1. Aerostatic Support
Aerostatic support is achieved when the vessel rides on a cushion of air generated by lift fans. These vessels tend to be lighter weight and higher speed vessels. The two basic types of vessels supported aerostatically are air cushion vehicle (ACV) and surface effect ships (SES). See Figure 1.
1.1 Air Cushion Vehicles (ACVs)
Air Cushion Vehicles (ACVs) or hovercraft continuously force air under the vessel allowing some of the air to escape around the perimeter as new air is forced downwards. They are usually propelled forward by airplane propeller type devices above the surface of the water with rudders behind the air flow to control the vessel.
Hovercrafts are very expensive for their size, but have the unique property of being amphibious. The Navy utilizes some hovercraft as LCACs (Landing Craft Air Cushion vehicles) because of this ability. Their use has opened over 75% of the world's coastline to amphibious assault compared with 5% with conventional landing craft.
1.2 Surface Effect Ship (SES)
The Surface Effect Ship (SES) or Captured Air Bubble (CAB) craft, are similar to ACV’s in that they use a cushion of air to lift the vessel. However, the SES has rigid side walls that extend into the water. This prevents the SES from being amphibious but reduces the air pumping requirements and makes them more directionally stable. The side walls also contribute to the hydrostatic or hydrodynamic support of the craft allowing the SES to carry more payload. They are usually propelled by water jets or super cavitating propellers.
2. Hydrodynamic Support
Hydro is the prefix for water and dynamic indicates movement. The two basic types of vessels supported hydrodynamically are planing vessels and hydrofoils.
2.1 Planing Vessels
Planing vessels use the hydrodynamic pressures developed on the hull at high speeds to support the ship. They are very fast, some capable over 50 knots. In smooth water they ride very comfortably. When moving through waves, planing vessels ride very roughly, heavily stressing both the vessel structure and passengers. This was particularly true of older types which used relatively flat bottom hulls. Modifications to the basic hull form, such as deep V-shaped sections, have helped to alleviate this problem somewhat. Planing hulls require much larger engines for their size than displacement hulls.
These factors above serve to limit the size of planing vessels. However, these ships are used in a variety of roles such as pleasure boats, patrol boats, missile boats, and racing boats.
At slow speeds the planing craft acts like a displacement ship and is supported hydrostatically.
2.2 Hydrofoils
Hydrofoil craft are supported by underwater foils, not unlike the wings of an aircraft. At high speeds these underwater surfaces develop lift and raise the hull out of the water. Bernoulli’s Principle is often used to explain how a wing develops lift. These vessels are very fast, reaching speeds of 40 - 60 knots and compared to planing boats, hydrofoils experience much lower vertical accelerations in moderate sea states making them more comfortable to ride.
The hydrofoil can become uncomfortable or even dangerous in heavy sea states due to the foils breaking clear of the water and the hull impacting the waves. If the seaway becomes too rough the dynamic support is not used, and the ship becomes a displacement vessel.
The need for the hydrofoils to produce enough upward force to lift the ship out of the water places practical constraints on the vessel's size. Therefore, the potential crew and cargo carrying capacity of these boats is limited. Hydrofoils are also very expensive for their size in comparison to conventional displacement vessels.
3. Hydrostatic Support
Hydrostatically supported vessels are by far the most common type of water borne craft. They describe any vessel that is supported by “Archimedes Principle”.
Word definition of Archimedes Principle
“An object partially or fully submerged in a fluid will experience a resultant vertical force equal in magnitude to the weight of the volume of fluid displaced by the object.”
In EN200, this force is called the “buoyant force” or the “force of buoyancy”.
Archimedes Principle can be written in mathematical format as follows.
3.1 Displacement Ships
Hydrostatically supported ships are referred to as “displacement ships”, since they float by displacing their own weight in water, according to Archimedes Principle. These are the oldest form of ships coming in all sizes and being used for such varied purposes as hauling cargo, bulk oil carrying, launching and recovering aircraft, transporting people, fishing, and war fighting.
Hydrofoil craft are supported by underwater foils, not unlike the wings of an aircraft. At high speeds these underwater surfaces develop lift and raise the hull out of the water. Bernoulli’s Principle is often used to explain how a wing develops lift. These vessels are very fast, reaching speeds of 40 - 60 knots and compared to planing boats, hydrofoils experience much lower vertical accelerations in moderate sea states making them more comfortable to ride.
