This guide describes what you see on screen, what every button does, and — most importantly — how to read what is shown. SeaData presents data that passage and work decisions are based on; knowing where a figure comes from matters as much as the figure itself.

1. The main screen

The chart fills the whole view. Everything else floats above it: the top bar, the side tabs on the left, Map layer top right, Tide stations bottom left, Overlay bottom right and the bottom bar.

The top bar

About «Fix age» — this is not a detail

The colour is a warning: green under 30 s, yellow under 5 min, orange under 15 min, red beyond that.

The figure shown is the sum of two ages: how old the last AIS message is, and how old the AIS service snapshot itself is. If the service stalls, the message looks fresh when it is not; adding them shows the real risk.

Buttons in the top bar

• ? — this user guide, with the Ask about SeaData search box on top.
• Clock — opens Data status: one ring per source (see below).
• Cog — Language, Position format, Speed unit, the Weather (Hs/Wind/Current) switch and AIS vessel (MMSI) to set your vessel. Your choices are remembered.
• Sun / moon — switches the colour theme.
• Eye — opens Overview (see below).

The bottom bar

• Cursor position on the left: the position under your mouse.
• Bullseye (Fleet+ and Fleet admin only) — pick your own spot on the chart instead of following the vessel. While it is on the button is yellow and the chart carries a yellow ChartPos — manual position — not a vessel position warning. Everything then applies to that spot, not to your vessel.
• Account type, e.g. Fleet admin.
• Three consensus rings: SeaWave consensus, SeaWind consensus and S.W.A.N consensus. Each shows how well the models for your position agree; the S.W.A.N ring compares SWAN's own wave height with the average of the other models. The button with the arrow picks the moment: now, +3h, +6h, +9h or +12h — fixed steps, because the models only publish every few hours.
• About SeaData — what SeaData does and which sources it runs on. The cookie preferences are at the bottom of that panel.
• Profile and Log out when signed in; Try it and Login when not.

Overview — the eye button

One look before you open a tab: WAVE, WIND, TIDE, a polar and CURRENT, each with the value now and a chart from 6 hours back to 24 hours ahead. The numbers are the consensus of all models covering your position; the chart shows the single best model, because an average of shapes flattens the peaks. Tide shows astronomical and actual water level side by side, current shows the pure tidal stream next to the total current. Next and Then are the next two turns of the tide in time order; Slack is the next slack water.

The polar points wind (blue), wave (yellow) and current (orange) inwards — where they come from — and your heading (HDG, solid red) and course over ground (CoG, dashed red) outwards. The difference between those two is your drift. The angles above it are relative to your centreline: 000 dead ahead, 090 starboard beam.

Data status — the clock button

One ring per data source, grouped as Wave, Wind + Warnings, Current, Tide, Observations and Radar. A ring is one refresh cycle: full and green just after a new run, draining towards red as the cycle runs out. Red also means missing or overdue. If a model is lagging, this is where you see it first.

Recently added: Met Office UKV, MET Norway MEPS, MEPS ensemble (30), DMI HARMONIE DINI, ICON-D2-EPS (20) and ECMWF ENS (50) under Wind, DWD EWAM under Wave, the DMI DKSS and RWS DCSMv7 current models under Current, and Meetplatforms, METAR offshore and Copernicus in-situ under Observations — the feeds behind the Platforms layer.

2. The map controls

2.1 Map layer (top right)

The dropdown picks the base map:

The SeaMarks layer (OpenSeaMap: buoys, fairways, lights) always sits on top of these.

Under the dropdown are switches. A dimmed flag or a grey button means that layer is off.

2.2 Platforms — measurements at sea

Platforms replaced the old AIS button in Map layer. It shows every fixed measuring station at sea with its latest wave, wind, current and visibility reading, merged from KNMI, Rijkswaterstaat, DMI, Copernicus Marine in-situ, Meetnet Vlaamse Banken, the Met Office, DWD wind stations (Alte Weser, Helgoland) and METAR/TAF from offshore platforms. The round logo says what it is: platform, buoy or mast / pile.

