GB2116926A

GB2116926A - Gear case for marine propulsion device

Info

Publication number: GB2116926A
Application number: GB08304275A
Authority: GB
Inventors: Philip J Mcgowan
Original assignee: Outboard Marine Corp
Current assignee: Outboard Marine Corp
Priority date: 1982-03-24
Filing date: 1983-02-16
Publication date: 1983-10-05
Also published as: IT1170321B; FR2523921A1; GB2116926B; FR2523921B1; AU1140083A; CA1199530A; SE8300769D0; GB8304275D0; IT8347873A0; DE3305278A1; AU561897B2; JPS58167295A; SE8300769L

Abstract

The marine propulsion device includes a lower unit which has a gear case 16 normally submerged in water and which is arranged to discharge engine exhaust gases through the propeller hub 54, the leading end of the gear case 16 having a tapered nose 18 and a trailing end of the gear case having a generally circular cross section and terminating in a generally vertical trailing edge 21 located forwardly of the propeller hub 54 and the outer diameter of the trailing edge 21 of the gear case 16 is substantially larger than the outer diameter of the propeller hub 54 to provide a sharp step down therebetween and the outer surface of the gear case 16 expands radially outwardly from the nose 18 to the trailing edge 21. In one embodiment, the nose 70 extends forwardly beyond the leading edge 17 of the drive shaft housing 14. <IMAGE>

Description

SPECIFICATION Gear case for marine propulsion device This invention relates to marine propulsion devices, such as outboard motors and stern drive units and, more particularly, to marine propulsion devices including through-the-hub exhaust and means for increasing propellerthrust. In another aspect, the invention relates to such marine propulsion devices including means for controlling hydrodynamic stall.

Outboard motors and stern drive units may employ a so-called through-the-hub exhaust system including an engine exhaust gas passageway extending through the drive shaft housing, the gear case, and an annular passage inside the propeller hub. Means for increasing the thrust delivered by the propeller of such marine propulsion devices is a continuing objective.

At higher speeds, cavitation and flow separation can occur along the outer surface of the gear case, causing air to reach the low pressure side of the propeller. When large quantities of air are present, there can be a sudden loss in the thrust and load carrying capabilities of the propeller, a condition commonly referred to as hydrodynamic stall.

Attention is directed to the following United States patents: The invention provides a marine propulsion device including a lower unit having an exhaust gas passageway terminating in a discharge port, a rotatable propeller shaft extending through the discharge port, a propeller mounted on the propeller shaft for common rotation therewith and including a hub which has a generally circular outer surface and which has an exhaust gas passageway through which exhaust gases are discharged rearwardly into the water, and a gear case rotatably supporting the propeller shaft. The gear case includes a leading end which has a tapered nose and a trailing end which terminates in a generally vertical trailing edge located forwardly of the propeller hub.The outer diameter of the trailing end portion of the gear case is substantially larger than the outer diameter of the propeller hub to provide a sharp step down therebetween and the outer surface of the gear case substantially continuously radially outwardly expands from the nose to the trailing edge.

In one embodiment, the tapered nose of the gear case extends some distance forwardly beyond the leading edge of the drive shaft housing so as to make the outer surface of the gear case more pointed, thereby reducing cavitation and the flow separation along the outer surface of the gear case at higher speeds.

One of the principal features of the invention is the provision of a marine propulsion device including a through-the-hub exhaust system and a simple means for increasing propeller thrust.

Another of the principal features of the invention is the provision of a marine propulsion device including a through-the-hub exhaust system and a simple means for controlling hydrodynamic stall.

A further of the principal features of the invention is the provision of a marine propulsion device including a gear case having an outer surface which is configured in a manner to produce pressure conditions in the vicinity of the propeller which are favorable for increased propeller thrust.

A still further of the principal objects of the invention is the provision of a marine propulsion device including a gear case which is configured to reduce cavitation and flow separation along the outer surface at higher speeds.

Other features, aspects, and advantages of the invention will become apparent to those skilled in the art upon reviewing the following detailed description, the drawing, and the appended claims.

The invention will now be described, by way of example, with reference to and as shown in the accompanying drawings, in which: Figure 1 is a fragmentary, partially broken away and partially sectioned, view of the lower unit of a marine propulsion device incorporating various of the features of the invention.

Figure 2 is a fragmentary view of the lower unit of a marine propulsion device including a gear case of an alternative construction.

Before explaining at least one of the embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawing. The invention is capable of other embodiments and of being practised and carried out in various ways. Also, it is to be understood that the phraseology and the terminology employed herein is for the purpose of description and should not be regarded as limiting.

Illustrated fragmentarily in Figure 1 is a marine propulsion device 10, which can be either a stern drive unit or an outboard motor, including a propulsion or lower unit 12 having a drive shaft housing 14 and a gear case 16 which is normally submerged in water. The drive shaft housing 14 has a leading edge 17. The gear case 16 has a leading end in the form of a tapered nose 18, a streamlined or torpedoshaped outer surface 19 and an aft or trailing end portion 20 having a generally circular cross-section.

