Why Charts Still Earn Their Shelf Space

At 30 knots you cover half a mile every minute. A chartplotter screen showing a comfortable blue channel can be hiding a 0.8 metre patch of rock that dries at half tide, and at planning speed you will not have time to read the fine detail once you are committed. Charts — paper or electronic — are the difference between a confident passage and a very expensive insurance claim.

The good news is that chart reading is a skill, not a mystery. Learn the symbols, understand the numbers, and you can plan a route through the Solent, Cardigan Bay or the Firth of Clyde with the same ease as a marina-to-marina run.

Depths, Datum and Contours

Every sounding on a UK chart is measured from chart datum, which is effectively the lowest astronomical tide. That means the printed number is the least water you should expect, not the average. A "5" on the chart means five metres at the worst case; at most states of tide you will have considerably more.

Read the numbers properly. Soundings are usually in metres and decimetres, so a small subscript figure — 5 with a small 4 — means 5.4 metres. Figures printed with a line beneath them are drying heights: the depth above chart datum that empties out at low water. A "1₂" in a channel you want to cross at half tide needs a tide table in your hand before you turn the wheel.

Contour lines do the heavy lifting when planning at speed. The blue tint on a paper chart typically shows the area shallower than 10 metres. Safe water is white. For a boat drawing 1.0 metre, a route hugging the 20 metre contour gives you a generous buffer; a route across the 5 metre contour is one you only take with a known tide height and a backup plan.

Symbols That Matter Most to a Powerboat

• Rocks and shoals: a cross means a rock that covers and uncovers; a star means a rock awash; a dotted outline is a drying area. Add a metre of swell and a planing hull, and these are not places to cut corners.
• Wrecks: a wreck symbol with a mast or depth figure is a known obstruction. Anything with "PA" means a position that is only approximate.
• Cables and pipelines: marked with a line and an anchor symbol — no anchoring, and no dragging a grapnel for a lost fender.
• Overhead cables: a line across a river or estuary with a clearance figure in metres. Check it against your antenna height, not just your hull.
• Restricted and practice areas: firing ranges, racing areas and anchoring prohibitions are all charted. In the Solent and around Plymouth you will meet these regularly.

Buoyage: IALA Region A

The UK uses IALA Region A, and the logic is simple once it clicks. When entering a harbour from seaward, red port-hand buoys stay to your port side and green starboard-hand buoys to starboard. Red cans are port, green cones are starboard. If you are leaving, it reverses.

Where a channel splits, preferred channel marks do the talking: red-green-red means the main route is to port, green-red-green means it is to starboard. Cardinal marks show where the safe water lies relative to the mark — north cardinal has two cones pointing up and black above yellow, east has cones base to base, south has cones pointing down, and west has cones point to point. Pass north of a north cardinal, east of an east cardinal, and so on.

Learn the rest at a glance: red and white vertical stripes is a safe water mark, black with red bands and two black spheres marks an isolated danger, and yellow marks special features such as racing marks or spoil grounds. Combine these with light characteristics — flashing, occulting, isophase, and their colours and periods — and you can identify a buoy at night long before the plotter tells you.

Paper and Electronic: Use Both

Paper charts are unforgiving in the best way. You cannot zoom, you cannot scroll past the hazard, and plotting a course by hand forces you to think about distance, speed and time. A 40 nautical mile passage at 25 knots is roughly 1 hour 36 minutes of running, plus a margin. Do that sum on paper and you will arrive with fuel and daylight in hand.

Electronic charts — vector ENCs or raster versions of the paper originals — add convenience: depth shading, route planning, alarms and instant position. But treat them as a second opinion, not gospel. Check that your chartplotter is set to the same datum as the chart, typically WGS84; a mismatch produces a small position offset that grows into a large problem in a tight channel. Zoom out occasionally to see the whole picture, because a screen zoomed to 100 metres will happily hide the rock two cables ahead.

Practising Before the Next Passage

Pick a harbour you know well and plan it on paper as though you had never been there. Note the soundings along your track, mark the tide height you expect, and identify every buoy you will pass. Then compare it with your plotter route and see where the two disagree.

On the water, run a familiar stretch at displacement speed and tick off the marks against the chart as you go. Bring a soft pencil, a set of dividers and a tide table, and keep the paper chart on the helm station even when the screen is on. That habit alone will make you a sharper, safer skipper — and it makes the fast passages far more enjoyable.