Canal & River Locks


Page 3: Details and Terminology

For simplicity, this section describes a basic type of lock, with a pair of gates at each end of the chamber and simple rack and pinion paddles raised manually by means of a detachable windlass operated by the boat's shore crew. This type can be found all over the world, but the terminology here is that used on the British canals. A subsequent section explains common variations.

The change in water-level effected by the lock. The deepest lock on the English canals is Tuel Lane Lock on the Rochdale Canal with rise of about 20 feet. A more typical (English) rise would be 8-12 feet (though even shallower ones can be encountered).

The level stretch of water between two locks (on a river, the corresponding term is commonly reach). The lock allows a boat to move between the pound above it (upper pound) and the pound below it (lower pound).

The main feature of a lock. It is a watertight (masonry, brick, or concrete) enclosure which can be sealed off from the pounds at either end by means of gates. The chamber may be the same size (plus a little manouevering room) as the largest vessel for which the waterway was designed; sometimes larger, to allow more than one such vessel at a time to use the lock. The chamber is said to be "full" when the water level is the same as in the upper pound; and "empty" when the level is the same as in the lower pound. (If the lock has no water in it at all, perhaps for maintenance work, it might also be said to be empty, but a less-confusing term for this is "drained".)

A narrow horizontal ledge protruding a short way into the chamber from below the upper gates. Allowing the rear of the boat to "hang" on the cill is the main danger one is warned to guard against when descending a lock, and the position of the forward edge of the cill is usually marked on the lockside by a white line. The edge of the cill is usually curved, protruding less in the centre than at the edges.

The watertight doors which seal off the chamber from the upper and lower pounds. Each end of the chamber is equipped with a pair of swinging oak, elm (or now sometimes steel) half-gates. When closed they meet at an angle like a chevron pointing upstream (this arrangement is often called pointing doors) and a very small difference in water-level squeezes the closed gates securely together. This reduces any leaks from between them and prevents their being opened until water levels have equalised. If the chamber is not completely full, the top gate is secure; and if the chamber is not completely empty, the bottom gate is secure (in normal operation, therefore, the chamber cannot be open at both ends). A lower gate is taller than an upper gate, because the upper gate only has to be tall enough to close off the upper pound, while the lower gate has to be able to seal off a full chamber. The upper gate is as tall as the canal is deep, plus a little more for the balance beam, winding mechanism, etc; the lower gate's height equals the upper gate plus the lock's rise.

Balance beam
A long arm projecting from the landward side of the gate over the towpath. As well as providing leverage to open and close the heavy gate, the beam also balances the weight of the gate in it socket, and so allows the gate to swing more freely.

The simple valves by which the lock chamber is filled or emptied. A paddle is simply a sliding wooden panel which when "lifted" (slid up) out of the way allows water to either enter the chamber from the upper pound or flow out to the lower pound. A gate paddle simply covers a hole in the lower part of a gate; a more sophisticated ground paddle blocks an underground culvert. There can be up to 8 paddles (two gate paddles and two ground paddles at both upper and lower ends of the chamber) but there will often be fewer.

Windlass ("lock key")
A detachable crank used for opening lock paddles (NOT the winding mechanism itself). The simplest windlass ia an iron rod (circular section, about half an inch in diameter and three feet long) bent into an L-shape, with a square socket at one end for fitting onto the "stub" protruding from the lock winding gear. Most have two sizes of socket, for different locks standards. There may be refinements such as an extensible handle (for stiff paddles), or a free-rotating cylindrical sleeve around the handle (to prevent blisters). On canals with well-maintained (easy-to-lift) paddle gear, crews often prefer to carry a smaller, lighter windlass made of aluminium. Most boats carry three or four windlasses of different types.

Winding gear / paddle gear
The mechanism which allows paddles to be lifted (opened) or lowered (closed). Typically, a square-section stub emerges from the housing of the winding gear. This is the axle of a sprocket ("pinion") which engages with a toothed bar ("rack") protruding from the top of the paddle. A member of the boat's shore crew engages the square hole of their windlass onto the end of the axle and turns the windlass perhaps a dozen times. This rotates the pinion and lifts the paddle. A pawl engages with the rack to prevent the paddle from dropping inadvertently while being raised, and to keep it raised when the windlass is removed. After the boat has gone through the lock, the pawl must be disengaged before the paddle can be lowered.

"Turning" a lock
This can simply mean emptying a full lock or filling an empty one (We entered the lock, and it only took us five minutes to turn it). It is used more often to refer to a lock being filled or emptied while you are not in it (The lock was turned for us by a boat coming the other way) and particularly when there is no boat in it at all (The lock was set for us, but the crew of the boat coming the other way turned it before we got there).

Page 1 - Introduction & use of locks...Go
Page 2 - Basic construction and operation...Go
Page 3 - Details & Terminology (Current page)
Page 4 - Variations...Go
Page 5 - Illustrations...Go
Page 6 - History & Development...Go
Page 7 - Use of water...Go
Page 8 - Alternatives...Go



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