CROSS-REFERENCE TO RELATED APPLICATION This application claims the benefit of Provisional Application No. 62/101,748, filed January 9, 2015, the entire disclosure of said applications is hereby incorporated by reference herein.

BACKGROUND

The three most common types of drive systems for powered watercraft are (i) outboard drives wherein the engine and drive unit are located off the aft end of the watercraft, (ii) inboard drives wherein the engine and transmission are located inboard and configured to power a propeller driveshaft that typically extends through and under the hull, and (iii) sterndrives or I/O drives wherein an engine is located inboard on the watercraft, typically near the transom, and a drive unit, referred to as an outdrive, extends through the transom with a gear box disposed outboard of the transom.

The original conception of the sterndrive has been attributed to Charles Strang as described by Jeffrey L. Rodengen in "Iron Fist: The Lives of Carl Kiekhaefer" (1990). The first patent disclosing a sterndrive is U. S. Patent No. 3,376,842, to J.R. Wynne, which is hereby incorporated by reference in its entirety. The first commercial embodiment of a sterndrive watercraft was the Aquamatic™, introduced by Swedish engine company Volvo Penta in 1959.

In a sterndrive the drive unit, or "outdrive," transmits power from an inboard engine through the transom and downward to a propeller below the waterline. The outdrive typically includes an upper subassembly comprising a driveshaft that connects the inboard engine to an upper gear box through the transom, and a lower subassembly connected to the upper gear box to a lower gear box that drives the propeller shaft.

A common problem with powered watercraft, and in particular high-performance watercraft operated in shallow waters such as the littoral zone and in rivers, is the risk of damage to the drive component, for example, the propeller(s) or water jet outdrives (sometimes referred to herein as a jet pump), and related components due to contact with the ground. Such risk is mitigated if the drive component can be operated higher in the water. Higher positioning of the propellers or other drive components would also enable the watercraft to safely operate in shallower waters, and would reduce the hydrodynamic drag. SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

A watercraft includes a hull having a transom, wherein the hull defines a downwardly open, stepped tunnel on the aft end of the hull. The watercraft further includes an inboard engine and an outdrive assembly. The outdrive assembly extends from the inboard engine, through the transom, and is drivably connected to a propeller or jet pump to provide motive power to the watercraft, wherein the propeller or pump is disposed aft of the stepped tunnel. A vent provides a flow path from the stepped tunnel to an atmospheric vent port.

In an embodiment the outdrive assembly includes (i) a first drive shaft drivably connected to the inboard engine and extending through the transom, (ii) an upper gearbox disposed outboard of the transom that engages the first drive shaft, (iii) a second drive shaft that engages the upper gear box and extends downwardly, and (iv) a lower gear box that engages the second drive shaft, wherein the propeller or jet pump are drivably connected to the lower gear box.

In an embodiment the stepped tunnel has a maximum width that is between 25% and 60% of the hull beam.

In an embodiment the tunnel length is between 10% and 25% of a length of the hull.

In an embodiment the vent includes a plurality of channels, each channel having a first port near the step.

In an embodiment the outdrive is configured such that during operation the propeller or jet pump is positioned to draw water from the stepped tunnel.

In an embodiment the outdrive is pivotable to move the propeller of jet pump between a lower positon in the water and an upper position out of the water.

In an embodiment the stepped tunnel extends along less than half the hull length overall.

In an embodiment the propeller or jet pump comprises one or more propellers. In an embodiment the hull is a Vee hull and comprises outboard stabilizing members mounted to hull side sheets. DESCRIPTION OF THE DRAWrNGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a perspective view of a watercraft in accordance with the present invention;

FIGURE 2A is a side view of the watercraft shown in FIGURE 1;

FIGURE 2B is a diagrammatic side view of the watercraft shown in FIGURE 1, illustrating the aft vented tunnel in the hull;

FIGURE 3 is an upper perspective view of the aft end of the watercraft shown in FIGURE 1; and

FIGURE 4 is a lower perspective view of the aft end of the watercraft shown in FIGURE 1.

DETAILED DESCRIPTION

A particular embodiment of a watercraft in accordance with the present invention will be described to assist in understanding the present invention. It will be readily apparent to persons of skill in the art that the invention is not limited to the details of the current embodiment, and the teachings herein may be readily applied to a broad range of watercraft.

A perspective view of a watercraft 100 in accordance with the present invention is shown in FIGURE 1. A starboard side view of the watercraft 100 is shown in FIGURE 2A. In this exemplary embodiment the watercraft 100 includes a rigid planing hull 102. For example, the planing hull 102 may be a deep V-hull (sometimes referred to as a vee hull). The hull 102 includes a pair of curved side sheets 103 that join at a forward end to form a bow 104. The side sheets 103 engage a transom 106 at an aft end. An optional and conventional cabin 1 10 is also shown. An inboard engine compartment 108 enclosing an engine 122 (see FIGURE 2B) is located aft of the cabin 1 10, near the transom 106. In a current embodiment the engine 122 is a conventional diesel or gasoline internal combustion marine engine, but alternative engine means are contemplated, including any suitable power plant, including, for example, electric, hybrid, or fuel cell powered motors. In this embodiment, the watercraft 100 further comprises an optional deck extension 114 that extends aft from the transom 106, and outboard stabilizing members or sponsons 1 12 mounted to the hull side sheets 103.

The inboard engine 122 is drivably connected to an outboard drive train or outdrive assembly 124. The outdrive assembly 124 extends from the engine 122, through the transom 106, and downward into the water to turn one or more (two shown) propellers 129 that drive the watercraft 100 through the water. Other drive means, for example, a jet pump or the like, may alternatively be use. The inboard engine 122 and outdrive assembly 124 are collectively referred to as an I/O drive or sterndrive 120. An exemplary sterndrive is disclosed in U.S. Patent No. 8,070,540, to West et al., which is hereby incorporated by reference.

