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Buy a long-keeled double-ender, mark the prop shaft so that you know when the prop blades are vertical, and turn the prop so that the blades are in line with the keel.
On the point about "hull speed", drag always slows the boat. At higher speeds the fractional contribution of the drag from the prop drops off as there is so much drag from the hull shape, but the drag from the prop is steadily rising.
With a lot of wind around, you won't notice it much, but it will be there.
At very low speeds I reckon you won't notice it much either, as the vast majority of the drag will be from surface drag, and the prop, though an obstruction, does not wet much area. It obstructs (from form drag), but do you not need a decent bit of speed to see that one? There should be a good graph to plot in there somewhere?
Next time I am out in light airs I will try to turn the prop in line with the keel, and then perp to the keel, and see if it gets noticed. It should be a good experiment.
Last edited by Rockter; 02-08-2009 at 08:53 AM.