The Mechanics of Hull Design

By November 3, 2015 Uncategorized No Comments

^ Parts of a Ship  – Image Courtesy of A12 at Lod Schema.png  Retrieved From https://en.wikipedia.org/wiki/File:Ship_diagram-numbers.svg

Necessity of Better Hull Design

Boat engineering is among the earliest forms of engineering thanks to water covering over 70% of the earth’s surface. Historical evidence points to the use of boats and waterborne vessels as far back as 4000 B.C.

Until 150 years ago when steam propulsion took root, the evolution of water vessels moved at a pace that would have made a snail look like a champion runner. And it was only some 120 years ago that we started to use steel hulls.

Hulls determine the hydrodynamic quality and the efficacy of the entire vessel system. Proper hull design addresses the two prime challenges confronting ship designers – cutting energy use without reducing design speed and boosting speed with the same available power.

Tightening exhaust emission norms, rising fuel costs, and falling charter rates and freights all make ship efficiency a critical factor, financially as well as legally. Refined hulls are a necessity in the process of building more environment-friendly vessels.

Hull Design Considerations

Dynamic interaction of the hull with surrounding water is usually the principal design parameter for a hull. Wider boats generally exhibit greater transverse stability and deeper boats are more robust.

Bilge Keel Cuts Rolling Motion Image Courtesy of Dj245 Retrieved From https://en.wikipedia.org/wiki/File:Bilgekeel.jpg

Bilge Keel Cuts Rolling Motion
Image Courtesy of Dj245
Retrieved From https://en.wikipedia.org/wiki/File:Bilgekeel.jpg

Although curtailing a hull’s wetted area brings down drag, other requirements demand larger hull area. Similarly, smoother hulls slash drag but building and maintaining such hulls is costly business. Drag is the resistance exerted on the vessel by the surrounding water.

Wave-Making Resistance is the greatest headache for the designers of large ships. Waves originate at different points on the hull and their resistance rises with the speed of the ship, for they travel at the speed of the ship.

You cannot normally run a ship at a speed that takes the speed-to-length ratio beyond 1.3 because this boosts the wave size and puts the ship in larger depressions (wave troughs). Trough is the lowest point of a wave while crest is its highest point.

Speed-to-Length Ratio is the ratio of ship speed in knots to the square root of her length at the waterline in feet. Smaller boats adopt planning techniques that are not practical to use for larger ships.

Planing raises the front part of the vessel way above the water surface and enables the boat to move at a very high speed. This is because its contact with water and therefore resistance from water gets reduced to a bare minimum.

Designers add the underwater bulb at the bow to create waves that will cancel bow waves. This happens when the crest of one wave coincides with the trough of another.

Center of Gravity (G), Center of Buoyancy (B), & Metacenter (M) for a Floating Body Image Courtesy of Life of Riley Retrieved From https://en.wikipedia.org/wiki/File:MetacentricHeight.svg

Center of Gravity (G), Center of Buoyancy (B), & Metacenter (M) for a Floating Body
Image Courtesy of Life of Riley
Retrieved From https://en.wikipedia.org/wiki/File:MetacentricHeight.svg

Hydrodynamic Resistance is more important than Aerodynamic Resistance because water is 784 times as dense as air and therefore offers greater resistance. High-speed crafts apart, all that other vessels do to cut aerodynamic resistance is round off and slope deckhouse surfaces.

Model Testing is the best way to estimate the propulsive power required to drive a ship as years of diligent research has failed to establish reliable principles of determining the same at the design stage.

Directionally stable ships i.e. those that deviate from the set course only under the influence of external force and movements use less energy to maintain the set course. But then, directionally unstable ships are easier to maneuver.

Striking a balance between these two extremes is a tough job reserved for the seasoned designer. If the area of the hull and its appendages in the underwater profile of the ship is concentrated towards the aft end, the ship is more likely to be directionally stable.

Bilge Keels decrease rolling motion of ships. Shaped like long and narrow fins, these keels project out from the intersection of hull sides with hull bottom. Rolling is the motion of the ship about the longitudinal axis. It is the most unwanted of all motions because it produces seasickness.

Antiroll Fins are most effective in cutting down roll. They measure about 10m in length and project transversely from the ship sides. These are rotated continuously to counter the rolling motion. Operating them is however somewhat expensive.

