Yieldable Blades For Propellers

Abstract

A blade design for propellers of watercraft wherein the blades of the proler are weakened in the line of impact but retain substantially all of their strength along the line of hydrodynamic load. Such a weakening permits the blades of a propeller to shear off or yield upon impact with an object without damage to the motor, shaft or pitch changing mechanism.

Description

The invention described herein may be manufactured and used by or for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

The propellers of watercraft are relatively unprotected and while underway the blades can impact on obstructions. Such an impact usually causes extensive damage extending from the prime mover through the power transmission system to the propeller itself. The instant stopping of the propeller with the inertia load of the rest of the rotating parts of the propulsion system is the primary cause of damage to propulsion machinery. A secondary cause of damage is the eccentric load imposed by the stopping of one blade which impacts on the obstruction.

The problem is particularly acute in Hazelton or tandem propulsion systems wherein the propeller hub is an annular ring the size and contour of the hull on which it is mounted. The blades mounted on such a hull protrude beyond the hull contour and are therefore more susceptible to impact. Also, the diameter of the propeller hub being so large the impacting blade forms a lever arm with the centerline of the propeller thus magnifying any forces generated therein. This force becomes important in a Hazelton propeller as the blade pitch changing mechanism and power transmission assemblies, as shown in U.S. Pat. Nos. 3,010,066 and 3,450,083, to Hazelton, are carried with the hub and susceptible to any impact loads thereon.

DESCRIPTION OF THE PRIOR ART

The prior art, as represented by current propeller technology, does not appear to include the concept of yieldable or breakaway blades. The concept of yieldable devices to absorb shock is well developed, with specific areas being yieldable bushings or clutches in the propeller hub, or mounted in the drive shaft. These devices have met with some success in that they protect the propulsion system beyond the resilient member. For example, a flexible coupling in the drive shaft would not protect the propeller, its hub, and that portion of the shaft before the coupling. Also, such devices lend themselves to conventional screw propellers with their relatively small diameter hubs and drive shafts attached thereto. The Hazelton type propellers with annular hubs and a multiplicity of relatively small blades spaced therearound do not lend themselves to an adaptation of current screw-propeller technology.

SUMMARY OF THE INVENTION

This invention utilizes the principle of weakening a propeller blade so that upon impact with an obstruction the blade will break off the propeller hub without damaging the propulsion system. The blades are weakened in a plane along which collision would normally occur, i.e. along the centerline of the blade in a fore and aft direction. The blade is not weakened in a plane along its hydrodynamic load axis which is at an acute angle to the aforementioned plane of collision. In this way the blade is not weakened, or nominally weakened, for its primary function of propulsion but is sufficiently weakened to shear off the propeller hub upon impact.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a top plan view showing a Hazelton propeller blade embodying the subject invention.

FIG. 2 is a side view of the Hazelton propeller blade of FIG. 1.

FIG. 3 is a side view of another embodiment of the subject invention.

FIG. 4 is a side view, with parts cut away for clarity, of yet another embodiment of the subject invention.

DESCRIPTION OF THE INVENTION

Referring now to the drawing, FIGS. 1 and 2 show a Hazelton type propeller blade assembly 10, which is mounted on the annular hub assembly 12, that is common to this type of propeller. The blade 10, is mounted on the hub 12, for rotational movement relative thereto to permit pitch changing of the blades. The pitch changing mechanism is not shown as it does not form a part of the invention and is mentioned herein to adequately describe the environment in which the subject invention is to be used.

The blades 10, have a general cross-section of an airfoil or teardrop as shown in FIG. 1. Therefore, the leading edge 14 will always cut into the water first and be subject to any collision with an obstruction. The blade assembly 10 has a base 16, with bolts 18, extending therethrough to attach the blade 10 to the pitch changing mechanism. Due to the entrance of leading edge 14, into the water to create the forward motion of a vehicle the hydrodynamic loads on the blade 10 will occur generally along the lines indicated at B. As can be seen, these lines of loading, B, occur on the side of and at an angle to the longitudinal axis of the blade 10. Therefore, the collision on the leading edge 14 causes a moment around moment axis D and the hydrodynamic loads cause a moment which has a major component around moment axis C.

The collision moment will cause a relatively high tension load on bolts 18 because of the short lever arm distance between line D and the trailing edge of base 16. The hydrodynamic moment will cause a relative small tension load on bolts 18 because of the long lever arm distance which is at least equal to the distance between the centers of bolts 18.

As the leading edge 14 of the blade 10, cuts through the water first, any collision will occur along a plane through axis C, and pivot the blade 10, relative to the hub 12, about the trailing edge of base 16. In order to aid the pivoting of the blade 10, about the trailing edge of base 16, with respect to the hub 12 upon collision, the base 16 can be tapered as shown at 20, rearward of the bolts 18 and their axis D. The bolts might also be made of a metal having a low specific strain to facilitate their breaking on excessive exertion. Thus on impact with an obstruction the blade will more readily pivot about the axis D and break the bolts 18.

In operation when the blade 10 is cutting through the water and the leading edge 14 thereof strikes an obstruction the load imposed by the collision will occur along a plane through axis C. There are no fastening bolts along this axis so a pivotal moment will be created about the axis D formed by bolts 18. The taper 20, will facilitate the pivotal movement and the bolts 18 will break. Thus the blade 10 will fall off without transmitting the large shock load of a collision to the pitch changing mechanism and/or power transmission system.

