How to amend your course when the weather is not behaving itself

In June 2022 we sailed from Australia to New Caledonia as the borders finally opened.  Unfortunately, we had been long awaiting Starlink and therefore did not buy Predict Wind with Iridium Go.  We are always on a budget because as you know we do all our sharing for free.

As the journey was only 4 days or so, we employed a weather router and did our homework on shore.  When we saw a suitable weather window which the weather router agreed with, we left.  The window did have a low which was tracking southeast at a good speed and it looked as though it was not going to affect us.

So, we left with only a Garmin Inreach and SSB. Brand new Gori props and good winds meant that we were doing over 200 miles per day whereas normally Impi is at about 180 nautical miles per day.

Here is what happened and how we learnt from that

The low stalled and as our weather router did not warn us, we sailed right into it in the morning of day 3.  We ended up in 6 m waves and 35 knots on the side of Impi, waves that looked like mountains.

I contacted New Caledonia MRCC on the emergency broadcasting channels on our SSB but had no reply on any channel.   Brent meanwhile was hand steering Impi through the breaking waves that washed across the bridge.  I wanted to know the location of the low which is why I contacted or tried to contact MRCC. We later found out that all SSB stations in New Caledonia have been decommissioned.

I contacted our weather router but no reply.  We later found out that a dog had priority and a long walk had been in order!

Anyhow, luckily, we could contact friends in Australia via Inreach who gave us the location of the low and we steered north to Koumac to find wider isobars at 1013hPa.  When we found those isobars, the winds dropped and the sea state improved.  Big sigh of relief!

What are isobars?

Isobars are the plain lines curving across a weather chart. They connect points with the same mean sea level air pressure. Wider isobars portray a weak pressure gradient typical of light wind situations.  Tightly packed isobars represent stronger winds.

How are isobars measured?

Isobars are measured in hectoPascals (hPa) which is the international unit for measuring barometric pressure. They are also sometimes expressed in millibars (mb) which is the same thing. For maritime use pressure is measured at sea level (SLP).

The central position of a shallow Low is above 1000hPa and of a moderate Low 980-1000hPa whilst a deep Low is below 980hPa.

The central position of a weak High is about 1015hPa, whereas an intense High is above 1030hPa.

Isobars also indicate the flow of air around a weather system.  The strongest winds are usually near cold fronts, low pressure systems, tropical cyclones and in westerlies in south Australian waters. 

Near a High’s center are light winds and sometimes areas of low cloud called anticyclonic gloom. Round the edge of a High, winds are sometimes strong.  Intense Highs squeeze isobars together creating strong winds. (ref. RYA weather handbook)

The bigger Highs are, the slower they move sometimes, blocking fronts that are trying to follow them. Winter Highs often bring frost whereas summer Highs may bring thunderstorms and hail.

In our case the barometer read 1016hPa on departure in Australia, so a weak High.  After two days the barometer dropped from 1014 to 1011hPa over 3 hours.  Of course, this always happens at night.  The wind increased from 20 knots to 35 knots and the waves were between 4 and 6 meters with breaking crests.

As we were watching the barometer we knew that by morning we would be in foul weather as it dropped to 1010hPa.  There is a rule of thumb that if the barometer drops by 3 points over 3 hours you should not continue to head south in the Tradewinds belt.(ref. Astrolabe sailing) So we turned North! There are other guidelines which can be applied.

Likely significanceSteady pressure drop over 6 hours
AlertLess than 3hPa
Caution3 to 4 hPa
Definite Warning4 to 5 hPa
Too late for forecastingMore than 5 hPa
table to read the barometer

What information goes into a weather chart?

In Australia the information of more than 700 automated weather systems goes into a weather chart. They add also the information of ships and meteorological weather buoys. In addition, international satellites supply information and monitor the upper atmosphere in terms of temperature and moisture profiles.

How can you get a weather chart when out at sea?

You can get Predict Wind, although it is an expensive subscription.

You can have a variety of charts sent to your phone including isobar charts. You will need a satellite connection to download the charts. For your connection you can use Iridium Go or Starlink, although the latter is as yet not fully operational.

For the different packages on predict wind consult their website

You can use your SSB to get a weather fax, but communications can be unreliable as in our case because many stations are inactive.

 Further research into the situation seems to indicate that after Australia it is hard to receive communications as the station in New Delhi India has weak transmission.

According to the Worldwide Marine Radiofacsimile Broadcast Schedules published in October 2022, the Naval Base at Simon’s Town South Africa is also no longer active.

