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Question forwarded from email

Please consider the following for publication in Waterlines.
Recently my wife and I negotiated the locks down the Aston Canal into Manchester, not a route we have taken for many years. We found the locks heavy with fountain-like leakage from the top gates on many of them. I had to make sure our boat stayed forward on the bottom gates or the back deck would get flooded by the leakage. This set me thinking about an aspect of boat behaviour when going down locks that I am at a loss to explain, despite being a Chartered Engineer with some knowledge of hydrodynamics. Perhaps WW readers might enlighten me?
When descending in a canal lock the boat initially move forwards onto to bottom gates as the paddles are opened; no mystery there as that's the way the water in the lock is moving as it begins to empty. However, when the lock is just about empty, the boat moves backwards towards the top gate and cill. I can't understand why this happens. It tends to happen on just about all types of lock regardless of single, double, gate paddles or ground paddles. It's as if water is flowing back into he lock when it's empty, but why? Often this effect is quite useful, particularly on single bottom gates as it moves the boat away from the gates ready to open them. On the Aston Canal though the unwary may get there back deck washed!
Can anyone come up with a convincing explanation? In case it matters, our boat is a traditional designed 58 ft. narrowboat.
Bill Root

Asked by: Robert Cowling   | 4.07pm, Thursday 10 November

WW says:

Your deduction is indeed what must be happening; when the lock is empty water flows back into it moving the boat back. This is because water is heavy and the flow out of the lock has momentum; the result is that the flow actually draws more water out of the lock than that required to make a level. Thus the water level in the lock actually ends up slightly lower than that outside for a short time before the flow reverses and evens up the levels.
Another way of explaining the effect is to apply Bernoulli's principle which states how the water flowing out of the lock creates an area of lower pressure in the lock tail; the level is slightly lower in this area than the rest of the pound and the water level in the lock finishes at this lower level before it all equalises.

Rupert Smedley  | 6.10PM, Thursday 10 November

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