Ship Handling, Manoeuvring and Stability
Ship handling and stability asks whether you can predict what hull, propeller, rudder, current and wind will do, and whether the vessel will come upright again.
Hydrodynamic effects
- Squat increases with the square of speed and is worst in shallow, narrow water; it reduces underkeel clearance and can shear the vessel onto a bank.
- Bank effect: the bow is pushed away and the stern sucked toward the nearer bank; the narrower the channel, the stronger it is.
- Transverse thrust from a right-handed propeller normally swings the stern to starboard going ahead and to port going astern; use it deliberately when docking.
- The pivot point sits roughly a quarter of the length from the bow when moving ahead, so the stern sweeps wide in a turn.
Stability
- GM = KM − KG. A small positive GM gives a stiff, snappy roll; a large one gives a tender, slow roll with big angles.
- Free surface in slack tanks raises the effective centre of gravity: FSC = (i × ρ) / Δ. Subtract it before judging GM.
- GM alone is not enough: the GZ curve gives righting energy, maximum lever and angle of vanishing stability. Deck cargo, water absorption and shifting cargo destroy that curve.
Worked example
KM 2.55 m, KG 2.10 m, so GM = 0.45 m. One partly filled tank gives FSC 0.07 m, so effective GM = 0.38 m. Loading 4 t of deck cargo at 3.0 m above the keel raises KG further and may push effective GM below the minimum in the stability booklet: recompute and, if needed, ballast low or shift cargo before departure.
Traps
Forgetting free surface; assuming a stiff ship is a safe ship; ignoring squat in a shallow canal; starting a turn too late with a long convoy.