Propeller combination for a boat propeller unit

Abstract

The invention relates to a boat propeller unit with double, counter-rotating propellers. The forward propeller is designed to function without cavitation, while the after-propeller is optimally cavitating by being made cupped and with a blade width of between 60% and 75% of the blade width of the forward propeller.

I claim: 
     
       1. Propeller combination for a boat propeller unit comprising a forward propeller and an after-propeller that rotate in opposite directions about a common rotational axis, characterized in that the forward propeller is so shaped as to function without cavitating while the after-propeller is so shaped as to function with optimum cavitation and has cupped blades and a total blade area of between 1/3 and 2/3 of the total blade area of the forward propeller, the blades of the after-propeller having as mean camber line whose maximum curvature is in its rear half, the blade width of the after-propeller being between 60% and 75% of the blade width of the forward propeller and the propellers having the same number of blades. 
     
     
       2. Propeller combination as claimed in claim 1, in which the radius of curvature of the mean camber line at the front portion of the blades of the after-propeller is at least three times longer than at the rear portion of the blades of the after-propeller. .Iadd. 
     
     
       3.  A propeller combination for a steerable boat propeller unit comprising a forward propeller and an after propeller intended to rotate in opposite directions about a common rotational axis, the forward propeller having a predetermined number of blades and a total blade area, and being shaped so as to function without cavitating and the after propeller having a predetermined number of blades, and being shaped so as to function with at least partial cavitation, the after propeller having a total blade area of between 1/3 and 2/3 of the total blade area of the forward propeller. .Iaddend. .Iadd.4. A propeller combination according to claim 3, wherein the blades of the after propeller have a predetermined chord and wherein the maximum curvature of the blades is in the rear half of the cord. .Iaddend. .Iadd.5. A propeller combination according to claim 3, in which the after propeller has a mean camber line and the radius of curvature for the mean camber line at the front portion of the blades of the after propeller is at least three times longer than that at the rear portion of the blades of the after propeller. .Iaddend. .Iadd.6. A propeller combination according to claim 3, in which the blade width of the after propeller is between 60% and 75% of the blade width of the forward propeller. .Iaddend. .Iadd.7. A propeller combination according to claim 3, in which the propellers have the same number of blades. .Iaddend. 
     
     
        .Iadd..  A propeller combination according to claim 3, in which the after propeller has one more blade than the forward propeller. .Iaddend. .Iadd.9. A propeller combination according to claim 8, in which the diameter of the after propeller is between 75% and 95% of the diameter of the forward propeller. .Iaddend. .Iadd.10. The propeller combination of claim 3, wherein the blades of the after propeller are cupped. .Iaddend. 
     
     
        .Iadd.11.  A steerable propeller drive unit comprising: an output shaft; a first conical gear secured to the output shaft; a second conical gear in constant engagement with the first conical gear such that the first conical gear drives the second conical gear in a first direction; a propeller shaft secured to and driven by the second conical gear; a third conical gear in constant engagement with the first conical gear such that the conical gear drives the third conical gear in a second direction which is opposite the first direction; a hollow propeller shaft secured to and driven by the third conical gear, the hollow propeller shaft being journaled concentrically to the propeller shaft; a forward propeller secured to one of the propeller shaft and the hollow propeller shaft; an after propeller secured to the other of the propeller shaft and the hollow propeller shaft such that the forward and after propellers rotate in opposite directions; the forward propeller having a predetermined number of blades, each blade having a predetermined width and a predetermined maximum thickness, the predetermined maximum thickness being no more than 0.08 times the predetermined blade width and the blade being shaped so that the forward propeller functions without cavitating; the after propeller having a predetermined number of blades, each blade having a predetermined width, the predetermined width of the after propeller being less than or equal to 0.70 times the predetermined width of the forward propeller and the after propeller being shaped for at least partial 
     
     
        cavitation. .Iaddend. .Iadd.12.  The steerable propeller drive unit of claim 11, wherein the blades of the after propeller are cupped. .Iaddend. 
     
