Ship Semi Planing Hull and Monohull Theory

Ship Semi Planing Hull and Monohull Theory
Ship Semi Planing Hull and Monohull Theory

Ship Semi Planing Hull Theory

Hulls for ships are, viewed from the shape of the hull under the water line, generally classified into three categories: a displacement type hull for a low speed range, a semi-planing type hull for a medium speed range and a planing type hull for a high speed range.

The displacement type hull is suitable for a low speed travelling. The ship of this type is travelled primarily under a hydrostatic pressure with a hull bottom submerged relatively deep under a water line. In an attempt to reduce frictional resistance and wave making resistance the hull bottom often is given a streamline shape, and a lateral crosssectional area under the water line as measured on a still water is maximal at the middle of the hull and is gradually decreased from midship toward the end of the stern. The hull bottom is curved upward from the middle of the hull toward the bow and stern, respectively. When the displacement type hull is travelled at high speeds, it is subjected to a dynamic water pressure tending to lift the front half portion of the hull upwardly, while at the same time it is subjected to a dynamic water pressure tending to pull the rear half portion of the hull downwardly. The hull, therefore, has a greater attack angle with the bow lifted upwardly and the stern pulled downwardly, increasing water resistance to the hull and making it very difficult to effect high speed travelling. To avoid such a situation attempt is made to, for example, move crew-members toward the bow of the ship, thereby shifting the center of gravity of the ship. This, however, provides no essential settlement to this problem.

The planing type hull is suitable for high speed travelling. The hull of this type has, in an attempt to provide a suitable attack angle during the high speed travelling period to the hull and effectively support a hull bottom by a dynamic water pressure, a substantially planar, wide planing surface at the bottom and a substantially vertical planar transom at the rear end of the stern. The lateral cross-sectional area under the water line of the hull is maintained substantially constant at the rear half portion of the hull. When the ship of this type is travelled at low speeds, water flowing along each side wall of the hull is turned inward behind the transom, creating an eddy current. The eddy current imparts a resistance tending pull back the hull rearwardly. For this reason, the planing type hull is subjected to a relatively large travelling resistance during the low speed travelling period, as compared with the displacement type hull, resulting in a prominently greater propulsion loss.

The semi-planing type hull is suitable for a medium speed range, i.e., a speed range intermediate between the displacement type hull and the planing type hull. The lateral cross-sectional area under the water line of the hull is slightly decreased from midship to the rear end of a stern where it shows a value intermediate between the displacement type hull and the planing type hull. In other words, the semi-planing type hull shows a poor performance in the low speed range as compared with the displacement type hull and in the high speed range as compared with the planing type hull.

As will be evident, an optimum designing speed range is determined dependent upon the type of hulls. In the case of the displacement type hull, a speed/length ratio (speed/.sqroot.water line length) is restricted to a range of below 1.5 kn/ft1/2; in the case of the semi-planing type hull, a range of 1.5 to 3.5 kn/ft1/2; and in the case of the planing type full, a range of above 2.5 km/ft1/2. If, therefore, the speed-length ratio is off the optimal range, a poor performance results.

Ship semi planing hull theory
Ship semi planing hull theory

The planing hull does not yield the solution to designing large fast ships. However, if the speed categories in relation to waterline length are examined, the semi-planing hull appears to offer attractive opportunities for fast sealift ships over a continuum of sizes of semi-planing hulls, small to very large. The monohull fast sealift (MFS) hull or semi-planing monohull (SPMH) design is the hull form which is widely used today in smaller semi-planing ships because it offers the possibility of using waterline lengths approaching that of displacement hulls and maximum speeds approaching that of planing hulls.

An advantage of a waterjet propulsion system in the semi-planing hull is its ability to deliver large amounts of power at high propulsive efficiency at speeds of over 30 knots and yet decelerate the ship to a stop very quickly. The system also largely eliminates the major problems of propeller vibration, noise and cavitation. A principal advantage of the integrated MFS hull or SPMH and waterjet system is that the shape and lift characteristics of the hull are ideal for the intakes and propulsive efficiency of the waterjet system, while the accelerated flow at the intakes also produces higher pressure and greater lift to reduce drag on the hull even further.

