After thinking about some of the oil burners I have been on, a SeeVee in particular, I think at lower speeds, 25 mph or so, they might actually get quite a bit better fuel burn. The down side is weight. In a 23' boat, a diesel is probably about max weight on the boat. With all that weight, you have to push it harder to get more of it out of the water. I am not sure that 23' SeaCraft hull really performs it's best with that type of power. The inboards do ok, but it is a different kind of boat/ride. More like a ship, than a boat. They plow, rather than slice.
It's hard to explain, unless you drive one.
These new outboards are really starting to get some gas milage, but it takes a lot of gas to pay for a 15,000 motor.
From what I have seen, 25' of boat is the break point for going to twin outboards, or diesel. The advantage of the diesel is milage and low center of gravity. The advantage of the twins is speed and redundancy, while giving up balance and fuel milage. If there was a perfect solution, there would be only one kind of boat
