GM in real time can be calculated from the rolling period:
The location of a vessel’s metacentric height (GM) is of paramount importance as a measure of a ship’s static stability and is a factor in the vessel’s response to dynamic loads and their distribution. (If for example there’s too much heavy cargo is loaded high up in the ship, it will become top-heavy and be in danger of capsizing.) When the ship is at sea, it is subject to wave motion causing rolling, pitching, yawing, or an infinite combination of all three. This motion can be severe. As a result, dynamically monitoring of GM, as well as roll, pitch, etc is of considerable importance with regards to safe maintenance of vessel and cargo. GM as provided by standardized tabular information on traditional hull forms and vessel types usually does not take into account the nominal changes arising cargo distribution and re-distribution. For vessels with traditional hull forms, the location of the ship’s metacenter, and ultimately its GM value, is estimated by standardized inclining experiments.

The dynamic GM is derived from the measured rolling period using a Fourier analysis calculation done by the program, and therefore the acquired GM value is not dependent on the given cargo weights. If, for example, the weight of containers are not correctly given, or are wrongly entered in the stability program, the resulting GM value as based on this can easily be less than a safe estimate. However, if the GM value is calculated from a measured rolling period, it is only dependent on the characteristics of the fitted vessel. Moreover, when the data is given a comparison between theoretical GM and measured GM, it will also provide the possibility to check if entries in the stability program are correct (or if the documented weights of containers, or other forms of cargo, are correct). Moreover, these calculations are made taking into account quantities of fuel, cargo and its distribution, and ballast water in the vessel. The International Maritime Organization (IMO) sets a minimum GM at which a vessel can sail. GM is one of the parameters that determine the ability of a ship to right itself, and is actually the distance between the metacenter of the vessel and the center of gravity (hence the abbreviation GM) of the vessel AND its cargo. Therefore, GM is one of the most important factors governing ship stability and seaworthiness, with a dynamic and real time estimate of GM being of considerable usefulness with respect to varying conditions of vessel and cargo. An inclination test can be used to calculate GM from changes in inclination while loading or transferring ballast, i.e. by flooding a ballast tank, or by pumping a tank from one side to the other. (The inclination test should also contain a checklist so that the conditions of the test will be checked.) Likewise, GM estimates from the inclination test can and should be compared with the dynamic real time GM while the vessel is at sea.

Parametric Roll and Resonance:
A ship moving through a series of waves in a head sea will undergo a change in metacentric height (GM) as it moves from the hogging to the sagging condition. If for example the wave encounter frequency is twice the roll frequency of the ship, then energy can be transferred from the waves into roll motion of the ship. The ship will rapidly develop an oscillatory or “parametric” roll motion which can result in capsized vessel. This phenomenon is known as parametric resonance. Parametric roll and resonance can occur suddenly and unpredictably, but usually happens when a ship (usually longer than 150 meters) is hit by frontal incoming waves or from waves overtaking the vessel. When a ship is hit in a frequency equalling 1, 1 ½, 2, ... of the rolling period, resonance can occur and the rolling angle can increase markedly. (There are known examples of an increase in rolling angles from 15° to 60° in less than 6 minutes!) SMC’s parametric roll function can be implemented to warn of the imminence of excessive parametric roll and resonance.

RP/PP:
Rolling period and pitching period. If the pitching period is ½ the rolling period the flair (forward or aft) of the ship is hit by waves in double the frequency of the rolling period. The ship danger of the ship coming in resonance increases drastically. The Alarm function is built around this effect.)

The Parametric Roll screen serves as a warning system for monitoring parametric resonance, as well as alarm the officer on watch when parametric resonance is imminent and therefore a corresponding evasive maneuver necessary. It is used to warn for the risks of resonance occurring between wave frequency in combination with ships course and speed, and to display roll and pitch of the vessel in a 3-dimentional picture. The parametric roll warning system can therefore help reduce the risk of excessive rolling due to resonance from wave action; thereby reducing the risk of cargo damage and related losses - or worse!