Stem drives for boats are well known and are popular among boat enthusiasts and the marine work force as well. Typical of these are units such as the Bravo 1, 2, 3, X, XZ and XR manufactured by Mercury Marine (Brunswick Corporation). Conventional stern drive units consist of an upper gear case housing which mounts on the transom of a boat for pivotal movement about a generally vertical steering axis. The stern drive unit also pivots about a generally horizontal pivot axis so the unit may be lifted or trimmed out of the water for inspection and trailering. The engine is normally mounted at the rear of the boat adjacent the transom. A shaft extends from the engine coupler through a gimbal bearing mounted in the transom assembly and connects to a U-joint which, in turn, connects to the input yoke shaft. The input shaft is connected to the pinion gear of the upper unit. The upper pinion gear, in turn, selectively drives the forward and reverse driven gears on the upper gearset. A clutch and spring assembly are stationary with the shift fork assembly centered around the clutch. The upper drive shaft extends through the center of both the forward and reverse driven gears. The clutch and spring are part of a gear, clutch, spring, and shaft assembly.
When the shift fork is moved by the shift cable, the clutch spins up or down on spiral splines on the shaft and engages a cup on top of the driven gear, which, in turn, engages the upper vertical drive shaft located in the upper case housing and which connects to lower gear case vertical shaft.
The lower gear case vertical shaft is supported by roller or needle bearings with race cups, a tab washer, pre-load shims, a pre-load spacer and O-ring above the bearings. Pinion gear height adjustment shims are located beneath the bearings. At the bottom of the lower case is the lower pinion gear. Power is transferred to the lower driven gear which, in turn, is splined to the horizontal propeller shaft which is supported by a bearing carrier that is held by a carrier nut. The propellor slides on the spline of the propellor shaft aft end and is held in place by the propnut and washer.
A significant problem with stern drive units of the general type described above is that the transmission provided by the original equipment manufacturers (OEM) of such units are limited in their power transfer capacity. If the boat owner wishes to modify or replace the marine engine increasing its torque, performance and horsepower, the transmission (upper gear, clutch, spring, bearing and shaft assembly) may be incapable of transmitting the increased horsepower and torque from the engine to the propeller shaft and propeller without damage to the transmission or the upper gear case housing support structure. Often the damage occurs to the transmission components such as fracturing of the upper gear case housing structure support. Another common problem is gear backlash due to the upper gear case housing flexing from increased torque, horsepower, heat growth factors and increased shock load and RPM. Such failures can be very expensive to repair requiring substantial replacement of the stern drive unit components, particularly the upper gear case housing and transmission assembly.
In view of the foregoing, there exists a substantial need for an improved stern drive unit which will accommodate increased engine power, torque and performance, and which can be provided both as an OEM boat builder option or an after-market unit.