Why Is Reducing Ship Resistance So Important?
Managing the dynamics of water resistance (both literally and figuratively) is one of the greatest challenges in ship design.
Simply put, any object moving through water or other fluids is subject to various forces known as drag. These forces primarily arise from: 1) the inherent physical properties of the fluid itself, commonly referred to as frictional drag; and 2) the waves generated in open bodies of water, commonly referred to as wave drag.
These forces can significantly affect a vessel’s propulsion characteristics, leading to increased fuel consumption, impaired speed characteristics, navigation and maneuvering difficulties, and an overall decrease in the vessel’s efficiency index.
Ship Resistance
Several sources, examining ship data metrics, report that approximately 80-85% of a ship’s input power is used to overcome drag!
For example, if we assume a ship, under ideal conditions, uses one ton of fuel to travel from point A to point B in an ideal, completely inviscid fluid, in real-world conditions, the ship would exhaust all of its fuel before even traveling a quarter of the distance!
Because drag is such an ongoing problem, people are constantly striving to find ways to reduce it. Although the terms “drag” and “resistance” are often used interchangeably, drag specifically refers to the frictional component of drag, not the wave component. The frictional component of drag is primarily due to the interaction between the fluid and the water surface, which occurs directly between the water surface and the ship’s hull.
The magnitude of this frictional drag depends on:
The properties of the fluid (temperature, pressure, flow dynamics, and, most importantly, viscosity).
The geometry of the vessel (hull shape, roughness, and the ship’s wetted surface area).
Due to the large wetted surface area and displacement, the frictional component of drag is greater than wave resistance (which increases proportionally with speed or Froude number) in large ships with diverse hull shapes (such as tankers or bulk carriers), which operate at lower speeds.
Thus, a significant portion of the mechanical energy generated by the propulsion systems of these ships is used to overcome the effects of drag acting on the hull surface.
However, to minimize the influence of external factors on wave formation and breaking on the high seas, the bulbous bow design has proven successful on nearly all tankers and bulk carriers in recent years.
Similarly, for high-speed ships, wave resistance is significant, while the drag effect due to hull shape is small enough to be insignificant.
Various methods and techniques for reducing ship resistance have a significant impact on wave resistance, which is a significant component of a ship’s total resistance. However, certain design and operational aspects, such as bulbous bow design and maneuvering speed values, are limited to wave resistance.
Improving Hull Form
Since the water medium is constant, ship design is also a variable and can be modified at any time. Improving hull form remains the traditional approach for designers, not only to reduce friction or drag effects but also to lower the overall drag coefficient, as wave-making resistance is largely dependent on the hull’s geometric properties.
For tilting ships, such as tankers or bulk carriers, the drag effect is greater due to their greater width, displacement, and resulting wetted surface area. The only way to mitigate drag is to modify the hull profile. This essentially means optimizing the hull’s curvature (below the waterline) without affecting the intended displacement.
Over the years, commercial vessels have made significant progress in adjusting the bow and stern shoulder profiles to maintain the angles of entry and exit from the hull within normal limits, thereby minimizing frictional drag or drag effects.
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Hull Form Optimization
In the bow area, the focus is on streamlining the flow pattern along the hull surface; in the stern area, the emphasis is on reducing flow separation effects caused by the boundary layer effects of the passing fluid.
In older designs or ships with hull forms that terminate abruptly at the stern, flow separation is more severe, resulting in turbulent or semi-turbulent “waves” in the ship’s slipstream, which in turn creates greater frictional drag and drag effects on the hull (these effects are affected by the
For inclined vessels, such as tankers or bulk carriers, the large beam, displacement, and wetted surface area create significant drag effects, and the only way to mitigate these effects is to modify the hull profile. This essentially means optimizing the curvature of the hull profile (below the waterline) without compromising the intended displacement.
Over the years, significant progress has been made in adjusting the shoulder profile of commercial vessels in the bow and stern areas to maintain normal approach and departure angles, thereby minimizing frictional drag and drag effects.
In the bow area, the focus is on simplifying the flow pattern over the hull surface profile; in the stern area, the emphasis is on reducing flow separation effects caused by the boundary layer effects of the passing fluid.
In older designs or ships with steeper stern ends, flow separation occurs more abruptly, resulting in turbulent or quasi-turbulent “waves” in the wake, which in turn generate greater frictional drag and drag effects on the ship’s form (which worsen resistance values).
However, improved designs not only incorporate a tapered stern hull but also a more refined stern (V-shaped) line, resulting in a more gradual and modular effect on the turbulence pattern in the wake. This reduces boundary layer irregularities, making them more modular and thus increasing the drag effect.
