US900797A - Screw-propeller. - Google Patents

Description

D. W. TAYLOR.

SCREW PROPELLER. APPLIOATION FILED NOV. 11, 1907.

4 SHEETS-SHEET 1.

Patented Oct. 13, 1908.

D. W. TAYLOR. SCREW PROPELLER. APPLIOATION FILED NOV. 11, 1907.

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Patented Oct. 13, 1908.

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SCREW PROPELLER.

APPLIOATION FILED NOV. 11,1907.

900,797. Patented Oct. 13,1908.

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DAVID W. TAYLOR, OF WASHINGTON, DISTRICT OF COLUMBIA.

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specificafionot Letters Patent.

Patented Oct. 13, 1908.

Application filed November 11, 1907. Serial No. 401,666.

tains 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. a

My invention relates to the method and means for preventing cavitation at propeller blades, and comprises essentially, the ex-' hausting of air from behind the blade at that part of the blade where cavitation occurs.

Referring to the drawings, in which like parts are similarly designated-Figure 1 is the customary developed cylindrical section through the ogival-shaped blade, the shaded portion showing approximately the area of cavitation. Fig. 2 is a section through a propeller shaft and hub on line 2-2 of Fig. 3, showing the developed outline of the blade in relation thereto, with the developed central cylindrical section as is customary, and showing my invention applied thereto. Fig. 3 is a section taken on the line 33 of Fig. 2. Fig. 4 is a view similar to' Fig. 2, showing a modifiedform of my invention. Fig. 5 is a section .on the line 5-5 of Fig. 4. Fig, 6 is a view similar to Fig. 4 showing a modification of the automatic pumppassage.

In the operation of a propeller, as its speed and thrust increases, a point is reached at which the phenomenon knownas cavitation appears. At low s'peeds and thrusts the propeller is surrounded, front and rear, by practically solid water. The driving face thrusts the water astern and the rear face sucks it astern. But, though thereis prob ably more or less eddying and disturbed motion in the water adjacent to the rear face,

' that is to say, the point at which the'suction of the back of the blade is so great that the water is unable .to follow the blade and breaks away from it, thus producinga cavity behind the blade somewhat as indicated by the shaded area Fig. 1. If this cavity were a vacuum, cavitation would not be nearly so objectionable. It would mean simply that the suction of the back of the blade would reach a maximum equal to the atmospheric pressure plus the water pressure due to the submergence of the blade. As a matter of fact, however, cavitation sets in at thrusts far below those which would be shown with ,vacua behind the blades, and the cavities occurring are not vacua, but filled with water vapor, whose pressure is of no importance in ractical cases, and with air which is drawn gem the water. The air is the cause of the trouble. Owing to its pressure in the cavity, there is little or no suction from the back of the blade, water is not drawn into the propeller disk, and practically the whole thrust is obtained from the face of the blade with excessive slip and low efficiency.

There have been at least two methods used to avoid cavitation with more or less success. The most common is the use of excessively broad blades with great surface. These, owing to their breadth, offer less suction at the back fora given slip and speed, owing to their finer entrance and more gradual acceleration and retardation of the water moving past them. Also for a given thrust and speed, the slip is less than fora -blade of ordinary width. The excessively broad blades, however, ofl'er excessive frictional resistance, and thus waste much power.

The second general method of avoiding cavitation is by the use of blades with sections specially shaped, so that the acceleration and retardation of the water are more gentle than with the ordinary types of sections, and hence the blades can be worked to higher thrusts before cavitation sets in,

than blades of the usual ogival section, while i are in communication with the cavities when ls, recesses or passages which open orcavitation setsup. Through these channels or passages I pump or otherwise exhaust the air as fast as it enters the cavities thus maintaining any desired degree of vacuum at the backs of the blades.

Referring more particularly to Figs. 2 and 3, the propeller shaft 5 is provided with the usual tapered end 6 on which the propeller hub 7 is securedsin any of the usual and well known manners. Now in order to providemeans for exhausting the air from the back of the blades, I format the root of each blade in the hub, a recess 8 which communicates by means of a passage 9 preferably but not necessarily, radial to the shaft with a central bore 10 in the shaft, which central bore 10 communicates with any suitable exhausting appliance and to this end may be connected with the ships condenser and thus with the main air pump or to a special pump or a jet of steam may be used for exhaustmg the air or any other well known method or apparatus for causing suction may be used." OED the-back. of each blade 11 and preferably but not necessarily, on the following half of the blade, I form a. cavity or recess 12 that may communicate directly or indirectly with one of the recesses 8 and in this particular construction I have shown the recess 12 communicating with a shorter recess 13 which in turn communicates through the root of the blade by a passage 14 with the chamber 8. I havethus showntwo connected recesses but it is obvious that I may vary the shape and outline thereof and to form a single recess as the requirements of construction of a particular shape of blade may permit. The larger recess 12 is covered by a plate 15 provided with perforations 16 and a plate 18 may or may not be perforated. The plates are countersunk in the blade 11, are-held in lace by screws 19 or any other suitable f stening devices and are shaped to give a fair contour to the back of the blade. The blade is rotated in the direction of the arrow shown at the top; of the figure and during the rotation the cavitation, which is greater at the following side of the blade, is reducedby drawin the air from the cavity behind the blade t rough the perforations 16 into the recess 15 in the blade 6, then through 13,

