Ocean Routing and Weather Routing

Prerequisites

This lesson builds on the Ocean Meteorology and Ocean Passage Planning lessons and on the Coastal Skipper and Yachtmaster Offshore weather syllabus. You should already be able to:

• Read a synoptic chart, identify fronts and lows, and apply the Beaufort scale.
• Explain the general circulation: trade winds, the ITCZ, subtropical highs and the westerlies.
• Describe how tropical revolving storms form and why they are avoided.
• Use a Pilot Chart or atlas of wind and current rose data at a basic level.
• Plan a passage in stages, with fuel, water, stores and crew endurance considered.

Learning Objectives

By the end of this lesson you should be able to:

• Use the standard reference books for ocean routing: World Cruising Routes (Jimmy Cornell), Ocean Passages for the World (NP136), Pilot Charts and Modern Marine Weather (David Burch), and say what each is good for and where it is weak.
• Choose a departure window by season for the main ocean crossings, and explain why seasons matter more than speed.
• Describe the classic trade-wind routes and the circuit of the North Atlantic.
• State hurricane and cyclone seasons for each basin and apply the usual rules for staying out of them.
• Explain how GRIB files are made, what they show and, above all, what they do not show.
• Explain isochrone routing, polar diagrams and why a computed route is only advice.
• Use ensemble forecasts to judge confidence, and name the main official products from NOAA OPC, NHC and the Met Office.
• Work through a complete Atlantic circuit and make sensible go or no-go decisions along the way.

Why Routing Is Mostly About Timing

On a long passage the most valuable decision you will make is not the course; it is the date you leave and the place you leave from. An ocean passage in the right season in an average boat is usually pleasant. The same passage in the wrong month can be dangerous in any boat. For this reason ocean routing has two layers:

1. Strategic routing is done months ahead from climatology (long-term averages). It decides the season, the general route, the stopovers and the places to avoid altogether.
2. Tactical routing is done day by day from forecasts. It decides how to sail round the next high, when to gybe, and whether to heave to or divert.

Climatology tells you what usually happens; a forecast tells you what is predicted to happen this week. Neither is enough alone. A good navigator reads climatology for the plan, forecasts for the decision, and uses their own eyes, barometer and judgement to check both.

The Reference Books

Ocean Passages for the World (NP136)

Ocean Passages for the World is Admiralty Publication NP136, published by the UK Hydrographic Office. It is the classic reference for routes in all oceans, written originally for sailing ships and steam vessels and updated since. For each ocean it gives:

• Recommended routes between major ports, split into seasons.
• Typical distances along each route, which are useful for first estimates of passage time.
• Descriptions of the prevailing winds, currents, ice, fog and tropical storm areas.
• The reasoning behind the route: why you head north or south of the rhumb line, and where the favourable current lies.

Its strength is the sound underlying logic: routes are given with their reasons. Its weakness is that it is written for large powered vessels in places, so some of its routes and passage times do not suit a small yacht. Treat it as the reference for the physical shape of each ocean, not as a sailing instruction.

Pilot Charts

Pilot Charts show monthly averages of conditions for an ocean region. They are produced by the US National Geospatial-Intelligence Agency (NGA) and are free to download, one chart per ocean, one panel per month. They typically show:

• Wind roses: the percentage of the time wind blows from each of eight directions, with mean force shown on the arrows.
• Percentage of gales (Beaufort 8 or more) and calms.
• Surface current arrows with mean speeds.
• Tracks of past tropical storms for that month.
• Limits of ice, the ITCZ position and typical barometric pressure.
• Great circle and recommended routes.

The data come from many years of ships' logs, so they favour the busy shipping lanes. Where few ships go, the data are thin. Remember also that averages hide extremes: a 10 per cent chance of gale in a month is a gale on roughly three days of the month on average, and it will not come when it is convenient.

World Cruising Routes (Jimmy Cornell)

World Cruising Routes by Jimmy Cornell is written for cruising yachts. It gives some 1,000 routes, each with a recommended season, distance, expected passage time, expected winds and currents, and notes on landfalls and hazards. It is the book you will actually see on yachts. Its great strength is that it is practical and yacht-sized. Its main weakness is that it is a summary: it does not replace the pilot book for a harbour or a current chart for a strait, and recommended dates are based on typical years.