You can get a Grib file via your Iridium phone.  In Australia, you can’t get a SIM card or subscription on your Iridium phone which works out of Australia. Go figure! Not much use to international sailors. It is worth to note that the north of Australia has very poor cell phone connection.

The Grib file sent to you on Iridium is readable via additional software which you have to install on your laptop. You sent a request via your email and then download the chart.  This worked quite well for us over the last 10 years of sailing but unfortunately it only gives one model, GFS which is not the preferred model in the Pacific.

If you find yourself in a low and have no weather chart what must you do to avoid it?

You can use Buys Ballot’s law.  What is this?  Well, in the Northern Hemisphere you can stand with your back to the wind, the low will be to your left, the high to your right. In the Southern Hemisphere the reverse would be true.

This movie by US Captains Training explains the law and how it can be used to avoid the dangerous sector of a low in the Northern Hemisphere. Remember that in the Southern Hemisphere, it is the opposite: winds around a Low would be going clockwise.

What did we do in our situation?

Well, our friends in Australia confirmed to us via Inreach that we had to sail North away from our course to Noumea to find the 1013 Isobar which was widely spaced. So, we used our barometer to find that isobar.

We like Inreach in that you can stop and start the payments on a monthly basis and that it is totally waterproof. You can hook the device to an App called Earthmate so that you can type with an Ipad or other device. Downside is that you can only get spot weather forecasts.

How can you estimate the wind speed by looking at the isobars?

You can guess the strength of the wind of a familiar area by looking at a weather chart. If you are in an unfamiliar area you can calculate the wind speed by using a mathematical formula.

Theory states that under stable conditions, wind speed over the open ocean and in areas away from the equator is proportionate to the pressure gradient divided by the sine of the latitude.  Because of surface friction, the wind direction is about 15 degrees away from the isobar lines towards lower pressure.  ( ref.  Thomas McCullough 2003 full article available on

To understand better why we need trigonometry formulae to calculate wind speed, have a look at this movie.

In order to find the sine of the degree of the latitude you need a scientific calculator. This movie explains how to do it.

What is a pressure gradient? Well it is the difference in pressure between high and low pressure areas. So as the pressure gradient increases, the speed of the wind increases.

What changes have we made in our communication and weather monitoring systems?

We have now an integrated barometer in our B&G systems as well as a barometer app (Marine Barograph) on a permanently assigned Iphone.

We have purchased Starlink here in New Zealand even though it will not work in a lot of countries West from here or in New Caledonia. We will probably have to combine this with Predict wind for crossings.

We have become a lot wiser! Now we will always ignore our destination and steer to widely spaced isobars to avoid the worst sea state and winds!


    1. Hi Brent & Anna,

      This is very good stuff. I have a question that is now about the conditions that you had. On it recent passage Phuket-Male we had 2-3 metre beam seas. Occasionally bigger, occasionally breaking. I found on that angle in those seas that we had a lot of pounding of waves breaking on the inside of the leeward hull. Some real bones shakers included. It makes me worry for the structural integrity over an extended period. I can’t imagine how much worse that must be in 6 metre breaking seas on the beam; not to mention the motion of the boat in those conditions. Do you ignore this or do you have a technique to minimalize the pounding.?


      1. Hi Wayne – great to hear from you again

        catamarans in particular being wide platforms in rough seas are prone to stresses that can either be ignored or attended to.
        One of our most important items to check regularly is boat structure (structural integrity) and if we find something showing signs of stress to immediately remedy it before it becomes a bigger issue. When we purchased our cat new from the factory one of the first projects on a brand new boat was to enhance the boat for world sailing to better cope with these conditions.
        On many cats the bridge deck is a lot thinner in materials than the hulls and for good reason because one wants to keep the weight of the craft down. On some cats one will observe ribs built between the hulls running along the bridge deck direction bow to stern and this is to compensate for the flex in this area. Because the glass is thinner here and spans a wide area it stands to reason that it will flex as waves fight for an escape route and this leads to the bone shaker effect inside the boat.
        the result of this is also not friendly to bulkheads or any other areas where there are ‘hard joins’ to the bridge deck from the inside because the material flexing ahead of and behind these ‘hard joins’ causes the glass to eventually crack and will result in bulkheads ‘letting go’ – it’s like taking a piece of wire and bending it back and forth and eventually it will break.
        For this reason on Impi over the years we’ve developed solutions for this effect eg. ribs internally going from the front of the main bulkhead forward and I’ve seen a few people on the 440 group now doing the same. There is a lot being made about Lagoons and bulkheads but actually this is a common issue across brands – some owners just haven’t looked and don’t know its happened.
        Just a few weeks ago I met another Leopard owner who was a boat builder by trade (He built Mustangs in New Zealand) who arrived across the pacific and had bulkhead failure. He discovered this by looking at his boats structure on arrival.
        He too concluded that catamarans in general are wide platforms moving around in heavy seas and will eventually have issues.

