Military


Deep-V Hull

Planing boats having a so-called deep-V hull configuration have been exceptionally popular due to their desirable riding and handling characteristics, particularly in rough water. In the "deep-V" design, the hull has a continuous surface from bow to stern with a ridge down the central portion thereof, forming a "V" shape when viewed from the stern. The deadrise of such a boat, that is, the angle between the hull surface and a horizontal plane, is generally twenty degrees or more.

Water craft and motorized water craft for a variety of purposes have long been known. These craft have different shapes and are of different weights, 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.

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.

The typical boat incorporates a V-shape hull extending from a forward point and widening to the rear of the boat allowing the hull to "cut" through the water. The depth of the V-shape dictates the performance characteristics of the boat. A flatter hull provides greater stability and is used on barges and fishing boats. A deeper V improves the ability of the boat to cut across the water with less drag and greater efficiency.

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.

It has been known for some time that boats having the deep-V hull type of hull configuration suffer from significant disadvantages. They require powerful, heavy, and uneconomic engines to achieve planing and to overcome the friction between the hull and the water. Deep-V hull configurations are not particularly amenable to the utilization of inboard or outboard power drives. The hull configuration makes it difficult to properly mount an inboard engine in such a fashion that the drive shaft angle to a propeller positioned below the keel of the hull is sufficiently small for efficient operation. On the other hand, conventional outboard power drives cannot normally be mounted on the transom of a deep-V hull so that the propeller extends below the keel line, unless a substantial portion of the upper part of the transom is cut-away. Yet, a high transom is a very desirable safety feature for sea-going, rough water boats.

A further disadvantage of known deep-V hull configurations is the substantial power input which is required in order to achieve planing speeds, due to the frictional resistance offered by the rather large wetted surface of the hull bottom. Attempts have been made to overcome this problem by the incorporation of one or more transverse steps in the hull bottom. These step configurations have a number of off-setting disadvantages and, accordingly, have not been widely used.

"Deep-V" boats have an undesirable tendency to pitch severely in rough seas in resonance with the frequency of the wave action. When moving with the waves, these hulls fall off one wave and plow into the next where their sharply inclined bow surfaces act like a rudder around which they rotate or broach. Yet their wide amidship bottom surfaces still pound against oncoming waves. Further, at high planing speeds, these hulls need to have their center of gravity substantially rearward of amidship in order to keep the bow up and reduce wetted area which reduces frictional drag, but such a rearward center of gravity causes excess bow rise at low speed.

The lifting characteristics of the continuous hull result in a non-level ride, and the boat exhibits lateral instability at rest. In high speed turns such a boat banks severely, and a large turning radius is required for low speed turns because the boat pivots on its bow. Trim tabs or similar devices are often necessary to provide the necessary lift at the stern area, depending on the orientation of the power unit. Further, if the angle of the V is not deep, these hulls tend to skid excessively in a turn.

Boats with deep-V hulls produce a large wake with a heavy spray, displace a great deal of water at all speeds, have a relatively high aerodynamic and hydrodynamic resistance, and generally have poor fuel economy.

One reason for the poor efficiency of deep-V hull boats is their tendency to ride at an angle to the water with the bow up high and the stern low. Thus, the hull presents a large frontal surface area to encounter wind and water resistance. In addition, visibility is reduced as a result of the high bow. Some boat designers have attempted to overcome this characteristic by adding trim tabs and/or lifting strakes to the hull; however, these additions cause an increase in drag and add to the cost and maintenance of the boat while reducing fuel economy.

Unlike displacement hulls which have upwardly curved sterns and curvatures at the bow, causing suction which sinks their center of gravity with forward speed (increasing their apparent weight), and unlike planing hulls having mostly flat undersurfaces and a CG which tends to rise with forward speed, the semi-planing hull usually has a Vee bottom and, for practical reasons, is heavier than a pure planing hull. Although the semi-planing hulls can generate the appearance of a "flat" wake at high speeds, their lift is generated by a combination of buoyancy and dynamic forces, which is inherently inefficient. These hybrids are longer and have lower volumetric coefficient compared to those of planing hulls, but are nevertheless much higher than for displacement hulls.

The borders of the wakes of semi-planing hulls, as seen from an aerial view, appear flat and join together at some distance behind the stern, generating a trailing "hollow" on the water's surface, which can be interpreted, from the viewpoint of a fish trained in hydrodynamics, as an virtual displacement hull of larger length than that of the dynamic waterplane of the operational semi-planing. hull. The conventional semi-planing hull is an inefficient hybrid: at slow speeds, it has excessive drag compared to a good displacement hull. It requires very large power to reach semi-planing speed, at which regime it is not as fast and is less efficient than a pure planing hull. On the other hand, a deep-vee semi-planing hull provides smoother ride for a greater payload in a rough sea, and is more seaworthy than a planing hull. However, it has a rougher ride than a displacement hull, with less favorable sea keeping characteristics, and is commercially not viable for most large maritime applications.

