(How they work)
The sequence to descend a lock goes thus: If the lock is empty, you must first shut the bottom gates and close the bottom paddles. Then fill the lock by opening the top paddles. When the lock is full you open the top gates and the boat can enter the lock.
With the top paddles down and the top gates closed, you now open the bottom paddles and the water level inside the lock equalises with that of the level in the pound below the lock. There is usually a cill underneath the top gates which extends out into the lock. Care must be taken to avoid getting caught up on the cill or 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 level 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 all gates and paddles are left closed unless on rivers, where the exit gate is often left open.
Ascending is the reverse of the above, taking care not to snag the boat on top or bottom gates.
The term paddle covers all the different types of devices for filling and emptying the locks.
Ground paddles
These are usually found at the top of locks and are a simple rack and pinion system. The water enters via the base of the lock chamber and causes the boat to surge backwards initially and then surge forwards as the level rises.
Gate paddles
These come in various designs, some rack and pinion via a gearing wheel, some hydraulic (found a lot on the K&A) and can sluice the water in above or below the water level. These need treating with respect, as opening fully, too soon, can cause the flooding of a boat which is well forward in the lock. With full length boats this is a major detail as they must position themselves to the front of the lock, because of their length. ( Dawn has her own name for some of the more difficult paddles.)