Ship Monohull

The hull of virtually all conventional ships and boats has a so-called monohull-configuration. The term “monohull” denotes a hull which is constructed of a single water displacing body. A monohull typically narrows in cross-section toward the front thereof to define a pointed bow which facilitates the ship’s ability to cut efficiently through the water. But a monohull is otherwise relatively wide and a good portion thereof remains submerged at all times below the water surface. This enables a monohull to withstand and to remain more stable in rough seas. However, because a large portion of the monohull is submerged at all times, a monohull produces greater drag at high speeds, i.e. resistance to motion, which results in a ship whose top speed is limited and/or which requires more powerful engines.

Traditional surface ship monohull designs have usually been developed from established design principles and assumptions which concern the interrelationships of speed, stability and seakeeping. Such sacrifices have to be made to achieve significantly higher performance than hitherto that current practical displacement monohull surface ship speed improvements are essentially stalled. For example, a major limitation of present day displacement hulls is that, for a given size (in terms of displacement or volume), their seaworthiness and stability are reduced as they are “stretched” to a greater length in order to increase maximum practical speed.

Traditional hull designs inherently limit the speed with which large cargo ships can traverse the ocean because of the drag rise which occurs at the “threshold speed” which occurs at a speed of about 1.2 times the square root of the ship’s length (in feet). For example, a mid-size cargo ship at about 600 feet length has an economical operating speed of about 20 knots or some 4 knots below its design threshold speed. In order to achieve higher operating speeds with commercial loads, it is necessary to increase ship length and size (or volume) in proportion, or to increase length while reducing width or beam, to maintain the same size and volume, but at the expense of stability. Naval architects have long considered the problem of achieving significantly higher ship speeds, without increasing length or decreasing beam, as the equivalent of “breaking the sound barrier” in aeronautical technology.

Over thousands of years, boat building has made considerable progress in ship propulsion for increasing speed and endurance: starting from muscle power in the time of the ancient Egyptians to wind power, and then to steam and oil and recently atomic energy. However during this revolution in terms of drive, hulls have hardly changed at all: monohulls have always been favored because of simplicity, light weight, and buoyancy, both static and dynamic, even though stability, at least in terms of comfort, is not perfect.

However, over the last few years, numerous novel designs have been studied and developed to take the place of the monohull with the purpose of achieving greater speed and better stability, with this being at the request both of navies and of shipping companies; shipping companies would like to become much more competitive than airlines by seeking to reduce the time required to cross an ocean to half or less while being capable of carrying many more passengers than can be carried by a large airliner. Nevertheless, it is clear that many very large vessels are probably going to remain monohulls for a long time yet, particularly in circumstances where neither stability nor high speed are considered as being major advantages, such as in transporting oil, for example.

Wave Resistance vs Froude Number
Wave Resistance vs Froude Number

In the nineteenth century, Froude first accurately measured and defined the phenomenon by which increased length is required for higher ship speeds because of the prohibitive drag rise which occurs at a threshold speed corresponding to a length Froude Number of 0.3. The length Froude Number is defined by the relationship 0.298 times the speed length ratio .sqroot..sub.L.sup.V, where V is the speed of the ship in knots and L is the waterline length of the ship in feet. Thus a Froude number of 0.298 equates to a speed length ratio of 1.0.

To go faster the ship must be made longer, thus pushing the onset of this drag rise up to a higher speed. As length is increased for the same volume, however, the ship becomes narrower, stability is sacrificed, and it is subject to greater stress, resulting in a structure which must be proportionately lighter and stronger (and more costly) if structural weight is not to become excessive. In addition, while for a given displacement the longer ship will be able to achieve higher speeds, the natural longitudinal vibration frequency is lowered and seakeeping degraded in high or adverse sea states as compared to a shorter, more compact ship.

