GB2248433A - Surface propeller located aft of transom by distance in the range 35% to 80% of propeller diameter - Google Patents

Description

IMPROVEMENTS IN OR RELATING TO DRIVE UNITS FOR WATER CRAFT The present invention relates to improvements in drive units for water craft, such as motor boats and particularly for fast motor boats.

Most commonly, boats adopting stern mounted water propulsion systems use either the Z drive gear transmission (also referred to as "stern drive" and sometimes "outdrive") or a water jet system.

The first of these propulsion systems is a unit, which is fitted on the transom of a boat, consisting of a submerged propeller driven through two sets of bevel gears hy an inboard engine. The uppermost horizontal drive shaft from the engine has a universal coupling therein. This permits the outboard components, comprising a vertical drive shaft and a lower horizontal propeller shaft, to be swung to the right or left (starboard or port) to steer the boat. Forward, neutral and astern propulsion is provided by a gear mechanism in the drive unit or by a gear box fitted to the engine.

The second alternative proFulsion system is the "water jet" which is a water pump unit driven by an inboard engine. The unit is fitted on or through the transom of the boat. Propulsion is achieved by an axial, mixed flow or centrifugal pump which draws water from an intake under the hull and ejects it astern through a nozzle.

The boat can be steered by altering the direction of the ejected jet of water. Similarly neutral and astern propulsion is achieved by partially or wholly deflecting the jet of water under the hull downwardly or forwardly respectively.

Both these propulsive systems have their drawbacks. The Z drive is a complex piece of machinery with a large number of moving parts which are subject to wear and which absorb power. It offers high appendage drag, particularly on fast boats, owing to the large frontal area of the propeller drive unit which houses the gearing. Despite manufacturers' claims, the water jet system generally has poor propulsive efficiency due to high internal losses. Also, because these propulsive systems necessitate accurate engineering tolerances in manufacture, they are expensive.

It has been proposed to provide a motor boat in which a propeller shaft runs directly aft through the transom of the hull from a driving engine to a surface propeller operating partially submerged. This arrangement takes account of the fact that the water level immediatly astern of a boat travelling at high speed is considerably lower than the general level of the water in which the boat is moving. This lower level may be termed the "planing wake level", and a propeller whose blade tips come above this level is operating partially submerged, though it will be understood that it may be totally immersed when the boat is stationary. The propeller shaft is downwardly inclined to the running lines of the boat at as small an angle as practible.It will be appreciated that although the propeller shaft is inclined it is possible with a surface propeller to achieve a horizontal thrust component by altering the design of the propeller. Since the rake of the propeller blade governs the direction of the thrust it is possible, by raking the blades aft, to cancel out the downward thrust component produced by the inclined propeller shaft. This is not possible with a fully submerged propeller where the general direction of the thrust from the upper half of the disc almost neutralises the general direction of the thrust produced from the lower half of the disc.

This drive system in its known form, however, has many unsatisfactory features. It is necessary to keep the angle betweeen the longitudinal axis of the propeller shaft and the running line of the hull of the craft as small as possible for reasons of efficiency as mentioned above. However, it is not possible to raise the propeller, because it is necessary for the propeller to be sufficiently immersed in water to provide a good degree of thrust. Furthermore, it is difficult to drop the engine very low with respect to the hull of the boat because of the nature of the complicated linkage between the engine and the shaft.

The result is that such a drive system is rather bulky and heavy and mechanically complicated. It also requires many steps in the manufacturing process and a comparatively high amount of manufacturing material. Due to its large number of components the drive unit is also complex to maintain, and is resultingly expensive to run as well as to buy.

The present invention sets out to provide a drive unit which is proportionally smaller with respect to the size of the host craft, is lighter, is simpler to make and maintain, involves the use of fewer materials to manufacture, requires the use of less components, and is, therefore, effectively cheaper and more efficient than known drive units. Furthermore, it is an object of the present invention to provide means whereby the drive from a surface propeller operating partially submerged may be applied to water craft.

The present invention provides a drive unit for a water craft comprising a surface propeller operating partially submerged and direct linkage means between a shaft of said surface propeller and a drive means of said drive unit, wherein said propeller shaft is relatively short so as to locate said propeller aft of the transom of the craft at a distance therefrom which is in the range of 35% up to 80% of the diameter of said propeller.

