There are two options for Power Routing - Minimum Fuel Consumption and Comfort.
Comfort Routing is based on boat motion modelling, which evaluates RMS roll, Vertical Acceleration and the Slamming Incidence.
The default comfort thresholds are set to avoid RMS roll greater than 4.0°, avoid vertical acceleration greater than 0.2 g, and avoid slamming incidence greater than 50%.
Whilst these recommended default settings may not be comfortable, the router will, where possible, avoid unsafe and very uncomfortable conditions. It is very important that you select the best departure day using the departure planning tool first.
When you request a route, the router will first attempt to calculate a route that stays within these comfort limits. If it cannot complete the route within those parameters, it will automatically rerun the route without comfort limits so you still receive a result.
You can adjust these default settings by turning on Advanced, we do not suggest decreasing the values.
It is important to set up Wave Polars for your boat so the motion modelling works correctly. In your routing preferences, you should enable Wave Polar and enter your boat dimensions, including the type of boat, displacement, LWL (waterline length), beam and draught. These parameters allow PredictWind to model RMS roll, vertical acceleration and the slamming incidence more accurately for your specific vessel.
Feedback from an experienced sailor, crossing oceans (Flemming 60, trawler style with ABT TRAC Stabilizers) 4 RMS Roll is a good default on safety around the boat for NON Stabilizers and 9 RMS Roll is about the correct number for boats with Stabilizers.
A little more detail
The router outputs three data points at each route point:RMS Roll, Vertical Acceleration, and Slamming Incidence.
RMS 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. An RMS roll limit of 4 degrees is often used to move around the boat and complete tasks safely. RMS Roll tends to be the largest in beam seas.
Please note: this is not 4 degrees roll to Port to upright then 4 degrees to Starboard, think of the RMS of 4 as just a baseline number.
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 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.