Anti-ventilation Fence For A Trim Tab

Abstract

Disclosed herein is a stern drive unit including a trim tab extending from a drive shaft housing behind a propeller and downwardly into an imaginary cylinder projecting rearwardly in concentric relation to the propeller axis from the path of the tip of the propeller, and a generally horizontally projecting barrier extending from the trim tab and located below the intersection of the imaginary cylinder and the trim tab.

Description

BACKGROUND OF THE INVENTION

The invention relates generally to marine propulsion devices such as stern drive units and outboard motors, and more particularly to trim tabs or exhaust snouts for such marine propulsion devices.

Trim tabs have been employed for some time in stern drive units to counter "steering torque," i.e., the tendency of a drive shaft housing to rotate in the direction opposite from the direction of rotation of the drive shaft, and thus, to cause an undesired steering effect. Trim tabs have concurrently served as exhaust gas discharge snouts and have also been employed with so-called "through-the-propeller-hub" exhaust gas discharge systems.

Examples of prior trim tab constructions are disclosed in the U.S. Shimanckas Pat. No. 3,000,183, issued May 19, 1965, and in the U.S. Holterman U.S. Pat. No. 3,537,419, issued Nov. 3, 1970.

Another prior trim tab construction is disclosed in the Canadian Kiekhaefer U.S. Pat. No. 687,868, issued June 2, 1964.

Still another prior trim tab construction is disclosed in the U.S. Granholm Pat. application Ser. No. 26,234 filed on June 26, 1972.

As pointed out in said earlier Granholm application, the amount of counteraction to the "steering torque" developed by a trim tab varies with boat speed. Often, the counteraction developed by the trim tab will approximately balance the "steering torque" at high boat speeds, i.e., during planning. However, the same trim tab arrangement may also overbalance the "steering torque" at boat speeds below planning and thus, disadvantageously serve to steer the boat out of its intended path.

It is noted that the low pressure area rearwardly of the propeller hub is relatively small during operation at high boat speeds but that, under conditions of acceleration at relatively low boat speeds, the low pressure area is larger.

SUMMARY OF THE INVENTION

The invention provides a stern drive unit with a trim tab having a generally horizontally extending barrier which projects from the trim tab in the area below the intersection of the trim tab with an imaginary cylinder projecting rearwardly in concentric relation with the propeller shaft from the tip of the path of the propeller blade.

Preferably, the barrier is located in more or less solid water above the low pressure area or void rearwardly of the propeller hub. One of the functions of the barrier is to control or reduce, or eliminate ventilation of the trim tab from the low pressure area or void and particularly at relatively low speeds when low pressure areas are larger as compared to when the drive shaft housing is moving through the water at relatively high boat speeds.

Control of trim tab ventilation is of particular significance in connection with "through-the-propeller-hub" exhaust gas discharge systems. The barrier of the invention serves to prevent migration upwardly along the trim tab of exhaust gases, which migration can otherwise susbstantially reduce the effectiveness of the trim tab. The vertical location of the trim tab can be varied to permit some area of ventilation, particularly during low speed operation, so as to reduce the effect of the trim tab and to obtain better balance between the trim tab action and "steering torque" at both relatively high and relatively low speeds.

The relative vertical location of the trim tab barrier also affects the action on the trim tab of the swirling water in the propeller wash and, if the barrier is located high enough on the trim tab, is believed to reduce the effect of the swirling water on the upper portion of the trim tab.

One of the principal features of the invention is the provision of a stern drive trim tab with a barrier for preventing or reducing ventilation of the trim tab.

Another principal feature of the invention is the provision of a stern drive trim tab including a barrier located below the intersection of the trim tab with an imaginary cylinder projecting rearwardly in concentric relation to the propeller axis from the tip of the propeller blade path.

Other features and advantages of the invention will become known by reference to the following drawings, general description and claims.

DRAWINGS

FIG. 1 is a fragmentary elevational view of one embodiment of a stern drive unit including a trim tab incorporating various of the features of the invention.

FIG. 2 is a sectional view taken along line 2--2 of FIG. 1.

FIG. 3 is a fragmentary elevational view of a second embodiment of a stern drive unit incorporating a trim tab in accordance with the invention.

FIGS. 4 and 5 are views similar to FIG. 2 illustrating modified trim tab constructions.

Before explaining 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 the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in the various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purposes of description and should not be regarded as limiting.

GENERAL DESCRIPTION

Fragmentarily shown in FIG. 1 is a stern drive until 11 including a drive shaft housing 15 having at the lower end thereof a gear box 17 which rotatably supports a propeller shaft 19 extending generally horizontally under operating conditions. Extending within the drive shaft housing 15 and into the gear box 17 is a drive shaft 21 which is connected to the propeller shaft 19 by a reversing transmission 23.

