GB2067709A - Improvements in or relating to propeller-driven water-borne craft - Google Patents

Abstract

A propeller-driven water-borne craft 1, particularly a sidewall-ACV, has the shafts 20 on which the propellers 5 are mounted inclined to the horizontal by a substantial amount. (8 DEG to 20 DEG ). To reduce propeller boss cavitation due to a local high velocity caused by high craft speed and/or high shaft speed, which condition is particularly exaggerated by the shaft inclination where the incident flow relative to any point on the propeller boss alternates rapidly in direction as the shaft rotates, pressurised air is discharged in the proximity of the propeller boss 5. The air is discharged at the outlet ends of conduits 31 attached to the "trailing" edges of the shaft- supporting brackets 21 by clips 40. <IMAGE>

Description

SPECIFICATION Improvements in or relating to propeller-driven water-borne craft Background to the Invention This invention relates to propeller-driven waterborne craft, that is to say, craft which at least in one period of operation, are supported, at least in part, by water and are driven by water-screw propellers.

As used herein, the term "water-borne craft" not only includes boats and ships but also gascushion craft for operation over water, wherein, in operation, at least part of the craft is immersed in water. An example of such a craft comprises a "sidewall" gas-cushion vehicle, one form of which is described in British Patent No. 1 ,400,035. (U.S.

Patent No. 3,977,491 of Fishlock corresponds).

A problem exists with propeller-driven waterborne craft wherein the shafts on which the propellers are mounted are inclined to the horizontal by a substantial amount (80 to 200).

The cavitation condition is due to a local high velocity which is caused by high craft speed and/or high shaft speed. The condition is particularly exaggerated by the shaft inclination where the incident flow relative to any point on the propeller boss alternates rapidly in direction as the shaft rotates.

The propeller bosses of such arrangements tend to be subject to erosion due to cavitation.

Summary of the Invention According to the present invention, this tendency is significantly reduced by discharging pressurised air in the proximity of a propeller boss.

Brief Description of the Drawings An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a side view of a sidewallgas- cushion vehicle, Figure 2 is an enlarged view of one of the water-screw propulsion units of the vehicle, Figure 3 is a view similar to that provided by Figure 2, and illustrates a modification, and Figure 4 illustrates, semi-diagrammatically, how the propeller unit of Figure 3 is provided with pressurised air.

In the figures, like reference numerals refer to like components.

Detailed Description of the Preferred Embodiments Referring first to Figures 1 and 2, a sidewall gas-cushion vehicle 1 is shown travelling over the surface 2 of water, supported by a cushion 3 of pressurised air formed and contained between the vehicle body 4 and the surface 2. The vehicle 1, which is substantially the same as that described in British Patent No. 1,400,035 (U.S. Patent No.

3,977,491 of Fishlock corresponds), is propelled by a pair of water-screw propellers 5 and is steered by a pair of rudders 6.

The sides of the vehicle-supporting cushion 3 are contained by a pair of laterally-spaced sidewall structures 7 extending longitudinally along the sides of the vehicle body 4 in substantially parallel array. The sidewalls 7 depend from the sides of the vehicle body 4 so as to dip into the water and form a cushion seal. Each sidewall 7 carries a water-screw propeller 5 and a rudder 6. Each propeller 5 is mounted on a shaft 20.

The front or bow end of the cushion 3 is contained by a flexible skirt 8 and the rear or stern end thereof by a flexible skirt 9. the skirts 8, 9, which are of hollow, inflatable form, and constructed from rubberised fabric, extend laterally between (and in the case of the skirt 8, beyond as well) the front and rear ends of the sidewalls 7. The skirts 8, 9 are attached to the vehicle body 4 and depend therefrom. The flexible skirt 8 is in light contact with the water surface 2.

The inflation and tension forces set up in the flexible skirt 9 ensure that it is maintained clear of, but in close proximity to the surface 2.

The front flexible skirt 8 is of two-stage form and comprises an inflated bag 10 from which depend a succession of independently-deflectable flexible wall or skirt members 11 of the form disclosed by British Patent Specification No.

