This was one of my favorite movies as a kid watching it on TV in the sixties and seventies. The really excellent miniature work captured my youthful imagination in much the same way as Jason and the Argonauts. The film stands as a textbook on how to stage model ship action with really good scenes under sail and particularly under cannon fire with some really fine destructive splintering of spars and hulls.
The sequence with the miniature French fortifications is particularly noteworthy.
Shot in England, 4 special effects personnel are credited, Harry Barndollar, and Arthur Rhoades, both American and George Blackwell and Cliff Richardson, well known British special effects men. George Blackwell was the miniatures specialist of the group.
According to IMDB Peter Ellenshaw is uncredited for the matte paintings.