^ Shipping is a Critically Important Element of Globalization – Image Courtesy of Digital Genetics at shutterstock.com
Putting Things in the Broader Perspective
Transporting as much as 90% of the globally traded merchandise, shipping is the chief instrument of international trade and commerce. That apart, it is also the paramount element of globalization so much so that shipping and globalization cannot march without each other.
But, lest we get carried away, shipping is among the major contributors of greenhouse gases (GHG). If it were a country, shipping would be the sixth largest emitter of carbon dioxide (CO2) discharging as it does around 3.3% of the average annual global emissions of CO2.
In all fairness, we have to mention that shipping is the most eco-friendly modes of transport spewing only 10 grams of CO2 to carry one ton of goods over one kilometer. This compares favorably with 470 grams for air transport, 59 for truck (tractor / Trailer), and 21 for rail (diesel train).
Ships spend an astounding 85% of their available energy in overcoming hydrodynamic forces. Even a slight improvement in ship efficiency can therefore work wonders for the health of global trade, commerce, and, of course, environment.
Elements of Ship Resistance
Frictional drag is the prime form of resistance particularly for merchant ships sailing at low speeds while wavemaking resistance is a greater concern for high-speed vessels.
Ship resistance can be:
- Frictional Resistance
- Wavemaking Resistance
- Pressure Resistance
- Drift Drag
Frictional Resistance makes up 60-70% of the total ship resistance and is a consequence of the hull not being perfectly smooth and water possessing viscosity. The hull therefore, drags along with it a layer of water just next to it. This layer is called the boundary layer.
Theoretically, boundary layer is the layer of fluid in the immediate vicinity of the bonding surface an object moving in the fluid where the effects of viscosity are considerable. Rising from zero at the bow, the boundary layer picks up width along the length and peaks at the stern.
In an article Wake Patterns by Donald C. Simon, the author guess estimates the width of the boundary layer along a 1,500 ton Columbian frigate he had photographed in October 1984. It stood at a startling 12-14 feet.
Wavemaking Resistance comes from the waves that a ship creates while sailing. The contours and the movement of a ship change the elevation and the direction of water flow around the ship. The larger this change in elevation and direction, the larger is the wave.
Pressure Resistance depends a lot on the shape of the hull. If the hull does not disturb the pattern of streamlines around it, pressure resistance is negligible. This is because pressure recovery occurs at the aft (stern) end.
Pressure recovery is the increase in static pressure of a fluid as it slows down again (after being accelerated first) when flowing past the object with relative motion to the fluid. Based on the Bernoulli Principle, this is why pressure increases after the flow leaves the stern end of the boat.
According to the Bernoulli Principle as applicable for steady flow along a stream line, the total energy remains constant. This total energy is the sum of energies on account of static pressure, velocity, and elevation.
At constant elevation, any increase in velocity is therefore accompanied by a drop in pressure. And when velocity falls at constant elevation, pressure rises.
In the diagram below, the circle encounters less pressure resistance than the triangle because it does not disturb the streamlines. Similarly, a smoother surface does not disturb the streamlines as much as a rough one does.
Drift Drag is the effect of waves, winds, and ocean currents. If a ship sets sail in calm waters and maintains a straight course at a certain speed and in a particular direction, it should arrive at a predetermined location after a given duration.
This is never the case as winds, waves, and water currents deflect it from the theoretical straight path. And the ship has to spend precious power to maintain course. Such deflection therefore constitutes resistance called drift drag.
Countering Ship Resistance
According to Clean North Sea Shipping’s (CNSS) website, the following techniques can lower a ship’s resistance. The table provides the average percentage reduction in ship resistance for all kinds of ships:
- Air Bubble Injection: brings down turbulence in the boundary layer
- Air Cavity in the Ship Bottom
- Air Films along the Bottom Plating
Frictional resistance depends on the:
- Submerged Area of the Hull: is also called as the wetted area of the hull. A larger area pulls more water with it
- Hull Roughness: a rougher hull pulls more water along with it
- Ship Speed: turbulent boundary layers offer greater frictional resistance. Boundary layers become turbulent on rougher hulls and fast moving crafts
Silverstream Technologies developed the air bubble injection system. Supplying air to the bottom of the vessel creates bubbles that cut frictional resistance more at the bottom than at the sides of the ship.