Refer now also to FIGURE 2B, which shows a simplified starboard side view of the watercraft 100, and illustrating a venting system in dashed line. Refer also to FIGURE 3, which shows a lower perspective view of the aft end of the watercraft 100, including details of the outdrive assembly 124. The outdrive assembly 124 includes an upper portion comprising a first driveshaft 121 that extends through the transom 106, and connects the engine 122 to an upper gearbox 123U located outboard of the transom 106. A second driveshaft 127 connects the upper gearbox 123U to a lower gearbox 123L. Typically the gearboxes 123U, 123L are 90-degree gearboxes. A propeller shaft 126 is driven from the lower gearbox 123L. In sterndrive systems the watercraft 100 is typically steered by pivoting the outdrive assembly 124 laterally (similar to an outboard watercraft). As in an outboard motor, no rudder is needed. The outdrive assembly 124 includes a mechanism, for example, one or more hydraulic actuators 1 19 for pivoting the outdrive assembly 124 upwardly for trailering or when the sterndrive is not in use, to avoid damage to the outdrive assembly 124. The outdrive assembly 124 may optionally be pivotable from a lower positon to an upper position, for example, when the watercraft is in very shallow water, to protect the outdrive assembly 124 from damage in the event of beaching.

A problem with conventional marine outdrive systems, including sterndrives, is that for acceptable performance the propeller(s) 129 must extend deep enough into the water to engage relatively "clean" water. The propellers 129 are near the aft end of the hull 102, and draw water from beneath the hull 102. Propellers in conventional stern drive systems are therefore positioned below the lowermost edge of the hull. The low position of the drive component increases the risk of damage to the drive system, limits the minimum safe operating depth for the watercraft, and increases the drag on the watercraft. The deeper the propellers 129 are positioned during operating, the more susceptible they are to damage from objects protruding from the waterbed. The risk to the propeller is compounded in high-performance watercraft that are configured to operate in a planing configuration because rotating the hull to the planing angle moves the propellers 129 deeper into the water.

The hull 102 of the watercraft 100 disclosed herein includes a short, stepped tunnel 130 at the aft end of the hull 102. The aft stepped tunnel 130 is positioned directly forward of the outdrive propellers 129. The stepped tunnel 130 allows the propellers 129 to be mounted higher in the water without encountering interference from the hull 102 because the stepped tunnel 130 provides a flow path for water to the propellers 129. The stepped tunnel 130 is short and narrow as discussed below, and therefore the outboard portions of the hull 102 adjacent the tunnel 130 provide desired buoyancy for the watercraft 100.

However, under conventional design practices it is counterintuitive to include a stepped tunnel on the aft end of a high-performance watercraft because a stepped tunnel will reduce hull performance. According to the Bernoulli principle, as the speed of a moving fluid increases, the pressure within the fluid decreases. During moderate- to high-speed operation of a watercraft having a stepped aft tunnel, water flowing through the stepped tunnel accelerates, generating a significant vacuum or low-pressure region in the tunnel. The vacuum pressure is very undesirable because low pressure in the tunnel pulls the hull deeper into the water, thereby increasing drag, thereby decreasing hull performance. Moreover, as the hull moves deeper into the water the propellers or jet pump nozzles also move deeper into the water, where they are more susceptible to damage.

In order to prevent the undesirable low-pressure region in the aft tunnel 130 region, the aft tunnel 130 in the present invention is vented to a location above the waterline, to maintain atmospheric pressure in the tunnel 130. Refer now also to FIGURE 4 and FIGURE 2B, which shows a lower perspective view of the aft end of the watercraft 100. The forward end of the stepped tunnel 130 is configured with one or more vents 131 extending between lower vent ports 132 (three shown) that open into the stepped tunnel 130 and atmospheric vent ports 134 that opens to atmospheric pressure above the waterline. For example, in this embodiment the lower vent ports 132 are fluidly connected to atmospheric vent ports 134 in the transom 106. Alternatively, the atmospheric vent ports 134 may be positioned on the side sheets 103 or in the deck, for example, into the engine compartment 108.

Therefore during operation of the watercraft 100 the pressure in the stepped tunnel 130 is maintained substantially at atmospheric pressure.

The stepped tunnel 130 in the current exemplary embodiment has a width between 25% and 60% of the watercraft 100 beam (conventionally defined as the maximum width of the hull 102). It is contemplated that in some applications the watercraft may have twin sterndrive 120 systems installed side by side (or more than one outdrive assembly 124 driven by a single engine 108). In the embodiment shown in FIGURE 1 the stepped tunnel 130 is about 44% of the beam. In multi-drive configurations, it may be desirable for the stepped tunnel 130 to be wider than 60% of the watercraft beam. Alternatively, it is contemplated that separate short tunnels similar to short tunnel 130 may be provided for each outdrive assembly 124.

The short tunnel 130 in the current embodiment has a length that is between 10% and 25% of the length overall (conventionally defined as the maximum length of the hull 102 measured parallel to the waterline), and in the current embodiment is about 15% of the hull length overall.

As discussed above, the vented aft stepped tunnel 130 allows the sterndrive and, in particular, the outdrive assembly 124 to be mounted higher on the transom, such that the propeller 129 (or jet pumps) may be located higher in the water, without significantly impacting the performance or other desirable properties of the hull. This provides the benefits of reducing drag and increasing performance and fuel efficiency. The higher mounting also reduces the risk of damage to the propeller or jet pump drive system, allowing the watercraft 100 to operate in shallow water. A higher propeller location also further protects the propeller when the outdrive 124 is pivoted upwardly for trailering, or for protection for example, in shallow waters.

While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.