Weather and a Simple Passage Plan

Prerequisites

• You have read the lessons What is the ICC? and Passing the ICC Practical Assessment, and you know that the coastal waters (C) category needs a knowledge assessment as well as a practical one.
• You can read a chart, measure a distance in nautical miles, and work out a time from distance and speed (time = distance divided by speed).
• You know what high water, low water, springs and neaps are, and roughly how a tidal stream atlas works. The ICC examiner keeps tidal questions simple, but you must understand the principle.
• If you are heading for the Mediterranean, the lesson Chartering Abroad covers documents and charter handover; this lesson covers the weather and planning side.

Learning Objectives

By the end of this lesson you will be able to meet these items of the ICC syllabus:

• ICC-4c Planning 1 and Navigation 5: name the sources of weather information in the UK and abroad, interpret a forecast, and judge its impact on a passage and on the sea state.
• ICC-4c Planning 2: prepare a simple passage plan for tidal and non-tidal waters, covering aims, hazards, tidal gates, timings, waypoints, ports of refuge and a plan B.
• ICC-4a and 4b Preparation, and Safety 2: show the examiner on the day that you obtained and assessed an appropriate forecast before setting out.
• Use the Beaufort scale and the official sea state and visibility terms, and explain why wind against tide makes the sea dangerous.
• Recognise the big local winds a charterer meets in the Mediterranean, such as the mistral, meltemi and bora.

What the Examiner Will Ask

The RYA's guidance to test centres puts the weather question in its simplest form: where did you get today's forecast, what does it say, and what does it mean for what we are doing? On the practical assessment you will be asked at the start of the day. On the coastal knowledge assessment you may also be asked to talk through a short passage plan on a sample chart.

You do not need to be a meteorologist. You need to show that you get a proper marine forecast from a reliable source, check it against a second one, understand the words, and turn it into a decision: go, wait, change the route, or stay in harbour.

Sources of Weather Information

Haz clic para ampliar

In UK waters

The Shipping Forecast. Issued by the Met Office and broadcast on BBC Radio 4 four times a day (check the current schedule in the almanac, because times and the long-wave service change from time to time). It covers the large sea areas round the British Isles for the next 24 hours, always in the same order: gale warnings, general synopsis, then for each sea area wind, sea state, weather and visibility. It is good for the big picture but says little about conditions close inshore.

The Inshore Waters Forecast. Covers coastal waters out to 12 nautical miles, in stretches of coast such as "Selsey Bill to Lyme Regis", with a 24-hour forecast and a further outlook. For most coastal passages by ICC holders this is the forecast that matters most. It is read on Radio 4 after the late Shipping Forecast and published on the Met Office website.

HM Coastguard Maritime Safety Information (MSI) on VHF. The Coastguard announces on channel 16 that a broadcast is coming and names a working channel, then reads the Inshore Waters Forecast and any gale or strong wind warnings. Routine broadcasts are made every three hours; times and channels are in the almanac. This works where the phone signal does not.

Navtex. A dedicated receiver that automatically prints or displays forecasts, navigational warnings and gale warnings. The international service on 518 kHz is in English; in the UK, 490 kHz carries inshore forecasts as well. Navtex keeps a written record, so you can read the forecast again at sea.

Online. The Met Office website and app give the same text forecasts, plus pressure charts and warnings.

Abroad

Every coastal country in Europe has an official marine forecast. The principles are the same; only the names and channels change, so look them up in the almanac or pilot book for your cruising area before you go.

• The national met service: Météo-France, AEMET in Spain, the Italian Aeronautica Militare, DHMZ in Croatia, the Hellenic National Meteorological Service in Greece. Most publish a marine forecast online, often with an English version.
• VHF broadcasts. In France the CROSS rescue centres broadcast Météo-France coastal bulletins (bulletins côte, out to 20 miles) at fixed times, announced on channel 16 with "sécurité" and read on a working channel such as 79 or 80. Strong wind warnings (BMS) are issued when Force 7 or more is expected. Croatia runs continuous recorded forecasts on dedicated VHF channels in several languages including English; Spain, Italy and Greece also have coastguard or coast radio broadcasts.
• Navtex works abroad too: 518 kHz in English everywhere in the system, 490 kHz in the national language.
• The marina office and the charter base. Most marinas pin up the day's forecast, and the charter base briefing is the best place to ask about local winds and harbours to avoid.

