How to compare departure times for a coastal passage

Start with the same route and two realistic times

Say you could leave at 08:00 or 11:00. Keep the route, waypoints and starting speed estimate the same for both options. Then calculate when you would reach each place. Changing the route and departure time together makes it harder to see what the later start actually changes.

Choose points that help explain the trip: the harbour exit, the start and end of an exposed stretch, and the destination. Add any place where you particularly want to compare conditions. The RYA includes weather, tides, currents, daylight and possible delays among the things to consider in a passage plan. Read the RYA passage planning guide.

A worked example: 24 nautical miles at 6 knots

This is an invented coastal trip with illustrative forecast values, not conditions for a real place or date. Every time below uses the same local clock on the same day. We assume an average speed over the ground of 6 knots, so the boat covers 6 nautical miles each hour. The 24 nautical mile route takes 4 hours.

For each leg, divide its distance by speed to find the travel time. Add the leg times together to get the elapsed time from departure, then add that to each possible start.

The key comparison is now clear. Look at the headland forecast for 10:00 and 13:00, then the destination forecast for 12:00 and 15:00. Reading every waypoint at 08:00 would describe the coast at one moment rather than the weather encountered as the trip progresses.

Compare the forecast at each arrival time

Imagine the headland forecast shows wind from the west at 11 knots at 10:00, increasing to 18 knots at 13:00. Illustrative wave heights are 0.7 metres and 1.2 metres respectively. At the destination, imagine wind from the southwest at 12 knots at noon and from the west at 19 knots at 15:00.

In this example, the earlier start reaches both places during lighter forecast winds. That is a specific difference worth recording. It does not make 08:00 a universal answer for this route. Another day's forecast could favour the later departure.

Keep wind direction, gusts, wave period and visibility beside the headline numbers. For waves, height alone leaves out useful detail. Our guide to wave height and wave period explains what the two measurements tell you.

Check the stretch between waypoints too, especially if a forecast changes during the hour you expect to spend there. An endpoint comparison can miss a change partway along a leg.

Put every forecast on the same clock

Check both the forecast issue time and its valid time. The issue time tells you when the product was published. The valid time tells you when the depicted conditions apply. NOAA explains this distinction in its Ocean Prediction Center FAQ.

Write down the time zone beside your departure options. If a forecast uses UTC and your trip notes use local time, convert before comparing. NOAA's marine forecast matrix documentation shows separate UTC and local time rows, illustrating why checking the label matters. See the NOAA forecast matrix explanation.

Also match the forecast area to the route. The Met Office inshore waters forecast covers named coastal areas and includes wind, weather, visibility and sea state. Use that wider description alongside forecasts for individual places. See the Met Office marine forecast guide.

Repeat the comparison with a slower trip

At 5 knots over the ground, the same 24 nautical miles takes 4 hours 48 minutes. The headland, 12 nautical miles from departure, comes after 2 hours 24 minutes. An 08:00 start now reaches it at 10:24 and finishes at 12:48. An 11:00 start reaches it at 13:24 and finishes at 15:48.

A delay before leaving shifts the whole schedule as well. Leaving 30 minutes late at the original 6 knots moves the earlier option's headland arrival to 10:30 and destination arrival to 12:30. Reopen those forecast hours instead of keeping the original weather notes beside the new times.

Keep a short note for each option

Record the departure time, headland conditions, destination conditions and slower arrival estimate in two columns. Include any arrival constraints, such as daylight or a harbour opening time, using the destination's published information. Refresh both sets of forecasts before departure so you compare the same forecast update.

You should be able to explain the difference in one sentence: the earlier option reaches the exposed stretch before the forecast wind increase, while the later option arrives after it. For choosing the broader period in which to make the trip, read how to find a weather window for a coastal passage.

Sources and further reading

• RYA passage planning
• NOAA Ocean Prediction Center forecast times
• NOAA marine forecast matrix explanation
• Met Office guide to marine forecasts