Conning the Ship: Wind, Current, Under-Keel Clearance, Interaction and Squat
At command level, conning is not just the mechanics of giving helm and engine orders — it is the continuous mental model of all forces acting on the vessel, and the Master's responsibility to anticipate rather than react.
Building the Picture Before You Move
Before any manoeuvre, establish the environmental forces. Wind and current are not problems to overcome after the fact; they are tools or hazards to quantify first. Determine the set and rate of any current from pilot books, tidal atlases, or observed transits. Assess wind strength and direction relative to the vessel's freeboard and windage. A light-displacement superyacht with high topsides and minimal draught will be dominated by wind; a deeper, heavier vessel will respond more to current. You must know which force dominates before you commit.
Current and Leeway
Current acts on the underwater body uniformly; it moves the whole vessel. Wind creates leeway, predominantly affecting the above-water profile. When stemming a current to hold position or approach a berth, you use the current as an anchor — approach into it, and you retain steerage and control. Running with a strong current reduces your ability to stop and your rudder effectiveness at slow speed over the ground. Always think in terms of speed over the ground for collision risk, and speed through the water for steerage.
Under-Keel Clearance
UKC is the vertical distance between the vessel's keel and the seabed, accounting for dynamic effects. Static draught is the starting point; from there, apply corrections for: water density (salt to brackish or fresh reduces buoyancy and increases draught), heel (a heeled vessel presents greater draught at the bilge), and dynamic effects — principally squat. A minimum UKC policy should exist in the SMS; in the absence of a port authority requirement, many operators use 10% of draught or a defined absolute minimum. You must know your vessel's figure and be able to justify it.
Squat
Squat is the bodily sinkage and trim change that occurs when a vessel moves through water. It is caused by the pressure drop beneath and around the hull as water accelerates to pass under and alongside — Bernoulli's principle in practice. The vessel sinks bodily and, depending on block coefficient, trims. Full-form vessels (high Cb) tend to squat by the bow; fine-form vessels by the stern. Squat increases with the square of speed and is dramatically amplified in shallow water (depth-to-draught ratio less than approximately 1.5) and in confined channels where the blockage ratio is high. The practical command decision: in shallow water, slow down early and positively — halving speed reduces squat to approximately one quarter.
Interaction
Interaction is the mutual attraction (and sometimes repulsion) between vessels in close proximity, caused by the same pressure-field distortion as squat. In a channel, a vessel is drawn toward a shallow bank (bank suction/bank cushion effect), and two vessels passing close experience bow repulsion and stern attraction — the classic interaction scenario that can cause a yaw impossible to correct at slow speed with limited rudder. The Master's response is speed reduction well in advance, maximum separation, and clear communication. Interaction is particularly dangerous at speed in confined waters and is a foreseeable risk the Master must plan for, not discover.