The hydrofoil can become uncomfortable or even dangerous in heavy sea states due to the foils breaking clear of the water and the hull impacting the waves. If the seaway becomes too rough the dynamic support is not used, and the ship becomes a displacement vessel.
The need for the hydrofoils to produce enough upward force to lift the ship out of the water places practical constraints on the vessel's size. Therefore, the potential crew and cargo carrying capacity of these boats is limited. Hydrofoils are also very expensive for their size in comparison to conventional displacement vessels.
3. Hydrostatic Support
Hydrostatically supported vessels are by far the most common type of water borne craft. They describe any vessel that is supported by “Archimedes Principle”.
Word definition of Archimedes Principle
“An object partially or fully submerged in a fluid will experience a resultant vertical force equal in magnitude to the weight of the volume of fluid displaced by the object.”
In EN200, this force is called the “buoyant force” or the “force of buoyancy”.
Archimedes Principle can be written in mathematical format as follows.
3.1 Displacement Ships
Hydrostatically supported ships are referred to as “displacement ships”, since they float by displacing their own weight in water, according to Archimedes Principle. These are the oldest form of ships coming in all sizes and being used for such varied purposes as hauling cargo, bulk oil carrying, launching and recovering aircraft, transporting people, fishing, and war fighting.
Displacement hulls have the advantage of being a very old and common type of ship. Therefore, many aspects of their performance and cost have been well studied. In comparison to other types of vessels the cost of displacement ships is fairly low with respect to the amount of payload they can carry.
Disadvantages of displacement vessels include their limited speed and at times, their seakeeping ability (how they respond to ocean waves).
3.2 SWATH
A special displacement ship is the Small Waterplane Area Twin Hull (SWATH). Most of the underwater volume in the SWATH ship is concentrated well below the water's surface as shown in Figure 2-1. This gives them very good seakeeping characteristics. They also have a large open deck and are therefore useful in a variety of applications requiring stable platforms and a large expanse of deck space. SWATH vessels are currently utilized as cruise ships, ferries, research vessels, and towed array platforms.
Two major disadvantages of SWATH ships are deep draft and cost. Additionally, these vessels present the designer with structural problems differing from other ships, particularly with respect to transverse bending moments.
3.3 Submarines
Submarines are hydrostatically supported but above 3 to 5 knots depth control can be achieved hydrodynamically due to the lift created by the submarines planes and body of the hull.
Submarines have typically been used as weapons of war, but lately have also seen some non-military application. Some submarines are being designed for the purpose of viewing underwater life and reefs, for example. Unmanned submersibles have been used for scientific purposes, such as finding the Titanic, as well as a wide variety of oceanographic research.
There are many differences between the engineering problems faced by the surface ship designer and those faced by the submarine designer. Many of these differences will be covered in the last chapter of this course.
Disadvantages of displacement vessels include their limited speed and at times, their seakeeping ability (how they respond to ocean waves).
3.2 SWATH
A special displacement ship is the Small Waterplane Area Twin Hull (SWATH). Most of the underwater volume in the SWATH ship is concentrated well below the water's surface as shown in Figure 2-1. This gives them very good seakeeping characteristics. They also have a large open deck and are therefore useful in a variety of applications requiring stable platforms and a large expanse of deck space. SWATH vessels are currently utilized as cruise ships, ferries, research vessels, and towed array platforms.
Two major disadvantages of SWATH ships are deep draft and cost. Additionally, these vessels present the designer with structural problems differing from other ships, particularly with respect to transverse bending moments.
3.3 Submarines
Submarines are hydrostatically supported but above 3 to 5 knots depth control can be achieved hydrodynamically due to the lift created by the submarines planes and body of the hull.
Submarines have typically been used as weapons of war, but lately have also seen some non-military application. Some submarines are being designed for the purpose of viewing underwater life and reefs, for example. Unmanned submersibles have been used for scientific purposes, such as finding the Titanic, as well as a wide variety of oceanographic research.
There are many differences between the engineering problems faced by the surface ship designer and those faced by the submarine designer. Many of these differences will be covered in the last chapter of this course.
No comments:
Post a Comment