1,043,351. (U.S. Patent No. 3,420,330 of Bliss corresponds). Air forming the vehicle-supporting cushion 3 is provided by a pair of centrifugal fans 12 disposed side by side. Air from the fans 12 passes through the flexible skirt 8 to inflate it before entering the space occupied by the cushion 3. The rear flexible skirt 9 is inflated by air supplied by a centrifugal fan 13. the fans 12 and fan 13 form separate means for supplying air to the space occupied by the vehicle-supporting cushion 3 and the skirt 9 respectively.

With additional reference to Figure 2, each propeller shaft 20 is supported at its propeller end, by a bracket 21, and is rotatable about an axis 22.

The axis 22 is inclined to the horizontal by substantially 150. The propeller shafts 20 are rotated by variable speed engines (not shown, but housed in vehicle body 4).

The vehicle 1 is provided with means for discharging pressurised air in the proximity of the propeller bosses, so as to reduce erosion thereof, caused by cavitation.

The means comprise an air compressor 25 (Figure 1) driven by a variable speed engine 26, and discharging air to passageways 30 (Figure 2) formed in the brackets 21 by way of flexible air conduits or lines 31 housed in the sidewalls 7.

The air may escape from the passageways at one or more points. For example, by ports providing exits to the space 35 between each boss of a propeller 5 and the adjacent surface of the associated bracket 21. Alternatively, or in addition, air is allowed to escape from ports disposed at or near the rear surface of a bracket 21.

As the air escapes, it flows over the propeller boss surfaces in the form of air bubbles. This flow of air cushions the implosive action of the collapsing water vapour bubbles as the water direction alternates around the propeller boss, which would otherwise cause erosion.

In a modification, the passageways 30 are dispensed with and the flexible lines 31 are disposed outside the associated brackets 21 , the outlet ends of the lines being disposed at the most beneficial points.

Figure 3 illustrates such a modification. Here a flexible line 31 is secured to the "trailing" edges of the brackets 21 by clips 40 attached to the brackets. The outlet end of the line 31 is adjacent the forward end of the propeller boss. The discharged air is indicated by dotted lines 42.

With reference now to Figure 4, pressurised air is supplied to each line 31 (one for each propeller 5), by a compressor 45 driven by a hydraulic motor 46. The compressor 45 is provided with a filter/intake 47. The compressor/drive motor unit 45/46 is provided with an oil tank 48. A pressure gauge 49 is fitted to the outlet side of the compressor 45.

Claims (8)

1. A propeller-driven water-borne craft wherein the shaft on which the propeller is mounted is inclined to the horizontal by a substantial amount, provided with means for reducing propeller boss cavitation by discharging pressurised air in the proximity of the propeller boss.

2. A craft as claimed in Claim 1, wherein the propeller shaft is supported by a bracket and wherein the pressurised air is discharged to the proximity of the propeller boss by way of a passageway formed in the bracket.

3. A craft as claimed in Claim 1, wherein the propeller shaft is supported by a bracket and wherein the pressurised air is discharged to the proximity of the propeller boss by way of a conduit attached to the exterior of the bracket.

4. A craft as claimed in Claim 1, 2 or 3, wherein the shaft is inclined to the horizontal by substantially 150.

5. A propeller-driven water-borne craft wherein the shaft on which the propeller is mounted is inclined to the horizontal by 80 to 200, provided with means for reducing propeller boss cavitation, by discharging pressurised air in the proximity of the propeller boss, the craft comprising a sidewall gas-cushion vehicle which, in operation, travels over the surface of water, and is supported thereabove by a cushion of pressurised air formed and contained between the vehicle body and the surface, with the sides of the vehicle-supporting cushion contained by a pair of laterally-spaced sidewall structures extending longitudinally along the sides of the vehicle body in substantially parallel array.

6. A craft as claimed in Claim 5, provided with two propellers, each propeller being carried by a sidewall structure.

7. A propeller-driven water-borne craft, substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

8. A propeller-driven water-borne craft, substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings, modified substantially as hereinbefore described, with reference to Figures 3 and 4 of said drawings.