High quartering and following seas may inflict immediate danger to a vessel. This module can produce a diagram that allows assessment of dangerous combinations of speed, course and wave period when sailing in quartering and following seas. Furthermore, an indication of the loss of stability is given for a wave with the same length of the vessel and a wave height of 1/50^TH of the wave length. The dangers of quartering and following seas can be categorized as follows:

Surf riding. When the speed of the vessel approximates the speed of the waves, the vessel will be accelerated by those waves, this reduces the rudder function and may cause broaching.
Loss of stability on a wave top. For a vessel with V-shaped cross sections the waterline breadths at the vessel's ends are reduced in the wave troughs, which reduces GM and may even lead to negative GM values.
Synchronous rolling or resonance.
Asynchronous rolling or resonance.

In order to be able to assess those dangers, LOCOPIAS can be equipped with a specialized module, which is based on the method of IMO paper SLF 39/3/3. This LOCOPIAS module presents a diagram (as depicted just below), which indicates all possible dangers instantaneously. It contains three elements which can help the crew to determine a possible change of course or speed:

A bar chart, indicating the dangerous speed zone for surf riding
A polar diagram, which indicates dangerous resonance zones for different courses and speeds. In this diagram the areas with possible synchronous or asynchronous resonance (resulting from the actual loading condition) are indicated in red.
An indication of the loss of stability, by showing the loss of metacentric height on a wave top with a height of 2% of the length between perpendiculars.

Diagrams indication dangerous quartering and following seas.

Surfriding and broaching

The upper left part of the graph is a bar chart, indicating the dangerous speed zone for surf riding and broaching. For every combination of wave period (not the encounter period!) and wave direction a separate bar is shown, see the legend on the right hand side of the graph. The bars correspond with a speed range that can be read from the left hand scale of the graph. The small ‘pie chart’ on the right hand side of the graph is just an indication of the course of the vessel relative to the wave direction in degrees.

Heavy rolling

When the encountered wave period is nearly equal to the natural rolling period of the ship, heavy rolling may occur (synchronous rolling). Large rolling motions also occur when the encountered wave period is about half the natural rolling period (parametric rolling). The lower part of the graph is a polar diagram, which indicates dangerous resonance zones for different courses and speeds for the actual loading condition. In this diagram the areas with possible synchronous or asynchronous resonance are indicated in red. The actual state of the vessel is the intersection between the heading of the vessel relative to the wave direction (the diagonal lines in the graph) and speed (the circular arcs, scale on the bottom axis). The red areas represent the dangerous combinations of speed and course. Left and right of every intersection in the graph, the encounter periods are printed for waves with a period of 6 and 9 seconds respectively. Note that the printed periods may differ from the ones above, depending on the length of the vessel.

Loss of stability

An indication of the loss of stability is shown in text, as the loss of metacentric height on a wave top with a height of 2% of the ship's length. Note that this kind of loss of stability is not included in the regular stability values of LOCOPIAS, because this loss of stability is usually temporary (only when balancing on a wave top). However, sailing in following seas with approximately the same speed as the wave can lead to a prolonged period with reduced stability, which may lead to dangerous situations.