US978677A - Screw-propeller. - Google Patents

Description

D. W. TAYLOR.

SCREW PROPELLBB.

APPLICATION FILED BEPT.10,1909.

Patented Dec. 13,1910.

3 SHEETS-$112111 1.

D. W- TAYLOR.

SQREW PROPBLLER.

'APPLI'OATION FILED 8112110, 1909.

s sums-sum 2.

D. W. TAYLOR.

SCREW PROPELLER.

APPLICATION FILED BBPT.10,1909.

Patented Dec. 13, 1910.

3 BHEBTB-BHEET 3.

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' plish the propellers and rated edge reinforced OFFICE 1mm warsorr TAYLOR, or THE 1mm sums NAVY.

SCREW-PBOPELLER.

Specification of Letters Patent.

Patented Dec. 13, 1910.

Application ma September 10, 1909. Serial No. 517,119.

To all whom it may concern."

Be it known that I, DAVID WATSON TAY- LOR, a citizen of the United States of America, of the U. S. Navy, have invented certain new and useful Improvements in Screw-Pro pellers; and I do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to make and use the same, reference being'had to the accompanying drawings, and to letters or figures of reference marked thereon, which form a part of this specification;

My invention relates to screw propellers, and has for its object a structure of the blade for these propellers to avoidthe harmful effects due to cavitation. This I accomby a novel structure of the blades of particularly of what is known as the backs of the propeller blades,

the specific structure herein shown and described being at the leading portions of the backs.

Referring to the drawings, in which like parts are similarly designated, Figures 1- to 4 are' sectional diagrams illustrating, cavitation, and are herein shown to more clearly illustrate the reason for the new structure of blade embodied in my invention. Figs. 5 and 6 are developed sections illustrating well constructed blades of accepted type now in use, with the cavitation areas in relation thereto. Fig. 7 is a viewsimilar to Fi 6, of a blade embodying my invention. Fig. 8 is a similar view of a modification. Fig. 9 is a section corresponding to Fig. 8, showing strengthening ribs on the back for the thin leading edge. Fig. 10 is a section on line 1010 of Fig. 9. Fig.11 is a developed outline of an ogival blade showing the back thereof and a developed section of a sery ribs.

A full understanding of the invention will be facilitated by a brief description of cavitation and its causes, reference being bad to Figs.'16 of the drawings.

In Fi 1 is shown a small circular cylinder 2 w ich is-supposed to be immersed in water that is flowing past it from left to but will part at the and come together again behind it at some right, the lines indicating lines of flow. If the velocity of the water is great enough it will not flow smoothly around'this cylinder, front of the cylinder leaving behind the cylinder a substantially vacuous space, as shown at b. If, instead of the cylinder 2 being at rest and the water flowing past it we move the cylinder with suflicient spee through water at rest, we shall have the same cavitation.

In Fig. 2 there is attached a portion 3 of wholly within that this blade 3 would have action, and that if an actual ler blade were made u front, and a blade of 51 section, it would have no propulsive action if it were rotated at such a speed that the main body or ogival section was always in point, as a,

no propulsive screw propel of a cylinder in e ordinary ogival water of the c lindrical portion constituting the-leading e e. Propellers are not made with their lea ing edges sections of cylinders, as shown in Fig. 2, but they are frequently made with their leading edges 4 rounded, as illustrated by the developed blade section 4, shown in Fig. 3. A screwpropeller blade having a roimded leading edge, such as shown in Fig. 3 will, if driven through the water at speed, inevitably produce cavities not only at the back of the blade as indicated at 0, but also in front of it, as indicated at d, over the driving face of the blade. These cavities or artial vacua over the face of the blade, w ere there should be the maximum pressures, are fatal to efliciency. When considering these matters we should remember that cavitation troubles appear when the actual velocities of ro eller blades through the water are very ig 10,000 feet a mlnute and more, or over a hundred miles an hour. Under these conditions a very small round at the leading edge of a propeller blade will produce large cavities. It is an improvement in ro eller blade construction to make the la es-with leading edges that are not rounded as shown in-Fig. 3, but sharp, but when the s eed ofthe pro eller through the water ecomesexcee 'ngly great, this construction fails with blades which I shall now explain.

In Fig. 4 I have shown a developed section of an ideal propeller blade, 56, of no thickness advancm through the water in a direction e-f.

to the cylinder p a propeller blade which lies the cavity 6. It is evident.

having sections hitherto used, for reasons are the angle 5, e, f is the cavity formed by the motion through the what is known as the slip angle. At some point 5 on the face of the blade the water in front of the face parts, a portion flowing around the leading edge at 5 to the back of the blade, and the rest fiowing along the face of the blade toward 6. No matter how small the slip angle, and in practice it is a very small angle, the point y will not reach the leading edge 5, as long as the slip angle has a value, and there will be no defect of pressure on the face of the blade. So while the ideal blade 5-6 of no thickness would show no ca 'tation over the face of the blade, it will show it over the back of the blade.