Planing hulls may be classed into two categories, namely those having (a) little or no deadrise for high efficiency on calm water, or (b) those having substantial deadrise, deep V's, to yield a smoother ride in rough water. It is well known that the high efficiency of flat bottom craft cannot be advantageously used at high speed in anything but a flat calm; thus, high speed hydroplanes are limited to running in calm water. The less efficient craft having deep V in vertical section offer a greater tolerance for rough seas but the high prismatic coefficient of lift forward in the bow curves tend to drive the hull upward as the hull enters a wave; moreover a high degree of shock experienced in reentering the sea often results in structural damage and personal injury. Increasing the angle of deadrise in these craft slightly decreases pounding, but greatly increases the surface area exposed to water friction and will dictate an increase in the exposed surface area needed to achieve lift, thereby reducing the overall efficiency of the hull. The trade-off of efficiency for a slightly improved ride would not be necessary if the hull were designed, as is the invention, to preclude the hull being launched into the air.

The bottoms of planing hulls are usually not stepped transversely because a step well forward of the center of mass will tend to accentuate lifting motion induced by passing through a wave. It is accepted that every unstepped hull will, in operation, induce longitudinal dynamic instabilities at an efficient angle of attack. On the other hand, longitudinal steps or strakes are often incorporated to release the hull from the drag of spray, but these cannot affect the angle of attack of the hull relative to the water. In general, planing surfaces of hydroplanes lack the proper angle of attack to the water when the hull has a zero angle of trim; therefore, the hull must trim up to get lift, unfortunately thus lifting the bow excessively well into the air. Poor visibility forward results when the hull is given a large angle of trim and the helmsman must either be positioned too far forward or inordinately high on the deck for the sake of visibility. To overcome this deficiency, bow sections of some hulls are removed, presenting negative shear of the decks, greatly increasing the danger of burying the bow into a steep wave. Such compromises made to seaworthiness, for the sake of visibility, will not be required if the relative hydrodynamic lifting surfaces are designed as the present invention with a built-in angle of attack to the water, the hull being at a zero angle of trim.

Planing hulls tend toward longitudinal hydrodynamic operational instabilities, such as porpoising, in calm water. This phenomenon is caused by a large proportion of the hull lifting into the air with only the after portion of the hull bottom in contact with the water. When the center of gravity of such craft is located well forward of the hydrodynamic lift area, the stagnation pressure line thus becomes the pivot point. When the thrust moment of such craft is less than the weight moment, the balance is lost and the hull falls into the water. At this critical instant the angle of attack is lost, the hull becomes wet, with consequent increased drag. All of this forces the bow deeper into the water until the lift on the forebody raises the bow again. Attempting to control and maintain this balance is much like attempting to ride a unicycle. Thus, operators of such craft must coordinate propulsive thrust and lift with screw trim and trim tabs to apply a moment to oppose falling or excessive lifting of the bow, again resulting in obvious loss in efficiency. The hull balance should be built in between two discrete areas of hydrodynamic lift wherein the forward planing surface is slightly forward of the center of gravity. Thus dynamic stability of a two planing surface hull is analogous to that of the stability of a bicycle having two points of support forward and aft of the center of gravity, thereby preventing longitudinal pivoting such as experienced in unicycle travel. Such is the objective of this invention.

In planing hulls the bow sections thereof are usually configured too blunt to soften the impact of launching upward over an oncoming wave face. These blunt bow sections thus tend to displace excessive volumes of water too rapidly and too far forward of the center of gravity, creating on the hull a large moment of pitch which again results in rapid and excessive pitch excursions. In an attempt to reduce such pitching, marine engineers have sloped the stem aft, in an effort to reduce the forward lifting moment of pitch by shortening the lifting arm, but unfortunately such a sloping stem will actually increase the ramping effect. A sloping stem is not required if the hull is designed, as the invention, to reduce the area of the bow sections, to displace less sea water less rapidly, so that by the time a significant volume has been displaced, the center of lift will be much closer to the center of gravity. Thus the moment of pitch will be diminished. The stem should preferably be vertical near the keel, to slice and push the water aside, rather than push the water downward. Thus the angle between the centerline and the chine of the forebody, in the plan view, is to be no less than 5 degrees and no greater than 15 degrees. Thus the full included angle between the chines of the forebody, which are symmetrical about the centerline, will range between 10 and 30 degrees in the plan view, as the sides initially rise from the bottom.

Shapes of planing hulls of marine craft are generally dictated by the demands of laminar flow and gentle changes in water particle velocity, associated with displacement efficiency. Graceful curves in the bow and stern sections are required for low speed displacement efficiency, but serve no purpose at planing speed. In fact, as the speed increases, the changes in curvature of the planing surface will become more detrimental. Such curved bow sections accelerate the water non-linearly, greatly increasing drag which requires an increase in propulsive power.

In many planing hulls, the afterbody comprises reduced vertical cross sections aft to reduce drag at displacement speed. Again, such construction is detrimental for high speed craft because the reduced sections aft create a suction effect. To avoid these deficiencies, some planing craft combines an unique, reinforced parallel afterbody with stepped, fluted planes on the bottom.



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