Model tests have demonstrated the capacity of the air bubble injection method to lower frictional resistance on the hull by up to 11% in ballast conditions and by 6% when fully loaded.
Trials of such a system on the tanker MT Amalienborg gave net average efficiency savings of 4.3% and 3.8% respectively in ballast and laden conditions.
Under construction at the Meyer Werft shipyard in Papenburg, Germany is Norwegian Cruise Lines’ Norwegian Bliss. Technicians are fitting the air lubrication system aboard the Norwegian Bliss.
In 2010, the Damen Shipyard Group completed the second phase of Project Energy-saving air-Lubricated Ships (PELS 2) and came up with the Air Chamber Energy Saving (ACES) system. The mechanism involves pumping air into specially formed recesses in the bottom of the hull.
Tankers and vessels transporting dry bulk goods can benefit from ACES. And the benefits include a 10-20% reduction in frictional resistance coupled with 15% cut in fuel use as also a 15% lowering of CO2 emissions.
They say, nothing succeeds like success. Taking cue from the success of PELS, the European Commission launched SMOOTH – Sustainable Methods for Optimal design and Operation of ships with air-lubricaTed Hulls.
Of course, the European Commission was already a part of PELS 2. SMOOTH aims to bridge the gaps in technology that hold back air lubrication systems from furnishing a 20% cut in frictional resistance to hulls.
SMOOTH plans on doing so by providing the necessary tools to successfully introduce air lubrication system. These include developing control mechanisms and paint systems for ships using air film, micro bubble (MB), or air cavity systems (ACS) for shallow draft vessels.
While fuel savings and emission cuts that air lubrication offers is welcome, we must not overlook the greater energy required to create the lubricating level. Then again, improperly applied air lubrication can escalate resistance – a pitfall of any largely unproven technology.
II. Hull Form / Shape Optimization: minimizes wave resistance without cutting down displacement volume significantly. It also curtails manufacturing costs while improving maritime safety.
Numerous designs are available. Usually, ship owners:
- obtain a standard hull form from the shipyard
- modify it using the line distortion method
- develop a novel design after several hydrodynamic, hydrostatic, and structural analyses
While zeroing on the final design, engineers consider not only ship resistance but also seekeeping, safety, and stability. They have to strike a sagacious balance between all these elements or risk failure.
Small ships are the prime beneficiaries of hull shape optimization because they produce relatively large wave-generating resistance in comparison to larger ships.
Hull Form Optimization can be:
- Fore Body Optimization
- Aft Body Optimization
- Appendage Resistance
Fore Body Optimization focuses on slashing the resistance on the forward part of the ship. This includes design of the forward shoulder, bulb, and waterline entrance.
Bulbous bows produce their own waves that are out of phase with the incoming wave. The destructive interference of the bow wave thereby negates the resistance created by the incoming wave. Bow shape is particularly important here and bulbous bows can cut resistance by 30%.
Designers V shape the base of bulbs in order to dilute slamming loads. The bulbs of bulk carries and tankers have a V-shaped entrance and a large cross section. This makes them act as conventional bulbs at the loaded draft while amplifying the waterline length at ballast draft.
Aft Body Optimization streamlines the flow around the stern and thereby slashes hull resistance. The focus here is on preventing eddies, lessening the intensity of stern waves, and bettering the flow into the propeller. Stern flaps improve flow around the stern.
Sterns can be cruiser, transom, or elliptical and each has its own set of merits and demerits. Engineers analyze these using CFD before selecting the most compatible one.
Appendage Resistance can contribute up to 2-3% of the total ship resistance. Rudders contribute around half of appendage resistance while bilge keels contribute the remaining half. Skegs reduce the rudder resistance most felt by directionally unstable ships.