the passage, 14, recess 8 and hole 9 to the central bore 10 in the shaft to the exhausting device. In-practice when the propeller is at rest the passages will fill with water and it will be desirable to provide a valve in the vicinity of the pumpv or other exhausting device for the purpose of shutting out the water. I have mentioned in connection with these two figures, special means for exhausting 5 the air produced by cavitation from behind the back of the blade towards its center and through its shaft, but Figs. 4 and 5, illustrate a modification in whlch I show recesses some? in the form of slots 20 connected together preferably at one of their ends, here shown as near the root of the blade at 21 and provided with a perforated countersunk cover plate 22 screwed or otherwise secured in place.

In place of radial channels 20 I may use a single cavity in the body of the blade. Adjacent the cavity or cavities 20, I provide a pump passage 23 passing substantially from end to end of the blade covered by an im erforate plate 24. This assage is prefera ly straight and radial and 15 open at both the root and the tip of the blade, the entrance being shown at 25 and the exit at the tip of the blade at 26. The chamber or'- the connected chambers or recesses 20 communicate with these passages 23 by means of one or more short passages 27 preferably inclined toward the tip .of the blade.

In orderto provide suflicient thickness at the tipof the blade for accommodating the cover plate 23, it becomes sometimes necessary to slightly increase the thickness of the blade at this point as shown by the shading at 28 in Fig. 4, however, this is not always necessary.

The operation of this structure is as follows: When the blade is in rapid rotation water will enter the pump passage 23 at 25 near the root of the blade and by reason of the centrifugal force set up will cause the water to be thrown from the channel 24 at the tip of the blade 26 by centrifugal force. The water thus moving through the channel 24' by centrifugal force draws or entrains with it the air through 27 and the recess or chambers 20 which in turn communicate through their perforations 16 with the cavity in the water behind the blade and thereby exhaust the air from said cavity by the centrifugal force set up by the movement of the propeller itself. Such a construction is automatic in its action and does not require special exhausting devices.

In certain cases 1t will be necessary to construct the channel or pump passage 23 as shown in Fig. 6, where this passage is shown as constricted or reduced at a point between the inlet 25' and the outlet 26 and at the point 30 where the assage 27 enters the pump passage 23. Sue a structure will reduce the pressure in 22, or in other words increase the suction of air.

I claim:

1. The method of preventingcavitation at screw propellers which comprises exhaustically and centrifugally discharging the air,

from the blades by their rotation 6. The method of, preventing cavitation at' screw propellers which comprises admitting the air from the cavity formed at the back of the blades into the body of the blades and exhausting it therefrom by an entraining current of watermoved by centrifugal force through the blade.

7. A propeller blade having a recess formed therein, a perforated cover for said recess and means for exhausting the recess, whereby the cavity formed in the water at the blade may be exhausted.

8. A propeller blade having a recess formed therein, a perforated cover for said recess, and a pump passage communicating with the recess whereby the recess may be exhausted.

9. A propeller blade having a recess therein, a perforated cover for the recess presented to' the water and means to automatically exhaust said recess by the movement of the propeller.

10. A-propeller blade having a recess on the back thereof, a perforated plate cover- 'ing said recess, a pumping passage leading portion.

substantially through the blade and ports connecting the pumping passage'and-recess.

11. A propeller blade having a recess in v the following half of the back thereof, a per- I forated cover plate for said recess a pump passage leading from the root to t e tip of the blade and, ports inclined toward the tip of the blade 7 connecting said passage and recess.

of connected recesses in the following half of the back of the blade, a perforated cover for said recesses, a pump passage passing radially through the blade and ports directed toward the tip of the blade and connect ing the recesses to the passage near the root of the blade.

13. A propeller blade having a recess formed therein communicating with the cavity formed in the water at the back of the blade and means carried by the blade for exhausting said recess and cavity.

14. A propeller blade having formed therein communicating wlth the cavity formed in the water at the back of the blade and means for automatically exhausting said recesa .15. A propeller blade having a recess formed therein communicating with the cavity formed in the water at the backof the a .recess blade and a pump passage through the blade restrictedQintermediate its ends and com municating with the recess at the restricted In testimony that I claim the foregoing as my invention, I have signed my name in presence of two subscribing witnesses.

. DAVID w. TAYLORQ Witnesses B. W. SOMMERS, HENRY ORTH, Jr.

. 50 12. -A propeller blade having a plurality

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