Apps and GRIB files, with caution

Weather apps and GRIB files (computer files of raw weather model output, viewed in an app or on the plotter) are popular and genuinely useful for seeing how the wind will change over the day. But they are computer model output, not a forecaster's judgement. They tend to:

• Show the mean wind, not the gusts. Gusts are often 30 to 50 percent stronger, and much more under squally showers or off high ground.
• Smooth out local effects: acceleration round headlands and between islands, wind funnelling down valleys, and sea breezes.
• Say nothing about wind against tide.

Use them alongside an official forecast, not instead of one. When two sources disagree, plan for the worse of them.

Understanding the Forecast

The Beaufort scale

The Beaufort scale describes the mean wind speed at 10 metres above the sea. The sea state column below is typical of open water once the wind has been blowing for some time.

A strong wind warning is issued for UK inshore waters when Force 6 or 7 is expected; a gale warning means Force 8 or more. Many charter companies forbid leaving harbour in Force 6 or above.

Sea state and visibility terms

Forecasts describe the sea by wave height, using fixed words:

Visibility: good is more than 5 miles, moderate 2 to 5 miles, poor 1,000 metres to 2 miles, and fog less than 1,000 metres. Timing: imminent means within 6 hours of the time of issue, soon 6 to 12 hours, later more than 12 hours. Always note the time of issue: a gale "soon" on a forecast issued at 0520 may arrive by lunchtime.

Swell is separate from wind waves. A long swell left behind by a distant storm can roll into an anchorage on a calm day, and breaks dangerously on harbour bars.

Wind against tide

When the wind blows against the tidal stream, the waves are shortened and steepened, and they break. A Force 5 against a 2 knot stream can produce a sea that feels like a Force 6 or 7 with the stream. The effect is worst where the stream is strongest: off headlands, in races and overfalls, and in narrow channels.

Haz clic para ampliar

The cure is timing. Pass headlands and narrows at slack water, or when wind and stream run the same way. This is why a tidal gate is not only about speed over the ground.

Turning the forecast into a decision

Interpreting a forecast means asking what it does to your passage:

1. Strength. Is it within the limits of this boat and this crew? A first charter with a family is not the day for a Force 6 beat.
2. Direction. Will you be beating, reaching or running? Is the wind blowing onto the coast (a lee shore) or into your destination harbour? Is your planned anchorage open to it?
3. Change. Is it increasing, veering or backing during the day? A front passing at 1500 matters if you plan to arrive at 1600.
4. Sea state. Will wind against tide or a swell make a headland or bar dangerous at the time you will be there?
5. Visibility. Will poor visibility or fog make pilotage into an unfamiliar harbour unsafe?

Then make a decision and be ready to change it. "We go, but we turn back at the headland if it is more than Force 5" is a good answer in an assessment.

Local winds abroad

Mediterranean weather is often settled for days, then changes violently. The big local winds have names, and an examiner may ask about the area you plan to visit.

Sea breezes everywhere follow the same pattern: calm mornings, wind building onshore from late morning to a peak in mid-afternoon, dying at dusk. In the Mediterranean, plan passages to arrive before the afternoon wind peaks, especially in meltemi season.

Preparing a Simple Passage Plan

A passage plan is the skipper's thinking done in harbour, so that decisions at sea are easy. The RYA's appraisal, planning, execution and monitoring framework (APEM) applies to a 10 mile hop as much as to an ocean crossing. For the ICC you need a plan that is simple, written down, appropriate to a small boat, and that answers these questions:

1. Aim. Where are we going, how far is it, and when must we arrive (daylight, marina office hours, a lock or sill)?
2. Weather. What do two sources say, and what are my limits for this passage?
3. Tides. Heights at departure and destination for any shallow areas or sills; streams along the route. In non-tidal waters, note any currents and the timing of the afternoon wind instead.
4. Tidal gates. Any headland, race or narrows I must pass at a particular state of tide. Plan the gate first, then work backwards to the departure time.
5. Route and waypoints. Tracks and distances, with waypoints in safe water, never on top of a buoy or rock, and clear of shipping lanes and traffic separation schemes.
6. Hazards. Rocks, shoals, overfalls, ferries and commercial traffic, firing ranges, fish farms, pot buoys, swimming zones.
7. Timings. Departure time, ETA at each waypoint, and a check of fuel range for the whole distance plus a reserve.
8. Ports of refuge. Harbours or anchorages along the way you can enter in the forecast conditions, at any state of tide if possible.
9. Plan B and decision points. The conditions that make you turn back or divert, and the places on the route where you will decide.
10. Pilotage notes for leaving and entering harbour: marks in order, depths, port entry signals, VHF channels. Small boats keep out of the way of large ships in channels.

