US1494760A - Screw propeller - Google Patents

Description

May 20., 1924.

D, W. TAYLOR 'scREw PROPELLER Filed Aug. '7

Patented May 20, 1924. f. mi'rso STAT s 1. igeifiegiae PA ENT QFFIQE;

DAVID w. TAYLOR, or wasnme'ron, DISTRICTQF COLUMBIA;

SCREW PROPELLEB.

Application filed August 7, 1922. Serial No. 580,318.-

To all whom. it may concern:

Be it known that I, DAVID W. TAYLOR, a citizen of they United States, and residing at lVashington, District of Colum bia, have invented certain new and useful Improvements in ScrewPropellers, of which the following is a specification.

My invention relates to screwpropellers and is intended andadapted, to secure greater efiiciency and propelling power for blades of a given width than has been-p051.

sible with propellers.heretofore used. It tends toprevent cavitationv on the face of the blade near-the, leading edge and re.- duccs it to a minimum. The-preferred-embodiment ofthe invention shown for pur-' poses of illustration includes a-blade in which the leading portion of the back. is helicoidal with the following portion rounded and in which the face and backv at the leadingedge make thesma'llest practi-.

cable angle with each other and in whichthe surface of the leadingportion of the face of the blade-from its leading edge isslightly concave in section and-0f increasing pitch for a certaindistance and is followed by a portion which is rounded or cons vex in section. The convexportion. may

terminate inadvance of thefollowing edge and be followed by a portion straight indeveloped section. The .novel and useful features of the invention'will be morefully understood from the following description and claims taken with theaccompanying draw- 1ngs.,-

In the drawings:

Fig. 1 is a section of a known form of propeller-blade on the lines of a cylinder concentric with the shaft axis showing it as unrolled or expanded into. one plane;

Fig. 2 1s a similar sectionshow'lngdiagrammatically a smallcavity upon the face of the blade at thev leading edge and a small:

cavity on the back of the blade at thefollowing edge .such as would be produced by a certain speed;

Fig. 3 is a similar sectiondi'agrammatically showing large cavities on the face and back of the blade such. as would be produced by extremely high blade velocity;

Fig. 4 is a similar section of a blade embodying one form of my present'invention showing in dotted lines the outlineof the usual ogival type of blade with the-same.

final pitch and Fig. 5 is a view similar to thatof-F-ig.

4'. showing a blade embodying my invention in a somewhat'difi'erent form. 1

Inord-er that thefunctions and advantages of the present invention may beeasily and accurately understood, the effects. and results produced by the commonly'used formofflpropeller blades will be first referred to andfiexplained. y

In a screw propeller working in open water, the forward thrust developed inaction-,is-approximately proportional to and measured by the sternward momentum impressed upon the wateracted upon by the screW.-- To obtain a given thrust ,we may impress a high-sternward.velocity upona small quantity of ,"water, or a low sternward velocity upon a correspondingly large quantity of water. efliciency is greater since there is relatively less energy. wasted in :gi-vingsternward ve-, locity to the water acted upon. If. there wereno changes. ofpressure in the water acted upon by the .propeller,' the forward producing sternward; momentum; in the water it actsupon,

Fig. 1 is a section of a knownform of propeller blade-on the lines of a cylinder concentric with the shaftaxis unrolled or expanded into one plane upon which we are;

supposed to lookdown. AB is the line of the shaft cr -the fore and aftline.

which develops .intothe. straight line FEL'.

Thepitch of the helicoidal face of this par- In the latter case the- FEL is a, section of the helicoidal driving face ticularpropeller. is 14: 39 -7", and the diameter 15 11".'

The section shown the radius in the vicinity where a large'proportion oftheworkof the propeller-is done. The line LC shows theinstantaneous direction of advance of the developed section for a slipof25 percent of the actual propeller. Theline, FDL is the sectionof the back of thebladewhich is of the ogival type. FDL isfor -itslinajor portion an'arc of a circle of large radius, but close tothe ends, as indicated, arcs of small radius are struck to eon-j nect L the leading edge, and F the. followin edge, with the main portions jofthe back;

Consider now what happens tothe water isita-ken at two-thirds in the immediate vicinity of the blade as it passes, and assume first that the water originally at rest remains at rest until influenced by the blade section, andthat what is known as cavitation is not present. The water which passes near the driving face LEF is diverted athwartships and astern, produclng excess pressure on the face of the blade. This pressure is a maximum near the leading edge, and falls off near the fol lowing edge as the excess pressure gives velocity to the water-{so that the water would leave the face with little final increase of pressure. The change of velocity is always astern.

