The invention relates generally to engines, and more particularly, to apparatus for preventing exhaust gases from flowing back into a propeller hub.
Outboard engines include an exhaust casing extending from a power head, and a lower unit secured to the exhaust casing. The lower unit includes a gear case which supports a propeller shaft, and a propeller is engaged to the shaft. The propeller includes an outer hub through which exhaust gases are discharged.
During operation, a region of low pressure is developed rearwardly of the propeller. A thin low pressure boundary layer around the hub can also develop. The low pressure condition rearwardly of the hub has a tendency to join with the low pressure boundary layer, and exhaust gas migrates forwardly along the propeller hub between the blades and along the front face of the propeller blades, thereby causing conditions of "cavitation" or "ventilation". Such conditions of cavitation prevent the propeller blade from biting into the water and result in an efficiency loss. In addition, excessively low pressure in the region rearwardly of the propeller hub results in a drag on the forward movement of the engine through the water.
Known propeller structures for preventing ventilation include diverging flare rings and converging rings at the rear end of the propeller hub. The rings affect the flow of water over the hub and prevent migration of the exhaust gases along the hub. Although the known ring configurations are effective in preventing ventilation, such rings can cause efficiency and speed loss due to increased drag.
It would be desirable to provide apparatus that is as effective as the known structures for preventing ventilation, yet avoid the efficiency and speed losses associated with such known structures.