The term "boat winch" can refer to a few different types of devices. The most common boat winch is the type used on a trailer that hauls a boat; the winch is mounted at the front of the trailer, closest to the towing vehicle, and it is used to help haul the vessel onto the trailer. Other winches can be used on board a vessel to feed the sails or otherwise move rope rapidly to help ease the function of some boat components. The device usually consists of a pulley wheel around which rope or cable is wrapped several times.
The pulley of the boat winch may be mounted to a motor that turns the pulley in either direction, or it may be mounted to a frame with a hand crank which allows a user to move the pulley manually. A motorized boat winch is more likely to be used on a trailer to help pull the boat out of the water and onto the trailer without the need for manual pushing or pulling by boaters. This device will mount directly to the frame of the trailer, and it will feed out cable with a hook at one end. The hook can be mounted to the front of the boat so it can pull the vessel effectively. Such winches must be rated by weight capacity; this will dictate how large of a vessel can be hauled.
Another purpose for the boat winch is securing a boat to a dock or preventing theft. The winch can be secured to the dock itself, and the cable or rope can be strung at various securing points on the boat, thereby creating a sort of lock for the vessel. During the winter months, some boat owners use a boat winch to haul the vessel out of the water to protect the hull from ice damage. This usually requires the use of a much larger mechanism that can handle the weight of the entire vessel.
Winches mounted on the boat itself are often used for controlling sails. The winch can be wound to pull the rope, thereby raising the sails, or let out rope to lower the sails. The advantage of using a boat winch for this purpose is speed: a user can raise or lower the sails far more rapidly than simply pulling on the ropes by hand, leading to more efficiency when sailing.