Everything wet is measured up from chart datum. Everything dry or overhead is measured down from MHWS.
| You want | Sum |
|---|---|
| Depth of water now | charted depth + height of tide |
| Depth over a drying patch | height of tide − drying height |
| Clearance under the keel | depth of water − draught |
| Clearance under a bridge | charted height + MHWS − height of tide |
| Range of the tide | HW height − LW height |
Springs: new and full moon, biggest range, about two days after the phase. Neaps: first and last quarter, smallest range. Compare the day's range with the spring and neap ranges printed in the curve panel and interpolate.
Mark HW height on the top scale and LW height on the bottom scale, join them with a sloping line. Time → height: enter on the time axis, up to the curve, across to the sloping line, down to the height scale. Height → time: the same picture backwards. Spring curve solid, neap curve dashed, interpolate between. Two times answer every height — say whether you mean the rising or the falling one.
1 · 2 · 3 · 3 · 2 · 1 twelfths of the range, hour by hour.
Half the range arrives in the middle two hours. It is a sanity check for a roughly six-hour, roughly sinusoidal tide — never for the Solent's double high water. Use the curve for the answer.
Work the standard port first. Interpolate the time difference between the two reference high waters (0000 and 0600) by your actual HW time. Interpolate the height difference between MHWN and MHWS by your actual HW height, and between MLWN and MLWS for the low. Lay it out as a table — standard, difference, secondary — and the marks follow the layout.
Work the answer, then take off your own clearance: commonly 0.5–1.0 m under the keel, more in a swell. State the margin you chose.
Enter checks, Enter again moves on. 1–4 pick an option. Alt+←/→ change stage. Progress is saved in this browser.