US3286681A

US3286681A - Propeller shaft support

Info

Publication number: US3286681A
Application number: US414927A
Authority: US
Inventors: Plum John
Original assignee: Individual
Current assignee: Individual
Priority date: 1964-11-30
Filing date: 1964-11-30
Publication date: 1966-11-22
Anticipated expiration: 1983-11-22

Description

Nov. 22, 1966 J. PLUM 3,286,681

PROPELLER SHAFT SUPPORT Filed Nov. 30, 1964 Q3 2 L w INVENTOR.

JOHN PLUM ATTY.

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AGENT United States Patent O 3,286,681 PROPELLER SHAFT SUPPORT John Plum, Montgomery County, Md. (5402 Tuscarawas Road, Washington, D.C. 20016) Filed Nov. 30, 1964, Ser. No. 414,927 5 Claims. (Cl. 115-34) 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.

The present invention relates to improvements in the installation of propeller shafts. One object of the invention is to increase the efficiency of the propeller. Another object is to reduce the vibrational forces which produce noise and fatigue failures in the propulsion system.

It should be realized that as a craft moves thru water, the shaft as well as the hull will be greatly affected by dynamic forces. This is especially true in the case where the propeller shaft is inclined toward the flow under the bottom of a high-speed craft. If the shaft is inadequately supported, the dynamic lift on the shaft will bend it up into a curve which is concave downwards. This will result in an angle of the propeller axis which is greater than the initial value of the shaft angle.

The detrimental effects of having the propeller axis at an angle to the inflow arise from the fact that this causes an increase in the angle of attack of the blades moving downward and causes a decrease in the angle of attack of the blades moving upward, which in turn will move the center of the thrust out of line with the center of the propeller. Furthermore, since the propeller has several blades, when it is rotating rapidly thousands of vibrations per minute will be transmitted to the hull through the propeller bracket, and there will be thousands of thrust-impulses tending to twist or bend the shaft in the vicinity of the propeller bracket. The many examples of fatigue failure of these brackets are, therefore, readily understandable.

Increasing the angle of attack of the blades moving down and decreasing the angle of attack of the blades moving up will not only reduce the effective area of the propeller but it may overload one side and cause cavitation.

It has been explained that a concave bend in an inclined shaft will aggravate all the detrimental effects which are produced by the intial angular infiOW. A convex bend will therefore have the opposite effect. In fact, a correct amount of convex curvature of an inclined shaft will:

(a) Distribute the thrust evenly between the two sides of the propeller, thereby increasing its effective area.

(b) Increase the cavitation-inception speed of the propeller.

(c) Eliminate the principal factor producing vibrationa1 forces and noise.

(d) Remove the obstruction of the inclined shaft from the inflow to the propeller.

The advantages of a convex curvature in an inclined shaft have been discovered accidently by many pilot-s of single-screw speedboats. This accidental discovery occurs when the propeller bracket fails at high speed. The failure of this strut will immediately permit the shaft to bend upward in the convex curvature of a cantilever, which will result in a sudden speed increase and a cessation of all vibration and noise.

The advantages resulting from a broken propeller bracket and the associated convexly bent propeller shaft have been recognized at page 127 of the 1961 volume of the Transaction of the Royal Institute of Naval Architects. It has been determined by the inventor that the dynamic lift on an inclined shaft which is convexly bent "ice (by, for instance, a failure of the propeller bracket) will enable the shaft to With-stand an additional buckling force which is four times the magnitude of the lift force, whereas, the dynamic lift on a concavely bent shaft will have the opposite effect. This explains the phenomenon that the shaft does not buckle when the propeller bracket breaks. However, the unsupported shaft will eventually undergo fatigue failure due to stresses on the shaft inside the hull caused by reverse bending.

It can therefore be concluded that in order to prevent buckling from a high thrust-load, a convexly bent shaft needs only a minimum number of points of support, Whereas a shaft installed straight requires a great many closely space-d struts in order to prevent concave bending at high speed.

It has been explained that a broken propeller bracket will produce the curvature of a single cantilever (similar to that shown in Figure 5, page 96 of Popular Boating of March 1962) whereas in the present invention a more uniform bending is obtained by installing the shaft with the curvature of a double cantilever. The shaft is initially supported at tWo points, i.e., at the propeller and at a suitable point forwardly in the craft. Then the shaft is prestressed downward by an intermediate strut located approximately midway between the two initial points of support. In this manner, two cantilevers are started in the middle region of the shaft, one of which will taper off toward the propeller while the other will taper ofi forwardly. The thrust of the propeller tends to hold the shaft in its convex bend, thus reducing stresses at the point of the intermediate strut. According to the invention bending stresses are reduced in the shaft by about fifty percent. Since fatigue failures are caused by alternating tension forces, the compression force produced by the thrust on the shaft-column does not contribute to fatigue. Ignoring the effect of torque, the result is that fatigue stresses will not affect the life span of a shaft of reasonable length.

Other advantages as well as objects and features of the present invention will be better understood by referring to the following drawings and accompanying specification in which like numerals represent corresponding parts and in which:

FIG. 1 is a simplified side view of a prior art Watercraft depicting propeller shaft disposition;

FIG. 2 is a simplified side view depicting propeller shaft behavior under certain conditions; and

FIG. 3 is a simplified side view of a propeller shaft arrangement according to the present invention.

It is to be understood that the principles of the present invention apply to surface-operating water craft of any configuration, e.g. hydrofoils, planing hulls and displacement hulls.

