We need to round up a closed face spin-ning reel, about 90 feet of 10-pound test line or stronger, a stop watch, and an old tennis ball. Using a piece of thin stiff wire with a tight loop bent into one end, thread the fishing
line through the tennis ball and then tie the line back to itself, thus securing the tennis ball to the line. Measure off 88 feet of line from the tennis ball and tie off onto the spool on the reel, remembering to thread the line through faceplate first.
We'll use the device in the following way. After getting the boat in the configuration and speed desired to measure, hit the release but-ton and let the ball drop into the water
. When the ball contacts the water, hit the start button on the stopwatch. When the line pulls tight, stop the watch. To determine our velocity through the water, we divide 60 by our measured time in seconds. (60/seconds = mph). Example: If we time 4.2 seconds then 60 divided by 4.2 seconds = 14.28 mph. The tennis ball can be quickly retrieved by reeling it in. Additional runs/measurements/verifications can then be performed.
The derivation of this method follows along with sample numbers arranged so that we can see the relationships. If an object is moving at a velocity of 60 mph, then it is trav-eling one mile/minute. (60 miles/60 minutes = I mile/1 minute). Since there are 60 sec-onds in I minute, and there are 5280 feet in 1 mile, we can divide 5280 feet by 60 seconds to find how far the object travels in one sec-ond at 60 mph (5280 divided by 60 = 88 feet/ second at 60 mph). We'll use this relationship to develop the following table. The table will demonstrate the relationship of speed, time, and distance (a constant in our calculation), so that we can have a feel for doing the speed calculations in our head
, interpolating when necessary. Of course, to get the really accurate value, you may need to actually do the division.d it!