A triangular sail is a type of sail used in sailing vessels. The three most common types of triangular sails are the JIB, SPINNAKER, and LATEEN. A JIB is a triangular sail that is set at the front of the boat and is used to help steer the boat. A SPINNAKER is a large, lightweight sail that is used to increase the speed of the boat. A LATEEN is a triangular sail that is set at the back of the boat and is used to help the boat turn more quickly. All three of these sails are triangular in shape and are used to help propel the boat forward.