• Hover for the name and the key readings (Hs, Wind, Current, Visibility) with their time.
• Click for every source at that spot with all its values; a METAR station also shows the raw METAR and TAF text.
• The layer refreshes every 10 minutes while it is on.

2.3 Tide stations (bottom left)

One flag per country switches its tide and measuring stations on or off: NL, FR, UK, DK, NO, BE, SE, DE, EE, PL, LV, ES, PT, IT, TW, ID, JP. All flags start off; a layer loads the first time you switch it on.

π 50km is on by default: it shows every station within 50 km of your position, whatever the flags say. The flag row dims while it is on. Switch it off to go back to the flags.

Clicking a station opens its detail; hovering gives a short summary. Ports and their tides are in SeaHarbour.

2.4 Overlay (bottom right) — Waves and Wind

• Waves — wave arrows coloured by significant wave height Hs. In full in SeaWave.
• Wind — streamlines coloured by wind speed, with isobars labelled in hPa. In full in SeaWind.
• The slider under each legend sets how many arrows are drawn. The spacing is measured on your screen, so zooming does not pile them up.
• Satellite — MEASURED wave height (dots, from ten altimeter satellites via Copernicus Marine) and wind (arrows, ASCAT via EUMETSAT) over the last 24 hours, in the same colours as Waves and Wind. These are tracks of real measurements, not a field: where no satellite passed there is nothing. Full = 0–6 hours old, half = 6–12 hours, faint = 12–24 hours. A measurement is 1.5 to 3 hours old when it arrives. Click a dot or arrow for the value, the satellite and the time.
• Overlay folds the block away without switching anything off; Tide stations does the same bottom left.

Wave height scale (Hs): below 0.5 m blue · 0.5-1 m green · 1-2 m yellow · 2-3 m orange · 3-4 m red · above 4 m dark red.

3. The side tabs (left)

Ten tabs. Seven open a panel; AIS, ForeCast and SeaRain are switches that put a layer on the chart. Everything applies to the position of the active vessel, or to your bullseye spot. Click the same tab again or the cross to close a panel.

SeaWave — waves

Three tabs: Models (every wave model covering your position, ranked as in chapter 5), Online (TidesAtlas Marine) and S.W.A.N. Models has Graphics, Tables and DPR (the three-hourly values of today from the best model, the way a Daily Progress Report takes them).

S.W.A.N runs the SWAN wave model right at your vessel's position and lists every quantity as a number: wave height and swell, all the periods, mean and peak direction with spread, wavelength and steepness, and the orbital motion near the seabed. It refines the swell from a coarser model onto the local depth, so it is a point calculation rather than a map layer. It runs when the page loads and refreshes every 30 minutes; Recalculate forces a fresh run. A traffic light shows whether the run is complete.

SeaCurrent — current

Now (tidal, total and residual current with their source, a compass rose, and a 3D profile down to 200 m where the model provides one), Prediction (per hour) and Graphics.

SeaWind — wind

Now, Charts, 7 days, DPR (wind, gusts and direction every three hours from the best model), Models, Forecast (the KNMI shipping forecast) and Typhoon (CWA cyclone data). Several models side by side, each in its own colour.

SeaTide — tide

Chart and Table. Two groups of sources: actual water level (CMEMS NWS, Met Office NWS, IBI, MED) and astronomical tide (EOT20, FES2022, DTU23).

SolarNav — sun, moon, space weather

• Space weather — Kp index over three days, F10.7 flux, solar wind, Bz, the NOAA G/S/R scales, the GOES solar image and the impact on GNSS and Starlink.
• Interference — GPSJam map showing GNSS interference.
• Sun & Moon — dawn, sunrise, noon, sunset, dusk, day length and the moon phase with illumination, plus a seven-day table.