The prior art hydrofoil configuration shown in FIG. I typically employs a straight inclined propeller shaft 11 carrying a propeller 12 and supported at its forward end either directly at a power plant 13 or by a shaft log 15, at an intermediate point by a strut 17, and at its aft end by a propeller bracket 19. When the craft of FIG. 1 moves in the water at high speed, the shaft 11 is bent under hydrodynamic forces and under the influence of the intermediate strut into an S shaped curve (indicated by broken lines 21). This S curve has a concave portion 23 and a convex portion 25. Thus, the propeller, located at or near the end of the shaft is caused to have a greater angle of incidence to flow (indicated in broken lines) reducing propeller efficiency and increasing vibration and noise. If there were no intermediate strut, the entire shaft would tend to be concave in shape, producing the same disadvantageous propeller orientation. Stresses similar to those described above would be produced in a propeller shaft mounted on other types of hulls, such as planing and displacement hulls.

As shown in FIG. 2, if both the strut 17 and bracket 19 failed or were deliberately removed, at high speeds the shaft would act as a single cantilever 27 and would be lifted by the propeller to the raised position shown. In FIG. 2 most of the shaft curvature is seen to occur in the region 29 of the propeller support which may be located either at the take-off 13 or at the shaft log 15. Thus, even with a flexible shaft in the arrangement of FIG. 2 there could be early failure of the shaft due to extreme stresses inside the hull at the region 29 of large curvature.

According to the present invention, a version of which is shown arranged on the generally indicated watercraft 37 shown in FIG. 3, there is provided a prestressed bent propeller shaft 41 having essentially uniform curvature. The craft 37 may be a planing craft, displacement hull, or a hydrofoil having foils 39, 40 as indicated. This uniform curvature is produced by a depending intermediate strut 43 of sufficient length to produce a doubly cantilevered bend along the central portion of the shaft. Of course, a bearing 45 of any suitable design is mounted at the tip of the strut 43 to carry the shaft. For example, a longitudinally grooved rubber bearing of the type manufactured by the Goodrich Rubber Co. may be used.

The extent of curvature imposed on the shaft depends upon the length and maerial characteristics of the shaft. For purposes of example, and not by way of limitation, it has been found that for a boat approximately 30 feet in length having an overall shaft length of about 12 to 18 feet, the curvature may be formed by displacing the portion of the shaft about 3 to 8 inches below a straight line joining the forward and aft central shaft support points. A shaft of Monel metal of 1% inches diameter may be employed.

It is to be understood that the propeller shaft support arrangement according to the present invention may be employed on watercraft of various sizes and configurations, for example, on multi-hulled craft and on larger ships.

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.

What is claimed is:

1. For a watercraft, propeller shaft support means comprising:

downwardly extending propeller shaft bracket means located at an aft portion of the craft;

power take-off means comprising at least one fiexible propeller shaft having a propeller at its aft end and having at its forward end a first load bearing point located at least substantially forwardly of said bracket and at its aft end a second load point at the bracket; means for supporting said at least one shaft in a 5 prestressed condition in a convex downward curve between said first and second load points, said means comprising a further strut extending from the craft to a position of said shaft intermediate said first and second load points to establish an intermediate load point for said shaft, said further strut having bearing means for carrying said shaft;

whereby the inflow of water to the propeller is essentially parallel to the shaft at the propeller location.

2. The watercraft according to claim 1 wherein the propeller is mounted forwardly of said bracket.

3. For a watercraft having a power plant mounted Within the watercraft substantially forward of the stern, said watercraft further having a rudder bracket located at the stern and a bearing in the bracket for supporting a propeller shaft coupled at its forward end to the power plant, a propeller shaft arrangement comprising:

a flexible propeller shaft inclined downwardly toward the bracket and supported at the bracket and power plant;

strut means depending from said hull to a position of the inclined propeller shaft intermediate the rudder bracket and the power plant;

bearing means mounted on said strut means for carrying said shaft;

said strut means being of such dimension as to displace the shaft downwardly to form a continuous convex downward curve between the bracket and power plant;

4. The propeller shaft arrangement according to claim 3 wherein said strut means comprises a single rigid strut.

5. The propeller shaft arrangement according to claim 40 3 wherein said watercraft comprises a hull.

References Cited by the Examiner UNITED STATES PATENTS FERGUS S. MIDDLETON, Primary Examiner. MILTON BUCHLER, Examiner.

T. MAJOR, Assistant Examiner.

Claims

1. FOR A WATERCRAFT, PROPELLER SHAFT SUPPORT MEANS COMPRISING: DOWNWARDLY EXTENDING PROPELLER SHAFT BRACKET MEANS LOCATED AT AN AFT PORTION OF THE CRAFT; POWER TAKE-OFF MEANS COMPRISING AT LEAST ONE FLEXIBLE PROPELLER SHAFT HAVING A PROPELLER AT ITS AFT END AND HAVING AT ITS FORWARD END A FIRST LOAD BEARING POINT LOCATED AT LEAST SUBSTANTIALLY FORWARDLY OF SAID BRACKET AND AT ITS AFT END A SECOND LOAD POINT AT THE BRACKET; MEANS FOR SUPPORTING SAID AT LEAST ONE SHAFT IN A PRESTRESSED CONDITION IN A CONVEX DOWNWARD CURVE BETWEEN SAID FIRST AND SECOND LOAD POINTS, SAID MEANS COMPRISING A FURTHER STRUT EXTENDING FROM THE CRAFT TO A POSITION OF SAID SHAFT INTERMEDIATE SAID FIRST AND SECOND LOAD POINTS TO ESTABLISH AN INTERMEDIATE LOAD POINT FOR SAID SHAFT, SAID FURTHER STRUT HAVING BEARING MEANS FOR CARRYING SAID SHAFT; WHEREBY THE INFLOW OF WATER TO THE PROPELLER IS ESSENTIALLY PARALLEL TO THE SHAFT AT THE PROPELLER LOCATION.