Thank you for your replies. Mostly they didn't address the issue. My simple question is: Two similar 45 foot displacement cats of different weights traveling at 16 knots. Why does the heavier one need more power? The best answer so far, I think, is that the heavier boat, making a bigger "hole" in the water has to push aside more water as it travels. But is the increased HP requirement proportional to weight? Will a cat weighing 50% more require 50% more horsepower? I don't think so. Perhaps in a barge it might, with all that frontal area. But a heavy cat can still have fine bows. There will be either a little more width or depth in the hulls. Gary