Modern Marine Weather (David Burch)

Modern Marine Weather by David Burch teaches you how to understand and use weather information at sea. It covers:

• How to read the weather around you: cloud sequences, pressure tendency, wind shifts.
• Handling pressure systems and fronts and how they move and develop.
• How to read and use GRIB files, weather fax and text forecasts, and how to judge how far to trust them.
• Tropical weather, including tropical cyclone avoidance rules.
• The role of the navigator in routing: knowing enough to question the data.

Burch's key message is that the sailor must be able to compare forecasts with what is happening outside, because the model output can be wrong and the sky and barometer are free and immediate.

Using the Books Together

Seasonal Windows

The reason ocean cruisers move in the same pattern each year is that the planet imposes a seasonal calendar. The idea is to be in the trade-wind belts when they are steady, and out of the tropics when cyclones are possible.

The diagram below shows the trade-wind belts on either side of the ITCZ. These are the engines of the classic low-latitude routes: the north-east trades in the northern hemisphere and the south-east trades in the southern, each blowing 15 to 20 knots for much of the year.

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Tropical Storm Seasons

Tropical revolving storms are the largest seasonal hazard. The seasons are broadly as follows. Check them against current Admiralty and NHC information because the edges of seasons are not fixed.

Note that the South Atlantic almost never has hurricanes. The seasons follow the sun: summer and autumn in each hemisphere, when the sea is warmest.

The diagram below shows which centre issues warnings for each basin. Always monitor the one responsible for your basin.

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Rules of Thumb for Windows

• Arrive in the Caribbean from the Atlantic in the hurricane season's tail only if you plan to go south of 12 degrees North or into Trinidad and Grenada area, which is rarely hit. Insurers often require yachts to be south of a stated latitude (commonly 12 or 13 degrees North) during the season, so read your policy.
• Leave the Canary Islands in November or December, not earlier, to avoid late storms, and to meet the established trades.
• In the Pacific, cross from Panama or Galapagos from March to May and be clear of the cyclone zone (Fiji, Tonga, Vanuatu) by November.
• In the Indian Ocean, make the Red Sea and Mediterranean transit with the monsoon calendar in mind; the south-west monsoon blocks most of the north Indian Ocean for sailing from June to September.
• In high latitudes, the weather window is short: late spring to late summer. Ice limits move through the season.

Trade-Wind Routes

The General Idea

Trade-wind sailing is the great ocean sailing success story. The wind blows from ahead of the beam only occasionally, so you sail downwind or on a broad reach for thousands of miles with gentle to moderate conditions. The classic passages are the Atlantic crossing east to west and the Pacific crossing from the Americas.

For each, the principle is the same: sail to find the trades, then turn and follow them. You often do not head straight to the destination; you head to where the wind is.

Atlantic: East to West

The usual routes begin from the Canary Islands, the Cape Verdes or Madeira and aim for the Caribbean, roughly 2,700 to 3,000 nautical miles from the Canaries to the Lesser Antilles.

Two approaches are common:

• The direct or rhumb-line route from the Canaries to Barbados or St Lucia, sailing slightly south of west. Simple, and good in a reliable season, but it can find lighter wind in the middle.
• The southern route by way of the Cape Verdes, going south to roughly 15 to 20 degrees North and then west. More reliable trades but it can feel like a long, sometimes rolly trade-wind sleigh ride. You may meet the ITCZ at the lower edge in some months.

Typical passage time for a cruising yacht averaging 5.5 to 6 knots is 18 to 25 days. The NE Trades are normally strongest in winter and early spring; the sea builds to 2 to 3 metres from the north-east with a longer swell on top.

