A handful of snippets about the process of designing the boat, which I have picked up from other senior members:
From Graham Knox: it is a mistake to think that Jack actually designed the boat, as such. Although he was one of the two leading dinghy designers of his era, Jack was first and foremost a boat builder, rather than a designer. When Group Captain Haylock commissioned the boat Jack had an idea. He then built the boat, using whatever wood he happened to have available, so certain details such as the shape and dimensions of the seat knees reflect what spare pieces of wood happened to be lying around in his workshop. Only after the boat was built did he produce the drawings.
From David Gilbert: Back in the days when there was a GP association members’ meeting at the London Boat Show, there was discussion about a rule change to permit the rudder blade to go down to make the front edge vertical. Jack attended, and after the meeting I recall overhearing Jack saying that this was a nonsense, as the draughtsman who drew the plans (note that Jack did not draw them himself) drew the rudder blade swept back only in order to fit it on the drawing!
From Dave Bower: He met Jack Holt at one of the Boat Shows, and asked him what he had done in designing the GP14 to make her such a superb sea boat. He got a one-word answer: Displacement!