Marine propulsion – Means for accomodating or moving engine fluids – Means for handling exhaust gas
Reexamination Certificate
1999-09-07
2001-07-31
Avila, Stephen (Department: 3617)
Marine propulsion
Means for accomodating or moving engine fluids
Means for handling exhaust gas
C416S09300R
Reexamination Certificate
active
06267634
ABSTRACT:
BACKGROUND OF THE INVENTION
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.
BRIEF SUMMARY OF THE INVENTION
These and other objects may be attained by a propeller flare having a periodic shape and configured for being secured to the aft end of the propeller hub. The periodic shape of the flare provides that the trailing edge, or aft end, of the propeller hub is increased only at locations where exhaust gases must be blocked from reaching the lower pressure region on the suction side of the blades. By only increasing the hub aft end at such selected locations, the efficiency and speed loss due to increased drag is minimized.
More particularly, and in one embodiment, the flare is configured for a three blade propeller and includes three flare sections. Each flare section has a helical cross-sectional shape, and ramps are located intermediate adjacent flare sections. Each ramp is aligned with one of blades, and each ramp has a pitch at least about equal to the pitch of blades. Reliefs are located adjacent each ramp, and each relief extends from an aft end of the ring toward a fore end of the flare.
Each ramp has a thrust surface, and the thrust surfaces align with each blade position and angle. The reliefs allow some bleed through of the exhaust gases, and the thrust surfaces provide thrust enhancement during engine operation. The ramps provide a local pressure increase and obstruct exhaust gases from entering into the propeller blades.
REFERENCES:
patent: 2948252 (1960-08-01), Alexander, Jr.
patent: 3356151 (1967-12-01), Strang
patent: 3542487 (1970-11-01), Knuth
patent: 3589833 (1971-06-01), Lancioni
patent: 3876331 (1975-04-01), DenHerder et al.
patent: 5158433 (1992-10-01), Cleary
patent: 5368508 (1994-11-01), Whittington
patent: 5527195 (1996-06-01), Neisen
Ewald Michael J.
Kreul Daniel J.
Werda Cy Gerard
Armstrong Teasdale LLP
Avila Stephen
Outboard Marine Corporation
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