Posted on Jan 18, 2024
Ryan Nicoll

Why seakeeping and maneuvering are integral parts of the same thing

It’s common to confuse a Portuguese Man o’ War with a jellyfish, but they are different in a very fundamental way. While a Jellyfish is a single life form, a Man o’ War is actually a colony organism. It’s made up of separate life forms, called zooids, that live and work together. A close look shows these zooids each have their own unique functions – one is for stinging tentacles, another for a gas bladder to keep the colony floating at the ocean surface, others for breaking down and digesting food, and so on. Each zooid has extreme differences from one another, yet each has a crucial role. They are all integral parts of the same thing.

Similarly, when it comes to the world of ship motion, seakeeping and maneuvering are integral parts of the same thing, too. A close look shows that these analyses are dramatically different from each other. But they both have unique functions, and they play a crucial role in evaluating the greater whole of ship motion and seaworthiness. But it pays to understand what each is and how it contributes to a better understanding of ship performance. So, what is seakeeping analysis?

Unlike a Jellyfish, a Man o’ War is a colony organism: made up of separate independent life forms. But they’re all part of a greater whole.

Seakeeping analysis is about understanding ship motion in waves

It’s typically performed by computer software based on the comprehensive details of a ship hull, appendages, mass, inertia, and a range of different wave conditions. After all, where there’s water, there’s waves, and through the lifetime of a typical ship, there’s going to be a wide scope of sea states it will need to navigate.

The Halifax-class Frigates are the Royal Canadian Navy’s go-anywhere, do-anything ships. Seakeeping analysis gives insight on motion and acceleration anywhere on the vessel, and its impact on the limits of safety for crew and equipment.

Why is seakeeping analysis important?

The goal of seakeeping analysis is to thoroughly understand the motion of a ship from all possible combinations of weather direction relative to the vessel. If there’s too much acceleration in certain sea state conditions, the vessel may not be able to operate safely. A seakeeping analysis helps verify what can be expected of a particular ship for safe operation – what speeds and what sea state conditions it can safely operate in. Designers can use this feedback to adjust some parameters of a vessel in the early stages of design or decide between different conceptual hull designs. For an existing ship, it can give operational feedback on what speeds are safe given an expected range of sea states for an upcoming cruise or marine operation.

Ship motion is a combination of linear surge, sway, heave, and rotational roll, pitch, and yaw motions. They combine to produce different motions and accelerations at different points on the ship.

How is seakeeping analysis performed?

The characteristics of waves change all the time based on the intensity of the wind and how long it has been blowing on top of existing wave propagation across the ocean. This means motion needs to be understood when waves may approach from all directions surrounding the ship. The ship’s forward speed has an influence, too – for example, it’s common for a boat racing forward to have a higher frequency of pitch motion as it drives faster over the prevailing ocean waves.

The motion analysis is often checked at many different points on the ship, too. There may be crew quarters, the bridge, and places with sensitive equipment that could be damaged by excessive accelerations. If all this sounds like a lot of calculations, it is! But most modern seakeeping analysis tools do an excellent job of summarizing ship motion performance with directional motion diagrams.

Generic Frigate RMS pitch motion in 3m 9s sea state across a range of weather directions relative to the hull. Pitch motion is minimum in beam sea (90 and 270 degrees) and maximum in bow quartering and head waves (around 180 degrees).

But seakeeping analysis isn’t about predicting how often and where these conditions will occur. That is part of an operability analysis that factors in the probabilities of different sea states concurring based on the region of operation. While seakeeping analysis is about characterizing ship performance in waves, maneuvering analysis is focused on what happens to the ship in specific operational scenarios.

What is maneuvering analysis?

Maneuvering analysis is about understanding how a ship handles in the ocean. Like seakeeping analysis, it’s typically completed by computer software based on fundamental parameters of the ship hull, appendages, mass, and inertia.

Predicting how a particular vessel handles is no trivial matter

There’s often a tremendous amount of inertia involved, and a lot of momentum can build up at even moderate speeds. Ships rely on the hydrodynamics of hull appendages (like the rudder!) to control the boat, and the nature of these effects change dramatically with speed, too. A maneuvering analysis factors in these physical effects and shows expected vessel performance in different kinds of operation. Standard maneuvering analysis includes the zig-zag test and a turning circle test. Both of these give an idea of how a vessel will perform and how quickly it may respond when needed.

Generic Frigate Zig Zag test. This test is an example if a maneuvering analysis to check how a ship handles and response to rudder commands.

Sometimes seakeeping and maneuvering are combined

Though they have different purposes, combining seakeeping and maneuvering is possible. This incorporates the added dynamics of a sea state on top of a particular maneuver. However, it’s not always part of a standard design process because it is not easy to systematically characterize and generalize the performance across a wide range of conditions and complex maneuvers.

Summary

Seakeeping and maneuvering are very different individually but are part of an integral part of ship motion analysis. They’re both crucial to getting a picture of the seaworthiness of a particular ship. Seakeeping gives a comprehensive picture of the motion of a ship across a wide range of ocean wave conditions. On the other hand, maneuvering is focused on anticipating how a vessel will handle operations from changing commands in propulsion and control. Both are critical to getting an expectation of a vessel’s performance, whether in the early stages of design or in evaluating an existing ship for an upcoming marine operation. Like each zooid in a Portuguese Man o’ War, they each have their own function but complement each other as part of a greater whole.

Next step:

An example of a seakeeping and maneuvering analysis tool is the ProteusDS ShipMo3D toolset. Read more about what you can do with it here.