Today, the maximum practical speed of displacement ships is about 32 to 35 knots. This can be achieved in a relatively small ship by making it long, narrow and light but also costly. To some extent it has been possible to avoid increased length above Froude numbers of 0.4, but this has been achieved in small craft design using semi-planing hulls for ships up to 120 feet long and 200 tons and improved propulsion units. In a larger ship, such as a fast ocean liner, the greater length allows a greater size and volume to be carried at the same speed which is, however, lower relative to its Froude number (i.e., 38 knots for an aircraft carrier of 1,100 feet waterline length is only a Froude number of 0.34). On the negative side, the larger size of these ships requires significantly larger quantities of propulsion power. There are major problems in delivering this power efficiently through conventional propellers due to cavitation problems and using conventional diesel or steam machinery which provide a very poor power/weight ratio.

Vessel hulls have traditionally been designed for specific uses, such as for use in shallow waters or in deep waters. Different hull designs provide for optimal operating characteristics for different uses. Shallow-draft vessels, for example, often have hulls that are relatively “flat” to maximize displacement and minimize draft, whereas deep-draft vessels often have v-shaped hulls that provide deep draft for desired seakeeping (e.g., good seakeeping providing low undesired motion, such as vertical motion or rocking).

More specifically, shallow-draft vessels are often designed with flat bottom hulls to provide the ability to navigate in relatively shallow waters, such as in shallow-water harbors, along rivers, along shorelines and in other bodies of shallow water. Shallow-draft vessels are also designed to maximize payload carrying capacity and to provide for simplified on-loading and off-loading of cargo. Examples of shallow-draft vessels include landing craft mechanized (LCM) and landing craft utility (LCU) that are often used by amphibious military forces to transport equipment and troops from sea to beachheads and/or to piers.

Shallow-draft vessels typically have relatively high water resistance due in part to large beam to length ratios, large wetted surfaces, and blunt water contact. Such characteristics provide for the generation of large amounts of resistance, such as turbulence and/or Kelvin wake, and high power requirements. Accordingly, shallow-draft vessels typically have poor seakeeping, poor ride, and poor handling characteristics. Due to these and other operational characteristics, shallow-draft vessels typically are not suited for use in deep water.

Alternatively, deep-draft vessels are often designed with v-hulls having relatively low beam to length ratios to provide the ability to navigate the vessels in deep waters, such as in the oceans and seas. Deep-draft vessels are often designed to provide desired seakeeping (e.g., good seakeeping providing low undesired motion, such as vertical motion or rocking) in high sea states. Deep-draft vessels, however, are typically not available for shallow-water use, such as docking in shallow harbors, river use, and navigation adjacent to shorelines, as the vessels may run-a-ground in these waterways.

A variety of operations require the use of vessels in both shallow and deep waters. As traditionally designed vessels typically have features that provide for optimized use in either shallow water or deep water, but not both, traditionally designed vessels do not provide optimal operating characteristics for both shallow and deep-water use.

The seakeeping characteristics of displacement hulls are well known. They are most evident in the classic lines of traditional sailing vessels with graceful curvature fore and aft to move easily under sail through the water and follow the waves. Those lines are little changed in the ocean-going ships of today, with their pointed bows, round bilges, rounded sterns, and amidships balance to ride as level as possible in meeting seas under all weather conditions.

Ships which carry a substantial quantity of cargo are typically constructed with a monohull having large displacement characteristics. This type of hull however is susceptible to the forces of waves encountered at sea, as well as those encountered while docked. These wave forces often cause the vessels to pitch, roll and heave to a large degree. The motion of the vessel often causes passenger and crew discomfort and increases the risk of dislodging and shifting of cargo. Such motion can damage the vessels as they move relative to off-shore rigs, docks and other ships to which they are moored. This lack of stability also hinders the loading and off-loading of cargo.

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