Two almanac figures become a height for any minute, a secondary port's tide, and the hour to cross a bar, round a race or make a tidal gate.
1 · The tidal curve
Any height, any time
How do I find the height of tide at a given time from the tidal curve?
Mark HW on the top scale and LW on the bottom one and join them; from the time go up to the curve, across to the line, and read the height: LW + factor × range.
On a standard port's tidal curve, enter the HW nearest your time in the box, so the time reads HW−4 or HW+2. Mark that HW on the top scale and the LW of the same half-tide on the bottom one, the LW before on the flood, after on the ebb, and join them. From the time go up to the curve, across to the line, and read the height: LW + factor × range.
Backwards, factor = (height needed − LW) ÷ range gives the time. Between the spring and neap curves go the fraction that today's range lies between the mean ranges; beyond either, use the nearer curve, never extrapolate.
2 · Secondary ports, the Solent
A standard port's curve, moved
How do I work out the time and height of high water at a secondary port?
Apply the secondary port's differences to the standard port: interpolate the time difference between its two tabled HW times and the height difference between MHWS and MHWN, then work on the standard port's curve.
A secondary port has differences on a standard port: time differences at two times of the standard port's HW, height differences at MHWS and MHWN, MLWN and MLWS. Interpolate each by today's value, then work the hour on the standard port's curve with the secondary port's times and heights.
Secondary port — high water
Standard port HW 0900 · 5.2 m
Time diff: HW 0600 −0030 · HW 1200 −0010
0900 is halfway: −0020, so HW 0840
Height diff: MHWS 5.8 −0.6 · MHWN 4.6 −0.2
5.2 is halfway: −0.4, so HW 4.8 m
From Swanage to Selsey high water is a long stand or a double high water, so these ports have curves drawn in hours from low water; pick the curve by the range at Portsmouth. Left of LW is the ebb, right of it the flood. Southampton holds near the top for two hours after the first HW; its five-hour fall runs harder than the seven-hour rise.
3 · Clearances
Under the keel, over the mast
How do I calculate the depth under my keel in a swell?
Charted depth + height of tide, minus draught, half the swell for the troughs and squat at speed: 5.4 + 1.2 − 1.0 − 0.3 − 2.2 leaves 3.1 m under the keel.
Depth = charted depth + height of tide; take off draught, half the swell for the troughs, and squat at speed. At anchor, depth now − the fall to come must cover draught and margin, and springs make tomorrow's LW lower. Least charted depth = draught + margin − height of LW. A sill gauge already reads the depth over the sill.
Overhead: room = charted clearance + HAT − height of tide. A power cable's safe clearance allows for arcing; never trim it by eye.
A forecast storm surge adds to the prediction and already holds the pressure effect: never twice. A positive surge bites on a spring HW; a negative one takes water off a shallow passage. North Sea: a deep low tracking SE, a northerly gale behind it.
Bridge on a surge
Clearance 15.0 m above HAT (6.0 m)
Predicted HW 5.6 m · surge +0.5 m
HW tonight: 6.1 m
Room: 15.0 + 6.0 − 6.1 = 14.9 m
Air draught 14.2 m: 0.7 m to spare
4 · Drying out and bars
The bottom comes up
When is it safe to cross a bar at a river or harbour entrance?
On the flood and in daylight: a bar breaks when the depth falls to about 1.3 times the swell height, sooner in a long swell, and is worst on the ebb against the swell.
To float over drying ground you need drying height + draught + margin of tide; she touches when the tide falls to drying height + draught. Arrive just before HW and let her settle, after sounding the berth: level, firm, clear. Heel a fin keel slightly to the wall, rig a fender board, and tend the lines all night. Go on while the tides are making; once they take off, the next HW may not float you.
A bar breaks when the depth falls to about 1.3 times the swell height, sooner in a long swell, worst on the ebb against the swell. Cross on the flood, in daylight; otherwise stand off or go to a harbour without one. Bars shift: trust the latest notice, and a closed bar is closed.
5 · Rates and tidal gates
How hard, and when
What do the numbers 18,34 on a tidal stream atlas arrow mean?
1.8 knots at mean neaps and 3.4 knots at mean springs; interpolate between them by today's range at the atlas's own reference port. With today's range halfway between the mean ranges, the rate is 2.6 kn.
