Canal locks: an overview

Because canals were man-made and created to cross the landscape, they inevitably had to deal with changes in the height of the land: hills, valleys and so on.

You might think this is no issue, just dig the canals up and down the landscape and job done – but think a little more and you’ll appreciate that as soon as you fill a canal like that with water, it will all immediately drain to the lowest point, leaving you with a dry canal higher up.

Unlike rivers, canals are not continuously re-filled by streams and tributaries – and, rivers don’t go across undulating landscape, they just slope gently downwards over their whole length.

And if you’re thinking “well why not have a continuous flow of water into the canal”, it’s because you’d have to have a suitable supply at every high point along the length of the canal, and there just isn’t one everywhere the canals needed to go.

The solution is that canals are built totally flat so the water doesn’t try to run out and there is no natural flow at all.

But that brings us back to the original requirement of canals needing to cover ground which varies in height. Consider that if you build one part of the canal at one height and another part further up a hill – both parts being entirely level remember – how do you then join those sections so that boats can actually travel the different bits?

The answer is an ancient Chinese invention: the lock.

Locks are a giant tub of water that have a lower portion of canal on one side, the higher portion of canal on the other, and a set of water-tight gates at each end which prevent the water all immediately rushing from the higher to the lower level. Conveniently, the pressure of the water against the gates holds them firmly shut at one end or the other.

Notice that if the lock is full then the water pressure holds the lower gates shut, while if the lock is empty, the pressure holds the upper gates shut. Also note that in both cases this means the other set of gates – at which the water pressure is the same on both sides – can easily be swung open. Thus you can always either open the top or bottom lock gates, as long as the lock is either full or empty.

The result is that by opening one set of gates (whichever is not held shut), taking a boat in, closing those gates, and then either filling or emptying the lock as needed, a boat can be lifted or lowered between the two water levels without losing more than one lock-full of water at a time.

Clearly you need some way to fill or empty the lock as well and this is done by sluices built into the lock wall which can be cranked open either by a hand-operated gearing system or in more modern locks, by hydraulics.

When the canal builders encountered very long changes in the landscape, they’d build several locks in succession to cover the rise or fall (it is not practical to build a single lock deeper than maybe 20 feet or so as the gates become ridiculously heavy and the water pressure against them quite phenomenal, not to mention the dangers of such deep tubs of water for the humans on the canal boats).

The longest such stretch of locks is at Tardebigge to the south-west of Birmingham on the way down to Worcester. There are 30 (yes, 30!!) narrow-width locks in very quick succession – just two and a quarter miles.

That’s because the land between Worcester and Birmingham is essentially one very long slope so in order for the Worcester and Birmingham canal to reach its two destinations, many, many locks were needed. In fact, once you get to the bottom of Tardebigge, another two flights of six locks each follow quickly thereafter and there are even more locks before you reach Worcester! At that point, the canal ends with a final lock out to the River Severn.

Since electronics and hydraulics simply didn’t exist when the canals were built, and retro-fitting such systems to them now would cost hundreds of millions of pounds[1], nearly all the locks on the English and Welsh canals are manually operated – by you, the boater.

True there are a very small handful at which Canal & River Trust staff are required to do them for you due to particular technicalities, and a much greater number at which you will often find officially-sanctioned and trained CRT lock volunteers (“lockies”) who will offer[2] to wind the sluices and open the gates.

By and large though, you will end up doing the locks yourself. This is easy if you have someone with you and a bit of a pain in the arse if you’re on your own (though it is entirely possible and even pleasurable if you’re not in a hurry. There are plenty of solo canal boaters. It just takes a bit longer and requires you to have a system in place)

Note: you may find that on some locks, typically in urban areas, you are unable to wind the sluices because they’ve been locked. This is an anti-vandalism scheme to prevent idiots opening all the sluices and letting the canal run dry (it really does happen). It’s worth ensuring you have a ‘handcuff’ or ‘anti vandal’ key, purchasable from the Canal & River Trust or some marinas. This just slots in and turns, releasing a spring-loaded lock. Make sure you re-lock the sluices when you leave!

Double trouble

It’s worth noting that on the narrow canals, the locks are only wide enough for one boat (though if you have a two very short boats it can be possible to squeeze them both in lengthways!).

On the wide canals, the locks are double width which means they fit either one widebeam canal boat or two narrowboats side-by-side. It is generally regarded as good practice to go up two-by-two if you can, as this is the most efficient use of the canal water supply (rather than each going up individually after the other, which clearly uses two locks-worth of water)

You will also encounter lock flights which is simply one lock within a very short distance of the next (sometimes as little as a few yards, sometimes longer).

Most confusing of all to the novice are what’s called “staircase” locks where one lock chamber is built directly onto the next one (so one lock’s lower gates are the next lock’s upper gates).

Unless these locks employ side pounds to handle the water flow, this means the water from one will drain directly into the next so it is essential to set the entire flight of locks before you start.

Put simply, if you’re going up a staircase, you need the bottom lock empty and every other one in the flight to be full (because each will empty into the one below as you go up). If you’re coming down a staircase you need the top lock full and every other lock below that to be empty.

You obviously need to check that no-one else is coming up or down the flight before you start, and keep an eye out for people who don’t know how it works trying merrily to come into the flight further along your path! (clearly, others can come up or down in the same direction as you but one lock behind and this is actually the most efficient way of doing things).

Generally, the CRT have lock volunteers at hand on the complicated flights such as the Bingley Five Rise though. The rules still apply, that at all times you are in charge of your boat’s passage and if you’re unhappy with how a volunteer is running the lock, you have the right to instruct them to stop. They may argue, especially at somewhere like the aforementioned Bingley Five Rise but I have a friend whose boat was nearly filled with water by a lockie who was careless on that flight so stick to your guns if you need to.

[1] Not to mention spoiling the originality and history of the entire system, which many boaters thoroughly enjoy.

[2] Despite what some volunteer lockies seem to think, they cannot insist on operating the locks for you if you would prefer to do it yourself, however they will usually assume their help is wanted. If it isn’t, you need to alert them and sometimes you need to be quite firm. Despite their training, it’s not uncommon for lockies to let the water in faster than you might like which results in the boat crashing around. Do not be afraid to remonstrate politely and even report poor lockmanship if it happens.