Bow thruster tunnels can add another 1-2% to the total ship resistance. Anti suction tunnels lower pressure variation across the bow thruster tunnel. Grid bars placed over the opening and at right angles to the flow direction minimize vortices and break up laminar flows.
Let us now look at hull form optimization in detail. As mentioned in the section on wavemaking resistance, the hull shape and its motion alter the direction and elevation of water around it. The greater this change, the larger are the waves.
Typically, such flow creates positive pressure at the bow, negative pressure at the section where the width of the ship is greatest, and positive again at the stern. Positive pressure creates wave crests (highest point) while negative pressure generates wave troughs (depressions).
Pressure rises at the bow as the bow deflects water away from the hull causing a velocity drop. Downstream from the section of greatest width on the ship, water is directed inwards. This hikes its velocity and brings down pressure.
At the stern, pressure recovery hikes the pressure again. This is in accordance with the Bernoulli Principle – pressure drops with rise in velocity and vice versa.
Displacement hulls are those which travel through water, not over it. They are so called because they displace sizable amount of water while moving through it. They rely on the buoyant force for floatation.
Bodies immersed in water (or any other fluid) experience an upward buoyant force that equals the weight of the water that the body displaces. The buoyant force counters the weight of the body. This is the Archimedes Principle.
And because they move water out of their way, displacement hulls create waves in water – one at the bow and one at the stern. Important here is the waterline length of the ship – the horizontal length along the hull at the water surface when the ship is carrying its usual load.
The waterline length is lesser than the overall length of the ship. And while not the only factor, it is the most important factor in determining the hull speed (HS), the theoretical upper limit of the ship’s speed.
Now, the speed of these waves is same as that of the ship. And the wavelength of these waves and their amplitude increases as the speed of the ship rises. As the ship speed mounts, there comes a point when the wavelength equals the waterline length of the ship.
When this happens, the crest of the bow wave supports the bow and the crest of the stern wave supports the stern. The ship can now ride efficiently with optimal power input. This speed of the ship is called its hull speed (HS).
Mathematical expression for hull speed in knots is given by:
where LWL is the Waterline Length in feet
Waves that the bow and stern generate are transverse. The wave speed (WS) in knots, according to the laws of physics, is given by:
where WL is the wavelength in feet
You can see that at the hull speed, the waterline length of the hull and the wavelength of the waves are equal. When a ship exceeds its hull speed, the bow wave becomes longer and larger.
This shifts the stern wave further aft and the rear part of the ship falls in the longer trough of the bow wave. The ship therefore has to climb the ascending part of the bow wave that is now higher and longer.
Meaning, the ship creates a mountain for itself to climb while also deepening the valley from where it has to climb up this mountain. To increase the speed of the boat any further, you need to put in a disproportionately large amount of power.
Talking of bow waves, these add to the wave resistance apart from damaging shore structures and creating an avoidable hazard for smaller boats. They also slow down ships by extracting energy from them. Faster moving ships with a blunt bow and large draft create larger bow waves.
Coming back to hull speed, vessels can exceed it. As the ship speed rises, so does its waterline length and, by extension, its top speed. Ships with large overhangs over the stern are capable of this as the overhangs extend their lengths.
Overhangs that exit the water at an angle of 15 degrees or less dampen the stern wave. This lessens the depth of the trough into which the ship falls. Such overhangs also amplify the buoyancy of the stern portion of the ship and limit its fall in the said trough.
And if a vessel with such a stern is lightweight and has a flat, shallow hull, it actually rises on top of the water instead of moving through it. It then becomes a planing hull, it is no longer a displacement hull.
Planing hulls rise in air when the boat sails at high speed. In marked contrast to displacement hulls that rely on the hydrostatic lift (buoyant force), planning hulls rely on the hydrodynamic lift.
This is similar to airfols where the tip bifurcates air flow. Greater air speed over its upper surface lowers the pressure. Reverse happens on the lower surface – lower velocity creates higher pressure.