Atlantic: West to East

Returning from the Caribbean is much harder because the same trades are now ahead. The standard answer is to go north to the Azores or Bermuda and then use the westerlies. The sequence is:

1. Leave the Caribbean in April to early June, ahead of the hurricane season.
2. Sail north-east to or via Bermuda.
3. Pass through or around the Azores High in the horse latitudes, where winds are light and variable. Expect to motor part of this.
4. Pick up the westerlies at about 35 to 40 degrees North and run east to the Azores.
5. Finish to the UK, Biscay or the Mediterranean, taking care with the often boisterous conditions north of the Azores.

Pacific

The Pacific crossing from Panama or Mexico to French Polynesia is some 3,000 miles to the Marquesas, normally taking 20 to 30 days. The cyclone-free window in the south Pacific is roughly May to October. Many yachts wait for the end of the south Pacific cyclone season and cross in March to May.

The Doldrums

The ITCZ is the band of light winds, squalls and heavy rain between the two trade systems. It moves north and south during the year. Passing it means:

• Crossing at its narrowest point where possible, and trying to cross at right angles.
• Being ready for sudden squalls, with sail reduced in advance, and for long hours of light, hot calm.
• Conserving fuel so that you can motor across the calm gap.
• Using weather information and your own barometer: in the ITCZ the pressure is flat, with a diurnal pressure variation of about 3 to 4 hPa.

Pressure Systems and Strategic Routing

On a long passage you are rarely sailing in a steady wind: you are working your way round pressure systems. Use these principles.

• Buys law: in the northern hemisphere wind circulates anticlockwise round a low and clockwise round a high. In the southern hemisphere the directions reverse.
• Crossing a high: you will be in light and variable winds near the centre. Skirt it where you can, staying on the side with better winds rather than the shorter one.
• Low-pressure systems in mid latitudes: the strongest winds are on the side nearest the track in the right-hand semi-circle in the northern hemisphere. Moving away from the low's centre early is nearly always wise.
• Weather at the edges: the area between a high and a low gives the strongest pressure gradient. Isobars close together mean strong wind.

The diagram below shows how geostrophic wind speed depends on isobar spacing and latitude. This tool allows you to estimate the wind from a pressure chart when no wind forecast is available.

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Hurricane and Cyclone Avoidance

Principles

You do not fight a tropical cyclone. You avoid being where one can reach you. The rules are:

1. Plan around the season. Be out of the area or in a recognised hurricane hole for the dangerous months.
2. Monitor early. Get the tropical weather outlook daily in the season. NHC issues a Tropical Weather Outlook for the Atlantic and East Pacific every six hours.
3. Move early. Disturbances become hurricanes in as little as 24 to 48 hours.
4. Stay out of the dangerous semicircle. In the northern hemisphere the right-hand (dangerous) semicircle, looking along the storm's track, has the strongest winds because the storm's forward speed adds to the circulating wind.
5. Keep your distance. Hurricane forecast tracks have an error of around 100 nautical miles at 48 hours and more beyond. Many yachts use the 34 knot wind radii forecast and keep clear of it by a large margin.

The diagram below shows the formation conditions for a tropical cyclone: warm sea, moisture, rotation and low shear.

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Identifying a Storm's Position and Track

Buys Ballots law helps you locate the centre. Face the wind in the northern hemisphere and the low centre is about 90 to 135 degrees to your right in a storm, depending on distance. It is closer to 90 degrees near the centre.

To work out which side of the track you are on:

• A steady wind direction with falling pressure and increasing wind means you are on the track.
• A veering wind (clockwise change) in the northern hemisphere means the right-hand (dangerous) semicircle.
• A backing wind (anticlockwise) means the left-hand (navigable) semicircle.

The diagram below shows the dangerous and navigable semicircles in the northern hemisphere.

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If caught: in the northern hemisphere, with the wind veering, put the wind on the starboard bow and run away from the centre; in the navigable semicircle (backing wind) put the wind on the starboard quarter. In the southern hemisphere, reverse the directions. For a yacht, the better plan is to be nowhere near the storm in the first place.

GRIB Files

What a GRIB File Is

GRIB (GRIdded Binary) files are compact numerical weather data. A file contains a grid of points, for example every 0.25 or 0.5 degrees of latitude and longitude, with values of wind speed and direction, pressure, sometimes wave height, sea temperature, rain and cloud, for a series of forecast times at, for example, 3 or 6 hour steps for up to 10 or 16 days.

