Halberry Roads
15 days · F 0.05 · semidiurnal
Springs and neaps: S2 beating against M2, and nobody drew that envelope.
- Height of tide
- 8.98 m
- State
- Ebbing
- Clock
- 00:00
- Next
- LW 0.90 m
1.38 m/h
0 h into the window
in 5 h 11 m
Tide prediction for small craft · 8 stations
Know the water before you are standing in it.
Wrackline gives skippers, pilots and harbour staff the height of tide at any minute of any day, at every station on the coast — offline, on the boat, with no account and no signal. Every figure is synthesised from the station's own harmonic constituents, and the constituent tables ship with the app so you can check the arithmetic yourself.
Fleet and harbour-authority licences: ring the duty watch on 0191 498 0272. Duty watch 06:00–22:00, every day of the year.
- 8 stations
- 8 constituents each
- ±0.11 m residual
- 0 requests offline
The prediction, running
Three coasts. Eight numbers each.
Same predictor, same code, same 900 samples. Only the amplitude and phase columns change. Scroll to run each window.
Skellow Bar
48 hours · F 0.06 · semidiurnal
Two high waters four hours apart. That is M4 — shallow water, not astronomy.
- Height of tide
- 2.57 m
- State
- Flooding
- Clock
- 00:00
- Next
- HW 5.71 m
1.30 m/h
0 h into the window
in 2 h 57 m
Nethermill Reach
48 hours · F 0.06 · semidiurnal
Four hours of flood against eight of ebb. Do not plan a passage on the mean.
- Height of tide
- 7.74 m
- State
- Flooding
- Clock
- 00:00
- Next
- HW 8.36 m
1.09 m/h
0 h into the window
in 1 h 03 m
What you get
Built for the three moments a tide actually matters.
Wrackline is a working instrument for small craft, not a reference book. It is sold to skippers, pilot boats, dive charters, harbour staff and survey crews, and it is priced so a single boat can buy one.
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Planning
Every station, every minute, fifteen days out
Height of tide at any instant, not a four-figure table you have to interpolate between. Highs, lows, ranges and the spring–neap position for every station on the coast, out to the limit of the analysis.
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On the day
A gate you can read at a glance
Set a least depth and a draught and Wrackline tells you the window: when the water makes your figure, when it goes, and how much of it you have left. It answers the question people actually ask a tide table.
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Offline
No signal, no account, no telemetry
Everything is computed on the device from the constituent tables, which ship with the app. It works with the aerial down and the phone in flight mode, and it does not report where you are, because it never needs to know.
Why it is right
Eight cosines. Nothing drawn, nothing fitted by eye.
A tide is the superposition of a small set of harmonic constituents, each a cosine at a period fixed by astronomy and identical at every port on earth. What differs from port to port is the amplitude and the phase lag — and that is the whole of what a tidal analysis produces, and the whole of what Wrackline stores.
Take S2 out and the fortnight goes flat: the spring–neap envelope is S2 beating against M2 at 14.765 days and nobody drew it. That is the test, and it is the second strip below. The full method.
Halberry Roads, 15 days
8 of 8 constituents · height in metres above chart datum
- Mean spring range
- 7.90 m
- Mean neap range
- 3.70 m
- Spring–neap beat
- 14.765 d
- Form factor
- 0.05
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M2
Principal lunar semidiurnal
12.4206012 h · 2.90 m · g 318°
One high water every 12 h 25 m, and every one the same height as the last. No port on earth looks like this.
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S2
Principal solar semidiurnal
12.0000000 h · 1.05 m · g 4°
Springs and neaps. Nobody drew that envelope — S2 runs 25 minutes a day faster than M2 and the two go in and out of step every 14.765 days.
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N2
Larger lunar elliptic semidiurnal
12.6583475 h · 0.56 m · g 300°
The springs stop matching each other. The moon's distance varies over a month and N2 is that variation.
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K2
Lunisolar semidiurnal
11.9672348 h · 0.29 m · g 2°
A small correction to the same beat, and the reason two consecutive spring tides are never quite equal.
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K1
Lunisolar diurnal
23.9344697 h · 0.11 m · g 122°
The first once-a-day term. The two highs of a day begin to separate.
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O1
Principal lunar diurnal
25.8193417 h · 0.09 m · g 348°
The lunar diurnal. With K1 it decides the form factor — here 0.05, which is semidiurnal.
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P1
Principal solar diurnal
24.0658902 h · 0.04 m · g 118°
Small enough to leave out of a rough prediction, and large enough to be missed in one.