Greater pressure on the lower surface creates a vertically upward force, a lift. Again, this happens according to the aforementioned Bernoulli Principle. This explains the lift in planing hulls.
With a large part of the hull not touching water, the wetted area and therefore the resistance falls. Then again, they displace less water thereby creating smaller bow waves.
Planing hulls are feasible for smaller crafts only. They require large power input. And, they behave like displacement hulls at low speed where they give their worst performance.
Bulbous bows cut wave making resistance by acting like a wave breaker – they minimize the size of the bow wave by ensuring that the bow wave and stern wave interfere and cancel out each other’s resistance.
Non-bulbous or traditional bows create a bow wave just ahead of the bow. Designers place the bulb of the bulbous bow below the waterline and ahead of this wave. This forces water over the bulb.
If the crest of the bow wave coincides with the trough of the wave flowing over the bulb, they cancel out each other and minimize the ship’s wake. While this creates a new wave stream and siphons off some energy from the ship, it also cancels out a larger wave and depresses wave resistance.
A bulbous bow amplifies the wetted area of the hull and adds to the drag. For large vessels moving at higher speeds, the bow wave is the chief source of resistance. And even if the bulbous bow magnifies the drag, it cuts down a greater quantity of resistance. Finally, its all about tradeoffs.
Which is precisely why bulbous bows are more viable for vessels that:
- are over 15 meters (49.2 feet) long
- run at or about their maximum speed for most of the time
III. Hull Weight Reduction: lighter hulls do not sink much in water. This diminishes the wetted area and this cuts ship resistance. Reducing weight can decrease resistance by 7%. But you cannot use too low-weight materials, for they can degrade the safety and strength of the vessel.
IV. Hull Coatings: smoothen the hull and thereby curtail ship resistance. Although the potential of coatings to cut ship resistance is only around 5% at present, the use of nanotechnology can magnify this to 15% in the near future.
Rough hulls pull along greater volume of water thereby escalating friction. And boundary layers become increasingly turbulent on rough hulls adding to the friction.
Fouling i.e. hull colonization by marine organisms is the biological cause for hull roughness. Physical causes include weld defects, corrosion, mechanical damage, improper coating, and wavy profile.
Thin boundary layers are more prone to the deleterious consequences of rough hulls. As mentioned, boundary layers are thicker at the stern making bows more vulnerable to the effects of roughness.
Apart from roughening the hull, fouling adds to its weight. The U.S. Navy blames fouling for a mind-boggling 40% escalation in fuel consumption. Some escalation that is!
Micro fouling or fouling by microbes such as slime and bacteria hike resistance by 1-2% while macro fouling by mussels and barnacles can add 40% to the resistance. Rough hulls provide these creatures a foothold and these organisms further roughen the hull.
Anti-fouling coatings serve to prevent such creatures from colonizing the hull. Non-toxic anti-fouling coatings such as tin-free, hydrophobic foul release, silicone-based, and hydrophilic coatings are replacing the toxic ones such as tributylin.
Foul release coatings must be smooth while possessing low surface energy and low adhesion strength. Other important considerations for these coatings include thickness and modulus of elasticity.
You can cut down drift resistance by putting in place a large, deep, and well shaped keel. Structural keel is the horizontal beam running from the bow to the stern. It forms the backbone of the ship and converts sideways force into a forward force.
Hybrid Lifting Body Ships utilize low-volume, hydrodynamic shaped lifting bodies with high lift-drag ratio to lift the parent hull. This is somewhat similar to planning and it diminishes resistance by 15-30% by (mainly) targeting frictional drag.
Wing-In-Ground crafts fly close to the water surface or land by letting out high pressure air. Such flight is faster and more economical than high speed ships. Hump Drag is a major deterrent in the wide use of these crafts, for it hinders the development of high speed needed for takeoff.
An area as critically important as this requires the highest level of meticulous treatment. With the shipping industry driving hard to cut its carbon use, such measures will gather further momentum in the near future.
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