[jyl]

It's even more complicated then, if the object is falling down a narrow hole and has to displace water which has a limited place to go? Think oil damped shock absorber.

Not to be a smarm, but I think you might turn to experimental physics rather than theoretical. Find a deep enough lake, attach (firmly) a tether of marked and measured length to the old bit, and see how fast it falls, time at 20 feet intervals, figure out when it stops accelerating and the velocity after that point. Then drop it through a steel pipe placed vertically in the lake, measure the difference in velocity with and without pipe, even if it isn't at terminal velocity before reaching the end of the pipe, you'll still get an estimate of how much the pipe slows it down. Sounds dumb but that's what I'd do, even if you have a calculated theoretical result it would be best to verify experimentally.