SeaHarbour — ports

• Port info (opens first) — harbour information from the World Port Index: every sea port in our areas that a vessel of 50 m or more can use, nearest first. Search by port, country or UN/LOCODE, pick an area (Europe, Asia, Japan), set your LOA and Draft, and tick Fuel, Water, Tugs, Ro-Ro or Medical. A port shows maximum vessel size, depths, pilotage, tugs, facilities, supplies and repair.
• Tide ports — all tide ports with the level now and the next HW and LW. measured is a tide gauge, model is FES2022 and less accurate in ports and rivers. Heights are relative to MSL. Clicking a port marker on the chart opens this list on that port.

AIS — other vessels (switch)

The AIS tab no longer opens a panel: it switches our own AIS layer on the chart on and off. Zoomed out you only see your own fleet; zoomed in you see every vessel in view. The count next to Platforms says how many are drawn, and when you are zoomed out it adds zoom in for all ships.

Colours follow the vessel type. A purple diamond is an aid to navigation (AtoN); an outline only means virtual (it does not physically exist), and red means off position.

ForeCast — warning areas by sea region (switch)

ForeCast puts the areas of the national weather services on the chart. Click an area for that service's bulletin; the popup closes by itself after 30 seconds.

SeaRain — precipitation (switch)

SeaRain replaced the separate radar buttons. A click on the tab switches our own precipitation layer on and off; the small cog in the tab opens SeaRain settings without switching the layer:

• Region — Europe, Asia or World. Switching on picks the region your vessel is in. World is a satellite estimate and the only one that shows anything on the open ocean; radar stops a few hundred km off the coast.
• Quality — Fast (2 km), Normal (1.5 km) or Sharp (1 km). Sharper costs your browser more memory.
• Storm cells — circles on convective cells above 40 dBZ: squalls, gusts, sudden poor visibility.
• Satellite — the hourly NOAA cloud image.

4. Workability

What can this vessel do at this spot? Workability is available on every account type, and on the demo screen without signing in. Nine sub-tabs, in the order you use them: Workability (opens first), Contract, Transit, Reporting, External weather, Vessel Online, Vessel, Limits and How it works.

The Demo vessel

Every account gets its own Demo vessel once: an example ship with a drawing, so you see a full result straight away. It is yours: change it into your own ship, rename it or delete it. It counts towards the number of vessels your account allows; once deleted it does not come back. On the demo screen (not signed in) it sits at the demo position in the North Sea and nothing can be saved or uploaded.

4.1 Workability — the polar

Pick a Moment (now up to +24 h) and press Calculate. Wave, wind and current come from the same sources the SeaWave, SeaWind and SeaCurrent tabs show for the position. The headline gives the answer (the best heading); the cards Waves, Wind and Current say why, each as Polar, Bars or Table.

• Colour per heading: workable below 0.8 of the limit, marginal 0.8-1.0, not workable above 1.0, and a separate mark for roll resonance. The ring is the limit.
• SWAN on / off (right of Calculate): with SWAN on, now uses a SWAN run at your position; for later moments, or with SWAN off, the best-ranked wave model is used. The line under the headline always says which, and why. While SWAN waits in the queue you see Waiting for computing power.
• What it calculated with lists every input: moment, wave source, Hs/Tp/direction, wind sea and swell, wind, current, the calculation point, wind area, sea state shape and station keeping.
• Confidence scores the answer (vessel data 50, environment 30, validity of the theory 20) and says what would improve it; below it a polar of wind + current + wave drift together.

4.2 Contract — can we work according to contract?

Per activity, per hour, up to 48 hours ahead: is the weather within the contract limits, and for long enough? Press Calculate; the headline says how many activities are workable now and when the next window opens for the rest.

An hour counts as workable only if every assessed limit is met and it falls inside a window at least as long as the activity's minimum. A quantity no model covers is Not assessed and said so in the line under the headline.

