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When I repowered my 1993 23 Walkaround , I went from old 150 2-smokes to a single Honda 250, going from around 800 pounds on the transom to 635 pounds. Also moved the 2 batteries from the transom , going to 3 batteries to the storage area under the helm station, moving them forward about 6 feet.
This week, went fishing with 4 men and 150 gallons of fuel on board, a total weight of about 6800 pounds , and managed 2.8-3.1 mpg running 17-30 mph. As a benchmark, my friends 21 Parker with brand new f200 Yamaha (4 cylinder) gets a best 2.75 mpg with 100 gallons of fuel and 2 people aboard.
Other boat test reports with the Honda 250 show around 30% better fuel mileage than other manufacturers engines on identical hulls, so I would at least give the Honda a look. It's also quieter than the New Yamaha 250 and 1st generation Etec 250, as I've been on boats at my marina with all of these engines.