How to plan a coastal passage

Start with the route and the legs

Draw the route you intend to follow, keeping off the shore by enough to clear hazards and give yourself room to manoeuvre. Then break it into legs at every change of course, every headland, every harbour entrance and every place where the conditions are likely to change. Measure each leg in nautical miles.

Legs matter more than the total. A 40 mile trip is not one block of weather: it is a run down an open stretch, a headland with its own stream, a bay where the sea breeze fills in, and an entrance with a bar. Each one has a different question attached to it, and the plan should answer each one separately.

1. 1Departure harbour, leaving at 07:00.
2. 2The headland is the tidal gate. The stream runs fair here from 07:30 to 11:30.
3. 3A bolt hole you can enter in the wind forecast for the day.
4. 4The last turn before the approach.
5. 5The destination inlet, with a bar at the entrance.

Speed, arrival times and the slow case

Time is distance divided by speed. At 6 knots, a 15 mile leg takes two and a half hours. Work out the arrival time at every waypoint, and write it next to the waypoint, because every later decision in the plan depends on those times.

Use the speed you actually make good over the ground, not the speed through the water on a flat day. A displacement powerboat or trawler holds its cruising speed well, so its times are reliable. A sailboat's speed depends on the wind angle, so use a conservative average and expect to motor some of it. Then add a fair or foul stream where you know it runs.

Then plan the slow case. Work the same route at about 20 percent below your planned speed, which covers a foul stream you underestimated, a sea that slows you down, or a late start. Every check that follows has to hold for both sets of times.

Tidal gates, pinch points and daylight

A tidal gate is a place you can only pass comfortably at certain states of the tide. Headlands, races, narrow sounds and inlets are the usual ones, because the stream accelerates where the water is squeezed. At a strong headland the stream can run at three knots or more, which turns a 6 knot boat into a 9 knot boat one way and a 3 knot boat the other.

For each pinch point on the route, look up the stream: NOAA tidal current predictions in US waters, a tidal stream atlas or the tidal diamonds on the chart in the UK and Europe. Note when it turns fair, when it is strongest, and when it turns foul. Then work backwards from the strictest gate to your departure time. If the headland in the example runs fair from 07:30 to 11:30, you want to pass it near the middle of that period, with the slow case still inside it.

Pay particular attention where the wind will blow against the stream. An inlet on the ebb in an onshore breeze can stand up into short, breaking seas even on a moderate day. Our guide to wind against tide shows how much current it takes.

Add daylight to the same timeline. An entrance you don't know, a bar, a crowded harbour or a field of fishing floats is far easier in daylight, so most coastal plans aim to arrive with a couple of hours of light in hand.

1. 1Daylight: sunrise 06:40, sunset 17:50.
2. 2The stream at the headland: fair from 07:30 to 11:30, foul either side.
3. 3The plan at 6 knots: leave 07:00, round the headland at 09:30, arrive 13:40.
4. 4The slow case at 4.8 knots: round the headland at 10:08, arrive 15:20.
5. 5Both headland times fall well inside the fair stream, and both arrivals come before sunset.

A weather window for the whole passage

Read the forecast for each leg at the hours you will be on it, from your departure to your slow-case arrival. A calm morning at the dock tells you only about the first hour. The question is whether the wind, the sea, the visibility and the risk of thunderstorms stay inside your limits on every leg, at the time you will be there.

Look hardest at the changes: the sea breeze that fills in after lunch, a front due in the evening, a swell building from a distant storm. Compare the models. When GFS, ICON and ECMWF agree on the timing of a change, you can plan around it with more confidence than when they spread it across half a day. Check for marine warnings on every zone the route crosses. Our guide to finding a weather window covers this step in detail.

Bolt holes, crew and boat

Mark every harbour along the route you could enter if the day changes, and note what each one needs: an entrance that faces away from the forecast wind, enough depth at the state of tide you would arrive, and somewhere to tie up or anchor. A bolt hole with a bar that breaks in an onshore wind is not a bolt hole on an onshore day. Note the time to reach each one from the legs either side.

Set limits for this crew and this boat, not for the toughest sailor aboard. A crew new to a sea will tire faster than you expect, and a passage that is comfortable for four hours can be hard work after eight. For a longer trip, plan the watches, the meals and when people will rest.

On a powerboat, check the fuel range against the route with a reserve. A common rule is a third of the fuel out, a third back and a third in reserve. Fuel use rises steeply in a head sea, so work the range at the slow case too.

The passage plan checklist

1. Route and legs. Plot the route, break it at every change of course or conditions, and measure each leg.
2. Speed and arrival times. Put an arrival time on every waypoint at your speed over the ground, and again at about 20 percent slower.
3. Tidal gates and pinch points. Look up the stream at every headland, inlet, bar and narrows. Set your departure around the strictest one.
4. Daylight. Check sunrise and sunset against your departure and both arrival times.
5. Weather window. Check wind, seas, visibility and thunderstorm risk on each leg for the hours you will be there, through the slow arrival, and look at model agreement and warnings.
6. Bolt holes. List the harbours you can enter in the forecast wind and tide, with the time to reach each.
7. Crew, boat and fuel. Agree the limits, plan the watches, and check the fuel range with a reserve.
8. Float plan. Tell someone ashore your route, your expected arrival and when to raise the alarm.
9. Final check. Before you leave, refresh the forecast, the warnings and the observations, and run the times again.

The final check before you leave

Forecasts are updated several times a day, so the plan you made last night was built on older numbers. Before you cast off, refresh the marine forecast and the warnings for every zone on the route. Then compare the forecast with what is happening now: in US waters the NOAA buoy network gives measured wind and waves, and in the UK the Met Office inshore waters forecast is reissued through the day.

If the observations are running ahead of the forecast, with the wind already stronger than it was supposed to be at this hour, assume the change will come early. Run the arrival times again against the new forecast. If the window still covers the slow case at every pinch point, you have a plan. If it doesn't, the choice is between a later start, a shorter destination and another day, and it is far easier to make that choice at the dock.

Sources and further reading

• RYA passage planning
• NOAA Tides and Currents: tide and tidal current predictions
• National Weather Service marine forecasts
• NOAA National Data Buoy Center
• Met Office guide to coast and sea forecasts