The combination of the direct linkage with a short drive shaft reduces the number of components necessary in the construction of the drive unit. Furthermore, the resulting drive unit is much smaller and, therefore, involves less material for manufacture. The drive unit is also lighter which leads to increased efficiency, particularly during acceleration. Furthermore, because fewer components are involved, the device is simpler to manufacture and maintain and is resultingly cheaper. In addition, because the drive shaft is shorter, reduced support is necessary for the unit, which is beneficial in that the weight of the drive unit is further reduced.

A further advantage of a drive shaft having such a reduced length is that the resulting range of propeller location offers a more rapid transition of the propeller from a fully submerged state to a condition where the propeller is only partly submerged whilst accelerating.

This reduces the possibility of overloading the engine.

Furthermore, when planing occurs there is less variation in the propeller immersion due to both speed and trim changes which affect the wake water height.

The short drive shaft according to the present invention has the advantage of reducing the liklihood of damage whilst the craft is docking. Furthermore, because the propeller does not project as far from the transom of the boat as in known drive units, the drive unit is safer in use.

Preferably an embodiment of the ivention comprises a surface propeller having a plurality of blades, at least one blade having a plurality of reverse camber surfaces on its back at or adjacent the trailing edge of the the blade. In a further preferred embodiment these reverse cambers are arcuate in cross-section and have clearly defined edges.

Further advantages and features of the invention will become manifest to those skilled in the art upon making reference to the following detailed description and the accompanying sheets of drawings, in which preferred structural embodiments incorporating the principles of the present invention are shown by way of illustrative example, and in which Figure 1 is a side elevation of a planing boat having a single drive unit according to the invention; Figure 2 is a plan view of the boat shown in Figure 1; Figure 3 is a rear elevation of the boat shown in Figure 1, showing a warped plane hull with a flat transom; Figure 4 is a plan view of a hull similar to the boat of Figure 1, having twin drive units; Figure 5 is a rear elevation of the boat shown in Figure 4, showing a warped plane hull with a flat transom;; Figure 6 is a rear elevation of a boat similar to that shown in Figure 1, differing in that it comprises a moderate or deep V hull; Figure 7 is a rear elevation of a boat similar to that shown in Figure 4, differing in that it comprises a moderate or deep V hull; Figure 8 is a side elevation of an actual drive unit; Figure 9 is a rear elevation of the drive unit shown in Figure 8; Figure 10 is a plan view of the drive unit shown in Figures 8 and 9; Figure 11 is an enlarged cross-sectional rear elevational view through the rudder stock shown in Figure 8; Figure 12 is a projected outline of the back of one of the blades of a surface propeller with two reverse cambers; Figure 13 is a section to a larger scale of the propeller blade shown in Figure 12 at 70% of the diameter with two arcuate reverse cambers on the back of the blade;; Figure 14 is a section corresponding to Figure 13 but wherein the reverse cambers are each flat with a cupped edge portion in cross section; Figure 15 is a section corresponding to Figure 13 but wherein the face of the blade is flat having a cupped edge portion in cross section; Figure 16 is a section corresponding to Figure 14 but wherein the face of the blade is flat having a cupped edge portion in cross section; and Figures 17 and 18 are sections corresponding to Figure 13 but wherein the blades have combinations of arcuate reverse cambers and reverse cambers being flat with cupped edge portions in cross section; Referring firstly to Figures 1 to 3, a drive unit 23 is mounted on the transom of a boat which has a hull 20.

The unit 15 is connected to an engine 21, and has an exhaust pipe 19 and a propeller shaft 22. The shaft 22 runs aft directly from the engine 21 through the hull 20 into the stern mounted drive unit 23 to a surface propeller 25. The drive unit 23 also carries a tunnel-like rudder 24. The exhaust pipe 19 runs aft from the engine 21 to the drive unit 23.

As can be seen from Figures 3 and 5 to 7, the boss or disc of the propeller 25 lies on the planing wake level 26 which is well below the general water level 27.

In the embodiment shown in Figures 1, 2, 3 and 6, a single drive unit is provided, whilst in that shown in Figures 4, 5 and 7, two drive units are provided. The boats shown in Figure 3 and 5 have warped plane hulls with flat transoms, whilst those shown in Figure 6 and 7 have moderate or deep V-shaped hulls.

