Perhaps you were thinking about making a ships log to measure speed.
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Speed of the ship is measured using the 'log' system. This derives its name from the old practice of casting a log or piece of firewood off the bow of the ship. As the ship moved forwards the navigator would follow the log to the stern as the ship passed it by. The ships speed could be determined from the pace he had to follow the log. By the beginning of the nineteenth century and more accurate system had been derived. This used a 'log ship', a triangular weighted piece of wood to which was attached a long rope. The log was cast off the stern of the ship and the line allowed to pull out as the log floated stationary in the water. The rope had a series of marks or knots every 48 feet. When the first mark was reached a small sand timer was turned which ran for 28 seconds. The number of marks in the rope that passed in this period was then counted. If exactly one mark passed then the ship was doing one nautical mile per hour or one knot! Knots are still used today for ship and aircraft speed.[/FONT]
Based on the photo and the technique a cork might not have enough mass. I suppose that, knowing the LOA accurately you could send a crewman to the bow to throw of a wine cork and yell start to you. You could then start a stopwatch and time how long it would take to pass the cork with the stern (or some pre-determined more convenient measured distance on the deck). You could then calculate the speed.
If I were you, I'd get a stopwatch and the next time I got any grief I'd put on my poker face and set my friends to trying this...it should at least be good for a few laughs