An atlas arrow marked 18,34 means 1.8 kn at mean neaps, 3.4 kn at mean springs. Interpolate by today's range at the atlas's own reference port; this line you may extend beyond springs and neaps. French tables grade a tide by its coefficient: 45 mean neaps, 95 mean springs. Near an amphidromic point the range is small but the stream strong: rates come from the atlas.
Rate for today
Atlas arrow: 18,34
Mean ranges: springs 5.6 m · neaps 2.8 m
Today's range 4.2 m: halfway
Rate = 1.8 + ½ × (3.4 − 1.8) = 2.6 kn
A leg: each hour in turn, added as vectors
Set = rate × time in the stream, so a slow yacht is set further. At 20 knots the stream hardly changes the time; wind against tide at each end decides.
Two yachts, one stream
Leg 30 M · beam stream 2 kn, steady
6 kn: 5 h in it, set 10 M
20 kn: 1 h 30 min, set 3 M
If the stream turns, the slow one wins back
Fuel against a stream: hours × litres/hour
A tidal gate opens only with the stream. Time to leave = time it opens − distance ÷ speed over the ground. Take an unavoidable foul tide where it is weakest, the fair one through the narrows.
6 · Races and overfalls
Where the stream breaks the sea
How do I round a headland with a tidal race safely?
Outside the race's charted extent plus a margin, wider at night and in fog, or through it at slack water. The inshore passage, a few cables off, is for settled weather, neaps and daylight.
A race forms where a strong stream sweeps past a headland and lies downstream, so it changes sides with the stream. Overfalls stand over an uneven bottom. Wind against stream steepens every sea: the water feels wind plus stream.
The inshore passage, a few cables off: settled weather, neaps, daylight, the pilot's time. Outside: the race's charted extent plus a margin, wider at night and in fog. Or through at slack: work back from the start of the fair stream, given as hours from HW at a reference port. Springs double the rate and shorten the slack; if the passage outlasts the slack, go at neaps, and a slow yacht arriving late waits a tide. Before a race: harnesses, hatches, briefing; planing boats slow down.
7 · Ocean currents
Off the shelf
Do I need to allow for tidal streams in mid-ocean, off the continental shelf?
Hardly: beyond the shelf the streams are weak and rotary and nearly cancel over a cycle. Plan with the routeing chart's mean current, check it by fixes, and apply a set that holds for half a day.
Beyond the shelf the streams are weak and rotary and nearly cancel over a cycle. Plan with the routeing chart's mean current, check it by fixes, and keep applying a set that holds for half a day. After a gale the wind drift sets right of downwind in the northern hemisphere. Off Portugal the Portugal Current runs south in summer with the northerlies; in winter a N-going current follows the shelf edge. In the Strait of Gibraltar the surface flows east into the Mediterranean, and the W-going streams run inshore.
Current in the DR
Tidal streams: rotary, up to 0.8 kn
Over 30 h they cancel out
Mean current NE 0.4 kn: 30 × 0.4 = 12 M
Apply it every hour; recheck at each fix
Practice questions
Questions from this topic, as the app asks them. Open one to see the answer.
Dover — today
HW 0412 6.6 m · LW 1105 0.9 m
HW 1638 6.4 m · LW 2330 1.0 m
Curve: hours HW−6 … HW+6
Wanted: height at 1400
You are working the height of tide at Dover for 1400 on the standard port's curve. Which high water do you write in the box under the curve?
1638 — then 1400 is HW−2 h 38 min
Either HW — the curve is the same both sides
0412 — the higher HW, 1400 is HW+9 h 48 min
1105 — curves start at LW: LW+2 h 55 min
Show the answer
A. 1638 — then 1400 is HW−2 h 38 min
Enter the HW nearest the time you want: 1400 is 2 h 38 min before the 1638 HW, on the rising side of the curve.
You are anchoring off Lundy for the night in your 24 m motor yacht, open to the swell. What is the least depth from the surface at which you let go now?
9.2 m
9.8 m
11.0 m
8.6 m
Show the answer
A. 9.2 m
At LW you need 2.0 + 1.0 + ½ × 1.2 swell = 3.6 m; the tide falls 6.5 − 0.9 = 5.6 m, so 3.6 + 5.6 = 9.2 m now.
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