Rotatably mounted in the gear case 16 is a propeller shaft 22 carrying a propeller 23 rearwardly of the gear case 16. Extending through the drive shaft housing 14 transversely to the propeller shaft 22 is a rotatably mounted drive shaft 24 which at the upper end is operably connected to an internal combustion engine (not shown) and at the lower end is drivingly connected to the propeller shaft 22 through a conventional reversing clutch ortransmission 26. Extending downwardly from the gear case 16 is a vertical fin or keg 28.

Extending through the drive shaft housing 14 is an exhaust gas passageway 30 which terminates an exhaust gas discharge port 32 at the trailing end 20 of the gear case 16.

The propeller shaft 22 includes an aft or rear portion 36 extending rearwardly of the gear case 16 and a forward portion 38 extending through the discharge port 32 and rotatably supported inside the gear case 16 buy a bearing retainer 40 suitably affixed to the interior the gear case 16. The bearing retainer 40 includes a central hub portion 42 carrying a roller bearing 44 and a lubricant seal 46. The bearing retainer 40 also includes an annular outer sleeve 48 engaging the interior of the gear case 16 and a plurality of circumferentially-spaced, radially extending spokes or ribs 50 interconnecting the hub portion 42 and the sleeve 48. The openings 52 defined between the ribs 50 form part of the engine exhaust gas passageway 30.

The propeller 23 includes a hollow hub 54 having a generally circular outer surface 55 and carrying a series of blades 56. The propeller 23 also includes an inner sleeve 58 which is connected to the hub 54 by a plurality of circumferentially-spaced, radially extending ribs 60. The sleeve 58 is mounted on the rear portion 36 of the propeller shaft 20 for common rotation therewith, such as by splines 62. The openings 64 between the ribs 60, together with the interior of the hub 54, define an axially extending exhaust gas passageway or duct 66 extending through the hub 54. The duct 66 has an inlet communication with the discharge port 32 and an outlet rearwardly of the propeller blades 56 through which the exhaust gases are discharged into the water. Such an engine exhaust gas arrangement is commonly called a "through-the-hub" exhaust system.

The forward thrust produced by the propeller 23 is increased by dimensioning the outer diameter of the circular trailing end portion 20 of the gear case 16 so that it is substantially larger than the outer diameter of the propeller hub 54, by arranging the trailing edge 21 of the gear case 16 generally vertical so there is a sharp stepdown from the trailing end portion 20 of the gear case 16 to the propeller hub 54, and by configuring the outer surface 19 of the gear case 16so that it substantially continuously radially outwardly expands from the nose 18 to the trailing edge 21. This specific arrangement produces pressure conditions in the vicinity of the propeller blades 56 which are favorable for enhancing the power transimitting characteristics of the propeller 23.

In the alternate construction illustrated in Figure 2, the tapered nose 70 of the gear case 72 extends forwardly some distance beyond the leading edge 17 of the drive shaft housing 14 to make the outer surface 74 of the gear case 72 more pointed. As in the embodiment illustrated in Figure 1,the outer diameter of the circlar trailing end portion 76 of the gear case 72 is substantially larger than the outer diameter of the propeller hub 54, the trailing edge 78 of the gear case 72 is generally vertical to provide a sharp stepdown from the trailing end 76 of the gear case 72 to the propeller hub 54, and the outer surface 74 of the gear case 72 substantially continuously radially outwardly expands from the nose 70 to the trailing edge 78. This arrangement, in addition to increasing propeller thrust, reduces the tendency for cavitation and flow separation along the outer surface 74 of the gear case 72 at higher speeds, thereby minimizing the amount of air present in the vicinity of the low pressure side of the propeller blades 56 which can cause hydrodynamic stall.

Various of the features of the invention are set forth in the following claims:

Claims

1. A marine propulsion device comprising a lower unit including an exhaust gas passageway which terminates in a discharge port, a rotatable propeller shaft extending through said discharge port, a propeller which is normally submerged in water and which is mounted on said propeller shaft for common rotation therewith, said propeller including a hub which has a generally circular outer surface and an exhaust gas passage way which has an inlet in communication with said discharge port and an outlet in communication with the water and a gear case rotatably supporting said propeller shaft, said gear case including a leading end which has a tapered nose, a trailing end which has a generally circular cross section with an outer diameter substantially larger than the outer diameter of said propeller hub and which terminates in a generally vertical trailing edge located forwardly of said propeller hub, and an outer surface which substantially continuously radially outwardly expands from said nose to said trailing edge.

2. A marine propulsion device according to Claim 1 wherein said lower unit includes a drive shaft housing having a leading edge and wherein said nose of said gear case extends forwardly beyond said leading edge of said drive shaft housing.

3. A marine propulsion device substantially as hereinbefore described with reference to and as shown in the accompanying drawings.