DEVICE WITH CAVITATIONAL EFFECT FOR PROPELLERS OF WATERCRAFT WITH A PLANING OR SEMIPLANING KEEL

Abstract: A device with cavitational effect for propellers used for the propulsion of watercrafts with a planing or semiplaning keel is described consisting of a duct (1) which enables air withdrawn from above the hull water line to directly reach the front part of the propeller (8).

Description: A. Technical sector This invention relates to a device with cavitational effect for propellers used for propelling watercraft with a planing or semiplaning keel. B. The technical problem One of the main categories of engine-powered watercraft comprises water crafts with a planing or semiplaning keel, ie a keel which enables the hull to plane when determined hydrodynamic support conditions are attained, ie when the weight of the hull is supported mainly by the dynamic action between the water and the keel plane. The watercraft thus rises on the water and slides thereon instead of simply floating thereon. These planing conditions occur only at high speed, and it is therefore extremely important to as quickly as possible pass through the transient conditions between the moment in which the hull simply floats, not having as yet attained the speed required for planing, and the moment in which it planes. It is apparent that the duration of this transient period depends on the ability to fully use the engine power. As these watercrafts use a propeller propulsion system, the problem of the duration of the transient period corresponds to solving the problem of optimising the propeller operation. Increasing the propeller speed is known to create certain problems, deriving from the fact that to develop maximum thrust the water must slide over the propeller blades with as little turbulence as possible so that a vacuum is created on the front face. If the propeller is accelerated such that this vacuum is less than that exerted overall by the water, the flow alters to give rise to the phenomenon of cavitation, which results in a rapid reduction in the torque absorbed by the propeller and an even more accentuated reduction in the developed thrust. C. The prior art As the propeller thrust is greater the greater the volume and speed of the mass of water traversing the propeller disc per unit of time and the lower its acceleration, many types of propeller and many types of propeller-engine coupling have been studied to optimise the propulsion efficiency of a watercraft in accordance with its keel shape and speed. In the case of planing or semiplaning heels, propellers of high speed type are much used to ensure the high speed required for planing is obtained. Before the watercraft has reached the speed required for planing, such propellers are not in a state of optimum operation because they are rotating at low r.p.m. and cannot be raised rapidly to the required r.p.m. Current marine propulsion units do not in fact comprise change-speed gears so that the propeller drive torque can only adapt to the resistant torque if the drive torque is always greater than the resistant torque. This means that the engine r.p.m. must be increased gradually, and only when the planing speed for the watercraft is reached can it operate under optimum maximum drive torque conditions. The above methods used to solve this problem are not however totally satisfactory and new solutions are continuously sought by experts of the art, especially for high-speed or competition craft for which the time required for attaining planing speed is very important. It has now been found possible to utilize the cavitational effect itself to solve satisfactorily, or at least better than in the case of previously adopted solutions, the problem of utilizing maximum engine torque and its use in attaining the planing speed for a watercraft with a planing or semiplaning keel within a short time. A device which enables the cavitational effect caused by the movement of a propeller to be utilized in reducing the time required to attain optimum engine power consists of a duct which enables air withdrawn from above the hull water line to directly reach the front part of the propeller. The air is drawn through said duct and on reaching the vicinity of the propeller it causes it to cavitate and reduce the water flow through the propeller, so also reducing the resistant torque and causing the engine r.p.m. to immediately rise. By gradually reducing the air quantity drawn through the device it is therefore possible to keep the engine r.p.m. at maximum torque level and to regulate the thrusting rate of the propeller, so using said maximum torque for the entire time required to attain planing and thus optimum operating conditions for the engine-propeller unit. In this manner the operating conditions are considerably more favourable than those permitted by conventional solutions in which the engine r.p.m. is gradually raised to attain maximum torque only when hydrodynamic planing conditions are reached. US-A-3,434,447 refers to a watercraft engine where, as mentioned above, a controlled cavitational effect is used during the starting up of the watercraft; according to one of the embodiments shown in U.S. Patent, air is introduced in the front part of the propeller through an air flow duct fixed by brackets to the engine. D. Detailed description of the invention. The present invention refers to a device with cavitational effect, comprising an air flow duct enabling air withdrawn from above the hull water line to directly reach the front part the propeller, where the air flow duct is readily movable and its outlet is positionally adjustable, along the diameter of the disk generated by the propeller, from a position above the propeller shaft to a position beyond the circunference of the propeller disk. If the device of the invention is installed in a watercraft with two or more propellers, it will comprise an air flow duct in correspondence with each propeller present, said ducts being rigid with each other to form an integral body. According to a further characteristic of the invention the ratio of the cross-section of the air flow duct to the area of the disc generated by the propeller is between 1:5 and 1:50. It is apparent to the expert of the art that a large number of embodiments of the device according to the invention are possible and these can be easily adapted to any engine type and power and to any form of planing or semiplaning keel. E. Brief description of the drawings Some preferred embodiments of the device according to the invention are described hereinafter with particular reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of a device mounted on the stabilizer and also showing the corresponding propeller; Figure 2 is a diagrammatic view of a device which is displaceable vertically relative to the corresponding propeller. F. Description of the preferred embodiments The device consists of a duct 1 rigid with a stabilizer 2 connected by the pin 3 to a support 4 which serves to fix the device to the hull of the watercraft (not shown in the figure). A hydraulic cylinder-piston unit 5 fixed to the stabilizer 2 by the pin 6 enables the stabilizer 2 to be moved about the pin 3 and thus the air flow outlet mouth 7 to be moved relative to the propeller 8. By suitably adjusting the inclination of the stabilizer 2 by means of the cylinder-piston unit 5 it is therefore possible to control at will the air flow drawn in above the water line by the movement of the propeller 8. In Figure 3 the device consists of a duct 30 which can move vertically so that, when totally lowered, it stands immediately above the shaft 31 of the propeller 32, and when totally raised it lies above the upper edge of the circle described by the propeller 32. The top of the duct 30 is always in communication with the atmosphere above the water line. The amount of air drawn in by the propeller 32 and flowing onto the front of said propeller is controlled by raising or lowering the duct 30.