Referring to Figures 8 to 11, which show the attachable drive unit in more detail, the propeller shaft 22 rotates inside a housing 28 which at the inboard end (nearest to engine 21) is fitted with a stern gland 52 and pressing sleeve 51 to prevent the ingress of water into the boat. The shaft 22, which is fitted with a half coupling 48 (on the engine 21 end), rotates in a bearing 30 which can be of the water lubricated type and is located in the shaft housing 28. The inboard end of the bearing 30 extends from just aft of the stern gland 52 to the rearward end which is in front of the propeller 25. This bearing is secured by means of grub screws 29.

The housing 28 has a water inlet fitting 31 to allow lubrication of the bearing 30. The propeller 25 is fitted to the shaft 22 in the usual manner with a taper and key or with a splined shaft and is held in place by a propeller nut 47. It is located at a distance L from the transom 56, which is is in the range of from 35% up to 80% of the propeller diameter D. The distance L shown in Figure 8 is measured from the rear of the boat in a direction parallel to the plane of the bottom of the boat and defines the distance between a point at the bottom of the transom 56 and the aft end of the axis of the propeller boss. Propeiler diameter D is indicated in Figure 9 and the distance of the propeller 25 from the transom 56 is expressed as a ratio where 0. 35 < L/D < 0. 8.

A V-shaped plate 32, which forms the bottom portion of the drive unit 23, assists the shaft housing 28 and shaft 22 to attain the necessary degree of rigidity. The V-shaped plate 32, is secured at its uppermost end to the bottom plate 33, which forms the lower part of the out-rigger torsion box. This box consists of a transom mounting plate 53 at the forward end (by which the entire unit is secured to the transom 56 by means of the through-fastening studs 54), two sides 35, a diaphragm 34, and a top member 36. It extends aft to encase the rudder bearing housing 40. The tunnel shaped rudder 24 partially surrounds the surface propeller 25 and it is secured at its top end to a rudder stock 37 and a doubler plate 43, and can rotate to the right or to the left by means of a tiller arm 41.The tunnel shaped rudder 24 is shown chain dotted in Figure 10 in the full starboard turn position 57 in order to deflect thrust to the right. Spacer washers 39 are placed between the tiller arm 41 and the rudder bearing housing 40 at the upper and lower ends between the bottom plate 33 and the doubler plate 43 in order to facilitate rotation. The tiller arm 41 is fitted to the rudder stock 37 by a taper and key and secured with a locking nut 42. The tunnel shaped rudder 24 may be operated mechanically or hydraulically by applying a lateral force at the forward end of the tiller arm 41, which extends through an aperture in the transom 56 of the boat 20.

The engine exhaust gases may be led aft to the drive unit 23 through an exhaust pipe 19 which is connected to a ring flange 44 on the exhaust outlet cone 45. The exhaust gases are emitted onto the upper part of the propeller 25 through a circular grill pattern 46 on the V-shaped plate 32, to which the outlet cone 45 is secured.

Figure 12 shows the outline of one blade 60 of a surface propeller 25 viewed from the front projected onto its back.

Figures 13 and 14 are two typical sections of such a blade, to a larger scale, taken at 70% propeller diameter along the circumferential line 70 shown in Figure 12. The surface propeller 25 has a boss 61, having a central axis 76 and a tapered bore or spline 75, to take a propeller shaft 22. It may have three or more blades 60, each with a cambered back 64 and a face 63. Each blade has a skewed leading edge 62 and a skewed trailing edge 65 on its face. Each blade also has a skewed trailing edge 66 on its back.

Each blade further has reverse camber surfaces 71 and 72, each of which may have straight or skewed trailing edges 73 and 74.

Figure 12 shows a right-handed propeller which will rotate counter-clockwise as seen from the front of the boat, to give forward thrust. When rotated in a clockwise sense, it will produce astern thrust, since the reversed camber areas 71 and 72 become pressure faces.

The trailing edge flat 67 forms an angle 68 with the pitch line 70 of the blade 60. This angle 68 is equal to the geometric pitch of the blade 60, which is measured between the pitch line 77 and a plane perpendicular to the axis of the propeller shaft. This is the case at any given radius of the propeller. It will, therefore, be seen from Figure 2 or Figure 3 that the trailing edge flat 67 has zero pitch along its entire length from the boss 61 to the tip circumference 69.