Now let us consider what happens to the actual blade: In Fig. 5 I have shown an actual blade section 7 advancing with a slip angle of 3, which is a reasonable slip angle value, especially for screw-propellers of fine pitch; but the angle between the face and back of the blade at the leading edge 5 is'25. Hence, while the water strikes the face of the blade at an angle of 3, itstrikes the leading portion of the back of the blade at 22. The back' overpowers the face, by reason of the water parting at somepoint on the leading portion of the back, as 1 and flowing around the leading edge, producing cavitation over the face of the blade, as in dicated at i. This cavitation on the face of the blade is fatal to efiiciency. Itis true that the water flowing around the curved back of the blade will,if the speed be great enough, fail to follow it, and cavities, as indicated at 7:, will be formed at the back of the blade, but these cavities contribute thrust unless they fill with air drawn from the air occluded in the water. If. they were complete vacuathere would be very little objection to them, and even if they are only partial vacua they do not reduce efliciency nearly as much as partial vacua on the face. It is not a difficult matter to avoid aconstruction that causes a flow from back to front, as in Fig. 5. j

' In Fig. 6 the face of the blade is rounded on its forward portion and the back is made tangent to the line of advance. In this case the water parts at some point, as Z,- on the face, and flows from face to back, as in the ideal blade of no thickness. There is no cavitation at the leading portion of the 7 blade face; but, as the blade face is rounded,

if the speed is high enough, the water is from this rounded portion, resulting in cavitation over the face of the propeller blade some distance back of the leading edge, as indicated at m, Fig. 6.

It is evident from my discussion as above stated, that if the angle of the blade, between surfaces of the face and back of the blade, at the leading edge is less than the slip angle, face cavitation can be wholly back adjacent to the leadin avoided. \Vith the usual ogival sections this is not possible with actual blade thicknesses, as slip an les are too small to give the thickness of b ade required for strength. Vith o ival sections the wider theblade, the sum ler the blade angle at the leading edge, and this is the reason for the recognized value of wide thin blades of the usual ogival section for preventing cavitation. Such blades, however, are objectionable from many points of view, and if forced to speeds sufficiently great will cavitate, and it is desirable to construct a blade that will absolutely prevent cavitation on the face, without the use of blades that are excessively wide or excessively thin. This I1 accomplish by means of blades in whose developed section the face is straight or nearly straight and whose back is hollow along the leading portion thereof adjacent the leading edge. The developed section of such a blade made in accordance with my invention is shown in Figs. 7 to 11.

The blade angle at the leading edge forms,

preferably, a sharp knife edge, as at 7 Fig. 7, and this angle is preferably not made greater than the slip angle, though itis hardly practicable to make 1 it less than three degrees. By makingthe propeller blade hollow, i; e.,with reducing or decreasing pitch at the leading portion of the back of the blade, as at 10, and 10, Figs. 7 and 8, this sharp edge can be employed without r'educing the strength cable limits.

With my improved structure of propeller blade as shown in the developed section thereof, Fig. (I, the water will part at some point on the face, as indicated at n, and flow around to the back of the blade by way of the leading edge 7, as in the ideal blade Fig. 4, and there will be substantially no cavitation on the face of the blade. There will be cavitation at the back of the blade, but if the cavities are perfect vacuathey will be helpful rather than harmful on 'the back, and if they are not high vacua the air can be exhausted from them as described in my Patent No. 900,797.

With a propeller blade having a hollow edge a straight or nearly straight face an an edge angle not greater than the slip angle, as shown in the developed section, Fig. 7, the cavitation over the face of the blade (a phenomenon which though not hitherto recognized, have found to exist, and to be cause of the reduction of efficiency accompanying cavitation) cannot occur any more than in the case of the ideal blade, ofno thickness. v I

While my novel type of hollow backed blade is particularly valuable for preventing cavitation over the blade face it is a structure of propeller blade of superior value for propellers operating ,under conditions of the blade below practi- I the chief ing speeds the usual conditions not to be feared. This, with such a construction practice over the whole which assists the thrust face action; while with ogival section of blade, the same t at produce cavitation when the propeller is her driven will below cavitatproduce abackward thrust over portion vof the face of the blade,

where cavitation is for the reason that of blade there is in blade-back suction that'is due to the the leading which is very detrimental to efliciency.

It is not necessary that the hollow back should be one continuous curved surface, and in Fig. 8, I have of a relatively thick propeller bla e11,whose face is extended by a thin sharp blade 12 secured to the face of the propeller blade forming a dis-continuous the leading edge and the customary curve continuous curved surface low bac at the leading portion thereof,-said plate 12 curve between its back and the back of the propeller blade 11 proper, to form the hollow lO of the. hollow back. To secure reater strength when it is necessary to ma e the leadin portion of the in, especially in are formed in a at these leading secure to or form integral with ribs 13, Figs. 9

r0 eller blade extremely t lila es whose hollow backs portlons, I the backs of such blades, and 10.