Consider now the action upon the water of the back F DL. This forces its ways into the water at a much greater angle than the face, so that the actual parting of the water is'not at the leading edge L, but at a point on the back behind L, though quite close to it, and at the actual leading edge there is a flow of water from front to back; The extreme leading portion of the blade back is evidently inclining forward rather than backward. Hence we have here sternward pressure or negative thrust onthe blade and forward momentum given the water. This of course is highly conducive of inefficiency; But as the water passes further along the blade back, the pressure changes to suction, a corresponding sternward velocity is developed and the water leaves the back at the following edge F with a greater sternward velocity than the water leaving the face.

All of the above is, as stated, upon the assumption that what is known as cavitation is not present, and also that the water was at rest before being acted upon by the blade. Now in the case of an actual rotating propeller as contrasted with an ideal section advancing linearly, the water reached by a blade has been already influenced more or less by preceding blades, resulting mainly in a certain sternward velocity. The result is to render less abruptthe action of the leading portion of the face of the blade since the water already has a certain amount of the sternward velocity which the blade face is striving to impart. The action is substantially the same as if the edge L instead of advancing along the line LC, were advancing along a line corresponding to less slip, such as L0 When we consider the action of the back of the blade, it-is obvious that'the initial sternward velocity of the water due to action of preceding blades will intensify the negative thrust of the leading portionof the back of the blade and cause negative thrust over a larger portion of the back. Nevertheless, if there is time for the suction of the following portion of the back of the blade to fully influence the water, it will leave the blade much as before, so that notwithstanding the eccentricities and perturbations of the action of the leading portion of the blade, the thrust is in the end little affected. The efficiency, however, is prejudicially affected since it calls for the application of greater force.

Suppose, however, the velocity of the blade through the water is steadily increased. We

have seen that for such a blade as in Fig. 1

tends to force it back into contact with the blade face. As blade velocity increases, a time will ultimately come when the pressure of the surrounding water is not adequate to keep the water flowing around L from back;

to front in contact with the propeller face, and a cavity is formed over the leading portion of the face of the blade. If the cavity is small, the major portion of the face FEL acts upon the water much as before and the thrust is not much affected. But as velocity increases the cavity necessarily gets larger, and in the end the face of the propeller has little or no action upon the water.

Fig. 2 small cavity 10 upon the propellerface at the leading edge indicated in section. This cavity is of course prejudicial to efficiency, but would not radically reduce thrust and efficiency. Fig. 3 shows similarly a large cavity 11 corresponding to higher blade velocity. The cavity onthe face such as shown in Fig. 3 is highly prejudicial to thrust and efficiency. I may remark here that it is generally believed, and for a long time I myself believed that these cavities contained air. I have found, however, by experiment that they are vacuous to a high degree.

Let us consider now what happens to the back of a propeller blade such as shown-by section in Fig. 1 as its velocity through theshows in a diagrammatic way a is reached and it can no longer keep the water in contact with the following portions of the blade back. j Then a cavity appears here.

Fig. 2 shows in ideal form a small cavity 12 on the blade back at the following edge and Fig. 3 shows alarge one 13.

Much experience has established that cavitation is very detrimental to the efficiency of propellers. The only palliative measurefound ofvalue so far is t make the propellerblades' very wide and correspondingly thin. This makes the angle of entrance of the leading edge small and reduces the disturbances which coarse, blunt blades, such as shown in Fig. 1, produce.

' ficient and more expensive tomanufacture.