• Timeline per activity — one block per hour (UTC). It starts at the last three-hour mark (00, 03, 06 … UTC) before now, so there is always some history; those hours are dimmed and count nowhere. The dashed now line moves along every minute. Hover an hour for the values; click an activity to see its weather below.
• Windows — per activity the next window that meets its minimum, its length, what limits it most, how many hours are workable, and Chance now.
• Weather against the limits — the line is the model average (visibility: the lowest), the band runs from lowest to highest model, the dashed line is the limit.

Working limits (the cog button) opens the table Working limit for safe 24h vessel operations on site: per activity Hs, wind, current, visibility (up to 12000 m) and Min. window. The limits belong to a vessel (the one selected under Vessel; all loading conditions of a vessel share them). A vessel without its own limits uses example values (Transit, Standby on Location, MBES, Crane), and an orange Example limits bar says so: set your own before you rely on the result. + Add activity, Reset to example values and Save and recalculate do what they say; with no vessel selected nothing can be saved. Near-limit margin (default 10 %, 0 = off) sets how close to a limit an hour turns orange.

The chance — information only

The thin cyan block under each hour is the chance that Hs and wind stay within the limits, counted over the members of two ensembles: ECMWF ENS (50 members, wind and waves, up to 72 h) and ICON-D2-EPS (20 members, wind, up to 48 h). Hours where current or visibility already exceed their limit get 0. The count becomes a chance with the rule of Laplace: (favourable + 1) / (members + 2), so it runs from 1 to 99 % — a full count is not a certainty.

4.3 Vessel — 2D and Config

Pick the vessel at the top (Vessel); two inner tabs follow.

• Config — the vessel data in four steps: Required (profile name, vessel type, Lpp, beam, draught, working speed) is enough to start; Stability booklet, Extended - motions and Station keeping (DP) each make the answer better and the confidence higher. Save uses the profile name and the loading condition: same pair, same profile. Every field has a what is this?.
• Upload stability booklet (PDF) — upload only. The booklet is stored with your account so we can look at it with you if you ask for support. It is not read automatically and nothing is filled in from it: enter the values yourself.
• 2D — the vessel in Top view and Side view, at the same scale. Choose what a click places (Thruster, CoG, Working point); positions are measured from 0,0,0 at stern, centreline and deck, as on a general arrangement. The working point is the calculation point for heave and accelerations; empty means the centre of gravity. What this layout can do shows surge, sway and yaw capacity after the worst case failure, without any weather.

Your drawing. Under 2D you can upload your own general arrangement as DXF: Top view (DXF), Side view (DXF) or both, max 5 MB each, then Upload drawing. The lines are drawn in the Top and Side view instead of the outline of the standard hull; nothing is built from them. The drawing is aligned to its aftmost and lowest line. If a stern platform or a thruster below the keel puts that off the transom or the baseline, enter the correction (Side: transom, Side: baseline, Top: transom) and watch the zero point move. Bow on the left and Top view mirrored fix a drawing that is the other way round.

4.4 Limits and How it works

Limits holds what is still acceptable for this work: roll, pitch, heave at the working point, transverse and vertical acceleration, an Hs ceiling, green water and slamming at the bow, and thrust utilisation. A Preset fills them in (SeaData defaults, NORDFORSK intellectual work, NORDFORSK heavy manual work); adjust afterwards. Statistic chooses significant values or the expected maximum in 3 hours. How it works explains both calculation chains and lists every source with its status.

4.5 Vessel Online — what the vessel itself sends

Everything above works on models: a forecast for your position. Vessel Online is the other direction — what the vessel itself reports, from its own instruments. A small program on board, SeaData.exe, listens to the ship's NMEA network and sends what it hears to SeaData about every six seconds over ordinary https on port 443. Outbound only: nothing has to be opened up on board, and nothing is sent back.

You need the API key of your account — that is what tells SeaData whose vessel this is. Key and program are both on the API tab of your account, and the vessel must be ticked Position via SeaData.exe in your fleet. Without that tick the server refuses the data, on purpose: otherwise you send for months to a vessel you only think is working.