The usual sources are the US Global Forecast System (GFS), the European Centre for Medium-Range Weather Forecasts (ECMWF) and others, such as the German ICON and the UK Met Office global model. You download a small file (a few tens of kilobytes) by email, satellite or the internet, and load it into a viewer or routing program.

What GRIB Files Do Well

• Give the large-scale pattern: where the highs, lows and wind belts are, and how they move.
• Show timing of a front or wind shift to within a few hours, often within a day or two.
• Provide data over remote ocean areas where no forecast text is written.
• Are free and small enough to receive by satellite phone.

Limitations

The most dangerous error is to treat a GRIB as the weather. A GRIB wind of 25 knots can mean 35 knots in a squall. A GRIB that shows no wind in a wide area may hide a ridge with a surprise line of storms. Always compare the file against what is outside, and update daily.

Good Habits

• Download at the model run times (00, 06, 12, 18 UTC) and keep old files to check how forecasts changed.
• Compare two models. If they agree, confidence rises. If they disagree, the real weather is probably uncertain.
• Look at the trend of forecasts over successive runs, not just the latest.
• Check the forecast against the actual conditions at least every watch.
• Remember that the file age matters. A GRIB downloaded 12 hours ago for a fast-changing situation is stale.

Ensemble Forecasts

A single model run is one possible future. An ensemble forecast runs the model many times (for example 20 to 50 members), each with slightly changed starting conditions and physics. The spread of the results shows how predictable the weather is.

• Tight spread: all members agree; confidence is high.
• Wide spread: members differ; the forecast is uncertain, and you should plan for more than one outcome.
• Ensemble mean: smooth, but it can hide the extreme outcomes. Look also at the individual members and the probability of strong wind (for example the chance of wind above 30 knots).

The best-known ensembles are the GEFS (US) and ECMWF's ensemble system. Burch advises using them to decide how much time margin to give a decision, not to find a single answer.

A practical use: when the operational GRIB shows a low crossing your route in six days, look at an ensemble. If 5 members in 30 bring the low close, you can proceed with a watchful plan. If 20 in 30 do, you should consider changing your timing now.

Official Products

The ocean sailor should know the public official sources. These are free and generally better than commercial summaries, especially around storms.

Many of these are available over HF radiofax, HF email (SailMail, Winlink), and by satellite. Learn how to receive these before you leave, and test your set.

Reading an OPC High Seas Forecast

The OPC text begins with warnings, then a synopsis (the position and movement of highs, lows and fronts), then a forecast for each zone for the next 24 to 48 hours, with winds and seas given as ranges. The text uses the following terms:

• Gale warning: wind of 34 to 47 knots.
• Storm warning: wind of 48 to 63 knots.
• Hurricane force wind warning: 64 knots or more.
• Seas are the significant wave height in feet or metres, and are the average of the highest third of waves. The highest individual wave may be nearly twice as high.

The UK convention is Beaufort force; the US convention is knots. You will need to convert both.

Isochrone Routing

The Principle

Weather routing programs search for the fastest route across a wind forecast using the isochrone method. An isochrone is a line of equal time: the outer boundary of all places the boat could reach in a given time.

1. Start from the departure point.
2. Over the next time step (say 3 hours), compute the boat's speed on each heading from the forecast wind and the boat's polar diagram.
3. Plot the farthest point reachable on each heading. Joining these points gives the first isochrone.
4. From each point on that isochrone, repeat the process for the next step, with the new wind.
5. Continue until one of the isochrones reaches the destination. Work backwards along the chain of points to get the route.

Polar Diagrams

A polar diagram shows the boat's speed at each true wind angle for several wind speeds. It shows, for example, that the boat makes 7.2 knots at 120 degrees true wind angle in 15 knots of true wind, and that it cannot sail within about 40 degrees of the true wind. It also shows the best VMG (velocity made good) to windward and downwind. The routing program uses these to make the computed route reflect what the boat can do, rather than a generic boat.

