Bob. A quandary here with your hypothetical question…
The destroyer would undoubtedly be faster that a flatty starting with a low power motor, but if the power was increased , it would reach a point where the flatty would start to plane and then IT would be faster. I think thats right….
My little 30 inch HMS Midge torpedo boat (1890`s variety), which I built with a my customary “Glynn Guest” style flat bottom, is long/thin and fairly light and ridiculously fast on a speed 400, 6v 4.5 Ahr gel battery and ordinary 30mm 3 blade brass prop. The front lifts out of the water at full pelt just like the Turbinia. Now and again for a larf I put my 7.2v 2500mAhr NiMh battery in , and it almost planes. If it had a square back, like turbinia, I am sure is would.
Ashley. Hooligan at heart.