Tidal currents: when to get in the water
Estimate the strength and direction of the current at your site, find slack water and choose your entry time.
Today's tide
High water time at the reference port and tidal coefficient, from the tide tables.
The current at your site
Speeds are read on the nautical chart (tidal diamonds) or in a tidal stream atlas.
Maximum current
The flood sets towards 090° E
High water slack
Current shape
Different flood and ebbestuary, headland
The ebb sets towards 270° W
Your dive
Entry time, duration, and the wind if it blows.
Wind
Current at entry
0.3kn
towards 090° E · 0.1 m/s · 8 m/min
WeakDuring the dive
Noticeable current during the dive
max 0.6 kn during the dive. Start the dive into the current so it brings you back.
Next tidal slack: 10:30 (in 15 min)
Estimated drift over 50 min: 260 m towards W
for a dive centred on slack water (10:30): max 0.4 kn instead of 0.6
Understanding tidal currents
How do you estimate a tidal current?
You need the high water time at the reference port, today’s tidal coefficient and the site speeds at springs (coefficient 95) and neaps (coefficient 45), read on the nautical chart or in a tidal stream atlas. The speed is interpolated from the coefficient, then the current shape is applied through the tide: the 50/90 rule or the rule of thirds.
Diving at slack water
Slack water is when the current stops before reversing. It does not always fall at high or low water: the offset depends on the site. The tool finds the next slack, suggests an entry time that centres the dive on it and lists the day’s calm windows under your comfort threshold.
Wind and drift
A steady wind adds a surface current of about 3 % of its speed. Tide and wind add up like arrows: the tool shows the resultant current, its direction and the estimated drift over the whole dive. To anticipate your gas consumption in a current, see the air consumption calculator tool; for the cold, the heat balance tool.
Frequently asked questions
What is the 50/90 rule?
With a gradual (sinusoidal) current, it reaches about half its maximum speed one hour after slack and 87 % (nearly 90 %) after two hours. It peaks at mid-tide.
How do you calculate the current for a given coefficient?
V = V(neaps) + (V(springs) − V(neaps)) × (coefficient − 45) ÷ 50. Example: 2.4 kn at springs, 1.2 kn at neaps, coefficient 70: 1.2 + 1.2 × 25 ÷ 50 = 1.8 kn.
Above what current should you not dive?
Common guidelines: under 0.5 knot you fin without trouble; from 0.5 to 1 knot it is tiring; above 1 knot you can no longer swim against it, drift dive with a pickup boat only. Your club may set other limits.
How do you read "12,06" in a French tidal atlas?
It means 1.2 knots at springs and 0.6 knot at neaps, in tenths of a knot. In Atlas mode, you can type "12,06" as is in the springs box: the tool fills both boxes.