Ships – Building – Antifriction surfaces
Reexamination Certificate
2002-04-30
2004-01-06
Morano, S. Joseph (Department: 3617)
Ships
Building
Antifriction surfaces
Reexamination Certificate
active
06672234
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of water craft and, in particular, to a high speed vessel which is supported by a combination of static air pressure, dynamic lift, and aerodynamic ram-effect.
2. Description of the Related Art
The idea of creating a thin layer of air between the water and the surface of a vessel's hull is not new. The intention is to reduce the friction component of its resistance. The difficulty has proven to be the distribution and control of the airflow. An alternative design is that the vessel also be carried on a pressurized air cushion, at the same time as fiction is reduced.
Many of the presented and patented inventions based on the above ideas lack a description of an overall concept for the invention. Certain inventions related to air cushion vessels demonstrate advantages within limited applications, but at the same time have disadvantages that make them unsuitable for commercial use. For example, they lack a description of how the claimed advantages could be maintained in the sea conditions that could be expected. Far too little consideration has been given to the combination of the requisite comfort, maintenance of speed in waves, reliability and limited maintenance, with high speed and low resistance—requirements that must all be satisfied to produce a commercially successful application.
Usual characteristics of previously presented solutions for air cushion vessels are that;
they focus on methods—often theoretical—for improving efficiency by reducing the frictional resistance of the hull when moving in still water
they assume a high level of aerostatic lift
the projected area of the air chamber is often larger than necessary for the displacement (because the size is determined by the arrangement)
the shape in plan view of the air chamber is usually rectangular
they require larger fans to maintain cushion pressure and airflow
the operation is sensitive to sea conditions and the method of sue, their motion is easily affected by passing waves
they have a shallow draught and large air leakage, particularly in waves
they have low hydrodynamic damping at speed
they have comparatively small reserve displacement to be able to counteract changes in trim
the hulls are extremely mechanically complex and are complicated to manufacture, which makes construction expensive
they require complicated control systems, and contain construction elements that lead to considerable maintenance costs and give less reliable operation
On both hovercraft and SES, the air cushions have been contained by flexible enclosures, called “skirts”. These have proven to be susceptible to wear and damage, and required regular replacement, in SES particularly the fore skirt. The small number of vessels built has led to spares being expensive. Using flexible skirts involves continuous air leakage under the skirt, although in controlled amounts. The airflow leads to additional energy consumption and an increase in the total power required for propulsion. Large variations in the leakage cause pressure fluctuations/pressure drop in the supporting air cushion, which can lead to considerable variation in the hull resistance and vibrations in the vessel (reduced comfort). A shallow draught when the vessel is cushion borne increases the likelihood of air leakage in a seaway. On hovercraft and SES, the forward skirt section projects an appreciable and blunt surface in the direction of motion, which, in a head sea and bow wave, can give rise to a relatively large increase in the resistance, i.e. a considerable reduction in speed. Waves that hit the skirt cause pressure variations in the air cushion, which are transmitted to the vessel and reduce the ride comfort.
Several inventions have been put forward in which flexible enclosures of the air cushion are totally or partially replaced by rigid ones, e.g. by Burg, Barsumian, Harley, Bixel, Peters and Stolper, for use on single and multihull vessels. Barsumian, Bixel and Harley combine a limited amount of dynamic lift (depending on speed, deadrise, trim and surface) on a (preferably forward) hull section in contact with the water, with aerostatic lift (air cushion), mainly located in the aft portion of the vessel. When planning, these inventions try to achieve a minimum of hull surface in contact with the water, in order to reduce the frictional resistance. Burg and Bixel extend the air chamber along almost the entire length of the hull at the water line. (Peter's idea is a conventional SES, in which the invention comprises a moveable, athwartships division of the cushion chamber, which can be activated for motion control.). In Stolper's idea, the intention is that the wind created by the movement of the vessel will be used for natural ventilation/air lubrication of planning, supporting surfaces, rather than the air cushion technique. The other inventions mentioned require forced ventilation (fans). A common factor of the above vessel solutions is that they have a minimum of hydrodynamic and hydrostatic (reserve displacement) lift and motion damping in relation to that which is needed to create a seaworthy vessel, and that they are primarily designed to reduce the wetted area, i.e. frictional resistance. Several inventions (in particular those of Burg) are considerably complex, demanding increased maintenance to provide reliability.
Vessels supported on air cushions are propulsively more efficient at high speeds than displacing and planning vessels. For devices intended for propulsion by contact with the water, high speeds involve hydrodynamic complications (cavitation, reduced efficiency, erosion damage), which can increase further due to air leakage from the cushion, which in turn is dependent on the vessel's position in the water/trim at speed. Sporadic ventilation of the propulsive device gives rise to torque and pressure fluctuations, which in turn can cause operating damage to the gears and engines.
Consequently, almost all of the ideas described above require propulsion using surface-piercing propellers (the blades of which have a fully ventilated suction side) as shown in figures in the respective patents. Unless stated otherwise, surface-piercing or air propellers are used. For moderate speeds, also a conventional propeller can provide propulsion.
Experience has shown that surface-piercing propellers have a shorter operating life than conventional propellers. Variation of blade immersion in the water can easily lead to load variations in a seaway, for both propeller and engine, which necessitates a dynamic regulation system. Precise regulation of the blade immersion is also necessary during acceleration and when passing the “hump speed,” not to overload the engine at low revs. Too high a torque can easily lead to the vessel not being able to reach full speed (“gets stuck on the hump”).
Only Burg refers in one of his patents to the possibility of using water jet propulsion, but puts the water inlet in an unsuitable position for its function, and does not develop propulsion alternatives further. The mixture of air into the water usually results in a greater likelihood of cavitation, thrust reduction, propeller ventilation and reduced propulsion efficiency. The surface-piercing propeller is the best choice, as it is designed for these conditions, and thus works at high speeds. However, it has poor characteristics during low speed maneuvering and when reversing, which increases maneuvering time in harbor. This, plus large load variations in the propulsion unit, has limited its commercial application. Ordinary propellers, with wing profile blade sections, can be used up to approximately 40 knots (oblique flow when mounted on an angled shaft usually leads to erosion damage due to cavitation). Higher speeds require a propeller shaft aligned with the direction of flow, modified blade sections or fully cavitating blades with lower efficiency. Contra-rotating propellers can be around 10% more efficient than a single propeller. They can work at speeds above 70 knots, but at
Effect Ships International AS
Knobbe Martens Olson & Bear LLP
Wright Andrew
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