Offshore navigation is an essential skill but it can be daunting. Jimmy Cornell explains how to do it, and how the parameters are changing

Every voyage starts with a dream, and a voyage on my own boat to the remotest parts of the world was my dream as far back as I can remember. That dream became a reality in the early 1970s when I started preparing for a world voyage with my wife, Gwenda, and our two young children, Doina and Ivan. The first step was to join a course on offshore navigation and it instantly turned me into a lifelong addict of that ancient art.

This early offshore navigtion training is how I learnt about the importance of pilot charts in voyage planning, and became my most useful source on weather conditions during our six-year-long round- the-world voyage.

In those days there were no weather forecasts available on offshore passages, and the monthly pilot charts provided information on the kind of sailing conditions to be expected by showing prevalent wind direction and strength, percentage of gale force winds, currents, and tropical storms and their tracks.

The first pilot chart was produced by Lieutenant Maury of the US Navy in 1853 and showed the prevailing winds and currents in the North Atlantic. The early pilot charts were based on records obtained from the logbooks of captains sailing the North Atlantic trade routes. The data was displayed numerically, but were superseded by easier-to-interpret charts in which the winds were shown figuratively, as in this British chart of the South Indian Ocean of 1856.

Crossing an ocean is a dream for many, but the reliable weather patterns are changing

Interpreting pilot charts

Pilot charts show the mean wind speed and direction for every month of the year. Each wind rose shows the distribution of the winds that prevail in that area from eight cardinal points. The wind rose arrows fly with the wind and their lengths show the percentage of the total number of observations in which the wind had blown from that cardinal point.

The number of feathers shows the force of the wind, which had been recorded most frequently from that sector. The wind force is measured on the Beaufort scale, with each feather being equivalent to one unit of wind force, so that four feathers represent Force 4 winds from that direction in that particular month.

In areas with prevailing winds, the resulting arrow would be too long to be shown in its entirety, in which case for percentages higher than 25, the percentage is shown numerically on the shaft. The figure in the centre of each wind rose gives either the percentage of calms in blue (less than Force 2), or the percentage of storms in red (more than Force 7).

Ocean currents are shown as green arrows indicating their prevailing direction and rate. Variable currents are shown as a dotted arrow, its direction being determined by the highest percentage of currents recorded to set in that direction.

Jimmy Cornell’s Garcia Exploration 45 Aventura IV in the Northwest Passage

Research and common sense

I have used pilot charts when planning the various transatlantic and round-the-world events I’ve been involved in. They were also a reliable source when updating or adding new routes in my book World Cruising Routes.

However, after a while I realised I should write a book dealing specifically with voyage planning. While researching this subject, I read this statement by the International Convention for the Safety of Life at Sea: ‘Voyage planning is common sense.’ Was this true, I asked myself? Is it indeed common sense when planning a voyage to be in a certain area at the safe time, and avoid being in the wrong place at the wrong time?

The north Atlantic pilot chart for April, showing the area of high pressure, and the location of the doldrums

Have I, unknowingly, followed this dictum? I had certainly done it during my first circumnavigation when we always left the tropics during the critical seasons and avoided high-risk areas. The safety factor continued to be my utmost priority on my voyages, which were all completed successfully. I would like to believe that common sense had been my guardian but perhaps it was just my cautious nature, or simply good luck.


While all those elements play a major part in successful voyage planning there is one more essential factor that cannot be ignored, and that is pragmatism. Perhaps flexibility might be a better term. Looking back on my long and eventful life I must admit that despite all my attention to forward planning, some of the most memorable highlights happened as a result of decisions that had been taken on the whim.

The first time it happened was in Panama when we reached the South Pacific and were planning to sail to French Polynesia. However, on our young crew’s insistence we agreed to make a detour to visit Paddington Bear’s country. So we turned left and sailed to Peru.

Even at the age of 80, Jimmy relishes a navigational challenge

Having sailed that far south we decided to extend our detour to Easter Island from where we sailed to the island of Pitcairn and finally to the Gambier Islands in French Polynesia. Similar unplanned detours happened throughout our time in the South Pacific as a result of which our planned three-year voyage turned into six!

Sailing with an open mind became the norm and I continued to act like that on all my subsequent voyages.

Having reached Tahiti with Aventura II, rather than continue around the world on my own, I decided to join the second round the world rally and use the boat as a convenient committee vessel. Aventura III’s world voyage was a lesson in flexible planning.

After a highly enjoyable sojourn in Antarctica, my son Ivan and I decided to sail from there all the way to Alaska. We completed that 12,700-mile transpacific marathon in six months, marked by innumerable highlights, from a wet landing on Horn Island and a cruise through the spectacular Chilean fjordland, to a return visit to Easter Island and the rarely visited Line Islands.

We completed the Polynesian triangle at its apex in Hawaii, having previously sailed to Rapa Nui (Easter Island) and Aotearoa (New Zealand).

