fine one, so it squats more at the same speed. None of this is visible from the bridge in the way a list or a trim is visible — it has to be worked out, which is exactly why it gets forgotten under pressure.

A moving hull sets up a pressure field around itself, and that field reacts against anything solid nearby — a bank, the channel bed, or another ship's hull. Run close to a bank on one side and the flow accelerating between hull and bank creates suction that draws the stern toward the bank, while the bow, meeting slower water and a building pressure wave ahead of it, is pushed away. The net effect is a ship that wants to swing her bow off the bank and drag her stern onto it — exactly the wrong direction to correct for instinctively if you are not expecting it.

The same mechanism operates between two ships. Meeting or overtaking close aboard in a channel, each hull's bow-wave and each hull's suction zone act on the other: bows are pushed apart, midship sections are pulled together. It is worse the closer the passing distance, worse the higher the speed of either vessel, and worse still when overtaking, because the ships remain close aboard for far longer than they do in a passing situation on opposite courses — plenty of time for a sheer to develop and for a slow correction to arrive too late.

• Bank effect — bow pushed off, stern drawn on, strongest in a narrow dredged channel.
• Bow cushion — the pressure wave ahead of an approaching or overtaking hull that pushes another ship's bow away.
• Interaction suction — the low-pressure zone amidships that pulls hulls together once bows have passed.
• Squat-interaction combination — shallow water and close passing together are worse than either alone, since both effects grow as the available water around the hull shrinks.

An anchor holds mainly through the friction and resistance of cable lying along the seabed, not through the anchor's flukes alone — which is why scope (the ratio of cable paid out to depth of water) matters more than the size of the anchor once it is on the bottom. More scope lays the cable flatter, pulling on the anchor at a lower angle and letting it dig in rather than break out. A common starting point in settled weather is cable of roughly four to six times the depth of water, increased well beyond that as wind or current build, since holding power comes from added scope and a longer catenary far more than from extra weight sitting on the bottom.

Tugs introduce their own hazard, distinct from anything the ship herself is doing: girting, where a tug made fast by a conventional towline is pulled round beam-on to her own tow and capsized by the very force she is meant to be applying. Escort tugs are built and operated to avoid this, working in a direct mode (essentially towing, force along the line) or an indirect mode (the tug's hull generates hydrodynamic lift at an angle to the line, steering the towed ship with far less risk of girting at speed). Either way, the figure printed on a tug's specification — bollard pull — is the static pull at zero speed, straight astern; the steering force she can actually deliver to the ship depends heavily on the angle of the line and the mode she is working in, and is often a good deal less than the headline number suggests.

Berthing is turning-circle theory and interaction theory applied at low speed and short range. A spring line run out and taken to a winch, combined with ahead or astern power against the rudder, lets the ship pivot on the wire and bring the bow or stern in without needing to fight the whole hull sideways — far gentler on the berth and on the ship than trying to crab in on thrusters alone. Wind and current on the beam matter more here than at sea, because windage acts on a hull that has very little speed to give the rudder any bite, so a berthing plan has to account for which way the wind is setting the ship onto or off the berth before the first line goes ashore, not after.

In open water and heavy weather the priorities shift to keeping the hull intact and comfortable rather than making a plan. Reducing speed in head or bow seas cuts slamming and green water on deck; altering course to broaden the angle of the sea does the same by changing the relative encounter period. Two specific dangers are worth knowing by name: synchronous rolling, where the wave encounter period matches the ship's natural roll period and the roll builds cycle on cycle, and parametric rolling, where large, rapid changes in waterplane area as a ship pitches in a head or following sea — most often a fine-formed hull with large bow and stern flare — pump energy into a roll that was never being directly excited by the waves at all. Both are avoided the same way: change the encounter period by altering course or speed rather than riding it out and hoping it settles.

Three fully stepped problems below: a squat-and-UKC speed limit, a turning-circle wheel-over check against available sea room, and a headreach scaling check for a crash stop. Work each one before reading the answer.