     
        .Iadd.13.  A steerable propeller drive unit comprising: an output shaft;   a first conical gear secured to the output shaft;   a second conical gear in constant engagement with the first conical gear such that the first conical gear drives the second conical gear in a first direction;   a propeller shaft secured to and driven by the second conical gear;   a third conical gear in constant engagement with the first conical gear such that the first conical gear drives the third conical gear in a second direction which is opposite the first direction;   a hollow propeller shaft secured to and driven by the third conical gear, the hollow propeller shaft being journaled concentrically to the propeller shaft;   a forward propeller secured to one of the propeller shaft and the hollow propeller shaft, the forward propeller having a predetermined number of blades, each blade having a predetermined width and a predetermined maximum thickness, the forward propeller being shaped so as to function without cavitating; and   an after propeller secured to the other of the propeller shaft and the hollow propeller shaft such that the forward and after propellers rotate in opposite directions, the after propeller having a predetermined number of blades, each blade having a predetermined width, each of the blades having a mean camber line whose maximum curvature is in its rear half such that the radius of curvature of the mean camber line at the front portion of the blade is at least three times longer than at the rear portion of the blades and the blades being shaped so as to function with at least   
     
     
        partial cavitation. .Iaddend. .Iadd.14.  The propeller drive unit of claim 13 in which the forward propeller has a total blade area and the after propeller has a total blade area and wherein the total after propeller blade area is between 1/3 and 2/3 of the total blade area of the forward propeller. .Iaddend. .Iadd.15. The propeller drive unit of claim 13 in which the forward propeller and the after propeller have the same number of blades and the predetermined width of the blades of the after propeller is between 60% and 75% of the predetermined width of the blades of the forward propeller. .Iaddend. .Iadd.16. The propeller drive unit of claim 13 in which the after propeller has one more blade than the forward propeller and the diameter of the after propeller is between 75% and 95% of the diameter of the forward propeller. .Iaddend. .Iadd.17. The propeller drive unit of claim 13 wherein the predetermined maximum thickness of each of the blades of the forward propeller is less than or equal to 0.08 times the blade width of the blades of the forward propeller. .Iaddend. .Iadd.18. The propeller drive unit of claim 13 in which the width of the after propeller is no more than 0.70 times the width of the forward propeller. .Iaddend. .Iadd.19. The propeller drive unit of claim 13 wherein the blades of the after propeller are cupped. 
     
     
        .Iaddend. .Iadd.20.  A steerable propeller drive unit comprising first and second propeller shafts intended to rotate in opposite directions about a common rotational axis, a forward propeller secured to the first shaft for rotation therewith and an after propeller secured to the second propeller shaft for rotation therewith, the forward propeller having a predetermined number of blades and a total blade area, and being shaped so as to function without cavitating and the after propeller having a number of blades, and being shaped so as to function with at least partial cavitation, the after propeller having a total blade area of between 1/3 and 2/3 of the total blade area of the forward propeller. .Iaddend. .Iadd.21. A propeller combination according to claim 20, wherein the blades of the after propeller have a predetermined width and wherein the maximum curvature of the blades is in the rear half of the width. 
     
     
        .Iaddend. .Iadd.22.  A propeller combination according to claim 20, in which the after propeller has a mean camber line and the radius of curvature for the mean camber line at the front portion of the blades of the after propeller is at least three times longer than that at the rear portion of the blades of the after propeller. .Iaddend. .Iadd.23. A propeller combination according to claim 20, in which the blade width of the after propeller is between 60% and 75% of the blade width of the forward propeller. .Iaddend. .Iadd.24. A propeller combination according to claim 20, in which the propellers have the same number of blades. .Iaddend. .Iadd.25. A propeller combination according to claim 20, in which the after propeller has one more blade than the forward propeller. .Iaddend. .Iadd.26. A propeller combination according to claim 25, in which the diameter of the after propeller is between 75% and 95% of the diameter of the forward propeller. .Iaddend. .Iadd.27. The propeller combination of claim 20 wherein the blades of the after propeller are cupped. .Iaddend.