Aventura Zero under spinnaker during the Elcano Challenge

In praise of pragmatism

Inspired by that voyage, my next project was a transit of the Northwest Passage on Aventura IV. Once again, pragmatism came on board. Unable to complete the transit from east to west because of it being blocked by ice that year, I took what I believed to be the logical decision: to sail south, transit the Panama Canal, and tackle the Northwest Passage from the other end, as ice conditions would be more favourable if we started from the Pacific. This was the case and I managed to sail successfully through that once-impenetrable waterway.

New possibilities

Global warming is already making such high latitude transits easier to accomplish, and my next project was a direct response to the changing climate conditions. The fifth Aventura, an Outremer catamaran, was conceived as a boat with zero carbon footprint by having electric propulsion and renewable electricity.

Sailing from Tenerife to Gibraltar in an unusual easterly, the boat easily cruised at up to 18 knots

The voyage started from La Grande Motte, in the south of France, where she had been built, and the plan was to sail around the world along the route of the first circumnavigation on its 500th anniversary in 2022. The initial part of the voyage went reasonably well but by the time we had reached Tenerife in the Canary Islands, I had discovered various shortcomings that needed to be put right before continuing.

By then the favourable seasons along the proposed route were so far advanced that I had to accept that even if those improvements were dealt with promptly, the voyage would be delayed by one year. While considering my various options, a depression passing close to the Canaries brought strong southerlies, which replaced the consistent NE winds that usually prevail in that area.

This change was so unusual and unexpected that I felt it to be an opportunity not to be missed. We set off immediately, sailed all the way to Gibraltar on one tack and completed the 1,540-mile passage in 10 days. It was December 2020 and within days Covid arrived and put an end to my plans.

Horta in the Azores is a popular stepping stone for west to east Atlantic crossings

Passage planning choices

Even the best-laid plans can be drastically affected by matters that are out of one’s control. This is another valuable lesson concerning voyage planning, and a useful reminder that when it comes to sailing no plan should be regarded as absolutely firm. For that reason it is always a good idea to have a Plan B and, ideally, a Plan C.

As for Aventura Zero’s passage from Tenerife, it shows just how important it is to be flexible, and even more so, to be prepared to be flexible. Having done all my early sailing at a time when there were no weather forecasts available on ocean passages, I had no choice but to take the weather as it came and make the best of it. Although nowadays, like everyone else, I do take advantage of the availability of GRIB files and offshore weather forecasts. But only within limits.

The two most sailed passages in the North Atlantic are the one from the Canary Islands to the Caribbean and the return passage to the Azores. When the times comes to set off on either of those passages, even the latest GRIB files and forecasts are only of limited use as they only cover the initial stages of those long passages. The same applies also to the actual course to be sailed, as the great circle route only indicates the shortest distance to the destination. However, that may not necessarily be the quickest in either case, as more favourable conditions may be found by sailing a different route.

Route options from the Canaries to the Caribbean. Green: great circle route Red: optimal route

Favourable winds

On the route from the Canary Islands to the Caribbean, the traditional advice is not to sail the direct route across the Atlantic, as the chance of better conditions would be higher by sailing an initial SSW course until the area of prevailing NE winds is reached. The suggested route passes close to the Cape Verde Islands, which lie in the area of prevailing NE trade winds.

This now appears to be an accepted tactic, as shown by the statistics from the ports of Gran Canaria and Mindelo. From 1,000 boats that left Gran Canaria on a transatlantic passage, around 75% sailed the longer but potentially more favourable route. Some stopped in Mindelo marina before resuming their voyage, and all reported encountering better winds on this route.

Similar considerations also apply for the route from the Caribbean to the Azores, where sailing the great circle route may not be the best option either. The usual tactic for boats returning from the Caribbean to the Azores was to sail a northerly course keeping to the west of the Azores high until the area of prevailing westerlies was reached, and then set a course for the desired destination. Some of those who took this route in the past called at Bermuda before turning east. This is no longer the case, as with reliable forecasts available it is quite easy to divert from the great circle route to sail through an area of more favourable winds in the initial stages of the passage, and turn east as the area of prevailing westerlies is reached.

Windgram of the winds between the Eastern Caribbean and Azores in April.

Making weather choices

Making sense of a large range of wind roses can make if difficult to surmise the relevant benefits of one route over another. To simplify the task of interpreting the expected wind conditions on a long ocean passage, the monthly pilot charts in the Ocean Atlas are accompanied by windgrams for the most commonly sailed ocean routes.

The windgrams summarise direction and strength along a hypothetical great circle route. The length and size of each wedge depict the proportion of winds from that direction, while the colours indicate the wind strength, from grey (calm), to dark blue (force 4), and red (hurricane).

Planning charts

Having completed my first circumnavigation with a sextant and paper charts, I still find small-scale charts useful for route planning. I also like to refer to them on longer passages as they provided a general picture of the entire route compared to even the largest computer or chart plotter screen.

The obvious drawback of using old paper charts is their possible inaccuracy due to them no longer being updated. This can be a serious risk, especially in some remote areas, where they may not tally with GPS positions.