• Position, speed and course — used everywhere in the platform: the map, the vessel selector, Transit. It sits on top of shore AIS, because a vessel knows its own position better than a coastal station does.
• Heading from the gyro, which AIS often does not carry.
• Roll, pitch and heave — the largest excursion per axis over the window, the measure your limits are written in.
• Wind, true and relative, and air pressure, temperature, humidity and dew point from the weather station on board.
• Depth once an echo sounder is connected.

It can replace AIS where AIS does not reach. Shore stations only hear you near the coast, and the satellite feeds are minutes to hours old. A vessel that sends its own position stays on the map in the gaps — and does so every six seconds instead of every few minutes. Shore AIS stays as the fallback: switch the program off and the vessel simply falls back to it.

The tab shows whether the link is alive (a ring that fills with every burst), what came in, the last six hours as a graph, the last message line by line, and any refused bursts with the reason. Six hours is the whole memory: older data falls off and nothing goes into the database. Today the platform uses the position; heading, motion and weather are kept so they can be looked at, and are not yet part of any calculation.

4.6 Reporting — the weather report you send

Contract gives the answer on screen; Reporting turns it into a report you can send. You pick a technique from the contract table of this vessel and get the weather for the running day and the day after: 48 hours, always starting at 00:00 UTC, not at the hour you happen to look. Three forms — on screen, as a PDF and by e-mail — and all three come out of the same calculation, so they cannot disagree.

The report says what is over the limit and by how many per cent, which quantity is the binding one, how long the adverse weather still lasts, when work can resume and how far the models are apart. It calculates nothing itself: the averaging and testing stay in Contract; the report summarises that outcome and draws it.

• The status band — 48 hourly blocks per quantity, in the same colours as Contract, so an hour that is amber there is not red here.
• The meteograms — every model as its own line in the fixed colour of that model (the same colour it has in SeaWave and SeaWind), with the limit as a red line. Visibility gets one line on purpose: fog is local, and the contract counts the lowest model there.
• The limit ladder — everything as a percentage of its own limit against a single 100 % line, so Hs in metres and visibility in metres can be read side by side.
• Confidence from the ensembles (ECMWF ENS, 50 members to +72 h, and ICON-D2-EPS, 20 members to +48 h), per period as high, moderate, low or very low. No ensemble for this position means the field stays empty — never an invented percentage.
• Valid at — the report need not be about now. Pick a three-hour moment in the window (00:00, 03:00, 06:00 …) and the report applies from that hour. The calculation is identical; only the windows, the percentage and "status now" count from there. The report says so throughout, and gets its own file name.

The PDF is the printed version, not a picture of the screen: white, with hairlines and a status letter per hour, so a greyscale printer does not lose the message. E-mail goes to an address you fill in, with the PDF attached; there is a cap of ten reports per account per hour, and it is not available from the demo screen.

External weather belongs with this: the forecast your supplier sends as a PDF (DTN, StormGeo, Infoplaza). You see what we read from it and release it with Use in SeaData; after that it appears as an extra line in the meteograms. It never changes the go / no-go.

• Upload or e-mail. Upload the PDF, or send it to weather@wx.seadata.online from a sender address you have set in External weather. An address belongs to one account only.
• The box: at most 10 reports (the oldest is removed), each for 72 hours, at most 2 MB per PDF — for uploads and e-mail alike.
• Refused (bounced by the mail server): mail from an unknown address, without a PDF, with more than 5 PDFs, with a PDF larger than 2 MB, or from a sender that cannot be verified. The reason is in Profile › Notifications; External weather then shows a red Error.
• Unknown supplier: the PDF is accepted and reported to SeaData. As soon as the layout can be read, it is read automatically.
• Distance from the vessel: up to 50 km it can be used (Use in SeaData, Reporting); from 50 to 100 km it is only plotted in External weather, with a warning; beyond 100 km it is read but not plotted.
• DPR shows 06, 12, 18 and 06 UTC the next day, straight from the supplier's table; PDF shows the pages as images.

