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A boom brake controls the motion of a boom, a pole or spar that extends sails on a sailboat by running horizontally along their bottom edge. Boom brakes manage sideways and upwards motion. The mechanism consists of a brake drum that rests between a pair of plates. A line wound around the brake drum and connected to a pair of guide plates gives the boom a limited amount of swing before the brake begins to function.
The position of boom brake is slightly below the boom. It is typically located aft, or near the rear of the boat. After it is wrapped around the brake drum, the line extends from both sides of the brake. Each end is secured to a side of the boat, typically with some kind of winch.
Much of the friction provided by the boom brake comes from the brake drum. It is a cylindrical object that is lined with indentations known as helicoidal grooves. These are evenly spaced so that the turning motion that precedes the resistance from the brake will be as smooth and even as possible.
The line around the drum can be adjusted, depending on the needs of the crew. With the line wrapped tight, it can provide maximum resistance to wind. In milder conditions, the line can be loosened slightly, so that it still manages the boom, but not with as high a degree of control. The brake may also be disabled entirely if the line is removed or completely loosened.
In order to properly adjust the boom brake, it usually must be tested with the craft in motion. It can then be adjusted to specifically suit current conditions. While a certain setting may suffice for most trips, it is possible that the boom brake will frequently need to be adjusted. If additional control is required, the line may be threaded through circular guides known as turning blocks. These pieces more evenly distribute the strain of resistance as well.
There are multiple reasons why a boom brake is an important part of sailboat rigging. From a safety perspective, it keeps the sail from swinging too fast and potentially causing injury to crew members and passengers. It may also prevent damage to the sails or the boat. Boom brakes also help to keep a sailboat on track by managing the motion, and thus the orientation, of the sail as it propels the craft.