Boats have what's called a "hull speed", calculated as a function of the waterline length.
"Hull speed or displacement speed is the speed at which the wavelength of the boat's bow wave (in displacement mode) is equal to the boat length. As boat speed increases from rest, the wavelength of the bow wave increases, and usually its crest to trough dimension (height) increases as well. When hull speed is reached, a boat in pure displacement mode will appear trapped in a trough behind its very large bow wave."
There is a pretty good article on Wikipedia from which I stole the above. With trawler-type boats, which do not get up on top of the water (plane) as you push them faster, the hull speed is the speed of best efficiency and usually the one that the boat will cruise at most economically. Very long hulls, because of their considerable waterline length, can achieve very high speeds in displacement mode- ie without having the hull lifted out of the water to plane on top of it. The design speed of the "United States" was in excess of forty knots, but that ship had nine hundred feet of waterline length.
To make small boats like ours go fast, it's necessary to lift the boat out of the water, so the hull is designed to create lift and elevate the boat partly out of the water as speed increases. To lift something that heavy out of the water using hydrodynamic lift takes a lot of horsepower, so the requirement for fuel goes up dramatically. But you get to go a lot faster.
I can't remember if your boat has turbos on the engines or not, but at idle speeds or speeds in the low rpm range, you aren't getting much boost from them, and the engines may be functioning more or less as nonturbos- not relying on boost to make horsepower. We have a number of members who choose to run at low speeds (ie below planing speeds) and just take longer to get where they are going. I don't think it hurts the engines at all as long as you speed up every so often and clear them out.
With the drop in values of older boats, a repower doesn't make any sense with your boat. Plus, it would make the boat much harder to sell when you decide to part with it and get something else, if you ever decided that. And fuel has gotten cheaper and may stay that way for a little while at least. I think you should stick with what you have, and if you decide you want to do a lot of long-distance cruising economically, start looking for a trawler. Like a Hatteras LRC.