Referring now to FIG. 3, a Hazelton propeller blade 22 is shown utilizing another embodiment of the subject invention. The blade 22, has the same cross section as the blade 10, as shown in FIG. 1. The blade 22, has a base 24, with a shaft 26, extending therefrom to connect the blade to the pitch changing mechanism. The axis of rotation of the blade 22, for its pitch changing mechanism, is shown by axis A', extending through the line of the shaft 26. As in the previous embodiment, the blade 22 has a leading edge 28 that will incurr a collision with an obstruction as it always cuts through the water first. Thus, a cut 30 is made through the blade from the leading edge 28, extending toward the axis A'. The cut 30, is filled with a filler, such as a plastic compound, as at 32, to fill the cut and maintain the contour and fairing of the blade 22. Similarly, a wedge-shaped cut 34, is made through the blade 22, from the rear of the blade toward the axis A'. This cut is also filled with a filler 35 to restore the contour of the blade 22.

In operation, as the blade 22 cuts through the water, if a collision with an obstruction should occur, the leading edge 28 of the blade will receive the impact. The cut 30, which weakens the blade 22, along its axis, will initiate and insure a rearward pivoting of the blade about an axis running through the axis of rotation A'. This will force out the filler 35, and permit the blade to completely pivot downwardly and shear off at the base 24. Thus, any load imposed by an impact of blade 22, on an obstruction will be absorbed and dissipated in the bending and breaking of the weakened blade. Any destructive forces normally transmitted through the propeller to the pitch-changing mechanism or propulsion system will be avoided.

Referring now to FIG. 4, a still further embodiment of a Hazelton blade assembly 36, embodying the subject invention is shown. The blade assembly 36, has a blade 38 attached to a shaft 40 which extends from a pitch-change mechanism. The shaft 40, is rotatably received through the usual Hazelton annular hub 42.

The shaft 40, at the end extending outwardly of the hub 42, has a cylindrical recess 44, with a narrow opening 46 extending thereinto. A groove or seat 48, with rounded edges extends diametrally across the end of the shaft, at a right angle to the longitudinal axis of the blade, to form a seat for the blade 38, and establish a pivot along an axis extending along this diametral axis. For example, the groove or seat 48 could extend completely across the shaft 40 or be a slot at the center thereof.

The blade 38, has a base 50, with a rounded longitudinal boss 52, on the bottom thereof with a contour adapted to be received in the groove or seat 48, whether it be a complete transverse opening or a slot as mentioned hereinabove. The blade 38, has a cross section like the blade shown in FIG. 1, with a leading edge 54. A high strength cable 56, is embedded in the blade 38 and attached to an anchor plate 57, along a line coinciding with the shaft 40. The cable 56, extends downwardly into the cylindrical recess 44, and has a piston 58 attached to the free end thereof. The piston 58 has a cross section to permit sliding movement within recess 44. A compression spring 60, is placed in the recess 44, extending between the piston 58 and the narrow opening 46 to drive the piston 58, downward and place the cable 56 in tension to maintain the blade 38 in an erect position by seating boss 50 in seat 48. Thus, the blade 38 is held rigid along a transverse direction but can rock in a fore and aft direction if the blade should impact on an obstruction. The blade can withstand the hydrodynamic loads, as shown in FIG. 1, at B, since these forces are not aligned with the longitudinal axis of the blade to pivot it about the boss 52 and seat 48.

In operation, as the leading edge 54 of the blade 38, cuts through the water, if the blade should impact on an obstruction the impact will occur at the leading edge 54. The force of the impact will pivot the blade 38 about the axis created by the seat 48 and the boss 52. The pivoting of the blade 38, will pull the cable 56 upward and compress the spring 60, between the piston 58 and the opening 46, thereby absorbing the shock of impact. After the impact load is removed from the blade 38, the spring 60, will drive the piston 58 back to its original position thereby pulling the cable 56, and re-erecting the blade 38, into its original position. The generally semi-cylindrical shape of the seat 48, and boss 52, assure that the blade 30, returns to the exact position relative to the shaft 40, as before the collision. It also insures the proper positioning of the blade with respect to the pitch changing mechanism when the shaft 40 rotates.

As can be seen, a simple but unique means to protect the propulsion system of a water vehicle has been developed wherein the blades absorb the shock and damage of collision and the propulsion system remains relatively undamaged.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims

What is claimed is:

1. A propeller for watercraft, comprising:

a hub;

a series of blade assemblies attached to the hub, wherein the assemblies are weakened along a longitudinal axis extending through the leading and trailing edges thereof without weakening the blade at an oblique angle to said axis wherein hydrodynamic loads occur;

each blade assembly has a blade portion and a base for attaching the assemblies to the hub;

the assemblies are attached to the hub by bolts extending through the base wherein said bolts are spaced rearwardly of the leading edge and outwardly of the longitudinal axis;

the lower surface of the base is tapered rearwardly of the bolts to leave a gap between said surface and the hub; and

each blade is mounted to extend outwardly from its respective base to rotate relative to the hub about an axis normal to the base.