Why Is It Important To Reduce Drag In Ships?

One of the biggest challenges in vessel design is navigating the resistance dynamics offered by water, both literally and metaphorically. 

Recapitulating, any body moving through water or any other fluid domain, is subjected to a variety of forces known as resistance forces. These forces primarily stem from: 1) The innate physical nature of the fluid itself, known more commonly as the frictional resistance, and, 2) The waves occurring in the open water bodies, known better as the wave resistance.

These components of forces impart a lot of opposition to the propulsive properties of the vessel, translating to greater fuel consumption, hindered speed characteristics, problems in seakeeping and manoeuvring, and the overall reduction in the efficiency index of the vessel. 

Drag In Ships
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Multiple sources, after studying vessels’ data metrics, have reported that about 80-85% of the vessel’s input power delivered is exhausted to overcome resistance forces! 

This translates to the fact that for instance, hypothetically if a vessel travels across a perfectly inviscid and ideal body of fluid under 100% ideal conditions from point A to B using 1 tonne of fuel, in a real-world scenario, the same vessel would exhaust all its fuel before even reaching 1/4th of the distance! 

Since resistance is always a problem, there are always efforts and ways to reduce it. 

While resistance and drag are often used interchangeably, drag more specifically alludes to the frictional component of the resistance more than the wave one. The frictional component of the resistance is mostly due to the fluid-surface interaction that takes directly between the water domain and the vessel’s structure.

The degree of this frictional resistance depends on:

  • The properties of the fluid (temperature, pressure, flow dynamics, and most importantly, viscosity) 
  • The geometrical nature of the vessel (hull form, roughness, wetted surface area of the vessel) 

As the wetted surface area and displacement volume are greater, for larger vessels with fuller forms like tankers or bulkers (having low speeds), a frictional component of the resistance is much more critical than wave resistance (that proportionately increases with speed or Froude number). 

Thus, for these vessels, a significant portion of the mechanical energy derived from propulsion systems is expended in overcoming the drag effects influencing their hull surface.                                                                                              

However, to keep the extraneous effects of wave-making and wave-breaking resistance to a minimum during high sea states, bulbous bow design implementation in almost all bulkers and tankers over the last several years has proved to be successful.  

On similar lines, for high-speed vessels, wave resistance is high and form drag effects are anyways low enough to be brought into much attention.   

The various methods and techniques to reduce the drag of the ship have significant effects on wave resistance since the latter is also an integral part of the overall resistance of the vessel. However, some ambits of design and operation are specific to wave resistance only, like bulbous bow designs and dealing with speed values. 

Hull form optimization

As the water medium is a constant property, the design of the vessel is a variable that can be modified from time to time. Hull form optimization remains the classical approach for designers in not only reducing frictional or drag effects but also the overall resistance quotient as the wave resistance also depends greatly on the geometrical properties of the hull. 

For bluffer form vessels like tanker or bulkers, having greater width, displacement volumes, and resultant wetted surface area, drag effects are high, and the only way to deal with assuaging them is to adjust the lines of the hull form. This essentially means optimizing the curvatures of the hull form (below the waterline) without compromising on the desired displacement volume. 

Over the years, commercial vessels have evolved significantly in fine-tuning the shoulder lines of the hull curvature at the fore and aft regions such that the angle of entrance and exit remains nominal to reduce the frictional resistance or drag effects to a minimum. 

Hull form optimization
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While at the bow region, it is more of streamlining the incident flow patterns over the hull surface lines, at the aft, it is about minimizing the effects of flow separation caused by boundary layer effects of the passing fluid. 

In older design ships or vessels with an abrupt ending of the hull form at the aft, the flow separation happens more drastically, leading to a turbulent or near-turbulent “wake” in the slipstream of the vessel, causing higher frictional resistance and form drag effects (exacerbating the drag values). 

However, in optimized designs with not only a tapered aft body but also a finer stern line (V-shaped), the effects of turbulent flow patterns at the slipstream of the vessel are much more gradual and nominal. This reduces the erratic nature of the boundary layer, keeping it more towards the nominal side which contributes to increased drag effects.  

Well-designed bow and stern forms also reduce the wave effects (contributing to wave resistance) though they are mainly attenuated by the presence of bulbous bows at the entrance. At the stern, an erratic flow pattern due to higher turbulence from rapid flow separation also affects the wash of the propeller which can create high stern wave resistance coefficients. 

