Question: Are water pipes square or are they round, and why??

If you have studied "Fluid Dynamics" there are cross-sectional flow diagrams in the textbooks that show that corners create drag ... along w/ adding surface area. It is the same for hull shapes moving through water/fluids.

Now, why would a flat bottom hull shape be more efficent?

It has to do w/ the fact that fluids are incompressable, with a specific viscosity ... so if you have a hull shape moving through a fluid, the fluid needs to move out of the way of the hull shape ... but ... it will only move so fast and no more. Now it has been discovered that if the hull shape is at the boundry layer (surface) between the fluid and gas/air and has a flat bottom (ie: planing hull shape), the hull shape will "rise" out of the fluid and into the air/gas since there is less friction/resistance when passing through the air/gas medium.

So at slow speeds, were fluids are in "laminar flow" a smooth round hull shape is more efficent ... now at higher speeds were the fluids are experiencing "turbulent flow" a planing hull shape will be more efficent as the hull shape rises up out of the fluid ... reducing the "wet surface area" and the associated resulting drag.

Now that is in ideal conditions w/ a smooooooth boundry layer between the two mediums (fluid/gas)..... now add waves ... waves of different amplitude and wavelength ... variations in speed/velocity of both the hull shape and waves ... materials used for the hull shape ... variations in hull shape attitude (pitch/roll) ... displacement of the hull shape ....

... very quickly you are dealing w/ engineering equations (calculus/diff eq) that are very long and complex ....

This is why it was easier from an engineering stand-point to get "a man" ... to the moon ... walk on it ... and return safely to earth ...

"Boundry Interface" problems are the toughest ....

Harry