Wide "blades also give more time for the Water pressures to'bring the water back-to contact with the front and rear faces "and hencenullify theefi'ects of cavitationn'ear the leading edge. Disregarding cavitation, however, a wide" blade is essentially less'ef than a blade of normalwidth. lVhile an extra wide blade which'does not cavitate is more efficient than a bla'depfrnormal width that does cavitate, it is inferior toa blade of normal width that does not cavitate.

My invention consists in a novel form of blade section of normal width which is more efficient than the extra wide blades of the commonly used forms and is less liable to cavitation particularly on the face of the blade where it is most harmful. Figures 4 and 5 show in full lines two blade section's embodying my invention and a corresponding section of the usual ogival type having the same final pitch being shown in outline in dotted lines.

To avoid cavitation on the face of the blade, it is important that the leading portion of the face shall be concave as shown at 14. and 15 in Figures 4 and 5 and thatthe leading portion of the back of a blade shall make a smaller angle with the line of advance than the leading portion of the face of the blade. For structural reasons it is necessary that this concave portion shall be followed by a convex portion, but this must be relatively long and merge gently into the hollow or concave portion and the following portion of uniform pitch, if any. The leading portion 16 of the back of the blade is helicoidal, i. e.. straight in developed section at the extreme leading edge, and curves gently into the usual round back 17 Its pitch is such that its inclination to the line of advance is as small as possible and preferably not over five degrees and, as above stated, its inclination tn the line of advance should not be greater than thatof the leading portion of the face of the blade.

The leading portion of the face of the blade makes with the back the smallest angle practicable with the materials available for construction, and the leading edge is sharp or rounded on the minimum radius practicable with the materials available for construction. Passing back on the face of the blade from the leading edge there is first a portion which is hollow or-concave in section, i. e., of increasing pitch followed by a portion which is rounded or convex in section, i. e., of decreasing pitch, and this convex portion may be followed by a portion straight in developed section extending to the following edge. In Fig. i the following convex or rounded portion 18 in developed section on the face of the blade ex tends to the following edge. In Fig. 5 the convex portion 19 does not extend to the oped-section and which extends to the following edge.

Having thus described my invention, what I clalmasnew and desire to secure by Letters Patent is:

1. A propeller blade in which the leading portion of the'back is straight in developed section and the leading portion of the'face is concave in developed section.

' 2. 'A propeller blade inwhich the leading portion of theback is straight in developed section and the leading portion of the face is concave in developed section, the angle between the faces at the leading edge being as small as practicable with the material used forming a sharp edge.

3. A propeller blade in which the leading portion'of the back is helicoidal having such pitch that its inclination to the line of advance is not materially over five degrees and in which the leading portion of the face is concave in developed section.

4. A propeller blade in which the leading portion of the back is straight in developed section and theleading portion of the face is concave in developed section and. of increasing pitch followed by a portion convex in developed section of decreasing pitch.

5. A propeller blade in which the leading portion of the face is'concave in developed section to the edge and at the smallest practicable with the material used angle to the leading portion of the back.

6. A propeller blade in which the leading portion of the face is concave in developed sectionto the edge and at the smallest practicable with the material used angle to the leading portion of the back, the concave portion of the face being followed by a convex portion of decreasing pitch.

7 A propeller blade in which the leading portion of the back is straight in developed section gradually curving into rounded form which extends to the following edge, its pitch at the leading edge being such that its inclination to the line of advanceis not materially greater than five. degrees, and in which the leading portion of the face is concavtil in developed section and o-f increasing pitc 8. A propeller blade in which the leading portion of the back is straight in developed section gradually curving into rounded form which extends to the following edge, its pit-ch at the leading edge being such that its angle of inclination to the lineof advance is not materially greater than five degrees, and in which the leading portion of the face is concave in section and of increasing pitch followed by and merging with a portion convex in section and of decreasing pitch, the angle between the face and back at the leadleading portion of the back, the concave portion of the face being followed by a convex 15 portion of decreasing pitch, and the convex portion by a portion straight in developed section extending to the following edge. a r 1 .11. A propeller blade in which he leading portion of the back makes a smaller angle 20 With the line of advance than the leading portion of the face of the blade.

In testimony whereof I hereunto afiix my signature. I

DAVID TAYLOR.

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