🌊 Tidal Stream Guide

Rule of Twelfths tide calculator, passage planning with tidal gates, spring vs neap explainer, and Beaufort scale reference.

High Water Time (HH:MM)

Tidal Range (m)

Tide Type

Chart Datum (lowest water, m)

Rule of Twelfths — Hourly Tide Heights

🌕 Spring Tides

Occur at new moon and full moon when Sun, Earth, and Moon align. Tidal range is largest — biggest highs and lowest lows. Strongest tidal currents. Occur twice monthly.

🌓 Neap Tides

Occur at first and last quarter moon when Sun and Moon are at 90°. Smallest tidal range — moderate highs and lows. Weakest tidal currents. Occur twice monthly between spring tides.

Passage Planning Notes

⚓ Tidal Gate: Plan your departure so you arrive at a tidal gate (shallow bar, narrow channel) at favourable tide state. Work backwards from the gate time.

🔄 Fair Tide: Always aim to carry the tide with you. A 2-knot tide saving adds or subtracts significantly over a passage. Counter a fair tide can double passage time.

📐 Tidal Diamonds: On Admiralty charts, tidal diamonds (♦ with letter) give tidal stream data: direction and rate for each hour around HW at a reference port.

Beaufort Wind Scale

Force	Description	Wind (kts)	Wave Height	Sea State
0	Calm	<1	0 m	Mirror glassy
1	Light air	1–3	0–0.1 m	Ripples, no foam
2	Light breeze	4–6	0.1–0.5 m	Small wavelets
3	Gentle breeze	7–10	0.5–1.25 m	Large wavelets, some crests
4	Moderate breeze	11–16	1–2 m	Small waves, frequent whitecaps
5	Fresh breeze	17–21	2–3 m	Moderate long waves, spray
6	Strong breeze	22–27	3–4 m	Large waves, extensive foam
7	Near gale	28–33	4–5.5 m	Sea heaps up, foam streaks
8	Gale	34–40	5.5–7.5 m	Moderately high waves, spindrift
9	Strong gale	41–47	7–10 m	High waves, dense foam, crests topple
10	Storm	48–55	9–12.5 m	Very high waves, sea white with foam
11	Violent storm	56–63	11.5–16 m	Exceptionally high waves
12	Hurricane force	64+	14+ m	Air filled with foam and spray