3 on a Meathook (1972)

William Girdler writes and directs this gritty slice of 1970s grindhouse horror. When four lithe young women set out for a weekend of partying and skinny dipping at a remote lake, their car breaks down and they accept the help of simple-minded farmer's son Billy (James Carroll Pickett). But when they get back to the farm run by Billy's father, Pa Townsend (Charles Kissinger), the party becomes a blood-spattered gorefest after Billy suffers one of his turns.