psneeld,
If a given boat of a trawler type requires 100 hp installing 200 hp or 50 hp is just stupid.
Unless---- as we have discussed before--- the market for your boat wants to go faster than slow at times, and slower than fast at other times using the same boat. Assuming you have the hull that can do it, which most semi-planing hulls like GBs and the like can, it can be very advantageous to a boater with a schedule to be able to go 15 knots to his destination, say Desolation Sound, then bumble around for a week or so burning hardly anything and enjoying the scenery at 8 knots, and then run back home at 15 knots.
The semi-planing hull lets you make a boat that will do this. A late-model GB42 with the two standard 400+ hp Cat diesels will semi-plane somewhat efficiently (if you consider 23 gph efficient) at 15 knots or so, but then that deep keel and hard-chine flat afterbody hull gives you a pretty stable ride at slower speeds at 7 gph at 9 knots or so.
If the goal is to move a boat through the water as efficiently as physics and hydrodynamics will let you do it, that's the absolute wrong way to go, I agree. An easily driven hull with an appropriately sized engine is the right way to go.
But the market trumps theory almost every time. So if your market wants to go reasonably fast as well as comfortably slow in a fairly good-sized boat, which pretty much defines GB's market over the last twenty years or so, you do what they have done.
It might be bad theory, but it makes for good sales. Theory doesn't make the house payments, put food on the tables, or gas in the cars for the people who own and work at the boat manufacturer.
We (Boeing) could make our planes a hell of a lot more efficient and quiet and less polluting and cheaper to buy and operate if we didn't have to make them big and strong enough to carry all those heavy seats and the heavy people who sit in them and their heavy luggage and Acme Freight Forwarder's heavy cargo. If our only goal was to make a plane that would fly as efficiently as possible we could do it very, very easily.
But nobody would buy one because it wouldn't do what people want to do with an airplane. So we compromise all sorts of aerodynamics and physics laws and we make them big enough for the airlines to jam a bunch of seats in them and carry tons of revenue freight in the belly and put in an incredibly heavy entertainment system (did you know the inflight entertainment system in a modern jetliner is BY FAR the most complex system on the entire plane?).
Which means we have to put a pair of gigantor 115,000 pound thrust engines on the damn thing to get all this crap up in the air and go somewhere at a speed that makes it worth what the people going and shipping stuff there are paying to do it. And airlines line up in droves to buy them because the plane will do exactly what they need to do to make money.
That's an extreme example of what GB and others are doing. The market drives the design and configuration and pure theory has to take a back seat.