Haz clic para ampliar

Tidal versus non-tidal planning

Telling someone

Tell someone ashore your plan and expected arrival time. In the UK you can register your boat and details with the Coastguard's voluntary safety identification scheme (CG66), which gives rescuers vital information if you call for help. Abroad, the charter base usually wants to know where you are heading each day.

Worked Example: A Tidal Passage

Aim. A 10 m yacht with four crew, Yarmouth (Isle of Wight) to Poole, about 18 miles, arriving in daylight. Boat speed about 5 knots under sail or engine.

Weather. The Inshore Waters Forecast at 0520: "South-west 3 or 4, occasionally 5 later. Slight, occasionally moderate later. Fair. Good." A weather app agrees on direction and shows gusts to 20 knots in the afternoon. Within this crew's limits.

Tides and the gate. The gate is Hurst Narrows, at the western end of the Solent, where the stream can reach about 4 knots at springs. From the tidal stream atlas, the west-going stream there begins at roughly HW Portsmouth minus 1 hour and is strongest a few hours later. Today HW Portsmouth is 1015, so the stream turns fair at about 0915. (These figures are for illustration; always take the real times and rates from the current almanac or atlas.)

Wind against tide. A south-west wind against a west-going stream means wind against tide in the Narrows and the Needles Channel. Two decisions follow: go through early in the fair stream, before it reaches full strength, and take the North Channel past North Head buoy rather than the Needles Channel, which is more exposed to the south-west sea and runs past the Bridge reef and the Shingles bank.

Timing. Leave Yarmouth at 0930; at Hurst, 3 miles away, by about 1000 with a fair stream building. North Channel to Poole Bar buoy is about 12 miles, roughly 2 hours with some stream help, then 3 miles up the harbour. ETA Poole about 1300, well before the wind increases later.

Hazards. Lymington ferries off Yarmouth, the fast stream and shingle spit at Hurst Point, the Shingles bank, pot buoys, and in Poole the Sandbanks chain ferry at the entrance, which has right of way: keep clear of it. Poole Harbour has busy channels with cross-Channel ferries and commercial traffic; the almanac shows the recommended small-craft routes alongside the main channel.

Ports of refuge. Yarmouth and Lymington before Hurst; Poole itself at the far end. Christchurch is nearby but has a shallow bar and fierce entrance stream: not for a deep-keeled visiting yacht.

Plan B. Decision point at Hurst at 1030. If it is blowing more than Force 5, or if the sea in the Narrows is breaking badly, or if the boat is running more than 30 minutes late, turn back to Yarmouth or into Lymington.

Communications. Listen to the Coastguard MSI broadcast on the way; VHF channel 16 and Poole Harbour Control's working channel noted from the almanac.

Worked Example: A Non-tidal Passage

Aim. A charter yacht in Croatia, Milna on the island of Brač to the Pakleni Islands off Hvar, about 10 miles, to pick up a marina berth by early afternoon.

Weather. The DHMZ forecast and the continuous VHF broadcast: "North-west 2 to 3 in the morning, maestral 3 to 4 in the afternoon. Sea slight. Tomorrow evening, risk of bura on the northern coast." An app shows the same and a gust warning for the following night.

Interpretation. Today is a pleasant passage, and the afternoon maestral will be on the beam or quarter for a southward passage. Tomorrow's bura matters for tomorrow night's berth: by tomorrow afternoon the crew must be in a harbour sheltered from the north-east, not a bay open to it. Check the pilot book for which harbours are safe in a bura.

Timing. No tidal gate. Leave at 1000, motor-sail across the Hvar channel in the light morning breeze, arrive about 1230, before the busiest time for berths and before the maestral peaks.