Large paper charts make planning easier, but ensure they are recent

I came across such a discrepancy in the Northwest Passage. From a navigational point of view, the most difficult stretch to navigate is between Cambridge Bay and Gjoa Haven. This is a poorly surveyed area of shallows and submerged rocks, with many blank areas even on the latest Canadian paper charts, where I was amused to read the notice: ‘The magnetic compass is useless in this area.’ Being very close to the magnetic north, the compass did indeed behave in an erratic manner and was unreliable, so we used a GPS-based one instead.

The other warning ‘Positions on the chart may be in error by as much as 5 miles’ was no exaggeration either, because at one point we found ourselves in very shallow waters, and suspected that we had strayed from the safe course and seemed to be about 6 miles off our presumed position.

Having learned my navigation long before the days of GPS, I managed to plot our position on the paper chart by taking bearings with the handheld compass of three conspicuous land features. The intersecting lines gave us a fix, which showed clearly that we had strayed far off our intended course. In this case the paper and electronic charts were both wrong.

Aventura IV rides out a storm at anchor. Severe weather is intensifying

Planning around climate change

In spite of all the advances in electronics and navigation aids, any prudent navigator planning an offshore passage or voyage should continue to refer to pilot charts as a valuable source of information on weather conditions that prevail in the intended area.

The pilot charts featured in the latest edition of Cornells’ Ocean Atlas are based on data gathered by NOAA (National Oceanic and Atmospheric Administration) satellites in recent years, augmented by observations obtained from meteorological buoys and other sources. The interesting thing is that long-established weather patterns are changing, and doing so at an alarming rate. Where the probability of wind strengths and directions, frequency of storms and ocean currents could previously be relied upon with a good degree of confidence, global weather systems are become more volatile and less predictable. This is the main focus of the latest edition – the intensification of the effects of global warming on weather throughout the world.

The area of the North Atlantic affected by hurricanes in November based on the tracks of all hurricanes recorded in the last ten years

Trade winds weakening

One of the most noticeable phenomena is the decrease in the regularity and reliability of trade winds, as witnessed by sailors on some of the frequently travelled ocean routes. This is now believed to be a consequence of climate change. As the polar regions are getting warmer at a faster rate than the low latitudes due to global warming, the poleward temperature gradient is weakening and affecting the strength and consistency of trade winds.

Cyclones intensifying

However, the most significant change has been the increased intensity and extent of tropical cyclones, both in the duration of the critical seasons and the areas affected. As this phenomenon has such a major impact on voyage planning, and safety generally, in order to provide a full perspective on the current situation, this new edition of the atlas highlights the areas affected by tropical storms on the relevant monthly charts.

Aventura I enjoying trade wind conditions in 1978

Safety factors

The safety factor in voyage planning is now even more important than in the past. With careful planning, and by being aware of the consequences of climate change, tropical storm seasons and critical areas can still be avoided.

Bearing in mind these changed circumstances, the following basic safety measures should be adhered to when planning a voyage either now or in the near future:

  • Arriving in the tropics too close to the start of the cyclone-free season should be avoided, and a safe margin should be allowed by leaving a critical area before the end of the safe period.
  • Cruising during the critical period in an area affected by tropical storms should be avoided. Those who plan to do so should monitor the weather carefully and make sure to be close to a place where shelter could be sought in an emergency.

Caution lies at the heart of voyage planning, and this atlas should make it possible to plan a safe voyage even in these changing times. For those who plan a voyage in the near future, it should provide a useful and reliable tool when planning, preparing, and bringing your journey to a safe and satisfactory conclusion.

Position fixing using a Yeoman plotter onto a paper chart – a method now rendered largely obsolete

Compass courses and time-keeping

If you prefer to stick to the great circle route the easiest way is to programme the autopilot to sail such a route, which means it will make continuous adjustment without any necessary input from the navigator. This is certainly a possibility on the latest autopilots linked to your chartplotter.

If you are doing this manually, you will need to plot a series of waypoints on your chart and join them together – these can be found, among other ways, by first plotting your route on a Gnomic projection chart before transferring it to a Mercator projection chart. While the Great Circle route will look strange, there are some significant distance savings to be had, and they should be weighed up against the weather along the route.

Your compass course will be derived each day from your midday position fix towards the next waypoint. You could of course follow a single bearing all the way across, but you may well sail much further.

Clock changes

Time also shifts gradually on an ocean passage at a rate of one hour every 15 degrees. Whether you adjust your clocks on board every day, or every 15 degrees, it is very much a personal choice. Some ships even maintain the time of their home port. As you do change the clocks, doing so during the afternoon dog watches, and splitting the difference between each of the watches, means that everyone gets a fair allocation of time on watch.

In my own case I always change the times as we progressed so that we are always on Meridian time. I always make sure that when we arrive we would be on the official local time, so that we would arrive during local working hours.

Enjoyed reading this?

A subscription to Yachting Monthly magazine costs around 40% less than the cover price.

Print and digital editions are available through Magazines Direct – where you can also find the latest deals.

YM is packed with information to help you get the most from your time on the water.

      • Take your seamanship to the next level with tips, advice and skills from our experts
      • Impartial in-depth reviews of the latest yachts and equipment
      • Cruising guides to help you reach those dream destinations

Follow us on Facebook, Twitter and Instagram.