Those later model looper “115”’s must be a lot stronger than the old 1975 cross-flow 115 that I had on my Seafari if it can turn that big a prop! I think I ran a 13.75” x 15”P OMC SST prop that gave me about 32-33 mph at WOT with a light load or 30 mph with a heavy Bahamas trip load. Used to cruise all day long at 4500 rpm/20 kts with those loads and averaged 2.8 mpg over 5 Bahamas trips, so I’d think your motor would do better than that.
Those old cross-flow motors weren’t particularly efficient but they were simple and easy to work on, plus they only weighed 300 lbs, so boat was well balanced and would plane easily at 12 mph! The one I had was extremely reliable; never failed to start once in the 31 years I owned it, and it was still running strong when I sold it!
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