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M4
Shallow water overtide of M2
6.2103006 h · 0.12 m · g 210°
Shallow water, not astronomy. The crest of the wave outruns its trough over a shoaling bottom, and the curve stops being symmetrical.
The eight strips are drawn at 34% of the sum panel's vertical scale and share one axis with each other, so P1 at 0.04 m really is that thin beside M2 at 2.90 m. At full scale M2 alone would be 147 units tall in a 60-unit strip.
Coverage
Six coasts that behave nothing like each other.
Two days at each, drawn from that station's own table. A double high water, a four-hour flood against an eight-hour ebb, a mixed tide where one of the two daily highs is not worth having — same eight constituents, different two columns.
Every one of these coasts is the same eight cosines. Shape is geometry; timing is astronomy.
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Halberry Roads
The reference station. Everything on this site is checked against it first.
spring range 7.90 m · F 0.05 · semidiurnal
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Skellow Bar
Two high waters, four hours apart, and neither of them is the real one.
spring range 5.26 m · F 0.06 · semidiurnal
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Nethermill Reach
Floods in four hours and takes eight to go. Do not plan a passage on the mean.
spring range 6.28 m · F 0.06 · semidiurnal
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Cargreave Sound
A mixed tide. One of the two daily highs is worth having and the other is not.
spring range 1.66 m · F 0.90 · mixed, mainly semidiurnal
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Ossett Head
The plain case. Semidiurnal, symmetrical, and nothing to argue with.
spring range 4.34 m · F 0.07 · semidiurnal
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Feltwater Ness
Half a metre on a spring. The semidiurnal wave has a node not far offshore.
spring range 0.74 m · F 0.35 · mixed, mainly semidiurnal
And two that are not tides
A predictor that cannot produce a flat line when the amplitudes are zero is broken, and the only way to know is to keep a station where they are. Both are carried in the same table as the six above and drawn by the same code.
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Brindley Dock
Impounded above a sill. There is no tide inside and the predictor says so.
all eight amplitudes 0.000 m · range 0.00 m
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Marnhead Test Gauge
One constituent and nothing else. A tide that could not happen, kept so the predictor can be checked against arithmetic.
M2 only, 1.50 m · seven amplitudes 0.000 m
What eight will not tell you
The prediction is astronomy. The weather is not.
A deep low can lift the whole curve half a metre for a day. A hard onshore blow will hold high water in for hours after the table says it has gone. Wrackline predicts the astronomical tide and publishes the residual — the difference between what it said and what the gauge recorded — because a figure without a residual is a guess with a decimal point on it.
- 0.11m
- median residual, all stations, 2025
- 0.54m
- largest surge recorded against prediction
- 96%
- of highs within ±0.25 m of prediction
Read the published residuals Wrackline is an aid to planning and is not a substitute for an official tide table. Do not navigate on it.
The seabed
Soundings, plotted. Never contoured.
A depth contour is not a measurement. It is a line drawn between two soundings by somebody who was not there. Every plate Wrackline ships publishes the figures the survey actually recorded and draws exactly one stroke — the coastline — and nothing else. 1,531 soundings across 7 sheets, and not one contour.
The seabed on these sheets is a worked model and not a survey record, and the page that carries them says so. How the plates are checked.
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SB/3 · Skellow Bar · 229 soundings
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NR/2 · Nethermill Reach · 265 soundings
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CS/1 · Cargreave Sound · 150 soundings
“We ran it against the gauge on the wall for a season. On the bar it was inside a tenth of a metre nearly every time, and on the two days it was not, the residual page had already told us why.”
- 8
- stations, controls included
- 1,531
- soundings published as figures
- 14.765 d
- spring–neap beat, not stored anywhere
- 0
- network requests once installed
Pricing
One price, one year, 8 stations.
No account, no subscription trap and no per-station charge. Tell us which plan and an invoice and a download link go out the same working day.
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Single device
£18 a year
One phone or one tablet. Everything offline.
- Every station and its constituent table
- All soundings at full sheet size
- Both control stations
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Most boats
Boat
£34 a year
Up to four devices on one vessel. The plotter build is in this one.
- Everything in Single device
- Four devices on one vessel
- The plotter build
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Club
£210 a year
Up to forty members, one invoice, and the sheets printable at A3.
- Everything in Boat
- Forty members on one invoice
- Sheets printable at A3
Harbour authorities, sailing schools and survey contractors: ring the duty watch on 0191 498 0272 or send the watch a message.
Take the whole coast with you.
All 8 stations, offline, on one device, for a year. No account, no telemetry, and the constituent tables in the download so you can check the arithmetic yourself.
Wrackline is an aid to planning and is not a substitute for an official tide table. Do not navigate on it.