How deep a ship sits determines where it can go. Draft is the vertical distance from the waterline to the lowest point of the keel, measuring how much water the ship needs in order to float clear of the bottom.

Draft changes with loading, ballast, and fuel consumption. Officers read draft marks painted on the hull at bow, stern, and amidships. Under-keel clearance—the margin between keel and seabed—must remain adequate in channels and at berths. Density of the water (salt, brackish, or fresh) also affects the reading.

Maximum permitted draft is set by the load line and by port and canal restrictions. Loading beyond safe draft is illegal and hazardous. Real-time draft monitoring helps prevent accidental grounding during cargo operations.

• Depth of the ship below the waterline
• Read from hull draft marks
• Limits access to ports and channels
• Must allow sufficient under-keel clearance