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Seaplanes are aircraft designed to take off and land on the surface of the water. Aircraft of this type fall into two distinctly different categories. Seaplane is the generic term under which various categories of flying boats, float planes, and outriggers are placed. The essential difference between seaplanes is in the fuselage itself.

  1. One category consists of conventional land planes that are mounted on floats, sometimes called pontoons, in place of a conventional landing gear with wheels. The float plane type has a main float attached to the fuselage by struts and an outrigger float under each wing. The term "twin float aircraft" is used to distinguish aircraft having a float located to each side of a center line aligned in a forward to aft alignment of the aircraft where the respective floats provide a substantial proportion at least of flotation for the aircraft and do not act merely as stabilisers. This then is to distinguish such aircraft from seaplanes which may have stabiliser floats depending from a respective wing.
  2. The other category consists of a basically different type of aircraft in which the lower part of the fuselage is shaped somewhat like a boat and which, at rest and low speed, floats on the surface just as a boat does - hence the term "flying boat." The flying boat is basically a seaplane with a boat hull fuselage.

Some seaplane types had two floats and no outriggers for buoyancy. The Grumman "Duck" had its main float joined to the fuselage and was a boat hulled seaplane. The term convertible was used to indicate that the standard landing gear may be removed and be replaced with floats for utilization aboard battleships or cruisers. An amphibian is a plane that can land on land or water.

Sea planes of various kind are well known for long period of time whereby certain wings or other parts are provided for hydrodynamic lift as long as the vehicle is on the water. Here then the wing is rigidly connected to the fuselage (or any other craft body) and under the development of a suitable flow pattern the boat will be lifted out of the water when otherwise i.e. without that particular flow pattern any added speed increase the water resistance and drag rather drastically. The wings then lift the boat out of the water and provide merely for hydrodynamic support so that he craft as a whole experiences drastically reduced water resistance.

In general, the amphibious aircraft is more complex than its land based counterparts due to the dual mission. Water landing requires the aircraft to be engineered to be water tight and withstand the added stress of water impact and docking. The propeller must be protected from the spray. The aircraft must have a safe method for boarding on both land and water. Desirably the aircraft would be designed so as to be able to use a standard boat dock and thus avoid the extra expense of building a special boarding facility. It also is desirable that the aircraft complexity does not degrade aerodynamic performance when compared to comparably sized land based planes.

These design objectives must be met in such a manner that the aircraft is stable on the water, on land and in air. Further, there is the consideration of designing the aircraft so that it is convenient and safe for the pilot and passengers, and yet is integrated so that the various aerodynamic, structural and design features blend together to provide an overall practical, efficient and economical aircraft.

Airplanes having a fuselage with a boat-like configuration are called "flying boats". The fuselage frames, or bulkheads, in the lower part thereof, corresponded to the cross-section of the boat shape and, in the upper part thereof, were adapted to the shape of the cabin. In a landplane, the cross-sectional shape of the fuselage remains unchanged over a relatively long extent of the fuselage area, and therefore similar frames or bulkheads are usable. This is not possible in the case of a boat shape of the fuselage, particularly because of the step-up distance. The manufacture of an airplane fuselage having a boat shape is, therefore, expensive and involves difficulties, especially in view of the fact that very special load conditions arise during take-off and landing which must be taken into consideration in the structural concept.

The use of a hydrodynamic step in a water hull for aircraft use is attributed by some to Glenn Curtiss. Early floatplanes designed by Curtiss had difficulty in becoming airborne, regardless of the amount of power applied or wing surface provided. It was discovered that the float or hull of the craft would tend to "stick" to the water surface due to the laminar flow of the water. The simple solution was to introduce a drag-inducing (and vortex generating) step at the rearward portion of hull or float to reduce or eliminate water adhesion and allow the craft to separate from the water flow. Unfortunately, this same step design also generates air vortices once the craft is airborne, thus contributing to aerodynamic drag.

Early aircraft utilized construction techniques resulting in high drag coefficients. Struts, wires, and braces in the airstream resulting in high drag designs. Thus, the amount of drag introduced by a hydrodynamic step contributed relatively little to the overall drag of the craft. However, modern aircraft are much more carefully designed to eliminate drag and have highly efficient aerodynamic designs. In such a design, a float or hull step may comprise a significant source of drag.

A seaplane hull is an integral part of the fuselage of a flying boat, or the floats on a floatplane. The design criteria of sponsons or floats mounted outboard on a flying boat which, arising from their displacement, commonly provide flying boats with lateral static stability, contrasts to the design criteria for seaplane hulls.

Since the inception of seaplanes, hull development has revolved around the planing hull concept, so much so that the hydrodynamic form to achieve efficiency when in displacement mode, that is to say below planing speed, has been generally ignored. By combining aspects of design established in racing catamarans and passenger catamaran ferries and innovating to enable such hulls to achieve seaplane design criteria, a new, highly original and advantageous seaplane hull has now been developed.

In plan form, conventional hulls almost universally have a resemblance to an elongated rain-drop with a rounded bow and an afterbody trailing to a point. Though this form is aerodynamically very efficient, it is wholly unsuited to motion on the surface of water. It creates a large bow-wave which creates high drag, and the Coanda effect draws it powerfully down into the water preventing it from taking-off in addition to creating a powerful nose-up pitching tendency. This is in contrast to the comparatively sharp bows and fine bow entry angles of recent catamarans.

The object of conventional seaplane hull design has been to minimise these hydrodynamic penalties while incurring as mild structural and aerodynamic penalties as possible. To prevent the Coanda effect sucking the hull down into the water, hulls are generally fitted with a transverse abrupt "step" below the center of gravity of the seaplane to separate the water flow from the hull. To enable them to climb over the bow wave, they feature hard edged forebody chines and powerful planing surfaces from the bow back to the step. The said forebody chines and the step are angled steeply across hydrodynamic and aerodynamic streamlines and create substantial vortices which both damage directional stability and create substantial drag. The result is undesirable qualities from the point of view of hydrodynamic loading and hydrodynamic resistance during take-off. They demand an aircraft with high power-to-weight ratio and result in limited payload capacity for a given size of aircraft.

Variable geometry hulls, hydrofoils, air blowing and other devices have been used to try and maintain a clean aerodynamic hull form. However, they all have limitations and add considerably to the weight and complexity of an aircraft. None of these attempted solutions have proved Certifiable and/or commercially viable though various ideas have been tested in prototype and experimental form.

Some improvements however have been demonstrated by the use of high length-to-beam ratio hulls and this was researched in some depth prior to 1950 and is well reported in "Development of High-speed Water-based Aircraft", by Earnest G Stout in the Journal of the Aeronautical Sciences Vol. 17 August 1950. This discusses tests on hulls with length/beam ratios of up to 12 though the advantages indicated were little exploited as there has since been little seaplane development work. The U.S. Navy's flying boat XP5Y-1 (first flown in 1950) had a length/beam ratio of 10. However, it had all the above-mentioned features of a conventional seaplane.

The top speeds of offshore racing multihulls and passenger catamarans approximately doubled in the 25 years to 1994. These hulls have also demonstrated major sea-keeping advantages, but despite the demonstration of the advantages of this type of hull, no seaplane has incorporated this type of design. The appropriate use of these types of hull for seaplanes is not obvious as there are two potentially problematic factors in their application to seaplanes. Firstly, they have a large wetted surface area at high speed which results in high frictional drag when approaching take-off speed. Secondly, there appears to be a widespread assumption amongst aircraft designers that the Coanda effect remains problematic on any hull without a central step.

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