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Hydroplane Planing Hull

It has long been the goal of naval architects to design and construct vessels with adequate internal capacities and accommodations, structural strength, stability and seaworthiness when the vessel is afloat and sufficiently small resistance to economize propelling power at high speeds. Ever since man first ventured onto the water in boats, he has tried to design hulls that increase speed without unduly sacrificing stability. This is of course, by definition, the main goal of designers of motor-powered speed boats, for which an increase in knots per horsepower is usually more important than an increase in load-carrying capacity.

One obvious way to increase speed is simply to increase engine power. There are several disadvantages to this solution, however. First, more powerful engines are generally heavier, which means that even more mass must be moved. Second, increased engine mass usually requires a redesign of the hull in order to provide for optimum balance under way, especially in the case of outboard engines. Third, more powerful engines also usually cost more and have greater fuel consumption than smaller powerplants.

In addition to inertia, two primary forces work against increased speed for boats, namely, the resistance of the air (aerodynamic drag) against the above-water structure of the boat and the resistance of the water (hydrodynamic drag) on the wetted surface of the hull. Many solutions have been proposed for reducing both forms of drag.

Hydrodynamic drag can be reduced not only by better streamlining the hull to minimize areas of turbulence and thus wasted energy, but also by reducing the wetted surface area of the hull. As is well known, one way to reduce the wetted area is for the hull to act as a hydroplane, such that the hull rises out of the water when at cruising speed. One way to enable hydroplaning is to mount hydrofoils on the hull, either near the bow, or both bow and aft. This of course adds structural complexity and greatly reduces the efficiency of the boat at non-hydroplaning speed.

One means to achieve high speed ships is the planing hull. The planing hull incorporates, typically, a combination of very high power, flat or concave "vee'd" bottom sections, often incorporating warped surfaces, with an angular section or "chine" at the conjunction of the sides and bottom portion, necessary for clean flow separation giving enhanced aquaplaning capabilities and imparting higher stability at very high speeds. It also characteristically features an extremely lightweight structure of wood, aluminum or fiberglass.

To date, this popular concept has been limited to a very short hull form, i.e. typically no more than 100 feet and under 100 tons. Boats of only 50 foot length are able to achieve speeds of over 60 knots (a Froude Number of 2.53 or a speed length ratio of 8.5). This is possible because the power available simply pushes the boat up onto the surface of the water where it aquaplanes across the waves, thus eliminating the huge drag rise which prohibits a pure displacement boat of normal proportions from going more than about 9 knots on the same length of hull.

However, at intermediate speeds of say 5 to 25 knots, before this 50 foot boat "gets onto the plane", a disproportionately large amount of power is required. If the 50 foot planing boat is scaled to the length of a frigate of 300 feet, these speeds scale to the precise range of 12 to 60 knots. Thus scaled, the power required for a 300 foot planing frigate to achieve its minimum practical speed (60 knots) would be about half a million horsepower; but currently such horsepower cannot be installed, let alone delivered in a ship of such small size and low displacement. Furthermore, the ensuing ride on this 300 foot ship would cause material fatigue as its large flat hull surfaces would be slammed at continuously high speed into the ocean waves inasmuch as it would be too slow to plane or "fly" across the waves as a much smaller planing craft would do.

Craft utilizing planing hulls have also been produced with waterjet propulsion. Due to limitations of size, tonnage and required horsepower, however, the use of a waterjet propelled planing hull vessel for craft over 100 feet waterline length or 100 tons displacement has not been seriously considered.

Hull designs using the concept of hydrodynamic lift are known with regard to smaller ships, e.g. below 200 feet or 600 tons powered by conventional propeller drives. The shape of this hull is such that high pressure is induced under the hull in an area having a specific shape to provide hydrodynamic lift.

The higher speed in a planing mode is possible as the resistance is significantly lower than it would be if the hull moved through water at constant draft. Still, a typical planing hull generates a great amount of waves and spray around and behind it at speed. Generation of waves, which represent a deflection, or displacement, of a certain mass of water, requires and absorbs a percentage of energy provided by the propulsive device in form of a single or multiple engines or motors utilizing a stored energy medium (fuel of any kind, accumulated electric power, or other) and driving a reactive device (water or air propeller, water jet, or other). The energy spent on generating waves is wasted, so far as the vessel designer and the operator are concerned, in addition to creating a disturbance for other boat operators and causing shore erosion. The first phenomenon causes an increased fuel consumption and cost of running a boat as well as a reduced potential speed. The latter two are subject to numerous regulations in force, which further limit the boat speed in certain areas.

It is a major disadvantage of conventional planing waterjet propelled boats that they readily spin-out when encountering natural or man made waves, particularly during a turn. This tendancy to spin-out is enhanced by the fact that the more power applied by the jet propulsion unit, the farther out of the water the boat transom rides. As the transom rises, the nozzle of the jet likewise rises and therefore loses its water expelling capability. This in turn results in a loss of the control normally afforded by the jet propulsion unit. Thus, when a conventional planing hull boat engages a wave, the waterjet will often be momentarily robbed of water thereby resulting in consequent loss of power and control, giving rise to a spin-out. Certain planing hull boats also have a tendency to dig in their bows at high speeds.

The introduction of fiberglass boatbuilding in the 1950s boosted mass production of planing hulls by the use of molds to provide hulls in almost any shape or form. Since then, extruded panels, steps, and chine lips have become common in multiple horizontal surfaces added to the basic vee-bottom. In what has been popularly accepted as modern styling, the extra angles and curves seem to create an illusion of speed, but speed is actually reduced by the increase in wetted surface. Improvement in trim may be claimed or implied, but there has actually been little change in planing performance.

A significant improvement in planing hull design occurred in 1959 when the so-called "deep vee" for ocean racing put a definite dihedral or deadrise angle in a planing hull bottom. The change became popular and was widely copied as it greatly improved directional stability for open ocean operation. However, the improvement had little efect on planing aspect or trim and thus did not minimize slamming or pounding by the forward portion of the hull. The latest models are still characterized by hard chines, vee-bottoms, and broad transoms to carry maximum planing surfaces farthest aft; and planing hulls still ride on their afterbodies, being notoriously rough in any waves.

While there has been no fundamental change in planing hull lines to achieve a desirable minimum angle of trim, trim tabs are commonly installed at the transom to offset an extreme squat. Similar to the former use of wedge-shaped blocks under the transom to force the water down and push the stern up toward a more horizontal position, external contrivances like trim tabs have only a limited effect, as they function at some expense of economy or speed. Any such projections from the hull proper, whether in attachments or extrusions, will reduce speed by adding to wetted surface and parasitic drag. Reverse curves or warped planes have the same adverse effect by increasing the area of skin friction and distorting the free flow of water past the hull.

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Page last modified: 07-07-2011 12:46:52 ZULU