Foil shapes for use in barge skegs and marine propeller shrouds

Abstract

An airfoil shape exhibiting low drag and a high lift/drag ratio has a low thickness/chord ratio and a low camber/chord ratio. The airfoil shape provides desirable results when used in barge skeg assemblies and in propeller shrouds for tugs used to tow ocean going barges.

1 . A skeg system for a barge, comprising:
 (a) at least a pair of vertically-oriented foil elements each having an upper end and a lower end; and   (b) a horizontal foil element attached to and extending transversely between the vertical elements, wherein each of the vertical foil elements has a chord length and a thickness and the thickness is related to the chord length by a ratio in the range of 0.13 to 0.20, and wherein each of the vertical elements has a maximum camber, with a ratio of the maximum camber to the chord length in the range of 0.065 to 0.080.   
   
   
       2 . A skeg assembly for a barge, including at least a pair of vertical foil elements each disposed in a vertical plane and a horizontal element attached to and extending horizontally and transversely between the vertical foil elements, wherein each vertical foil element has a ratio of thickness to chord length (t/c) in the range of 0.13-0.20, and a ratio of maximum camber to chord length in the range of 0.065 to 0.080 and has a maximum thickness located at a distance from the leading edge in the range of 0.30 to 0.50 of the chord length. 
   
   
       3 . The skeg assembly of  claim 2  wherein said vertical foil elements are oriented at an angle of attack in the range of −5° to +5° relative to a vertical plane parallel to a longitudinal centerline of the barge. 
   
   
       4 . The skeg assembly of  claim 2  wherein the horizontal foil element is oriented at an angle of attack in the range of −5° to +5° relative to a plane parallel with a raked counter portion of a hull of the barge. 
   
   
       5 . The skeg assembly of  claim 2  wherein the horizontal foil element has a chord length greater than the chord length of the vertical foil elements. 
   
   
       6 . The skeg assembly of  claim 5  wherein the horizontal foil element has a chord length at least 1.25 times as great as the chord length of the vertical foil elements. 
   
   
       7 . The skeg assembly of  claim 5  wherein the horizontal foil element has a chord length twice as great as the chord length of the vertical foil elements. 
   
   
       8 . The skeg assembly of  claim 2  wherein the lift/drag ratio of each vertical foil element is at least 170 at a speed in salt water in the range of 5-15 knots. 
   
   
       9 . The skeg assembly of  claim 2  wherein the horizontal foil element is oriented at an angle of attack, relative to a flow path of water already of the horizontal foil element beneath a raked counter portion of the barge, in the range of −5 to +5°. 
   
   
       10 . The skeg assembly of  claim 9  wherein the angle of attack relative to the flow path is in the range of 0° to +5°. 
   
   
       11 . A propeller shroud for a marine propeller having a central shaft and plurality of radially extending blades, the shroud comprising:
 (a) a tubular nozzle structure having an interior surface shape and an exterior surface shape, the exterior and interior surface shapes cooperatively defining a foil section shape as seen in a radial plane extending through the tubular structure, the foil section shape having a thickness to chord length ratio in the range of 0.13 to 0.20, a camber to chord length ratio in the range of 0.65 to 0.080, with maximum camber located between 35% and 55% of the chord length from the leading edge; and wherein   (b) the foil section shape has an angle of attack in the range of 0° to −8°.   
   
   
       12 . The propeller shroud of  claim 11  wherein the angle of attack is in the range of −4° to −8°. 
   
   
       13 . The propeller shroud of  claim 11  wherein the foil section shape of the tubular nozzle structure has a lift to drag ratio of at least 100 at an angle of attack within the range of 0° to −8° and operating at a Reynolds number in the range of 1×10 6  to 18×00 6 . 
   
   
       14 . The propeller shroud of  claim 11  wherein the drag coefficient (Cd) is in the range of 0.01-0.007 at speeds in salt water in the range of 5 knots-18 knots.