Carefully designed bow and stern shapes can also reduce wave effects (which increase wave-making resistance), although these effects are primarily mitigated by the convex bow at the entrance. At the stern, the irregular flow pattern caused by the increased turbulence caused by rapid airflow separation affects the propeller flow, resulting in a higher wave resistance coefficient.
Artificial Air Lubrication System
For decades, the prevailing design philosophy affecting bulk carriers and tankers has been to reduce the effects of drag by allowing the water in the bow and stern areas to flow along thinner lines. However, in this optimization process, cargo capacity within the hull remains the most limiting factor, and for whatever reason, it cannot be reduced (because these ships operate at high efficiency, displacement being the most important criterion).
Because drag is largely dependent on the roughness of a ship’s deck, various techniques are employed to reduce surface roughness. Lubrication essentially involves artificially smoothing the hull surface, significantly reducing the friction coefficient of the passing fluid. According to research and data, an effective air lubrication system can reduce friction by up to 20-25%.
Common methods include:
Bubble Lubrication
This method is one of the oldest methods for reducing the effects of surface friction. It involves continuously generating tiny bubbles on the hull surface, either through a jet or, more traditionally, using a special wire.
The bubbles essentially form a buffer layer between the hull surface and the surrounding fluid. This buffer layer significantly improves surface smoothness over a large area, thereby reducing shear stress and friction.
Furthermore, the air pockets formed by these bubbles mitigate the viscous effects of the surrounding fluid, which can significantly increase drag.
The bubbles also reduce boundary layer turbulence, particularly in the stern region, thereby reducing the severity of irregular flow patterns. The adhesive properties of the bubbles also delay flow separation in the stern region.
While air bubbles remain the most commonly used method, they are ineffective if the bubbles are not of the right nature. Furthermore, this method is ineffective in adverse weather conditions with extremely rough and irregular sea conditions.
Air Stratification
This method is an improvement on the bubbling method. A layer of air is continuously blown around the underwater structure using a pump or compressor. This not only mitigates the turbulent effects of the boundary layer but also creates a permanent air barrier, significantly suppressing the dynamics of fluid-surface interaction. Using an efficient high-pressure injection system, the formation of an air layer is not a problem, even in adverse weather conditions.
However, this system is less effective for V-shaped structures or very delicate structures, as air tends to drift around small or sharp edges.
The air cavity system is a highly modern system that features very fine cavities designed into the ship’s hull (below the waterline). Air is expelled at high speed via powerful blowers or air jet systems. Bubbles continuously form and become trapped within these cavities, helping to reduce air resistance.
Various antifouling and water-repellent coatings can counteract the viscous effects of liquids.
Other modern technologies:
Hydrophoils are only used on high-speed vessels and help slightly raise the hull, lower the waterline, and reduce wetted surface area.
Wall vibration is still a topic of ongoing research. At certain design levels, forced vibration of the hull can be used to reduce the overall drag effect. However, this approach has limitations, such as affecting the fatigue strength and stress levels of the hull itself.
Why is air resistance reduced?
Reducing ship drag is crucial for the following reasons:
Improving ship speed characteristics: Due to friction and wave resistance, a ship experiences opposing forces that affect its motion. Simple physics based on Newton’s laws of motion states that a ship’s speed gradually decreases, and in order to move forward, it requires a greater force to resist these opposing forces. Reducing ship speed means reducing time, and in the maritime sector, whether commercial or civilian, time is money, and safety is money.
Fuel Consumption and Efficiency: To overcome the opposing forces, ships require greater power, which places greater strain on their propulsion characteristics and increases fuel consumption. This, in turn, creates two major, intertwined issues:
Economics: Higher fuel consumption leads to higher sailing costs, negatively impacting the entire supply chain and increasing exponentially.
Environmental Issues: Higher fuel consumption leads to higher emissions. This is a serious issue facing the world and all industries. Because global trade relies primarily on maritime transport, ships are a major source of harmful carbon emissions and greenhouse gases, causing staggering damage to the environment.
The International Maritime Organization (IMO) has issued a series of regulations and guidelines, most of which have already been implemented, and released an ambitious new strategy, the “Greenhouse Gas Strategy,” which aims to reduce shipping emissions by 30% by 2030, 70% by 2040, and close to zero by 2050.
Therefore, as pollution is a serious warning sign for the shipping industry, all ships must implement best practices to reduce emission levels. To avoid this, regulations are constantly limiting the fuel efficiency of ships to certain levels, and in order to successfully meet these standards, the effect of drag must be minimized as much as possible.
