Gary

"Now you follow the same procedure, you continue a bit further, turn around and then paddle in the opposite direction against the current. What will the time be on this up current leg to pass the bobber?"

3mph - 1mph=2 mph(2.93 ft/sec) 18 ft/2.93=6.14 seconds.

I was expecting, Harry, that the example I gave would get you to recognize the error in this computation but you missed the opportunity. I wrote before:

"Now let's consider a case where there is a current. Same 18 foot canoe and procedure, only this time you have a 1 mph current moving in the direction of the first test run, you are traveling with the current on the first pass by the bobber. Since you are traveling with the current your speed is now 4 mph, 6 feet per second, so you pass the bobber in 3 seconds. Now you follow the same procedure, you continue a bit further, turn around and then paddle in the opposite direction against the current. What will the time be on this up current leg to pass the bobber?"

You have been doing the math as though the bobber were fixed to the bottom and not free to move with the current or using a landmark on shore as your reference point. But the method you gave to determine the current, that started this discussion, was to time the movement of the raft past a piece of jetsam that certainly was not fixed to the bottom of the ocean so was free to move with any current. Using my canoe example, with the canoe going with a 1 mph current, its speed

over the bottom would be 4 mph. But the bobber would also be moving with the current and so was also moving with the current at 1 mph

over the bottom so the relative speed between the canoe and the bobber is still only 3 mph, the speed that you are paddling the canoe

through the water so the time would have been the same as in the example given with no current, 4 seconds not the 3 seconds I gave in the current example that I thought would tip you off to the problem in your original statement.

The answer to the test question "What will the time be on this up current leg to pass the bobber?" is the same 4 seconds because the relative speed between the canoe and the bobber will still be 3 mph. It works out like this, the canoe is going at 3 mph through the water against the 1 mph current so its speed

over the bottom is only 2 mph in the upstream direction. The bobber is still moving downstream with the current at 1 mph

over the bottom in the downstream direction. Putting these two speeds

over the bottom together you find that the relative speed between the bobber and the canoe is still 3 mph so the time will still be 4 secends.

You can work out other examples for yourself and draw diagrams if necessary and you will find that the time to pass the bobber remains exactly the same no matter what the current is so it is impossible to determine the current using the method you gave.

gl