Claims: A device with cavitational effect for at least a propeller (8, 32) used for the propulsion of watercrafts with a planing or semiplaning keel, said device comprising an air flow duct (1, 30) which enables air withdrawn from above the hull water line to directly reach the front part of said propeller (8, 32), characterised in that said air flow duct (1, 30) is readily movable and has an outlet which is positionally adjustable relative to said propeller (8, 32), the air flow duct outlet being positionally adjustable along the diameter of the disk generated by said propeller (8, 32) from a position of said diameter above the propeller shaft to a position beyond the circunference of said propeller (8, 32) disk. A device as claimed in claim 1, characterised in that the ratio of the cross-section of said air flow duct (1, 30) to the area of said propeller (8, 32) disk is between 1:5 and 1:50. A device as claimed in claim 1 and suitable for at least two propellers (8, 32), characterised in that said air flow ducts (1, 30) corresponding to each of said propellers (8, 32) are rigid with each other to form an integral body. A device as claimed in claim 1, characterised in that said air flow duct outlet (7) is positionally adjustable by pivoting said air flow duct (1). A device as claimed in claim 4, characterised in that said air flow duct (1) is affixed to a flap (2) connected to the watercraft hull by a pin (3) enabling said air flow duct (1) to be moved to modify the position of said air flow duct outlet (7) relative to said propeller (8). A device as claimed in claim 5, characterised in that further comprises a hydraulic cylinder-piston unit (5) operatively connected to said flap (2) and to said watercraft hull, said hydraulic cylinder-piston unit (5) moving said flap (2) and said air flow duct outlet (7) connected Therewith. A device as claimed in claim 1, characterised in that the outlet of said air flow duct (30) is positionally adjustable by a vertical movement of said air flow duct (30). A device as claimed in claim 7, characterised in that said vertical movement of said air flow duct (30) to a lowest position places said air flow duct outlet immediatly above the shaft (31) of said propeller (32), while the movement of said air flow duct (30) to an uppermost position places said air flow duct outlet above the the upper edge of said propeller (32) disk, the top of said air flow duct (30) being in communication with the atmosphere above the water line.

Inventors: BUZZI, FABIO
Applicants: BUZZI, FABIO
Application year: 1989
Main classification: B63H1
IPC codes: B63H1