Real polars are always an estimate. Cruising boats fully loaded with stores sail more slowly than the design polar, so most routers allow a percentage reduction (for example 85 or 90 per cent), and extra reductions for sea state.

Why Isochrones Are Not the Answer on Their Own

• Garbage in, garbage out: the route is as good as the forecast, and a small error in a forecast front timing can ruin the optimal route.
• Fastest is not safest: the quickest route may go very close to a front or the edge of a low. Add constraints: maximum wind, maximum wave height, avoid squalls.
• Comfort and crew: a route that keeps the boat on a hard reach for three days in 28 knots is fast on paper but tiring.
• Land and hazards: the program may not know all dangers, and the weather near coasts is poorly modelled.
• Sea state and current: many routers ignore or simplify swell and current.
• Extreme sensitivity: tiny forecast changes can swing the optimal route by hundreds of miles. Re-run the routing with every new GRIB.

The practical advice is to compute several routes under different constraints and compare them. If the routes are similar, the choice is robust. If they differ greatly, the weather is uncertain, and you should pick the route that is safe in all the likely outcomes.

Professional Weather Routing

Some yachts, especially racing and delivery, employ a shore-based router, who sends daily advice by satellite. The skipper remains responsible. A router is a source of information, not a replacement for judgement. Make sure you give the router accurate boat polars, your constraints and the position and condition of the boat at each check-in.

Decision Making at Sea

Plan the decision points in advance.

• Set go and no-go criteria before departure: for example, no departure if a gale (Force 8) is forecast within 72 hours on the route; no departure if the tropical outlook shows a disturbance within 1,000 miles.
• Define a bail-out: know which ports are within reach at each stage, and what they need (draught, entry conditions).
• Check forecasts regularly: at least daily, and before each watch change if the weather is changing.
• Record observations in the log: pressure every hour (or at each watch), wind, sea state, cloud. A fall of 6 hPa in 3 hours warns of a gale; a fall of 1 hPa an hour is a strong signal in the tropics.
• Compare the forecast with reality: if the difference grows, trust reality and treat the forecast as suspect.

The barometric pressure thresholds below are a useful quick reference for the tropics, where a normal diurnal variation of about 3 hPa is expected.

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Climate Change

Pilot Chart averages are drawn from decades of data. The climate is changing and some patterns, such as the location of cyclone genesis, the strength of the trades and the position of the ITCZ, are shifting. Use recent seasonal summaries alongside the Pilot Charts, and keep flexibility in your dates.

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Worked Example: An Atlantic Circuit

A crew plans a one-year circuit from the UK to the Caribbean and back in a 14 metre cruising yacht with a typical passage speed of 6 knots (144 miles a day) and a fully loaded polar at about 85 per cent of the design.

Strategic Plan

Calculating the Atlantic Crossing

The crossing from Las Palmas to St Lucia is about 2,700 nm. At 6 knots average, the time is:

2,700 / 6 = 450 hours = 18.75 days

Allowing for lighter patches, and the fact that routes are not straight lines, round to 20 to 22 days. If the fleet routinely sails 5.5 knots, that gives 2,700 / 5.5 = 491 hours = 20.5 days. The planner provisions for 30 days of food and 40 days of water for a crew of four, which is a safety margin of 50 per cent on the passage time. Fuel is carried for 150 hours of motoring in case of calms or an emergency.

Day 0: Go or No-Go

On the morning of departure, the skipper checks:

1. The GRIB (GFS and ECMWF) for the next seven days: both show the NE trades 15 to 22 knots with a ridge of high pressure to the north. A low in the north Atlantic is far from the route.
2. The OPC and Met Office offshore forecasts for the area: no warnings for the route.
3. The NHC Tropical Weather Outlook: no disturbances (and the season has ended on 30 November).
4. The ensemble: the spread is small for days 1 to 4; wider for days 5 to 7.
5. The boat: sails, rigging, rig tension, safety gear, liferaft service date, EPIRB registration and battery, and crew readiness.

The decision is to go. The skipper writes the decision criteria in the log: for example, if the forecast winds over 30 knots appear for the route within the next 48 hours, heave to or divert to the Cape Verdes.