Carried on the propeller shaft 19 for common rotation therewith is a propeller 25 including a central hub 27 and one or more propeller blades 29. In the construction illustrated in FIG. 1, the hub 27 includes an exhaust gas discharge passageway 31 which communicates with an exhaust gas passage 33 in the drive shaft housing 15. Although the invention has particular significance with respect to "through-the-propeller-hub" exhaust gas discharge arrangements, the invention is not limited to such exhaust gas discharge arrangements.

Above the gear box 17, the drive shaft housing 15 has a rearward extension 41 which projects above the propeller and supports a trim tab 43 located rearwardly of the propeller and above the propeller shaft. While the construction disclosed in FIG. 1 is a "through-the-propeller-hub" exhaust gas discharge arrangement, the trim tab 43 can constitute an exhaust gas discharge snout. Also supported by the drive shaft housing 15 is a horizontally projecting cavitation plate 51 which extends close to the bottom of the rearward drive shaft housing extension 41.

The trim tab 43 projects downwardly from the drive shaft housing extension 41 toward the propeller shaft axis and preferably terminates slightly above an imaginary cylinder projecting rearwardly in concentric relation to the propeller shaft axis from the outer surface of the propeller hub 27.

In a horizontal cross section, (as shown best in FIG. 2) the trim tab 43 preferably has an air foil or modified air foil shape and includes a pressure side 53 located for impaction by the swirling water of the propeller wash and a low pressure side 55. Thus, in the embodiment shown in FIGS. 1 and 2, propeller rotation is such that the direction of the upper portion of the propeller rotation is from the top toward the bottom as seen by the arrow 57 in FIG. 2.

In accordance with the invention, the trim tab 43 includes a relatively thin, generally horizontally projecting barrier or plate 61 which is located between the bottom of the trim tab 43 and the intersection of the trim tab 43 with an imaginary cylinder extending rearwardly in concentric relation to the propeller axis from the tip of the path of propeller blade rotation.

It is preferred to locate the barrier 61, as shown in FIG. 1 intermediate the cylindrical projection intersection and the bottom of the trim tab 43. Thus, assuming a 13 inch diameter propeller located with the propeller shaft axis about 7 inches below the drive shaft housing extension 41, the trim tab 43 shown in FIG. 1 is located about 2 inches below the drive shaft housing extension. However, at least some of the advantages of the invention can also be obtained when locating a barrier 73, as shown in FIG. 3, at the bottom of a trim tab 75 extending downwardly about 3 inches from a rearward drive shaft housing extension 77.

Various trim tab cross sectional configurations and various barrier configurations can be employed as shown in FIGS. 4 and 5. While the barrier 61 preferably extends continuously around the trim tab 43, as shown in FIG. 2, and while a barrier 79 extends continuously around a trim tab 80, shown in FIG. 5, at least some of the advantages of the invention can be secured when, as shown in FIG. 4, a barrier 81 extends only from the low pressure side 83 of a trim tab 84, i.e., from the side of the trim tab 85 not subject to impact by rotating water.

While other dimensions can be employed, in the construction shown in FIG. 1, the barrier 61 extends outwardly from the trim tab 41 for a maximum distance of about three-quarters of an inch.

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

Claims

What is claimed is:

1. A marine propulsion unit including a drive shaft housing having, at the lower end thereof, a gear box, a propeller shaft supported by said gear box, a propeller mounted on said propeller shaft, a trim tab extending from said drive shaft housing behind said propeller and downwardly into an imaginary cylinder projecting rearwardly in concentric relation to said propeller shaft from the circumference of the path of said propeller, a generally horizontally projecting barrier extending from said trim tab and located below the intersection of the imaginary cylinder and said trim tab and exhaust gast discharge means located below said barrier thereby to reduce ventilation of said trim tab above said barrier by exhaust gas.

2. A marine propulsion unit in accordance with claim 1 wherein said trim tab has an air foil shape in a generally horizontal plane.

3. A marine propulsion unit in accordance with claim 1 wherein said barrier is located adjacent to the bottom of said trim tab.

4. A marine propulsion unit in accordance with claim 1 wherein said barrier extends continuously about said trim tab.

5. A marine propulsion unit in accordance with claim 1 wherein said trim tab includes a low pressure side and wherein said barrier extends from said low pressure side.

6. A marine propulsion unit in accordance with claim 1 wherein said propeller includes a hub having an outer surface and wherein said trim tab terminates above an imaginary cylinder extending rearwardly in concentric relation to the propeller axis from said outer surface of said propeller hub.

7. A marine propulsion unit in accordance with claim 6 wherein said barrier is located adjacent to the bottom of said trim tab.