About Sailseer: how the passage planning works and where the data comes from

What is Sailseer?

Sailseer is a free web app for passage planning at sea and on inland waters. It routes from harbour to harbour through a network built on water depth data, reads nine weather models, sea state, tides, currents and official warnings for each waypoint at the estimated time of arrival, and rates whether and when the trip is worth making.

Sailseer is designed for cruising sailors and motorboaters with their own or a chartered boat, from a day sail to a multi-day trip or an offshore crossing. It runs in the browser at sailseer.com, in twelve languages, and was called “Segelwetter” until September 2026.

Who makes Sailseer

Sailseer is an independent project by Calvin Asmus from Hamburg, without advertising. Questions, bug reports and notes on incorrect harbour data go through the contact form, and details of the provider are in the legal notice. Next to every measuring station there is also a “Station looks wrong?” link for reporting implausible readings.

The sailing area

Sailseer reaches from the Caribbean to Svalbard and from the Atlantic to the Indian Ocean. Around 8,800 harbours, marinas and landing stages have their own page with harbour info (Harbours and cruising areas).

• North Sea and Baltic with Scandinavia, the Baltic states and Poland, the English Channel, Great Britain and Ireland
• The Atlantic coast down to Agadir, Madeira, the Canaries, the Azores and Cape Verde
• The Faroe Islands, Iceland, Greenland, Svalbard and Norway up to Kirkenes
• The whole Mediterranean and the Black Sea (Libya, Syria, Gaza, Ukraine and Russia with weather only)
• The Red Sea with the Suez Canal
• The Caribbean from Trinidad to the Bahamas and Bermuda, and the western Caribbean with the Panama Canal
• The U.S. East Coast from the Florida Keys to Maine with the Intracoastal Waterway, Chesapeake Bay and New England
• The Gulf of Mexico from Florida's Big Bend to Texas with the Gulf Intracoastal Waterway, and Mexico's Gulf coast to Yucatán
• Atlantic Canada from the Bay of Fundy to Newfoundland and the St. Lawrence River up to Montréal, with Saint Pierre and Miquelon
• The Indian Ocean from the Seychelles, north-west Madagascar and Mauritius via the Maldives and Sri Lanka to Phuket, Langkawi, Singapore and the Gulf of Thailand with Koh Samui
• Inland waters: 102 lakes such as Lake Constance and waterways from Mecklenburg and Berlin via the Göta Canal and the Canal du Midi to the Shannon and Saimaa

How the forecast is made

Sailseer reads nine weather models: ICON from the Deutscher Wetterdienst (DWD, Germany's national weather service), IFS and the AI model AIFS from the ECMWF, GFS from NOAA, the models of the Met Office and Météo-France, HARMONIE-AROME from DMI and KNMI, and MET Nordic from MET Norway. Regional models only count where and for as long as they actually run; AIFS does not output gusts and only counts for wind speed and direction. The model series come via Open-Meteo, plus the DWD's MOSMIX station forecast and, in the Caribbean, AROME Antilles.

For each hour Sailseer forms a consensus: the median of the models, their spread and how closely they agree. The ensembles of the ECMWF (51 runs) and ICON-D2-EPS (20 runs) provide the probability of strong gusts. The forecast covers seven days and the outlook for each harbour ten days, from the sixth day onwards as a trend.

Sailseer also checks which model was right most recently: the previous day's forecast is compared with the measurements from the nearest stations. The best model delivers, for each hour, the value of the most accurate model that is still running at that hour. More in the guide Weather models.

How the route is made

The route runs through a graph of harbours and waypoints on the water grid of EMODnet Bathymetry (west of 36 degrees west from GEBCO), and on rivers and canals along the waterways from OpenStreetMap. In the weather analysis, the boat comes into play:

• Boat speed comes from a polar diagram, scaled to the boat profile; in a calm Sailseer calculates with the engine.
• If the next waypoint lies too close to the wind, Sailseer lays out tacks in open water (Beating to windward).
• The tidal stream goes into the speed over ground, and against too much stream the plan waits for the turn of the tide (Tides and tidal streams).
• At bridges and locks the boat waits for the next opening, with times from EuRIS, ELWIS (the German waterways information service), Rijkswaterstaat FIS and the operators; the route avoids fixed bridges the mast does not fit under (Bridges and locks).
• No route leads through restricted and protected areas with a navigation ban. Sailseer checks the crossing angle at traffic separation schemes under COLREGs Rule 10, and the departure and destination harbours against the draught.

Each leg gets 100 points minus deductions, for example for gusts above the boat's limit, headwinds, sea state, warnings or night; the weakest leg counts, green from 70, yellow from 45. The same calculation for every full hour of the next 120 hours shows the best weather window.

Where the data comes from

The source and licence of every item are listed in the app under Profile, Sources and status. If a source fails, the site says so.

What Sailseer costs

Sailseer is currently free and can be used without an account: planning, checking the weather and navigation mode all work without signing up. For alerts, the morning briefing and the float plan all you need is an email address. An Android app is in closed testing.

Privacy

Sailseer shows no advertising and does not embed any third-party trackers or analytics services. Boat, favourites and saved routes stay in the browser. Sailseer's own server counts usage only as daily totals, without positions, and you can switch this off in the cookie banner and in the settings. An email address is only stored for alerts, briefings and the float plan, for as long as you use them. More in the privacy policy.

Important note

Sailseer is a planning aid and not intended for navigation. Official charts, marine forecasts, warnings and Notices to Mariners always take precedence. Responsibility lies with the skipper; in an emergency, use VHF channel 16 or DSC.

Terms such as consensus, turn of the tide or chart datum are explained in the glossary.