This post has been arround a while, so I don't know if you got an answer yet or if you still want one. In any event,the dynamic pressure in water(pounds per square foot ) is very close to the square of the velocity in feet per second.So that 50 mph becomes 73.3 fps. The square of this number yields almost 5400 pounds per square foot.Flat plate drag coeficients are as high as 1.28, but in your case,due to cavitation would be somewhat lower.Even so your 40 sq.in., which is 0.278 sq. ft.would create a huge 1500 pounds of drag.