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Weather Routing - Using Wave Polars
Weather Routing - Using Wave Polars
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Written by Support Team
Updated this week

With PredictWind Weather Routing, you can set your Wave Polar not only to define the performance of your boat in various wave conditions but also to view the Roll, Vertical accelerations and Slamming Incidence that your boat will experience through the route.

Setting up the Wave polar can only be done on the PredictWind Website at; once set up, all the PredictWind Apps will use these settings saved on your account. The Apps (PredictWind App and Offshore App) cannot modify the Wave polar settings. They can only turn it ON or OFF.

To set up Wave polar:

  1. Go to the weather routing page, then click Routing Preferences.

  2. Press the Wave Polar button in the Routing Settings section

  3. You will first need to turn ON Wave polar

  4. Next, turn OFF Advanced Wave Polar (if it isn't already. We will explain Advanced Wave polar at the bottom of this article)

  5. If you use a Predefined Polar, make sure the checkbox indicating "Use Predefined Boat Dimensions" is ticked. The boat dimensions of our Predefined Polars will be automatically defined using this setting, saving you the effort of looking them up.

  6. If you're not using a Predefined Polar or think the values given are incorrect, uncheck the box, then fill out the fields, Displacement, LWL, Beam and Draught. If you're unsure what these values mean, click on the link: "What are Displacement, LWL, Beam and Draught?" The image below will be shown on screen.

The router will use these supplied dimensions to alter the performance of your boat based on the influence of wave conditions.

Note: For sailboats the the roll damping effect of the mainsail is accounted for. The mainsail forms a large part (around 40 – 60%) of the roll damping. So, resonant roll motions may be approximately doubled with no sails up.

If motoring, it has been assumed that the mainsail is hoisted.

Roll, Vertical Acceleration & Slamming Incidence

(Only available on the Professional Subscription. Upgrade Here )

There are three new data points output by the router at each route point: Roll, Vertical Acceleration and Slamming Incidence.

Roll is the root-mean-square (RMS) roll amplitude in degrees. When roll motions are large, moving around the boat and performing tasks becomes difficult. Objects not strapped down are likely to move. A roll RMS limit of 4 degrees is often used to move around the boat and complete tasks safely. Roll tends to be the largest in beam seas. For sailing yachts, which have a heel angle, the roll is the change in heel angle due to the waves.

Vertical acceleration is pitching or a vessel's up and down motion. This is characterized by the rising and falling of the bow and stern. Vertical acceleration is the root-mean-square vertical acceleration in 'g's' (1.0 = Earth's gravity 9.81 m/s^2). Vertical acceleration is a good indicator of the potential for the crew to get seasick. The algorithm considers the primary, secondary and tertiary swells to calculate the overall vertical acceleration for the boat. The higher the acceleration, the more likely the crew will get seasick.

A vertical acceleration limit of 0.2g is often used for safely performing tasks and avoiding seasickness.

"Slamming Incidence” is the likelihood of experiencing at least one slamming event per minute. For monohulls, slamming is measured near the bow (10% LWL from the forward extent of the waterline). When this part of the boat emerges clear of the water and then impacts the water with a high relative vertical velocity, it is called a slamming event. For sailing catamarans, slamming is measured at the cross-deck structure (bridge deck), and again is based on relative vertical velocity. For either monohulls or catamarans, a slamming incidence of 50% is considered excessive and may lead to hull damage or injuries to the crew.

There are three new graphs on the graphs tab of the route results, one for each new variable; the data is also given in a tabular format on the wave tab.

Set up Advanced Polar:

Although not recommended, it is possible not to use the automated Wave polar and instead define your own Wave polar Table. This does not give you the Roll, Vertical Acceleration or Slamming incidence outputs.

  1. Turn ON Advanced Wave Polar.

  2. You will see a table of values as shown below:

Each cell defines the performance factor to be applied to your polar when your boat experiences the given wave conditions. E.g. the highlighted cell will define the factor applied to your polar when the wave conditions are 5m high, at an angle of 0 degrees to your boat (boat heading with the waves), while the TWS is 0 knots. 100 refers to 100%, so a reduction in performance is a number ranging from 1-99.

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