Hazards. Ferries and fast catamarans crossing the Hvar channel; rocks and shoals among the Pakleni Islands, where the chart and pilot book must be studied before entering any channel; buoyed swimming areas off beaches; many boats on mooring buoys.

Ports of refuge. Milna (behind you), Hvar town harbour and the Pakleni Islands marina. Each has a different exposure, so check against the wind direction.

Plan B. If the marina is full, a mooring buoy in a sheltered bay for one night, moving to a bura-safe harbour the next morning. If the bura arrives early, stay in Milna.

Common Mistakes

• Using one source, once. A forecast heard at breakfast is out of date by lunchtime. Check the official forecast before leaving and listen to updates during the passage.
• Taking the app's mean wind as the whole story. Expect gusts and local acceleration well above it.
• Ignoring the time of issue, so "soon" and "later" are misread.
• Forgetting wind against tide when choosing the time to pass a headland or narrows.
• Planning from the departure time forwards instead of from the tidal gate backwards.
• Putting waypoints on buoys or rocks. With modern GPS you will hit them. Offset waypoints into safe water.
• No plan B, or a plan B that is no use in the forecast wind, such as a harbour open to it.
• Arriving in the Mediterranean afternoon wind peak in meltemi or mistral season.

Summary

• UK sources: Shipping Forecast, Inshore Waters Forecast, Coastguard MSI on VHF, Navtex and the Met Office online. Abroad: the national met service, coastguard VHF broadcasts, Navtex, the marina and the charter base.
• Apps and GRIB files are model output: use them with an official forecast, expect stronger gusts, and plan for the worse source.
• Know the Beaufort scale, sea state terms (slight, moderate, rough) and the timing words (imminent, soon, later).
• Wind against tide makes short, steep, breaking seas; pass headlands at slack water or with wind and stream together.
• Know the big local winds where you are cruising: mistral, meltemi, bora, jugo.
• A simple passage plan covers aim, weather, tides, gates, route and waypoints, hazards, timings, refuges, plan B and pilotage, and is worked backwards from any tidal gate.

Check Your Understanding

1. Name four sources of a marine forecast in UK waters.

Answer: Any four of: the Shipping Forecast on BBC Radio 4, the Inshore Waters Forecast, HM Coastguard MSI broadcasts on VHF, Navtex, and the Met Office website or app.

1. You are chartering on the French Atlantic coast. Where would you get the forecast?

Answer: Météo-France online, the CROSS VHF coastal bulletins (announced on channel 16), Navtex on 518 kHz in English, and the forecast posted in the marina office.

1. Why should a GRIB file or weather app not be your only forecast?

Answer: It is raw model output showing the mean wind. It underestimates gusts and smooths out local effects such as headland acceleration and sea breezes, and says nothing about wind against tide.

1. What wind speeds does Force 5 represent, and what sea state is typical?

Answer: 17 to 21 knots, with moderate waves and many white horses.

1. A forecast issued at 0520 says "gale soon". When might it arrive?

Answer: Between 6 and 12 hours after the time of issue, so roughly between 1120 and 1720.

1. Why is a Force 5 against a 2 knot tidal stream more dangerous than a Force 5 with it?

Answer: Wind against tide shortens and steepens the waves until they break, so the sea is much rougher, especially off headlands and in races.

1. What is the meltemi, and how would it affect your passage planning in Greece in August?

Answer: A strong summer northerly in the Aegean, typically Force 5 to 7, strongest in the afternoon and lasting days. Plan short passages, sail early, arrive before the afternoon peak, and choose harbours sheltered from the north.

1. Why do you plan a tidal passage from the tidal gate backwards?

Answer: The gate fixes the time you must be at a particular point. Working back from it with distance and speed gives the departure time; working forwards risks arriving at the wrong state of tide.

1. List six items a simple passage plan should include.

Answer: Any six of: aim and distance, weather and limits, tidal heights and streams, tidal gates, route and waypoints, hazards, timings and fuel, ports of refuge, plan B and decision points, pilotage notes for departure and arrival.

1. How does choosing a port of refuge differ in the Mediterranean compared with the Solent?

Answer: In tidal waters it must be enterable at the state of tide when you need it. In the non-tidal Mediterranean depth is not the issue; it must be sheltered from the forecast wind direction, for example not open to the north-east if a bora is coming.