Day 5: A Tactical Decision

By Day 5 the yacht is at 22 degrees North and 25 degrees West. The new GRIB shows that the NE trades weaken to 8 to 10 knots over the next three days to the south-west and that a stronger flow lies further south, near 18 degrees North. The routing program, using the yacht's polar at 85 per cent, shows two options:

The skipper checks the ensemble. Most members support the stronger southern wind, but a few show it weakening. The skipper picks B, with a plan to review in 24 hours and a note that the squalls near the ITCZ require reefing before dark. This illustrates a general rule: a slightly longer route in stronger and steadier wind is often faster than a shorter route in light airs.

Day 12: Using the Barometer

On Day 12 the barometer is steady and has the expected diurnal variation of about 3 hPa. This confirms the trade-wind regime. On a different day, if the barometer fell 4 hPa below the normal diurnal curve and the wind backed and increased, the crew would suspect a tropical disturbance. Out of season this would be unusual, but it would still prompt a check of the NHC advisories by Sailmail or satellite.

Day 21: Arrival

The yacht arrives at Rodney Bay, St Lucia, after 21 days at an average of 5.4 knots over the ground for the 2,830 nm actually sailed (the extra 130 nm being the southern detour). The log shows that the actual result differs from the GRIB-based prediction by about a day, which is within normal error.

The Return

In May the crew leaves the Caribbean for Bermuda, 900 nm to the north, keeping to the west of the NE trade belt and watching for the early tropical systems that can develop in May and June, especially in the Gulf Stream area off the US coast. The crew crosses the Gulf Stream with care: a north-going Stream with a north wind is steep and dangerous, so they plan to cross when the wind is southerly or light. They use the NHC outlooks and OPC fax charts.

The Bermuda to Azores leg is about 1,800 nm. The Azores High usually lies north of the route in June, and the crew goes round it on its southern side then north-east into the westerlies. They expect to motor for 24 to 36 hours in the light winds, and carry the fuel for this.

The final leg from the Azores to Plymouth is about 1,200 nm. In July it is mostly a reach or run in westerlies, with the possibility of fog near the UK, and the possibility of a deep summer low that must be seen coming several days ahead.

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Common Mistakes

• Treating a GRIB as truth. It is a forecast, with gusts, squalls and wave details missing. Always compare with observed conditions.
• Relying on one model. Use at least two models and look at trends in successive runs.
• Leaving to a calendar rather than the weather. Departing because a rally leaves, or because you have to be somewhere, is a leading cause of trouble. Have go criteria and stick to them.
• Ignoring the season edges. Storms occur at the start and end of seasons. A season that ends on 30 November is not a guarantee.
• Trusting the router. A computed route is advice. Check it for squalls, fronts, sea state and the crew's endurance.
• Following the rhumb line through a high. The shortest line often passes through a ridge with calms. The best route is often longer.
• Neglecting the barometer. It is the cheapest and most reliable tool, and the one that does not depend on a signal.
• Using stale data. A GRIB or fax chart more than 12 to 24 hours old may be wrong in a changing situation.
• Under-estimating the return passage. West to east is a different passage with different weather, with fronts and gales. Do not assume a gentle trade-wind passage.
• Not planning communications. Decide how you will get weather (HF fax, SSB email, satellite), and test it before departure.

Summary

• Ocean routing has a strategic layer (season, route, stopovers) and a tactical layer (daily decisions from forecasts).
• Use the reference books together: World Cruising Routes for yacht routes and seasons, Ocean Passages for the World (NP136) for the shape and distances of the oceans, Pilot Charts for monthly averages, and Modern Marine Weather for understanding forecasts.
• Seasons dominate: be in the trades when they are steady and out of tropical storm areas in the storm season. The North Atlantic hurricane season runs 1 June to 30 November, peaking mid August to mid October.
• Trade-wind routes run downwind; return passages go north to the westerlies via Bermuda and the Azores, crossing the Azores High.
• Avoid tropical storms by timing, early monitoring and distance. Know the dangerous semicircle.
• GRIB files give the large-scale pattern but not gusts, squalls, or small intense features. Compare models and check against reality.
• Ensemble forecasts show the spread of outcomes and therefore how much confidence to have.
• Isochrone routing uses a polar diagram and forecast winds to find the fastest route, but the answer is sensitive to forecast error and ignores comfort and safety unless you add constraints.
• Use official sources, including NOAA OPC, NHC and the Met Office, especially near severe weather.
• Set go and no-go criteria in advance, and keep the log and barometer as your check on the forecast.