The inclusion of the skewed leading edge is advantageous because it prevents the leading edge from effecting a sudden entry into the water. By making the leading edge skewed, this edge does not enter the water substantially at the same time along the whole of its length, but instead enters gradually along its length. This leads to a reduction in vibration and reduces stress. By further extending the blade face in the direction of the trailing edge, it is possible to increase the surface area of the face of the blade, thereby effecting increased thrust without inducing excessive vibration and the associated damaging stresses. The result is that there is no loss in efficiency or increase in fuel consumption. As a consequence, performance is improved and the life of the blade is extended.The shape of blade face which best embodies the abovementioned features is shown in figure 12 and is essentially scimitar shaped in profile.

The concave reverse camber surfaces 71 and 72 on the backs of the blades provide a thrust surface during reversing. The result of this is that the performance of the craft is improved greatly during reversing and also during acceleration.

The reverse camber surfaces 71 and 72 can be given any appropriate cross-sectional profile. For example each surface may in cross-section be arcuate or be substantially flat with a cup at one edge. Examples of various cross-sections are shown in Figures 13 to 18.

In particular, Figure 13 shows arcuate reverse cambers, Figure 14 shows flat reverse cambers with cupped edge portions and Figures 17 and 18 show combinations of cross-sectional profiles. However, it should be understood that many other cross-sectional profiles and combinations of such cross-sectional profiles will suggest themselves to those skilled in the art upon making reference to the foregoing description.

Furthermore, the face of each blade may be arcuate in cross-section, as shown in Figures 13, 14, 17 and 18, or flat with a cupped edge portion, as shown in Figures 15 and 16.

Performance of the craft will generally be further improved if the shaft of the propeller is inclined at an angle of no more than 10. to the running lines of the craft. The propeller shaft may, when the unit is mounted on the craft, pass through the transom and be driven by an engine within the craft. Alternatively, the propeller may be driven from an engine forming part of the unit.

The above description is given by way of example only; many further embodiments and modifications of the invention will become apparent to a person skilled in the art upon making reference to the foregoing specific description which is not intended to limit the scope of the invention in any way.

Claims (16)

Claims.

1. A drive unit for a water craft comprising a surface propeller for partially submerged operation and direct linkage means between a shaft of said surface propeller and a drive means, wherein said shaft is relatively short so as to locate said propellor shaft of the transom of the craft at a distance therefrom which is in the range of from 35% up to 80% of the diameter of said propeller.

2. A drive unit according to claim 1; wherein said propeller comprises a plurality of blades, at least one blade having a plurality of reverse camber surfaces on its back at or adjacent the trailing edge thereof.

3. A drive unit according to claim 2; wherein at least one blade comprises two reverse camber surfaces on its back at or adjacent the trailing edge thereof.

4. A drive unit according to claim 2 or 3; wherein at least one of said reverse cambers is arcuate in cross-section and has clearly defined edges.

5. A drive unit according to claim 2 or 3; wherein at least one of said reverse cambers comprises a substantially linear portion and a substantially arcuate end portion.

6. A drive unit according to any preceding claim; wherein the shaft of the propeller is inclined at an angle of no more than 10 degrees to the running lines of the craft.

7. A drive unit according to any preceding claim; having means for connecting the propeller shaft to an engine mounted within the hull of the boat.

8. A drive unit according to any one of claims 1 to 6; further comprising an engine for driving the propeller shaft.

9. A drive unit according to any preceding claim; further comprising a tunnel rudder.

10. A drive unit according to any preceding claim; further comprising exhaust ventilation means for the propeller.

11. A drive unit for mounting on a water craft, substantially as described herein with reference to any one of the embodiments shown in the accompanying drawings.

12. A water craft comprising a drive unit according to any preceding claim.

13. A propeller blade comprising at least one front surface having an arcuate leading edge and having an arcuate trailing edge.

14. A propeller blade according to claim 13; comprising at least one rear surface having an arcuate leading edge and having an arcuate trailing edge.

15. A propeller blade substantially as described herein with reference to any one of the embodiments shown in the accompanying drawings.

16. A drive unit according to any one of claims 1 to 11 having a propeller comprising a blade according to any one of claims 13 to 15.