In rough weather, waves slam against the hull and create two very destructive phenomena:

  • Whipping is when the bow rises above water and reenters it to produce very high local stresses
  • Springing is when the frequency of a slamming wave matches the natural frequency of vibration of the hull

Springing is more difficult to control but there are no recorded incidents of failures on account of it. Whipping on the other hand is more lethal but can be controlled.

Involved Principles

For a boat to operate smoothly, the:

  • Buoyancy / Buoyant Force that acts vertically upward must be equal to or greater than the Weight of the vessel if the vessel is to float. For this to happen, the overall density of the boat has to be lower than the density of the water
Buoyancy / Hydrostatic Force Image Courtesy of Yupi666 Retrieved From https://en.wikipedia.org/wiki/File:Buoyancy.svg

Buoyancy / Hydrostatic Force
Image Courtesy of Yupi666
Retrieved From https://en.wikipedia.org/wiki/File:Buoyancy.svg

Also called Hydrostatic Lift, buoyant force acts vertically upward through the Center of Buoyancy

  • Thrust or force imparted by the propeller must exceed the Drag by a sizeable amount

Drag is the resistance to the motion of the boat exerted by the water

  • Pitch must be zero i.e. the diverse forces must produce a turning movement whose net value of zero. Such turning movement is generated about the center of gravity of the boat

Normally, the center of buoyancy is located (approximately) midway between the bow and the stern. Any sudden shift in its position can cause the vessel to capsize.

Center of Buoyancy (B) is the Center of Gravity (CG) of the liquid that is displaced by an immersed body. CG is of course the imaginary point inside any body through which all its weight acts vertically downwards.

Stable & Unstable Equilibrium of a Ship Image Courtesy of Cmglee Retrieved From https://en.wikipedia.org/wiki/File:Ship_stability.svg

Stable & Unstable Equilibrium of a Ship
Image Courtesy of Cmglee
Retrieved From https://en.wikipedia.org/wiki/File:Ship_stability.svg

Whenever a body is immersed in a fluid, it displaces the fluid. The volume of the displaced fluid equals the volume of the immersed part of the body. And the buoyant force will equal the weight of this displaced fluid. This precisely is the famed Archimedes Principle.

For a floating body (i.e. body not completely submerged in the fluid), the center of buoyancy will be usually located at the CG of the immersed part of the body.

If you provide some angular displacement to a floating body, it tilts. The CG maintains its position but the center of buoyancy changes location. A fictitious vertical through this new center of buoyancy (B’) meets the original imaginary vertical (through CG and B) at the Metacenter (M).

A floating body is stable if its metacenter is located above its CG. On the other hand, an immersed body is stable if its center of buoyancy is located above its CG.

Tankers are particularly prone to instability because they carry liquids. When the tanker tilts, so does the liquid it is carrying. This alters the position of the overall CG of the entire tanker in the direction of its new center of buoyancy.

In order to minimize this hazardous shift of the CG, designers break down the cargo space in tankers into a number of small compartments. This lowers the movement of the liquid inside the ship and therefore the CG shift.

Metallic Coin Floats in Mercury: Mercury is Denser than the Coin Metal Image Courtesy of Alby (talk) Retrieved From https://en.wikipedia.org/wiki/File:Pound-coin-floating-in-mercury.jpg

Metallic Coin Floats in Mercury: Mercury is Denser than the Coin Metal
Image Courtesy of Alby (talk)
Retrieved From https://en.wikipedia.org/wiki/File:Pound-coin-floating-in-mercury.jpg

Coming back to the magnitude of the buoyant force and Archimedes Principle, it all comes down to density. If the overall density of a body:

  • is lower than that of a fluid, the body will float in the fluid
  • equals the density of the fluid, the body will completely immerse in the fluid but not sink
  • is greater than that of the fluid, the body will sink in that fluid

Ships use hulls made from steel, aluminum, and other alloys. Their densities are greater than that of seawater and freshwater. Yet, they do not sink because of the large, vacant, air-filled onboard spaces that bring down the overall density of the structure to below that of water.

Finally

Hull design is a nuanced, complex, and iterative process that has evolved over centuries and has gathered pace over the past century and half. With the present compulsions for creating environment-friendly ships, hull design may soon witness groundbreaking developments.

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