5. The rankings — which model leads, and why

This is the heart of SeaData and the least obvious part. In SeaWind, SeaWave and SeaCurrent the models are not in a fixed order: the model on the left fits your position best, the one on the right least. The order therefore changes as you sail.

The ordering is decided on the server, so no panel can invent an order of its own. Workability and Contract use the same ranking.

The three rules

• Rule 1 — if a model does not cover your position, it is dropped. Showing nothing is better than a grey area that implies certainty.
• Rule 2 — sort by resolution, finest on the left. A 2 km model sees more detail than a 13 km one.
• Rule 3 — a penalty for being near the edge. Near the edge of its domain a model is driven by the boundary conditions it receives from a coarser model, not by its own physics. So a fine model 5 km from its edge is worse than a coarser model well inside its own domain.

The score is resolution in km times penalty, and that is what is sorted on, lowest first.

This does by itself what «the country's own model» would do: AROME sits deep inside its domain off Brest and outside it off Hoek van Holland, and HARMONIE the other way round. No separate country rule is needed.

The wind models

Visibility (used by Contract) comes from the wind models that provide it: HARMONIE, HARMONIE DINI, ICON-D2, ICON-EU and GFS. The ensembles ECMWF ENS and ICON-D2-EPS are not ranked: they only feed the chance in Contract.

The wave models

A ranking of wind models says nothing about waves — hence a separate list.

How to read the ranking

Each model states the reason for its place:

• well inside its domain (180 km) — no penalty, full value.
• only 12 km from its domain edge — full penalty; here this model is driven by its boundary conditions.
• global model — worldwide, no edge problem, but coarse.

In practice: if the leftmost model differs markedly from the rest, read its reason. If it sits close to its edge, take it with a pinch of salt — which is exactly why the reason is there.

6. Provenance: measured, official or computed

Every sea level figure carries its origin, and the order is not arbitrary:

Official sources take precedence over our own computation. If the service publishes it, SeaData shows that — not its own sum. Grey means there is no reliable official source for that station.

Differences between models are shown, not smoothed away. If models disagree, the forecast is uncertain. You are entitled to see that.

7. Accounts and vessels

An account follows one or more vessels by MMSI. The type of account determines how many slots you get and how freely you can change them. Workability is included in every type.

Viewers (Fleet+ and Fleet admin): view-only logins for crew, office or client, made under Profile › Viewers.

While no vessel is set

Without a vessel there is no AIS position to base the data on. SeaData then falls back to a fixed spot in the North Sea. Everything you see — tide, waves, wind, current — applies to that spot and not to you. Enter an MMSI and the view follows your vessel again.

The hold period on Fleet standard

Remove a vessel and that slot stays reserved for seven days. Your account page shows this, with the date it becomes available again. Without that hold the number of slots would mean nothing: you could remove a vessel and add another straight away, changing as often as you like. The other types have no hold period.

Own position on the chart

Fleet+ and Fleet admin have the bullseye button in the bottom bar that lets you pick any spot on the chart instead of following your vessel (see chapter 1). On the other types the data follows the active vessel.

Without an account

Try it in the bottom bar opens a page where you enter an MMSI and see that vessel as your own for half an hour. It only works for a vessel currently in our AIS reception; otherwise the page says so. Without signing in, the Workability tab shows the Demo vessel at the demo position.

Registration

Every account is reviewed and released by hand. For information or a demo account, write to the address shown in the top bar (admin@seadata.online). You get a message once the account has been approved; you can sign in after that.

8. Disclaimer

• Always check against official sources: charts, tide tables and notices to mariners.
• Astronomical tides contain no surge from wind or air pressure.
• AIS is incomplete and delayed and is not suitable for collision avoidance.
• Workability and Contract are model-based estimates; the decision to work stays with the vessel.
• SeaData accepts no liability for the use of this data.

The terms are at Terms; the full attribution with holder, licence and required credit is at Attribution.

9. Quick fixes