Does size matter?

As my boat is presently in the parkland near home in Herts I hope you will forgive me for using photos from previous years to illustrate a continuing concern that I have. That concern relates to wastage or at least extravagant use of water when taking my boat through a lock. My boat is a mere 41 foot long (yes no masts, Ronnie). Some are up to 72 foot. Mine is 6 foot 10 wide (beam), some are nearly 14 foot wide, although not in the canals pictured below, I hope. So the perpetual worry in my mind is that every time I go through a lock by myself am I using a disproportionate amount of water compared to a bigger boat? My boat weighs less, about 14 tons, so displaces less, so there is more water in the lock when I go up and down. That water always comes from the upper pound (stretch of water between the locks)  and flows inevitably to the lower pound. Imagine a boat as small as a matchstick. If you dropped that through a lock, effectively a whole lock of water drops through. But a boat of the width of the lock and as deep as the canal would only drop a fraction of that water wouldn’t it? I mean take a look at the wide lock below. All that extra water alongside  my boat was dropped.


The picture below is of a set of interconnected locks called a staircase. It also has side pounds were the water is let in and out of mini reservoirs at the side, which you must not allow to overflow. The lock keeper  taught me how to do it properly. He said you operate the red  and white paddles in the correct order and you can’t go wrong. He said “Red before white and you’ll be alright” …   or was it “Red after white and you’ll be alright” . I know it rhymed.


Anyway the point is when you use these locks you can see that as the boat passes from one chamber to the next it obviously displaces the water from in front to the rear chamber. The bigger the boat the more it displaces.  When the boat exits the top lock water runs in behind the boat, the bigger the boat the more water. So when a big boat goes up more water is needed to fill the hole in the water left behind. And when it goes down it leaves more water behind. So now we seem to have contradictory ideas. I love to watch water, to see how it flows, see how it can pile up when under strong forces etc. I wanted to be a pioneering hydraulic engineer once, but realised the maths (pre computer days) would be beyond me.

Now for those who are still with me on this very real concern I have a third observation. Once the boat is in a lock, no matter how big or small, to raise or lower it the height of the lock, shall we say 6 foot, will always take exactly six foot of water. It is like adding or removing the bottom slice of a sponge cake. The top half is the boat floating on water, we are just adding or removing the bottom slice of water. So once in the lock it always takes the same amount of water to fill or empty assuming the boats are floating (not always true around here as we get a least one sunk in a lock each year! Then you really get the gongoozlers turning up ).

So now I have got it (badly you may add). When my boat enters a lock it leaves 14 ton of water behind going up or down. Double the size would double that amount left behind. Hence assuming most boats go up and down in equal measure there is no difference in that usage of water. Furthermore it always takes the same to empty or fill a lock one the boats are in and floating. So that means I can stop worrying, as long as I remember to share a lock to reduce the number of times it is filled or emptied. I’m glad I sorted that out!


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