The answer to the U-boats was going to be Howard Hughes's Spruce Goose, until they were defeated by Allied Naval and Air Power with the crucial help of the Code Breakers. You just have to do the Maths to see that this project was not the answer to getting the massive build up of men and material to the UK ready for D-Day. Perhaps ACM Bowhill had more personal reasons for having his SiL attempt this hazardous crossing?