Vessel Emergency Repairs 2 . Locks 3 . Dams 4 . Security 5 . These are used to move boats up or down from one level to another.
Locks operate on a simple premise.
You drive your boat into a chamber with gates at each end. To raise the boat, water is let into the chamber until it is the level of the water to be entered. When the water reaches that level the gates on the new elevation side are opened and you drive out. To go to a lower level, you enter the chamber and the gates are closed and water is let out until you are even with the lower level. The gates on that side are opened and again you drive away.