Many years ago on a dark March in the Atlantic we were hit by a large wave on the beam – all went quiet as the yacht was rolled; luckily, I was in a quarter berth and woke up safely on the deck head. After a short while we rolled back upright leaving a badly bent mast and a shaken crew, with the helmsman having been washed out of the cockpit at the end of his tether and scooped back in again as we came upright. As you can imagine it was messy below, with the cooker dangling on the end of its rubber hose (I now always check the stove cannot come out of its gimbals).

It is water – not wind – that usually causes damage. In our case it was a long period of south-westerly winds building up a big sea followed by an active cold front. As the front came through it veered the wind by around 90° and this new wind arrived with big breaking seas, which was our undoing. We jury rigged the mast to get us into Horta and safety.

But while it is waves that often bring danger, most of our forecasting and weather routing is based on wind forecasts.

Options suggested by different models using the wave routing function in PredictWind. The warning triangles mark areas where levels are deemed uncomfortable for your particular boat.

The importance of waves in relation to routing has been recognised for a long time, and weather routing for commercial ships has used wave models to avoid damage. But for yachts, our routing is usually done for speed and relies on wind speed and direction combined with the boat’s performance data. At the press of a button onboard electronic charting software will produce a route to follow.

Alternatively, without the software on board – or for comparison purposes – there are companies that, for a small subscription, will perform ‘cloud’ routing. A computer ashore will hold your yacht’s polars, you provide your start and finish points, and an optimum route will be sent back. This has the advantage of being able to use multiple models and, as only the solutions are sent rather than all the data from the models, it uses considerably less data which is a big bonus when using slow and expensive satellite communications.

Speed v comfort

When racing we want to maximise speed, while cruising we likely want to maximise comfort. In the past, particularly with heavier boats, the stronger the wind the faster we go but more modern, lighter boats will tend to maintain a higher average speed if winds are not too extreme. We also need to start to slow boats down if the waves get too big to avoid slamming and damage.

This is where using wave forecasts can help. We have been able to route around maximum wind speeds or wave heights by changing the parameters in our routing program for some time. It is, however, tricky to predict what is going to be safe or comfortable for your particular boat and what will be on the dangerous side.

While wave heights can appear daunting, it is the period between waves that is important. A long period between waves means that the size of the wave is less important, it is when they are close together with a short period that they tend to break, not allowing time for the boat to rise and go over the waves.

Ocean waves can be split into swell and wind waves (sometimes just called sea). Swell waves are not created by the local wind blowing at the time, but due to winds that are– or were – blowing a distance away. Wind waves describe the waves being generated in our local area by the wind blowing at the time. Significant wave height is another term often used, this is the average height of the largest third of the waves.

In general, all quoted heights are for significant wave heights, and a timing or period is given in seconds, which is the time taken for crests to reach you. Wave length is sometimes used but usually models and reports are in heights and period.

Swell can be anything from an oily swell from a storm long gone or the first indications that a storm is on its way as the swell travels a considerable distance from the storm centre.

The wave routing output data, including likelihood of slamming, swell and rolling

As the swell and wind waves can be from very different directions, we can get an uncomfortable corkscrew motion or a roll if the swell is on the beam. With a westbound transatlantic crossing, while the direct or ‘northern’ route may be quickest from the Canary Islands, a persistent northerly swell can make this option uncomfortable. In these conditions a southerly ‘traditional’ route will be more comfortable and less wearing on boat and crew.

Wave models are therefore very useful for comfort and safety and, while it is not unusual to be able to download wave models in GRIB file format or in the form of charts, there is some analysis necessary to make sense of them and in deciding what is safe or not. This is not simple as it will depend on the size and shape of your boat and the direction relative to your course.

Auto wave routing

Recently PredictWind has launched automatic wave routing. As with cloud routing, you input your boat’s characteristics and routing solutions offered will include the combined sea state (swell, wind waves and direction), wind waves, and swell.

Swell is divided into primary, secondary and tertiary swell height, period and direction along your route. By using your boat’s characteristics a figure for roll, vertical acceleration and incidence of slamming is computed.

Roll is fairly self-explanatory and makes a great deal of difference to comfort on board, and particularly to safety. We all know how difficult it is to even put the kettle on if the boat is rolling along, and on a long passage a number of days of excessive roll can be very tiring to the crew, and potentially damaging to the boat if everything is not well secured.

PredictWind wave forecasting for the same period shown in the table above

Using PredictWind routing, roll is given as the route-mean-square roll amplitude in degrees and a limit of 4° is suggested as a maximum for safety.

The second variable is vertical acceleration which is stated as being a good indicator for the potential to get seasick, with a safety limit suggestion of 0.2 g (the root mean square vertical acceleration in g-force).

The final variable given is a % incidence of slamming. This is the likelihood of experiencing at least one slamming event per minute. It is measured differently for catamarans and monohulls, and the higher the number the greater the risk to both sailors and boat. A figure of around 50% or upwards could be considered excessive.

There are other considerations; a good helm will reduce the incidence of slamming by pointing up a wave and bearing away at the top but keeping that level of concentration on a long passage short-handed is difficult. Autopilots and less experienced crew will not be so successful. Within the routing program there is an option for motor boats, which are also susceptible to wave action.

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Wave models

Using wave and swell models in passage planning makes a great deal of sense and even if you cannot avoid an area of an uncomfortable sea state it does give time to prepare or to decide if a different route would be better. When running routing there will often be routes that are similar in time but follow quite different paths, although our routing program will opt for the fastest as a default.

By analysing a route rather than just following it, a marginally slower route may be safer and in the long run quicker.

The analysis of routes is something that top navigators will do to balance risk and reward. This is easier to do with onboard routing, however cloud routes will give multiple routes based on different models. Comparing the waves on each route may help in deciding just where you want to go but as the PredictWind wave model used is ECMRWF (European Centre for Medium-Range Weather Forecasts) comparing one model’s wave output with another model’s wind data may give some interesting results.

The more information that we have the better our decision making should be. By combining boat characteristics with wave models and putting numbers to it, it gives us something to consider and compare.

While boats differ in what is comfortable and safe, we should soon be able to decide where our own levels are and what is acceptable. The new function from PredictWind is easier to use than downloading the raw information and trying to analyse it ourselves. Improving communication, along with new wave modelling, is a great tool for navigators and will help in avoiding adverse conditions.

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