        It’s a long reply but I think it’s important for us to remember whats happening to the boat as we sail in these conditions because the forces at play, longitudinal, width and torsional forces are massive.
        This is one of the reasons on Impi we don’t sail with a tightly set rig – the rig tension plays a massive role in how the boat structure will ‘feel’ these forces at sea. You probably do what we do – set the rig up tensioned enough but not heavily so and a bungee chord between the leeward shrouds takes up the slack so it has a shock absorber effect and not a ‘snapping effect’. This is because as the boat moves over waves and the apparent wind shifts, the sail can momentarily stall causing the mast to whip back and forth further hammering away at not only the rig but also the boat itself.
        Rig tension in these conditions is massively important.
        Following onto this, in big seas we reef, reef and reef again. Sailing by the numbers (reffing numbers) is fine in normal seas but when seas get abnormally rough we take the ‘punch out of the sails’ and will sometimes engage engines to assist the boat through these conditions. There’s nothing worse than a boat moving down a wave and suddenly being stopped by another wave with the inertia of the mast moving forward – both on the rigging and the structure of the boat. It gets worse when this happens to either bow separately because of the torsional forces exerted through the boat.
        For this reason we also hand steer in these conditions. I’ve heard people saying an autopilot does a better job at steering than a person and whilst that may be true in most conditions, the autopliot cannot see or anticipate the odd very big wave that pops up and runs toward the boat, so we always handsteer when we feel conditions are not right and we steer off waves that are breaking bringing it back afterwards. But then there’s night time and one doesn’t see much on a dark night except hopefully the white runs of breaking waves – so all we can do is our best in any situation. As you know in really bad seas where the safety of the boat and ourselves is compromised to that extent, we will deploy a warp with a mooring ball on the end as a drogue of sorts and let the boat go with the weather at slow speeds. This assumes we have the ‘sea room’ to do this .
        So yes, in summary we do have techniques to reduce ‘hammering the boat’ but ultimately when all this is down there’s not much more to do.
        The issue with rudder position on the boat plays a role because where the rudder is set aft of the prop, the prop wash will allow rudder steering at low speeds which one sometimes wants in rough seas especially seas ahead of midships. The disadvantage is that rudders set far back like this ‘catch air’ as the stern lifts through the back of a wave in following seas. In our case where the rudder is ahead of the prop, due to no prop wash we need a certain speed (in rough seas that would be 4knots minimum) to maintain sufficient water flow over the rudders for steering. The advantage we have is in following breaking seas we don’t loose the rudder to air as soon so we have better steering in those conditions. This FORCES us to sometimes go with the weather as opposed to ‘across it’.
        Interesting topic – so much more one can share but hopefully others will engage and share away too !


      2. Thanks Brent! It is a big subject. I always take time to go downstairs and listen to the boat talking when we are in a seaway. As an ex-aircraft maintenance engineer I have a lot of sympathy for machines. Luckily CathayOz hasn’t exhibited any of the telltale bulkhead issues that have received so much publicity elsewhere. Some folks say that the pre-2008 boats were stronger, but I have no evidence of this. On this last passage I did notice some noises made by creaking panels around the hull-to-bridgedeck join area in the starboard aft cabin. While we are here in the Maldives I’ll do some crawling around the rear crawl space with a torch and also check out the rear of the bulkhead in the engine room to see if there are any cracks. I would be surprised if there were any as I would expect this area to be the strongest, and lightest loaded of the bulkheads, but you never know. Really fascinated to read your thoughts on rig tension. When I was racing trailer sailors we always changed the rig tension for the conditions. I also loved having runners on my old farrier tri, which gave great options whilst under way. Never thought much about it on this boat. In fact, the seagull striker cable was replaced two years ago, and I actually have no idea what tension the riggers put into the rig. Blows me away when I read you comments on this. Bloody hell, more knowledge to suck up! Thanks again buddy, your dedication to helping and informing the sailing community is mega.


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