Why Ships and Boats Run Aground: Causes and Responsibility

A grounding is almost never a mystery about where the water was. The charts showed it. The tide tables predicted it. The question is why a ship with all that information ended up somewhere the water was not deep enough.

Under-keel clearance

Every passage through shallow water starts with a simple sum: the depth of water available, minus the ship's draft, minus a margin for things that reduce clearance. That margin has to cover squat (the way a ship sinks lower as it moves faster in shallow water), waves and swell, the ship's trim and any list, chart inaccuracy, and the difference between predicted and actual tide.

Many companies and ports set a minimum under-keel clearance. When a ship grounds, the first check is whether anyone did that calculation for the transit, and what number they got.

Squat

Squat catches people out because it grows with speed, and the effect is stronger in a narrow or shallow channel. A large, deep-draft ship can lose a surprising amount of clearance simply by going faster than the plan allowed. Pilots and masters know this. Groundings happen when the speed creeps up to make a tide window or a schedule.

Tide and current

Tide predictions are predictions. Wind, river flow, and atmospheric pressure can all push the real water level below the forecast. Prudent mariners compare predictions with real-time gauges where they are available. Current also matters, because it sets a ship off her intended track, especially in bends and at channel entrances.

Bars

River bars and harbor bars are among the most dangerous water any ship crosses. Swell builds as it moves into shallow water over the bar, and a ship pitching and rolling in that swell loses clearance on every downward motion. Many bars have pilots who specialize in them and set conditions for when ships may cross.

The passage plan

A good passage plan marks the shallow areas, sets safety margins, identifies abort points, and lays out what the crew will monitor along the way. When I review a grounding, I want to see the plan as it existed before the transit, not a version updated afterward. A missing or generic plan tells its own story.

Electronic charts

ECDIS can alarm when a ship approaches water shallower than a set depth. That only works if the safety contour and alarms are set correctly for the ship's actual draft, and if the officers do not silence alarms they find annoying. The equipment's settings and alarm logs are worth preserving.

What to collect

• Passage plan and under-keel clearance calculations
• VDR, ECDIS, and AIS data
• Draft readings at departure and any changes during the voyage
• Tide, current, and gauge data for the time of the grounding
• Pilot card and master and pilot exchange records
• Bridge audio and VHF recordings

About the author

Captain Mitchell S. Stoller is a Master Mariner Unlimited, former Exxon tanker master, and former Los Angeles Harbor pilot. About Captain Stoller