Offshore passage planning and strategy

What changes offshore

Lesson 32 planned a coastal day passage round the island. An offshore passage uses the same plan, but more of it matters at once. You are out of sight of land for hours, so your position rests on the log, the compass, the tidal predictions and the electronics. The tide turns more than once on the way, so you plan one course for several hours of stream (Lesson 36). You may meet shipping lanes, the night, tired or seasick crew, and weather that changes before you arrive. And the end of the passage — the landfall, where you close the coast again — is often its most dangerous part.

Offshore planning is about timing: when to leave, so that the tide, the daylight, the lights and the crew all work for you at the landfall.

The crossing: Port Morvane to Port Kerran

On the night of Wednesday 2 June 2027 we take the course yacht (11 m, draught 1.8 m, 6 kn planning speed, a crew of four) from Port Morvane on the mainland to Port Kerran: 48.0 M of open water from WP1, off Morvane fairway buoy, to WP2, the landfall waypoint north-east of Gull Point, then Lesson 32’s route in reverse round Gull Point and into Kerran Bay — about 63 M berth to berth. Between the mainland and the island runs a traffic separation scheme.

Tidal strategy: one course for the night

An eight-hour crossing sees most of a tidal cycle, so we don’t correct the course hour by hour. As in Lesson 36, we lay off every hour’s stream end to end, then steer one course for the whole crossing: streams setting opposite ways partly cancel, and we sail fewer miles through the water than if we corrected for each hour in turn. The passage chart has its own diamonds, E, F and G, referred to HW Port Kerran like A–D, and each hour is read from the diamond nearest to where we expect to be.

The tidal gate at Gull Point

The departure time is fixed by the landfall end. From WP2 we go south past Gull Point and The Teeth, where diamond B runs at up to 2.1 kn tonight. The flood there sets north-east — foul — until 0607; then the stream goes slack and from 0707 the ebb runs south-west, fair; it stays fair or slack until about 1225. So we want to reach WP2 soon after 0607, in daylight, with Gull Point light seen before dawn as a check.

Leave earlier and you wait off Gull Point in the dark for the stream to turn; leave later and Gull Point light rises after dawn, so you lose its check on your position (the stream stays fair until about 1225). 2200 gives the light at about 0423, before dawn (0431), and the landfall at 0629 as B turns: WP3 0657, WP4 0757, berthed about 0824. We slip from Port Morvane at 2125.

Crossing the traffic separation scheme

Lesson 20 introduced Rule 10. At the chart table it becomes part of the plan: where and when you cross, how long you will be in the lanes, and what the stream does meanwhile. Our scheme has two lanes of 3.0 M either side of a 1.5 M separation zone: 7.5 M in all. Its lanes run east–west, so the heading at right angles is 180°T.

Landfall: aim off, and time it

With GPS working you simply steer for WP2. But a landfall plan must still work if the electronics fail, and after 8.0 hours on estimated positions the error can be large: a stream prediction just 0.25 kn out every hour adds up to 2.0 M, before any error in the log, the steering or leeway. If you aim straight at Gull Point and see nothing, you won’t know which way to turn.

So aim off: steer deliberately to one side of the target, by more than your likely error, so that you know which side of it you are. Choose the side away from danger (with no danger either side, the up-tide side is a good choice: the stream then helps you after the turn). Here danger decides: east, seaward of The Teeth — aim for a point 2.0 M east of WP2 (49°49.4'N 004°00.5'W), about 3.8 M east of Gull Point light: more than the likely error. When you identify Gull Point light or the land, you know the island lies to starboard, to the west.

Then time the landfall: close an unfamiliar coast in daylight, or at night only on lights you can identify and expect to see. Two checks decide which lights count. First, when should it rise? As in Lesson 34, its rising distance is the sum of the distances of the horizon for the light and for your eye — and you see it no further off than its nominal range. Second, is it actually in sight? The line of sight must clear the land.

When a light first appears, its bearing and the rising distance give a rough position — rough, because refraction varies and your height of eye changes as the boat moves. If it shows when you stand and disappears when you crouch, it is right on the horizon. Often the loom of a powerful light, its glow in the sky, appears first.

Watches, fatigue and seasickness

Through the night someone must always be on watch: steering or minding the autopilot, keeping a lookout, checking the course, and logging the position every hour. A watch system shares that out so everybody also sleeps. There is no single right system; these are common examples.

For our four we might run watch A 2100–0000, B 0000–0300, A 0300–0600 and B 0600–0900, with the skipper called for ships whose bearing doesn’t change, any drop in visibility, a change in the wind, the lanes, the first sight of Gull Point light, or anything the watch is unsure about. At each change, hand over the position, the course and why, ships in sight, the lights expected and the weather.

Fatigue builds quietly: start the passage rested, eat hot food and drink regularly, keep warm, and use the time off watch to sleep or at least rest. Attention is usually lowest in the small hours — one more reason to save the landfall and pilotage for daylight. Seasickness affects most people at some time. Seasickness tablets work best taken before you sail, as the instructions or a pharmacist advise, and some cause drowsiness. Fresh air, the horizon, small plain snacks and water help; reading, the chart table and the galley usually make it worse.

Go or no-go: the weather window

Lesson 38 covers forecasts; here, the decision. A weather window is a spell of acceptable weather long enough for the whole passage plus a margin for delays — so look at the forecast beyond your ETA, not just for the start. Set your limits in advance, while you are calm and ashore: for example, no more than force 5 (17–21 kn) for the crossing, visibility moderate (2–5 M) or good, no gale warning, and no strong wind against the stream at Gull Point. Read the timing words with your own timetable: “force 6 later” in a forecast issued at 1700 means after 0500 — right at our landfall. Make the decision at a fixed time (here 1800) on the latest forecast — go, delay or cancel — and don’t let a schedule talk you out of it.

Contingencies and ports of refuge

Decide now what you would do if the plan stops working, and where you could go at each stage.

Leaving the plan ashore

Before you go, give a responsible person ashore the boat’s name and description, who is on board, the route, ports of refuge, departure time and ETA, and how to reach you — with a time to call the coastguard if they haven’t heard from you (ours: 1200 on the 3rd). Tell them when you arrive or change plan, so no search starts by mistake. Where the coastguard runs a voluntary safety scheme for recording your boat’s details, join it: rescuers will know what they are looking for.

The plan on one card

Everything above, as you might write it for the chart table and the crew briefing:

Work the plan backwards from the landfall: the gate, the daylight and the lights set the departure, and everything else follows.