Anchoring & piloting

Tides & clearance

Estimate tide height by the Rule of Twelfths, then check clearance under the keel.

Rule of twelfths

Low tide height

High tide height

Duration low → high (hours)

Hours since low tide

Works the same rising or falling — for a falling tide, swap the low/high figures and count hours since high water instead.

Height of tide—

Hour	Rise this hour	Cumulative	Height

Charted depth

Height of tide

Vessel draft

Safety margin

Water depth (charted + tide)—

Clearance under keel—

Use the same units throughout (feet or metres) — the calculator doesn't convert.