Roll, Vertical Acceleration & Slamming Incidence

The router outputs three data points at each route point: Roll, Vertical Acceleration and Slamming Incidence. For safety reasons, reviewing these after running a route is important.

For instructions to generate this data for your route and to view it, please see this article: PowerBoat Routing Set Up and Use.

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 for safely moving around the boat and completing tasks. Roll tends to be the largest in beam seas.

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 takes into account the wind wave, 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.

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.