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Worldwide Patrol Boats - Design

These craft have different shapes and are of different displacements, depending upon the use of the craft. The traditional hull shape is the displacement hull, which is supported by buoyancy. However, due to the large surface area in contact with the water, the speed of such craft is limited.

Speed boat development began in the early 1900's with the development of the first gasoline powered piston engines. These engines were large and heavy. Boat hulls were long narrow round bottomed displacement hulls. As engine design improved, the Vee bottomed, hard chined planing hull and the stepped hull were developed. Drive systems included the direct drive, Vee drive, stern drive, and surface drive. The performance of each combination of hull type and drive system can be illustrated by a graph of Performance Factors. These Performance Factors take into account the running weight, engine horsepower, and measured maximum speed.

In an effort to overcome the disadvantages of the displacement hull, the planing hull was developed which lifts most of the hull out of the water during travel. Ships with this sort of hull travel very rapidly in smooth water. But in waves, these ships are subject to pounding or slamming, so must be driven at lower speeds. One method for improving performance of the planing hull is the deep-V design, which cuts through the waves to reduce pounding.

To date, the intended use of the ship has determined both its weight and its hull shape. Thus, racing boats are generally as light weight as possible, in order to improve the ship's speed, and include a deep-V hull in an attempt to reduce pounding. On the other hand, patrol boats and other ships, which are subject to slamming on rough seas, are built with relatively thick protective walls and are, therefore, much heavier, and are traditionally made with a flatter semi-displacement hull which is very fast in quiet waters but which tends to slam in rough seas.

Furthermore, patrol boats and other relatively heavy boats are generally propeller driven. The conventional drive system includes high speed, fast engines which have a high power/weight ratio. There are known racing boats with water jet propulsion systems, but such systems are relatively new and more expensive than traditional propeller propulsion systems, as well as being less efficient in fuel consumption at certain speeds.

In operating a boat, it is desirable to have control of the bow of the boat so that the bow will not ride up too high to cause instability of the boat or to ride too low in the water to cause excessive drag forces on the boat. Attempts have been made to use tabs, flaps and the like to stabilize boats. At present, high speed on rough seas has been achieved by hydrofoils of certain design and some hovercraft. However, the price of manufacturing and maintaining these craft is very high.

Steps in hulls reduce the area of engagement between the hull of a boat and the water, thereby increasing the speed of the boat. Such hulls have been used on racing boats, on plane floats, and on amphibious plane hulls. However, these steps are not directly exposed to the atmosphere, and require a certain minimum speed, a very powerful engine and expenditure of substantial amounts of energy to get the hull up "on the step" sufficiently high to allow air under the hull to break the vacuum, or water-to-hull contact in the area immediately behind the vertical portion of the step.

This phenomenon is apparent from the observed fact that it takes a float plane a much greater distance to take off from the water when the water is calm than when it is choppy. If the water is choppy, the step is exposed to the atmosphere in the valley between the waves, thus breaking the vacuum or the contact between the float and the water much sooner, thereby increasing the speed as the wetted area of the float decreases, and enabling a much shorter takeoff distance. The friction of a hull in water is a function of the wetted area of the hull, and the function of the step is to reduce the wetted area. Unfortunately, in most of the hull designs which have been proposed up to the present time, the boat hull is not easily brought up onto the step and this limits the usefulness of the stepped hull concept.

Boats which are capable of operating in a planing mode, especially near the stern where the weight of the boat is supported in the planing mode, have a hull that usually "V" shaped in cross-section, each arm of the "V" being virtually flat and subtending an angle of less than 25 or 30 degrees to the horizontal, in order that planing can be sustained. For a planing boat, the volume of water displaced at rest is usually at least five or ten times the volume of water displaced when the boat is planing.

