Join Date: Feb 2000
Location: Annapolis, Md
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I think your description of winglets does fit my understanding of the purpose of winglets relative to tip vortex generation. To explain my understanding of tip vortex generation, foils (whether we are talking about keels, rudders or sails, and whether they are operating in a compressible fluid like air or an incompressible fluid like water) have a high pressure side and a low pressure side. Lift is generated by the suction created as the fluid flows over the foil. As you approach the tip of the foil, some of the low and high pressure 'leaks' over the tip of the foil and where the high and low pressures meet, a highly turbulent area exists that leaves the tip as a vortex. The force of dragging this vortex through the water adds considerably to the drag of the foil.
The larger the tip of the foil in relationship to its span, the greater the tip vortex and the greater the drag that is produced at any given speed and side load. This one of the main reasons is why long keels are not as fast or do not go to weather as well as higher aspect ratio, shorter tip length keels.
While it is true that the loading on an underwater foil is ellptical, that does not mean that the keel itself needs to be elliptical to be efficient.
Winglets do two things, they are intended to keep pressure from leaking from the high pressure to the low pressure side of the tip (end plate effect) making the ajdacent area of the keel more effective. They also reduce the size of the tip and so reduce the size of the tip vortex. In keels they are typically made of lead which lowers the vertical center of gravity adding to the stability of the boat as well.
Winglets do not work all that well on rudders since the rotation of the rudder places the winglets across the flow adding enormous drag for little or no benefit. Instead designers of modern rudders use very high aspect ratio foils with pointed tips intended to minimize the tip vortex generated.
As far as I know surface tension has little or nothing to do with the winglets.