US2686568A - Screw propeller - Google Patents

Description

Aug.- 17, 1954 G. c. ENGSTRAND SCREW PROPELLER Filed Aug. 24, 1949 Patented Aug. 17, 1954 UNITED STATES PATENT OFFICE SCREW PROPELLER Gunnar C. Engstrand, Staten Island, N. Y. Application August 24, 1949, Serial N 0. 112,053

4 Claims. 1

My invention relates to a screw propeller adapted to work in a fiuid medium, and it is especially adapted for marine transportation. When propellers were first invented, the idea was that they should be made as large as it was possible to have them in order to minimize the backwards slip which was already then accepted as a necessary loss in the propulsion of ships.

Propellers designed with this end in view have, however, been particularly ineiiicient especially when used at high speeds by reason of the setting up of an excessively large friction resistance when they are driven through the water. A screw driven into wood encounters considerable friction, and screw propellers driven through the water also encounter a great deal of friction. Experiments have shown that even at moderate speeds from to '20 per cent of the total power of the engine is consumed in overcoming the friction. For the modern outboard motors, for instance, the working speed is 4,000 R. P. M. and higher, the blade friction becomes the main factor to be considered in the design of the propeller, and it is of the utmost importance to keep such propellers down in size and to proportion the blade sections so that no excess blade area exists in the propeller.

The results of many years of experimenting have evolved a standard. propeller wheel with uniform pitch and blades elliptical in shape set at right angles to the shaft, or slightly raked aft from the perpendicular, according to the individual fancy. In" the propeller herein disclosed, nothing in the design is left to individual fancy, and when the diameter and pitch are decided upon the propeller becomes determinate in substantially every detail. The importance of my invention lies in the fact that every blade element is made of minimum size to accelerate just its part of the water column that forms the propeller race. In my propeller, the elements on the suction or front sides of the blades all have the same pitch as thecorresponding elements on the pressure or rear sides, and the blade width is determined so that half of. the water column is accelerated by the suction side while the other half is accelerated by the pressure or rear face or side of the propeller. The front or forward face is of course presented to theship having the propeller, and the rear face is presented to the opposite direction. The propulsion is, therefore, evenly divided on the two opposite sides of the propeller and the blade width required is reduced to substantially half of that for the conventional propeller, and the friction thereof becomes correspondingly reduced.

2 Further, in my propeller, I prefer to incorporate a variable pitch that is so determined that the backward thrust is equally distributed over substantially the entire bladev area, and each ele-.

ment on both sides or faces thereof will perform its full share of the work'so that a smaller propeller will suflice to do the work of the larger. conventional propeller, which, as a matter of course, has to overcome a greater frictional resistance.

Finally, the leading edge on the suction side and the trailing edge on the pressure side ofa blade conform to the actual path of the blade so that the blade cuts the water and leaves it with the least possible disturbance.

In the drawing:

Figure 1 is a fractional ferred marine propeller.

Figure 2 shows the blade sections of the propeller, as if out by cylinders of successively greater radius, all concentric with the axis of revolution.

Figure 3' is a side view of the propeller with the outlines of the helices described by the blades.

Figure 4 shows a plan of a blade before receiving its pitch.

In the drawing where like reference characters denote corresponding parts, I is my preferred high speed marine propeller, which is composed of the hub 3 with the three blades 4, arranged as shown. The propeller is keyed to the shaft 2 and is preferably provided with a fairing locknut IS. The three blades are provided with the leading edges M and the trailing edges i5, each edge conforming to and being in the line of one of the three generating helices 5, 6 and i, all as shown in the drawing. The longitudinal cross section 8 of the blade shows an increase of thickplan view of my preness at the root thereof to withstand the bending The construction 'of my preferred propeller will' be understood from the following discussion. Fundamentally the propeller is an inclined rotating plane which is pushed straight backwards.

by the ship resistance.

the backward'motion which, therefore," is equal for all the cylindrical'sctions of the propeller.

From the geometry of the screw you note that the closer a blade section is located to the propeller axis,'the greater is its inclination angle The screw propeller is a rigid unit so that any part thereof partakes of aesases with the propeller plane of rotation. As any pressure must act vertically on a surface, it is evident that a rotary component must be larger for an inner section in order to properly balance the constant backward pressure component.

The design steps are as follows:

First translate the ship speed together with the required backward slip into true pitch and strike a circle q throughthe pitch point with the propeller center of mass at C as center of the circle. The radius of the circle equals the efiective or true pitch divided by 2w, as is well known for the developed propeller sections; this circle also lying in a plane containing the axis of ro tation H; in this case the planenof the paper on which the construction is illustrated and at right angles to the plane of rotation. Also the efiective or true pitch is determined in the usual-manner when the required twist is known. Then draw a tangent t to the circle from each point H on the radial center line l8 of the blade. The distance between the point C and the points where the tangents intersect the propeller axis, give the pitch of the corresponding blade section when multiplied by 211'. dure is illustrated. in the'Figure 2 of the drawing. The radius ofthe quarter of a circle q shown on the drawing equals the required pitch divided by 21, and all pitch lines shown in the drawin aswell as the quarter circle are located in the developedplane'of the blade sections, i. e., that of thepaper.

In the case at hand, we have an original rotary speed of the blade element of R/cos A, which stream velocity is reduced by the rotary slip S to R, and the rotary component becomes in which expression we recognize the factor RPSW/Zg as the backward reaction of the section, and the. rotary component together with this component exerts a vertical pressure on the section. It is to be noted that the common term 25.- has been omit't'ediin the above expressions.

