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Why the EGA shows aggregate effects of ship motion on people

The elephantnose fish lives in such murky waters that it can’t see anything. Yet it always senses what’s going on around it. These fish thrive in maritime climates like river deltas. But the problem with water in these areas is they can be highly turbid: mud, silt, and sand, all mixing around in the water. It can be so bad you can’t see your own hand in front of your face. But an animal like the elephantnose fish doesn’t really need eyes. It uses electrical pulses to perceive what’s going on around it. This is crucial because it helps them avoid predators. But it can also help them hunt for food. What’s really remarkable is when elephantnose fish hang out in groups.

What happens in this case is their electrical fields interact with each other. With this interaction, they can effectively extend their individual sensing range – reaching as far as three times their normal range compared to when they are on their own. So when they are alone, they can find their way around and get by. But things work far better when they are around others. The summation effect of their electric fields makes them far more effective. They get a lot of clarity from these effects in aggregate.

Similarly, when faced with a problem, we may have a lot of separate pieces of information. What’s needed is a way to put this information together in aggregate to get clarity on a solution. In the world of ship motion analysis, the Effective Gravity Angle does just this: you get clarity on what you can and can’t do at sea on a ship when factoring in all the cumulative effects of different ship movements. In this article, we’re going to cover what the EGA is and how it gives insight into operability limits in ship design. So what is the Effective Gravity Angle (EGA)?

The EGA is a calculated value from the local accelerations at a specific point on a ship

These accelerations form while it is moving in waves. It’s a value indicated from vertical. Normally, gravitational acceleration always pulls straight down. So your EGA sitting in your chair while reading this article is zero. However, on a moving platform like a ship, the EGA is a value calculated from the lateral and vertical accelerations at that point. The greater these vertical and lateral accelerations, the greater the EGA angle is. So why is the EGA important?

The EGA correlates to tipping and sliding

From the perspective of an individual, it directly correlates to the sensation of tipping or falling over. At a certain threshold of EGA, people start to reach out to grab something, or they may fall over. The EGA is then a single number you can use to gauge motion-affected comfort from tilting or sliding on a ship in a seaway.

Of course, it also gives insight into whether unsecured equipment may tip or slide, so it is not only about motion-affected comfort of an individual. Regardless, what’s so valuable about it is that it is a single parameter distilled from all the different motions of the ship.

How is the EGA specifically calculated?

The EGA is a function of the vertical and lateral accelerations. The angle is created from the instantaneous lateral and vertical accelerations (including the vertical acceleration from gravity).

Of course, the lateral and vertical accelerations are themselves a function of all degrees of motion of the ship: the linear surge, sway, heave, and rotational roll, pitch, and yaw motions. These accelerations may be estimated with empirical formulas, measured from test tank or field trials, or more commonly, calculated from a ship motion prediction program.

For example, the EGA is computed by ProteusDS to evaluate motion-affected comfort at sea. This is done in tandem with calculating the ship’s motion in a specific sea state from the vessel RAO.

When do you use the EGA?

The EGA is one parameter that’s used to gauge the dynamic performance of ships at sea. It’s useful in the ship design stage, but it can also be helpful to assess the operational condition of a specific vessel. In the context of the design of a ship, the earlier you can get feedback on motion performance, the easier it is to make adjustments to the design.

The challenge with the EGA is that it isn’t something you directly measure

This makes it hard to validate because it’s a calculated value that distills multiple pieces of information into a single value. To validate the EGA value calculated for a specific location on the ship, you need to measure the vertical and lateral accelerations and then complete the calculation of the measured values.

Another disadvantage is that there are often different levels of uncertainty when using ship motion prediction programs. For example, there is often more uncertainty when calculating roll motions than when calculating heave or pitch motions. This might mean there is a variable amount of uncertainty in the EGA values in positions with accelerations that are more sensitive to roll motions than heave or pitch. This usually correlates with regions near the beam or at positions much higher than the CG of the vessel.

We covered a few facets of the EGA and now it’s time to look at an example

The ISO 22834 standard provides a framework for calculating a motion-affected comfort rating of large yachts at sea. The EGA is an essential component of this calculation. The ISO standard requires an evaluation of EGA in a bow quartering condition to ensure a robust assessment of comfort. But it also means that heave, pitch, and roll all contribute to the motion experienced at each place on the yacht.

The 50m Generic Yacht sample project was used to evaluate ship motion performance and EGA values in a 2m, 7s JONSWAP sea state in bow quartering condition. The locations used to evaluate the EGA values are illustrated below. The five locations correspond to the Owners Cabin, Dining Area, Wheel House, Crew Area, and Beach Club.

In a bow quartering condition, without any anti-roll or other stabilization technology, the overall ship motion in this sea state is an RMS roll of 10 degrees, RMS pitch of 1.3 degrees, and RMS heave of 0.3m. Each of these contributes in different ways to the vertical and lateral accelerations at each location. However, generally the affect of these motions should be lowest near the CG. Roughly, roll affects locations vertically farther from the CG, and pitch affects locations longitudinally farther from the CG. The resulting EGA values for all locations are listed below. The result is a single number indicating the tilt effect from multiple sources of motion.

Table 1: Summary of EGA values in 2m, 7s JONSWAP Bow Quartering sea condition for each location

Location	OC	DA	WH	CA	BC
EGA (degrees)	3.0	2.5	4.5	1.5	2.0

It’s summary time

Evaluating the effects of ship motion at sea is not easy. The multiple degrees of freedom of ship motion – surge, sway, heave and roll, pitch and yaw – all contribute to the motion experienced at each location on the ship. The Effective Gravity Angle (EGA) is a concept that factors in the cumulative effect of all these motions from the vertical and lateral accelerations they produce. The single EGA value then provides insight in motion-affected comfort and safety by directly correlating to what a person might experience at a specific location. The higher the EGA – the more likely someone needs to reach out and grab hold of something, or otherwise may fall over and get hurt.

By adding these ship motion effects in aggregate, the EGA gives you clarity on what’s going on. It gives you a way to move forward when things seem murky – much like the elephantnose fish finding its way through turbid waters.

Next step

The EGA is a crucial component of evaluating passenger motion-affected comfort on ships. Incorporating motion-affected comfort in the early stage can help improve the design. Read more about how the ProteusDS Ship Motion toolset calculates the EGA as part of an ISO-compliant comfort rating for large yachts here.