These words mean "without the necessary power, capacity, or ability" to perform some particular act."The speaker was incapable of making himself heard in the rear of the hall." "The speaker was unable to make himself heard in the rear of the hall." Unable is always followed by to, incapable by of. Incapable usually refers to a permanent or long-standing lack of some sort; unable refers to a temporary inability: "This truck is incapable of carrying more than 2 tons." "For several minutes he was unable to breathe normally."
Dictionary of problem words and expressions. Harry Shaw. 1975.