The terms used hereand in the drawing are as follows:

Rk-Reduced rotary speed S-Backward slip" S -Rotary slip" P-Eil'ective pitch V'Speedofprogress A--Inclination angle of section W-Unit weight of water g-Accelerationof gravity It now remains to determine the blade width: of the cylindrical blade elements, and the following. considerations are helpful. The wider the blade elementis, the more Water will it accelerate, and also the greater the velocity with which the water sweeps over the blade element, the more water is acceleratedthereby. The total water in the propeller race that a' blade element can handle equalsthe clearance between adjacent'blade. paths timesthe velocity or the passing water. Suction ona propeller, of course, leads to cavitation. and reduction in work performed while pressure on the pressure side may build up- Butin my propeller,.where.the bladesectionsare The design p-rocecost.

dimensioned so that the suction accounts for precisely the half of the fluid passing through the propeller and pressure the remaining half, cavitation will not set in before the suction equals the air pressure, and as the pressure at this very moment reaches two atmosph s, the resultant propeller thrust per square inch of projected blade area at the start of cavitation must aproximate twice the atmospheric pressure or twenty-eight pounds per square inch or more than double the now acepted Barnaby cavitation value of eleven or twelve pounds per square inch. in actual tests, this propeller has developed not less than twenty-five pounds per square inch, without. cavitation. Thus in my propeller where the work is evenly divided on the two sides of the propeller, the active-width of the propeller blade becomes equal to one half the clearance between the spiral paths traced by any point on one blade and the corresponding point on an adjacent blade at the same radial distance from the axis of-r'evolution, andthe blade width (seeFigures'2 and becomes substantially constant for the entire length of the blade, the variation in such width being less than 5 When my propeller works in an elastic medium like air, the weight of air displaced by suction i substantially equal to that displaced by pressure, although the relative volumes are different due to the elasticity of the medium. There-- fore, as the work of the suction and of the pressure is a function of propeller'spe'ed andweight' displaced, the expected worlivalues are substantially the same for both suction and pressure. with the propeller of my U. Si Patent No.

1,767,786, the propeller of the'appl ic'ation is suit--- ed equally well for air as for water.

As compared with the propeller of my early- Patent No. 1,767,796, the propeller of this appli cation, due to the blades having a width of one" half the distance or space between the helical paths that the blades describe, and other differences in design, not only reduces friction; but also for equal speeds of operation, cuts down the slip loss of'th'e' propeller; and gives an increase,

out of all proportion to'the lessened friction, and amounting to about 40 per cent, in the power'or thrust of the propeller. Further, the narrower blades save material and lower both weight and The reduction in friction with narrower blades could be foreseen, but thesaving in slip and increase in power constitute a wholly un-' expected result. By' all rules of propeller design heretofore accepted, reduction in the width of the blades is regarded as producing less power, and increase in such width has been relied upon to step up the power.

Also by the propeller described herein, an in crease in speed of revolution in water; withthe" same engine power is achieved. This increase amounts to nearly 15%, mainly due to therela= tive small width of the blades. Friction loss is never more than 20%, and in an ordinary propeller' a reduction in width of blade, decreasing the friction by half, would not give an increase in speed of more than 3 5% under the same con-- ditions.

It is to be noted that the pitching'of the blade for the innermost part of the: propeller is made" constant from the point-Where the inclination angle is 60. Of course, in some propellers a" larger hub may be used so that the entire bl'ad'e area becomes pitched as described.

In the shoumpropellenthe entering. and trailing' edges of the blades. are shown pitched to correspond with the helical paths of the blades, so that the blades will cut through the water with the least disturbance.

It will be clear from the above that, as stated in the beginning of this specification, the various features of the invention are applicable to propellers for aircraft and for fluid circulation in general, and such propellers come within the scopes of the appended claims.

In any propeller there are certain necessary losses that cannot be avoided. In the propeller of this application these inherent losses are kept at a minimum. It is by making the blade width half of the clearance between blade paths, thus dividing the work equally on the suction and the pressure sides of the propeller blades, cutting the friction in half, and giving the blades equal pitch on both sides, that a much superior design of propeller is attained.

I claim:

1. A propeller having blades all of substantially equal width and substantially the same length along a radius intersecting the axis of revolution, the pitch of said blades varying continuously and in the same degree from end to end, and bein substantially the same on each blade at all points radially equidistant from said axis, and the helical paths described by substantially the blades being separated by equal distances between all points on adjacent blades, the width of the blades being substantially constant and substantially equal to half said distance, the pitch on both faces of each blade being always the same at any point in the radial length of said blades along a transverse line curved concentric to said axis, so that said lines on opposite faces of said blades at said points are parallel to each other.

2. The propeller according to claim 1 with each blade having bevelled portions on its leading and trailing edges, said bevelled portions lying in said helical paths.

3. A propeller having blades all of substantially equal Width and substantially the same length along a radius extending from the axis of revolution of the propeller, the pitch of each blade being the same and increasin continuously and to the same degree from the outer to the inner ends of said blades, the pitch of each blade at any point of its length being the same as the pitch of all the other blades at the corresponding point, and the helical paths of the blades being separated by substantially equal distances at all points on said blades, the width of each blade being substantially constant and substantially equal to half said distance, and the pitch on both faces of each blade at the same distances from said axis being equal.

4. A propeller having blades all of substantially equal width and substantially the same length along a radius extending from the axis of revolution of the propeller, the pitch of each blade being the same and increasing continuously and to the same degree from the outer to the inner end of said blades, the pitch of each blade at any point of its length being the same as the pitch of all the other blades at the corresponding point, and the helical paths of the blades being separated by substantially equal distances at all points on said blades, the width of each blade being substantially constant and substan-- tially equal to half said distance, and the pitch on both faces of each blade at the same distances from said axis being equal, each blade having bevelled portions on its leading and trailing edges, said bevelled portions substantially conforming to said helical paths.

Number Name Date Engstrand June 24, 1930

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