Any boat engineer knows that a flat bottom planes quicker...that's why we ski on flat sticks and not tubes. Boats hydroplane because they put flat "skis" under them and skiffs plane quicker than deep-V bottoms...So...why has no one built a pontoon boat where the tubes are round up front but morph into a flat surface as you go more aft? Wouldn't this make pontoons get more speed per horsepower?
Your observations are astute as far as mono-hulled boats. But pontoons are multi-hulls. The tubes--or AMAS as a naval architect would call them in multihull-speak--have a lot less surface area than a monohull. Because they would displace even less water than round tubes, flat bottomed tubes ( or amas or hulls, pick your word) would provide less ability to carry a load, like the gang and all their gear, and carrying power is one of the great strengths of pontoon boats.
Stability would also be an issue. While it might be easier to achieve plane, and to go faster, a 'toon with flat bottomed tubes would be dicey to handle, if not dangerous, at those higher speeds. At least if you ever want to turn.
And then there's the pounding of a flat bottom at high speeds...
Now it could work as a fast displacement boat. After all, some flat bottom ships can do 30-knots. But it would have to be BIG, as in long, since a displacement hull's speed is dictated by its length. Thats ships do it.
For all of that, there are pontoon boats that have incorporated tube geometry and added lifting strakes that provide increased performance for a given horsepower.Check out these links:
Thanks for the question.