What precautions does a ship take in heavy weather?

Heavy weather precautions start long before the first big sea comes aboard, and the ships that suffer damage are usually the ones that prepared late. The sequence is forecast, prepare, then handle the ship.

Preparation while the weather is still workable: complete or suspend deck work; check and double cargo lashings in accordance with the Cargo Securing Manual, particularly containers, vehicles and heavy lifts; lash or strike below everything loose on deck, in the engine room, galley and stores; close and dog all weathertight and watertight doors, hatches, vents and deadlights; rig lifelines on exposed decks and then ban access to them; press up or empty slack tanks to cut free-surface effect, and ballast for a seakindly draught with adequate stability but not an excessive GM, since a very stiff ship rolls viciously; secure the anchors with extra lashings and cement the spurling pipes. In the engine room, prepare for rolling: tanks and sumps at sensible levels, standby pumps and generator ready, and the engine ready for manoeuvring.

Handling in the storm is mostly about speed and heading. Reducing speed eases slamming, pounding and propeller racing as the stern lifts; altering course changes how the ship takes the seas. The watch must recognise and avoid the dangerous phenomena: synchronous rolling, when the wave encounter period matches the ship's natural roll period; parametric rolling, notably in large container ships in head or following seas, which can build to extreme angles within a few cycles; broaching and surf-riding in following seas; and heavy slamming in ballast condition. IMO guidance on avoiding dangerous situations in adverse weather gives the speed and heading combinations to shun; when rolling builds rhythmically, an early and bold alteration of course or speed breaks the resonance.

Throughout, monitor forecasts and use weather routing to avoid the worst sector of a system, remembering that avoiding action taken a day early costs little while hove-to survival costs a lot. Keep the crew informed, feed people early, and enforce one hand for the ship. Log every precaution taken: if cargo is damaged, the record that lashings were checked and speed was reduced matters. Afterwards, inspect lashings, hatch covers, vents, anchors and the forecastle, because heavy weather damage found in port is far cheaper than damage discovered in the next storm.