Fig. 9 is a section identical with 7 showing the rib 13 formed thereon. he

ribs are preferably placed short distances apart as shown in Fig. 11 and these ribs are referably perpendicular to the back of the blade and at their roots they-merge in beveled or curved surfaces 14 into the surface of the back. The leading ends of these ribs are inclined to the back as indicated at 15,-

1 11 and are beveled or sharpened. The following ortions of these ribs merge into the back of the blade.

In Fi 11 I have illustrated such a holed blade, relatively thin, having ribs 13 sharpened at their leading ends 15 and the leading edge of the blade is serrated there being a rib extending to the point of each tooth 16 of the serrated leading The ribs of Fig. 11 may be omitted and the serrations will, enable the use of slightly blunter blades but when a rib 13 is run on the back of the propeller blade to the point of each tooth 16 there results a construction of superior stren th.

The essential eature of my invention is the hollow or concave back extending from referably merged into (i or convex back, serrated or not serrated, at the leading edge and sup orted -or not supported by ribs. This hol owback may be combined with a face of any usual type but for the best results to avoid cavitatlon at the most extreme speeds, I prefer that the face he a true helicoid of uniform pitch, showing as a straight shown a develo ed section ed e.-

face and a line in the developed sections Figs. 2, 3, 4,

5, 7, 8, 9 and 11' or it may be slightly rounded toward the leading edge. I claim 1'. A rigid propeller blade having in its developed "section a substantially straight face and a curved back havin a hollow formed therein adjacent a lea ing sharp edge.

2. A rigid propeller blade having in its developed section a substantially straight curved back of decreasing pitch adjacent the leading edge.

3. A- screw propeller blade having the angle between theface and back of the blade'at each point of the leading edge substantially equal to the angle of sli and the back of the blade adjacent the lea ing edge of decreasin pitch.

rigi propeller blade having in its developed section a substantially straight face extendin to a sharpened leading edge, and'a curved ack meeting the bevel on the back of the blade that forms said sharp leading edge.

5. A propeller blade having in its developed section a substantially straight face extendin to a" sharp leading edge,'and a back formed by a compound curve concave to the face in the immediate vicinity of the leading edge, and convex over the major portion of the back.

6. A screw propeller blade having its back concave and a continuous curved surface, adjacent the leading edge, the leading ed e serrated and ribs on the back of the bla e extending from the serrations to the body of the blade and concentric with the center of rotation of the blade.

7. A screw' pro angle between the ace and back of the blade at each pointof the leadin edge substantially equal to the angle of s ip, the back of the blade adjacent the leading edge of decreasing pitch and the leadlng edge serrated. Q

8. A screw propeller blade having a hollow formed in the back of theblade along and adjacent the leading edge, the leading edge serrated and ribs extending from the serrations to the body of the blade.

' 9. A- propeller blade having in its developed section a substantially straight face extendin to a sharp leading edge, and a back formed by a com ound curve concave to the face in the imme iate vicinity of the leading edge and convex over the major ortion o the back, and ribs, concentric wit 1 the axis of rotation of the blade only onthe leading portion of the back of the blade.

10. A screw propeller blade having the angle between the face and back of the blade at each point of the leadin edge substantially e ual to the angle of s ip, the back of the bla e adjacent the leading edge of decreasing pitch, and strengtheningribs on the hack of the blade. 7

11. A screw propeller blade having the angle between the face and back oft-he blade at each point of the leading edge substantially equal to the angle of slip, the back of the blade adjacent the leading edge of decreasin pitch and strengthening ribs on the back of the blade extending from the leading edge toward the center of the blade and merging into its back. 7 V

12. A screw propeller blade having the angle between the face and back of the blade at each pointof the leading edge substantially equal to the angle of slip, the back of the blade adjacent the leading edge of decreasing pitch and strengthening ribs on the back of the blade extending from the leading edge toward the center of the blade and merging into its back, said ribs beveled or sharpened at their leading ends.

13. A screw propeller blade having a hollow back adjacent the leading edge, the

leading edge serrated and ribs on the back of the blade extending from the point of each serration to and merging into the back.

14. A pro eller blade having a plate secured to am projecting beyond the body of the blade to form a thin leading edge, the angle bet-ween the front -and back surfaces of said plate being substantially equal to the angle of slip.

15. A rigid propeller blade having the back of the blade adjacent the leading edge of decreasing pitch and convex to the face of the blade over the major portion of the back and strengthening ribs extending from the leading edge on the leading portion of the back of the blade.

In testimony that I claim as my invention, I have signed my name m presence of two subscribin witnesses.

v DAVID WATSdN TAYLOR. \Vitnesses v 'C. W. FOWLER,

HENRY On'rH, Jr.

the foregoing

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