The Role Of Hydrodynamics In Modern Ship Design

A vessel must always operate in the fluid domain. Hence, hydrodynamics plays a vital role in the multidisciplinary design process of a vessel that also includes engineering elements ranging from mechanical to electrical to structural. 

In this article, we shall have a brief outlook about what are the aspects of hydrodynamics that are relevant in the vast realm of vessel design and naval architecture as a science without delving deeply into the granularity of applications. 

Before we go any deeper, it is important to understand what hydrodynamics is. 

Hydrodynamics, in simple terms, is a branch of physics (more specifically a discipline of fluid mechanics) that deals with the motion and action of fluids and the study of their effects on bodies directly interacting with them.

Hydrodynamics
Image for representation purposes only.

It is a more advanced domain of fluid mechanics that comes after understanding the physics of fluids at rest, better known as hydrostatics (velocity = 0). In real-world problems, it is not practically feasible to attain environments where a fluid domain under interest is under perfectly static conditions or has zero motion. 

Thus, to study practical applications of fluid mechanics in the real world, any problem or system is better understood considering the hydrostatics and hydrodynamics of it discretely, that is observing its behaviour both under ideal static conditions and the theoretical dynamic conditions. 

As any form of vessel, partially immersed like a ship, or fully immersed, like a submarine is always interacting with the fluid domain: water, hydrodynamics in a ship mostly deals with the following important parameters: 

  • The nature of fluid flow past the bodies 
  • Force dynamics of the fluid on the vessel 

The former deals chiefly with the motion and control behaviour of the vessel while the latter deals with the response of the vessel to the various types of loading imparted by the fluid flow. 

When we speak about the nature of fluid flow, we essentially focus our attention on how the fluid domain creates an environment for the vessel to: 1) travel from one point to another, 2) Remain stable under various circumstances, and 3) Change its course as and when required. 

In a more technical sense, these three requirements give rise to some of the most critical areas of naval sciences and ocean engineering that are influenced by the ambit of practical hydrodynamics. 

While the tendency of a vessel to travel from point A to B is governed by various problems in hydrodynamics like resistance and propulsion, the stability (dynamic) of a vessel is measured in terms of seakeeping (static stability and flotation of a vessel are governed by the fundamental principles of hydrostatics, the other basic domain of fluid mechanics), and directionality is considered a subject of manoeuvring and course-keeping, all of these mostly separate but often interrelated to one another. 

Resistance is the index of opposition offered to a vessel from the water domain. The total resistance (wave + frictional) of a vessel indicates the power required by the vessel to move forward at a given speed after overcoming these opposite forces.

Ship Propellor
Image for representation purposes only.

Now, speaking of propulsion, designing a propeller for a particular vessel involves studying the flow patterns as well as the maximum hydrodynamic forces that should act on the propeller. The study of resistance and propulsion is thus further related to the engine selection for the vessel (based on power expenditure requirements) along with the fuel consumption metrics. These, in turn, further then act as crucial determinants for aspects like fuel tankage that are again related to basic design key elements like general arrangement, tank capacities, and stability. 

Hence, hydrodynamics is relevant at the stages of the basic design itself when the hull form is envisaged. 

With the mission requirements and derivatives like basic parameters, displacement, and tonnage capacities acting as constraints, the hull form design is optimized based on how to minimize the resistance such that all critical parameters like speed, fuel consumption and efficiency are improved to the best feasible extent. 

For instance, for a low-speed large vessel like a tanker or bulker, frictional or viscous resistance is more important than wave resistance whereas for a high-speed vessel, wave resistance is much more critical than frictional resistance. Hence, in the former case, to keep the other less important component (wave) to almost negligible limits, design features like bulbous bow are incorporated. Moreover, the hull lines are optimized especially at the bow and aft shoulders such that the flow patterns are lesser adverse to exacerbate the effects of frictional resistance. 

Bulbous Bow
Image for representation purposes only.

Similarly, in high-speed crafts, the hull form design is made of the planing type such that apart from creating the necessary hydrodynamic lift required to increase the speed of the vessel, the wetted surface area, chiefly responsible for causing frictional effects of resistance is kept at a bare minimum.   

Modern vessels have perpetually better surface designs that are conducive to the hydrodynamics pertaining to a vessel. 

Any floating vessel has six degrees of freedom: Surge, sway, roll, pitch, heave, and yaw. 

These motions are solely dependent on the hydrodynamics of the flow patterns affecting the vessel and are studied separately under the discipline of seakeeping and dynamic stability. 

While roll, pitch, and heave are more relevant in the context of seakeeping which mostly deals with the response of the vessel structure in the vertical or the Z-plane, sway, surge, and yaw are subjects of interest under the manoeuvring or course-keeping domain that measures the vessels’ ability to veer off course under uncongenial sea states when the flow patterns are dramatically altered and erratic. This further ramifies into another very critical ambit of naval sciences: rudder design, something which is done in the basic design stage again. 

The rudder is nothing but a hydrofoil section at the aft that pivots about an axis and rotates at various angles to create counteracting moments that correct the effects of drifting and bring back the vessel to its intended course. Rudder action is another critical aspect that is intricately related to hydrodynamics and works in conjunction with the hydrodynamics of flow around the vessel.  

The nature of the flow also determines the forces that enact on the ship structure, thereby becoming the basis to assess the seakeeping ability of the vessel. 

Force dynamics of the fluid are related to the strength considerations and are mostly dealt with along with the structural domain of the vessel. For example, when we are designing a hull structure, the study of hydrodynamics gives an idea about the feasible degrees of loading that are expected to be incident onto the ship structure during diverse fluid flow patterns around the vessel. 

In a more specific manner, during direct strength analysis of a vessel during the basic design phase of a vessel, the forces generated from the waves and currents are taken into consideration apart from the lightweight dumped on the vessel structure itself. These forces then act as key factors for structural design at both the local as well as global scales such that the hull structure, as a whole, remains within acceptable margins of safety.  

The loads from waves and currents are mostly interpreted as the wave bending moments as well as the distributed wave pressure loads which are critical parameters used in designing the hull structure. 

The exact nature of the wave loading is better understood from a very complex subdomain of hydrodynamics known as the Fluid-Structure Interaction (FSI) which is nothing but a detailed numerical, experimental, computational, and theoretical analysis of the behaviour of the structure (ships in this case) in response to the time-varying action of the fluid flow and vice-versa.

The advent of Computational Fluid Dynamics in the realm of hydrodynamics has further helped to simulate and predict the exact nature of flow about a structure and with the introduction of modern software-based capabilities, aspects like FSI are better visualized with clarity. 

Hydrodynamics is a vast field and includes numerous other eclectic areas as well. For example, underwater noise and acoustics is another crucial aspect of hydrodynamics that deals with studying the noise and vibration levels emanating from vessels and how they affect the aquatic ecology, something increasingly important underscoring the ubiquitous environmental question in today’s time. Moreover, noise and acoustics, in the defence sector, have another level of importance in terms of the stealth and security capabilities of a vessel. 

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About Author

Subhodeep is a Naval Architecture and Ocean Engineering graduate. Interested in the intricacies of marine structures and goal-based design aspects, he is dedicated to sharing and propagation of common technical knowledge within this sector, which, at this very moment, requires a turnabout to flourish back to its old glory.

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Disclaimer :
The information contained in this website is for general information purposes only. While we endeavour to keep the information up to date and correct, we make no representations or warranties of any kind, express or implied, about the completeness, accuracy, reliability, suitability or availability with respect to the website or the information, products, services, or related graphics contained on the website for any purpose. Any reliance you place on such information is therefore strictly at your own risk.


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