How Canal Locks
WorkHow Canal Locks
WorkLocks are part of the very essence of canals in
the UK. There are few canals which do not have locks which allow the canal to
be raised and lowered according to the lie of the land. Originally, canal
engineers would survey the best route for a canal that involved the fewest
locks, because they slowed the progress of commercial traffic. In this leisure
age, however, they are all part of the enjoyment to be had from travelling on
the canal system.
The earliest locks on navigable rivers were little more than manual sluice gates, above which a head of water built up and which could be raised to provide a surge of water to assist craft over shallows. Hills could not always be circumvented, though, when it came to building canals from scratch. Some canal engineers went to unbelievable lengths to avoid building locks, as travellers on the South Oxford Canal will know!!! The construction of locks vary from canal to canal. Some, like a couple on the Kennet & Avon, are turf-sided locks, but most are built of brick or stone. Most have a single top gate but some have twin gates. Almost all have twin bottom gates because of the greater water pressure against which the boat has to push.
Locks vary in depth from the monster lock at Denham on the Grand
Union to ones like Kent Green on the Macclesfield, where the drop is a mere six
inxhes. Some have gate paddles, and some have ground paddles and some have both
or a combination of the two. Gate paddles raise sluices set into the gate
itself, allowing water to dlow directly into or out of the lock. Ground
paddles, on the other hand, are almost always found on top gates, and have a
sluice set into the top side of the lock: Water flows from there through a
crescent-shaped underground culvert to the exit partway down the lock chamber
below the lowest normal water level. This picture shows my father and one of
the ground paddles common on stretches of the Grand Union Canal, this example
at Tatton flight near Leamington.
Some
locks require back-breaking effort to raise the paddles, like the Rochdale
Canal in Manchester, whereas some newer examples of lockgear are almost
effortless, like the leading-edge ones installed on some locks on the Kennet
& Avon Canal. Newer hydraulic ones have replaced many of the older ones
which require more maintenance and are more prone to being raise in mischief by
the local urchins. Some locks still use the original type of ground paddle
where an enormous wooden lever is pulled up from the lock edge to uncover the
entrance to the underground culvert. This picture shows a typical kind of gate
paddle found throughout the system. The windlass is put onto the square
tapering bar in the foregound and turned to raise the paddle.
Layout of a LockThis is the layout of a typical lock. The light
blue shows the upper level and the dark blue shows the lower water. The twin
lock gates are oversized so that they meet at an angle to strengthen them
against the water pressure above the gate. Even a difference in water level of
2" is sufficient to prevent the gates from being opened. The oversize
balance beams act as a counterweight for the gates and facilitate opening them.
Gate PaddlesThese are the most common type of lockgear. By a series of connecting rods, and gears to make the winding easier, the shaft attached to the paddle is raised to uncover the sluice. Because the paddle is under water except for when the lock is completely empty, an indicator post above the lockgear indicates whether the paddle is up [open] or down [closed].
Needless to say, it is only in exceptional
circumstances that both sets of paddles (ie: on both top and bottom gates] are
raised at the same time. Similarly, all the gates should be closed before any
of the paddles are opened. In particular, the paddles on the bottom gates
shouldn't be raised if the top gates are open: Apart from the fact that if left
like that, the pound above the lock will be slowly emptied, if the top gates
subsequently swing closed, it will be with such force that they can be
seriously damaged, with a similar result!
The sequence to descend a lock goes thus: If the lock is empty, you must first shut the bottom gates and fill the lock by opening the top paddles (and closing the bottom ones). When the lock is full you can open the top gates and the boat can enter the lock.
With the top paddles now down and the top gates closed, you now
open the bottom paddles and the water level inside the lock equalises with that
of the lower level in the pound below the lock. There is always almost a cill
underneath the top gates which extends out into the lock for a couple of feet.
Care must be taken to avoid getting caught up on the cill, else disaster can
overtake the unwary. The furthest extent of the cill is normally marked in
white on the side of the lock and can be up to 6 feet or more from the lock
gates.
Lastly, once the water inside
the lock is the same as that below the lock, you can open the bottom gates and
the boat sails off into the distance (normally, but not always, after letting
the bottom paddles down and closing the bottom gates again).
One last word: It is not good practice to rope up
when descending a lock unless directed to do so by a lock-keeper (more
typically on large river locks such as on the Thames, Severn, Trent, or Avon
Rivers. If you do have cause to do so, take a single turn around a bollard only
rather than tying any sort of knot. That way the boat can pull more rope free
as it needs to do when descending the lock This will prevent you from tearing
cleats out of your decking or doing more serious damage. I know of a fibreglass
boat which was effectively toppled on its side before ripping half of its deck
away when it was tied to two bollards before letting the water out!
The sequence for going up a lock is essentially the reverse of this. First, equalise at the lower level, put the boat in the lock and open the top paddles. When the water level is equalised at the higher level you can open the top gates and leave the lock. Going up a lock care must be taken to be either right at the back of the lock or (only if you're experienced and in a narrow lock)to be right at the front, actually touching the front gates or cill. If you stay in the middle, the water turbulence will tend to slam you forwards against the front gates with great force. I once saw this happen where there were twin top gates. The front of the boat forced the gates partially apart and the water pressure behind the gates then held it fast there. A huge column of water sprayed out and drowned the boat completely in about 20 seconds!
Whichever of the above (ie: right at the front
or back of the lock) you choose, care must be taken that the boat does not get
caught up on any part of the cill, lock gates, or paddle gear, as if this
happens, the rising water level will force that part of the boat under water.
When ascending a wide lock alone, going right to the front of the lock is not an option to choose unless you have no alternative. For experienced users, my own preferred method is to "straddle" the lock, with my stern right back against one of the bottom gates and my bows against the opposite wall of the lock; Then only the paddles on the same side as the bow are opened to start with. The reason for this is so that the water entering the lock is either hitting the empty wall and rebounding towards the boat (keeping it there) in the case of ground paddles, or travelling across the lock almost parallel to the "open" side of the boat in the case of gate paddles. Note that if you open the paddles on the other side, the bow of the boat is invariably forced across the lock at some speed and an uncomfortable thump results, usually causing havoc in the galley!
For more inexperienced users of the canals, I would suggest that you rope up, by tying your longest bow rope to the bollard closest to the stern that it will reach. You should still be towards the back (near the bottom gates) of the lock, but as with a narrow lock, keep checking to ensure that the boat isn't getting hung up on anything.