Check Your Understanding

1. What does Ocean Passages for the World (NP136) provide, and what is its main limitation for a yacht?

Answer: It gives recommended routes, typical distances and descriptions of winds, currents, ice and storm areas for each ocean, with the reasoning for each route. Its main limitation is that it was written largely for commercial vessels, so its routes and passage times do not always suit a small yacht.

1. What information does a Pilot Chart show, and why should the data be used with care?

Answer: It shows monthly averages of wind roses, percentages of gales and calms, surface currents, past tropical storm tracks, ice limits and typical pressure. It is based on ships' log data, which are sparse off the shipping lanes, and averages hide extremes, so any month can bring conditions far from the average.

1. Give the main hurricane season for the North Atlantic and the peak period.

Answer: The season runs from 1 June to 30 November, with the peak from mid August to mid October.

1. Why is the passage from the Caribbean back to Europe not simply the reverse of the outward trade-wind passage?

Answer: The trade winds blow from the east, so heading east would be against the wind. The usual route goes north to Bermuda or the Azores, crosses the Azores High's light winds, and then uses the westerlies at about 35 to 40 degrees North. The passage involves fronts and depressions, so it needs more weather vigilance.

1. Name four things a GRIB file does not show well.

Answer: Any four of: gusts, squalls and thunderstorms, small intense features such as tropical cyclones, local wind effects near coasts, detailed sea state such as wind against current, and errors that grow with forecast time.

1. How can you tell which part of a northern hemisphere tropical cyclone you are in from the wind shift?

Answer: A veering wind (clockwise shift) with falling pressure means the right-hand, dangerous semicircle. A backing wind (anticlockwise) means the left-hand, navigable semicircle. A steady wind direction with falling pressure and rising wind means you are on the storm's track.

1. What is an ensemble forecast and how do you use it?

Answer: It is a set of model runs, typically 20 to 50, started from slightly different conditions. A tight spread means high confidence; a wide spread means uncertainty. You use it to decide how much margin to build into your plan, and to estimate the probability of strong winds, rather than to find a single forecast.

1. Describe the isochrone method of weather routing in outline.

Answer: From the start position, the program uses the forecast wind and the boat polar to find the farthest point reachable on each heading in a time step, forming an isochrone. It then repeats from each point on that line using the next forecast, until an isochrone reaches the destination, and works back along the chain of points to give the fastest route.

1. Why might the fastest computed route be the wrong choice?

Answer: It depends on forecast accuracy, so small errors can change the best route; it may go close to fronts, squalls or heavy seas; it may ignore comfort and crew endurance; and it may not include all hazards. Add constraints, compare several routes and use judgement.

1. A GRIB forecasts 25 knots of wind. What should you expect in practice?

Answer: 25 knots is the mean wind. Gusts may be 30 to 50 per cent higher (about 32 to 38 knots), and squalls may produce more. The sea state may also be bigger than modelled if the wind is against a current. Reef for the gusts, not the mean.

1. You are 5 days into a crossing and the new GRIB shows a weak-wind area ahead on the rhumb line. What do you do?

Answer: Compare two models and the ensemble, run the routing for both the direct and a longer route to stronger wind, check the fuel for motoring, and choose the route that is safe and acceptable in the likely outcomes, often the longer route in steadier wind. Review it every day as new data arrive.

1. Why should you set go and no-go criteria before departure?

Answer: Because pressure to leave on a date, with a rally or a crew arrival, can cloud judgement. Criteria agreed in advance, such as no departure with a gale forecast on the route within 72 hours, make the decision objective and are written in the log.