Artificial air lubrication systems 

These common design philosophies affecting bulkers and tankers over the last several decades have cut down the drag effects by allowing the water flow at the bow and stern regions to enter and depart along finer lines. However, in this optimization process, the most critical constraint remains the tonnage volume of the hull, which can never be reduced on any grounds (since these vessels are workhorses trade and the displacement tonnage capacities are the most crucial parameters). 

Artificial air lubrication systems 
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Since drag is also greatly dependent on the surface roughness of the vessel, various technologies are also employed to assuage the surface nature. Lubrication essentially means using artificial methods to smoothen the surface roughness on the hull such that the frictional coefficient of the passing fluid can be significantly reduced. As per studies and data, efficient air lubrication systems can damp down the frictional coefficient values to as much as 20-25%. 

Some common methods employed are: 

Bubbling 

This has been one of the oldest ways to reduce skin friction drag effects and involves the constant generation of microscopic level bubbles at the hull surface either by means of some jets or more traditionally using special wires that can lead to effervescence. 

Bubbling essentially creates a coating or layer between the hull surface and the surrounding fluid domain. This aggregate buffer layer, on a macroscopic scale, induces a great deal of surface smoothness reducing the shear stress and frictional coefficient

Moreover, these bubbles create pockets of air cushion that attenuate the viscous effects of the surrounding fluid domain that highly contributes to the drag. 

Also, the bubbles lead to a decrease in the turbulence of the boundary layer, especially in the stern regions which further reduces the intensity of the erratic flow patterns and the adhesive nature of the bubbles delays the flow separation at the stern regions

While bubbling remains the most common method, it remains ineffective if the nature of the bubbles is not proper. Furthermore, this method also does not work in very severe weather conditions when sea states are very critical and erratic. 

Air Layers

This is an advancement over bubbling and involves constant projection of an air film or stream through pumps or compressors around the immersed hull that not only assuages the turbulent effects of the boundary layer but also provides a permanent air buffer that resists the fluid-surface interaction dynamics at a dramatic level. With efficient high positive pressure injection systems, during rough weather conditions also, generation of the air film is not a problem. 

Air Layers
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However, this system is not very effective for V-shaped or very fine-form hulls as air tends to drift away from fine or sharp edges. 

Air Cavity Systems are very modern systems where the outer hull of vessels (below the waterline) is designed with very fine recesses through which air blows out at very high rates from a high-power blower or air-jet system. The air bubbles constantly form and get trapped inside those recesses, which helps decrease the drag values.  

Various kinds of antifouling and hydrophobic paints that resist the viscous effects of the fluid.

Different other types of modern technologies-

  • Hydrofoils are applicable for only high-speed ships and help in somewhat lifting the hull, that is lowering the waterline and decreasing the wetted surface area. 
  • Wall vibration is something that is still under research and study, where the overall drag effects are reduced by induced forced vibration of the structure at some design level. However, this has limitations like affecting the fatigue life and stress levels of the structure itself. 

Why reducing drag is necessary? 

Thus, it is important to reduce drag in ships because of the following reasons: 

  • Improving speed characteristics of the vessel: Because of both the frictional and wave resistance, the vessel is essentially imparted with opposition forces that act against the motion of the vessel. The simple physics based on Newtonian laws work in a way that the speed of the vessel gradually diminishes and the vessel, in order to move forward, needs to impart greater force countering these opposing forces. Speed reduction in a vessel is equivalent to reduced time, and in the maritime domain, whether commercial defence or civil, time is money and security.
  • Fuel consumption and efficiency: To overcome the opposing forces, the vessel needs to impart greater power and this amounts to an increased load on the propulsive properties, exacerbating the fuel consumption. This, in turn, leads to two important bifurcating problems:
reducing drag
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  • Economy: A higher fuel consumption leads to greater costs incurred for a voyage, having a diverse ripple effect on the overall supply chain in compounding amounts. 
  • Environmental question: Greater fuel consumption leads to augmented emission levels; something being seriously flagged across the world in any ambit. Since the bulk of global trade is dependent on maritime transportation, vessels are crucial sources of carbon and greenhouse noxious emissions that are alarmingly harmful to the environment. After a slew of regulations and guidelines, most of which have been implemented, IMO has released the new ambitious strategy to curb emissions from maritime transport by 30% by 2030, 70% by 2040, and almost zero by 2050, in what is highlighted as the Greenhouse Gas Strategy (GHG).

Thus, as pollution is a serious red flag across the shipping domain, it is imperative that all vessels resort to best practices such that the emission levels are cut down to a bare minimum. Hence, to avoid this, the fuel consumption efficiency of the vessel is always capped to certain levels by regularization and for adhering to such yardsticks without fail, the drag effects need to be minimized as much as possible. 

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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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