Finding a weather window: when is the best time to leave?

A good weather window is not simply the day with the least wind. It is the period in which wind, waves, current and light suit exactly your passage, your boat and your crew. Check them one by one and you will less often cast off at the wrong hour.

First: your limits

Before you look at the forecast, decide what is acceptable: up to which gust, which wave height and how many hours at a stretch. A well-drilled crew on a large yacht has different limits from a family with small children. In Sailseer they live in the boat profile: size, boat type, draught, mast and, if you like, your own limits for gusts and waves.

Wind direction relative to your course

The same 15 kn feels very different on a beam reach from dead on the nose. Upwind you have to beat, the distance gets much longer, the crew gets wet and the waves slam into the bow. On a broad reach the same strength is pleasant, whereas dead downwind in strong gusts it is rolly, with the risk of an accidental gybe. Because the wind shifts on the way, Sailseer works out the wind angle for every hour of every leg and lays out tacks when the course is upwind. How that works is explained in the guide Beating to windward.

Gusts, not mean wind

The gust is what counts for the decision. A mean of 14 kn with gusts of 28 kn is a different day from a steady 18 kn. Look at the models' gusts and at the probability from the ensemble; more on this under Gusts and thunderstorms.

Waves and comfort

Height alone tells you little. A short, steep sea on the bow stops a small boat in its tracks, while a long swell from astern merely lifts it. A cross sea of wind waves and swell makes every watch tiring. Sailseer estimates the comfort of each leg from the angle of the waves to the course, the wavelength relative to the boat's length and the steepness, in four levels from calm to very rough.

Tidal stream

In the North Sea, the English Channel and along the Atlantic coast, the stream can dictate the whole passage. Against 2 kn of foul tide, a boat making 5 kn through the water does only 3 kn over the ground, and wind against tide builds a short, steep sea. Sailseer includes the tidal stream in the speed over ground. If the boat cannot make headway against the stream, the plan waits for the turn of the tide, and at shallow tidal inlets it checks whether there is enough water on arrival.

Daylight

It is better to enter an unfamiliar harbour in daylight. Plan your arrival with time to spare before sunset, especially in autumn and further south, where dusk is short. Sailing in the dark costs points in the rating, and the sailing window alert can be limited to daylight departures.

How far ahead to plan?

The further ahead, the coarser the forecast. Sailseer's hourly forecast runs seven days ahead, and the departure comparison checks the next 120 hours. The 10-day outlook for each harbour, from ECMWF and GFS, only answers whether next weekend might be worth it: it gives a confidence level from the ensemble and from day six onwards shows only a trend. Firm plans should wait for the model runs of the last two or three days before departure. How the models differ is explained in the guide to weather models.

Comparing departure times

Instead of guessing a time, Sailseer works through every full hour of the next 120 hours as a departure, with wind for each place and hour, waves, current, waiting times at bridges and locks, and daylight. Each departure gets a score counting down from 100, and the weakest leg counts: green from 70, yellow from 45, red below that. A gale warning, thundery conditions, waves above the boat's limit or visibility below 1 km set it to zero. Underneath you can see how the score comes about, so you can check the reasons yourself. Plan your passage on the map and look at the departure comparison.

Getting notified

• Sailing window alert: for a passage, Sailseer notifies you by email or push as soon as a window of 55, 70 or 80 points or more opens, with 48, 72 or 120 hours' lead time and, if you like, only for daylight departures. When no window is left, a single “Window closed” message follows.
• Morning briefing: at your chosen hour between 05:00 and 10:00 local time, an email for one harbour covering morning, midday, afternoon and evening, the warnings and, where there are tides, high and low water.

Before casting off

Shortly before departure: read the official marine forecast and warnings, look at the radar and the measured wind at the nearest station, and compare the latest model run with your plan. If something no longer fits, staying in harbour is a good decision. Sailseer is a planning aid, not for navigation; official charts, marine forecasts and warnings take precedence.