What is the primary function of the keel on a sailboat?

The primary function of the keel on a sailboat is to stop it from sliding sideways through the water. When a sailboat is under sail, the wind fills the sails, creating force that propels the boat forward. However, this force also has a tendency to push the boat sideways due to the angle of the wind on the sails. The keel counteracts this lateral movement or "leeway," providing stability and allowing the boat to maintain a proper course. By creating lift, similar to an airplane wing, the keel helps to ensure that the sailboat moves forward rather than sideways, which is vital for effective sailing.

The other options serve different purposes related to sailing but do not describe the primary function of the keel. Controlling the angle of sails to the wind primarily involves the rigging and sail management. Steering while under sail is primarily accomplished with the rudder, which directs the boat's course. Raising or lowering the mainsail is managed through the mainsail halyard and doesn't involve the keel at all.