Brief Intro to Nautical Charts

A nautical chart is the seaman’s map. Its main job is to help move safely by showing where the water is deep enough, where hazards are, and how to navigate from place to place.

Water Depth

• Depths are shown as numbers called soundings, measured from a reference level called the chart datum (often Mean Lower Low Water in the US).
• Units may be feet, fathoms, or meters — chart notes specify which.
• If the chart shows 6 ft, the water is usually at least 6 ft at datum; actual depth may be more or less depending on tide.
• Safe passage requires: charted depth + tide level + vessel draft + safety margin.

Bottom Contours

• Contour lines connect areas of equal depth, like elevation lines on a land map.
• Lines close together indicate a steep drop-off; lines far apart indicate a gradual slope.
• Channel-like contours suggest a deeper route through shallows; shoals, bars, and humps are potential grounding hazards.
• Experienced navigators often steer by contour patterns, not just isolated soundings.

Bottom Type

• Charts note seabed material with abbreviations: S = Sand, M = Mud, Sh = Shell, R = Rock, Co = Coral, G = Gravel.
• Mud and sand generally offer better holding for anchoring; rocky ground should be avoided.

Aids to Navigation

• Aids to navigation (marks) are man-made markers guiding traffic or warning of danger: buoys, beacons, daymarks, lights, ranges, and fog signals.
• Charts show each mark’s position, shape, color, light characteristics, and number or name.
• Light characteristics are abbreviated on charts — for example, Fl R 4s means flashing red every 4 seconds.
• Red markers are nun-shaped buoys (pointy top) or square daymarks, with even numbers. Green markers are can-shaped buoys (flat top) or triangular daymarks, with odd numbers.
• Daymarks are shaped opposite to what you might expect. Use color, not shape.

Red Right Returning

• “Red right returning” is the US rule: keep red channel markers on the starboard (right) side when returning from sea toward harbor, upriver, or into port.
• Green markers (cans) stay on the port (left) side when returning.
• “Returning” is based on the official direction of buoyage — on some coasts, rivers, and inlets it is not simply “heading toward land.” Check the chart.
• On the ICW, red nuns are on the landward side and green cans are on the open-water side (“Keep tents on shore and shipping containers at sea.”) The implied direction of “returning” is southbound.

Planning a Safe Route

• Never trust a single sounding alone — check nearby depths, contour trends, tide state, recent local changes, buoy positions, and notices to mariners.
• Follow deep-water contours or marked channels; avoid shoals, rocks, wrecks, and obstructions.
• Allow for tide and current when plotting your course.