Such a planing boat suffers from several disadvantages. Firstly, it is uncomfortable to ride in, especially in high sea states. This is because when the bows hit a wave it has a tendency to "slam" into that wave, causing discomfort and possible injury to passengers. Secondly, this slamming behaviour can significantly reduce the life of the structure of the boat owing to the high structural stresses that it can cause. Thirdly, the boat may not behave uniformly well at all speeds. At very low speeds it may behave as a typical "displacement" boat. As such, it may ride fully in the water without displaying any planing behaviour. At high speeds, it may behave fully in a planing mode. However, at intermediate speeds, as it attempts to surmount its own bow wave in order to achieve the planing mode, its behaviour may be less predictable, since it may intermittently achieve and then fail to achieve the planing mode. Finally, for a given weight of boat, the power required to surmount the bow wave and achieve the planing mode is very significant. This power requirement can be a serious limiting factor on the amount of fuel carried and hence on the range of the boat.

An alternative to a planing boat is one which behaves in a displacement mode, in other words, a boat which behaves according to the displacement rule. Such boats do not display any significant planing behaviour, but rather have a maximum speed which is limited by the drag losses of the boat hull as it moves through the water. The displacement rule postulates that the maximum achievable speed (in knots) is proportional to the square root of the length of the boat (in feet). The constant of proportionality has been found empirically to lie usually between 1.3 and 1.6. Hence displacement boats have a speed which is limited by their length. The speed of such a boat is insufficient for the present requirement of a light manoeuvrable fast patrol boat or the like.

An alternative to a boat which behaves either in a displacement mode or a planing mode is one which behaves in a so-called "semi-displacement" mode. Such a boat is relatively slender (say, having a length to beam ratio (a "slenderness ratio") of greater than 5 to 1, 6 to 1 or even 7.5 to 1). On the one hand, it does not have a maximum speed limited by the displacement rule, whilst on the other hand it does not exhibit any significant planing behaviour since it does not generate a bow wave of any magnitude over which it can rise. Hence boats which behave in a semi-displacement mode do not in general suffer from the disadvantages of boats which behave either in a displacement or a planing mode of, on the one hand, lack of speed, or, on the other hand, lack of range.

The major disadvantage with a long slender semi-displacement boat is its tendency to be unstable, especially in roll. Such roll instability is commonly caused by two factors. Firstly, the very slenderness of the boat means that there is little resistance to roll. Secondly, such a boat tends to have a hull surface which is deliberately designed to avoid planing, and hence has few sharp edges such as might increase roll damping due to vortex shedding off those sharp edges.

One solution to such a problem has been adopted in catamaran or trimaran configurations. Here stability in roll is achieved by connecting two or more slender semi-displacement hulls so as to be spaced apart from each other. Catamarans or trimarans suffer from two main disadvantages of relevance. Firstly, they can be awkward to maneuver and handle. Secondly, they can have no passive self-righting capability since they are equally stable inverted as they are the correct way up.

Marine vehicles can operate in a surface effect condition by entrapping a cushion(s) of artifically pressurized gas between the vehicle and a water surface and/or a ram effect of ambient air that is sandwiched between the vehicle and a water or other surface at higher vehicle speeds. The first are most commonly called hovercraft or Surface Effect Ships (SES's) and the latter Wing in Ground Effect (WIG), Wing in Surface Effect Craft, or simply wingships. The common thread of all of these is that the pressurized gas disposed between the vehicle and the supporting medium carries most of vehicle weight. In any case, overall efficiencies of the SES are much greater than conventional marine vehicles and overall efficiencies of the WIG are much greater than commercial aircraft.

The Displacement-Length Ratio (D/L) is a nondimensional expression of how heavy a boat is relative to its waterline length. A D/L ratio is calculated by dividing a boats displacement in long tons (2,240 pounds) by one one-hundredth of the waterline length (in feet) cubed. Because the displacement in long tons can be expressed as a volume (cubic feet of seawater), and since length-cubed is also volumetric, the result of the D/L formula is nondimensional, i.e., it is no longer tied to a particular physical size. The significance of the displacement-length ratio is that the lighter a boat is relative to its waterline length, the higher its speed potential, especially when in displacement mode.





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