Whipsy, You might want to re-calculate as your milage seems way out of wack for 15 kts of speed which I would guess as WOT!
It is 15NM from Point Roberts to Sucia as the crow flies, which was 2.5 hours on my old Honda 50 @ 3700rpm 6 kts. I could usually get 4.5-5 hours on my 25 litre tank at that speed & rpm. It would be a tough squeeze to get the return trip (30NM) on my one 25 L tank but I might make 25NM on 25L which works out to 1 NM per Litre at 3700 rpm 6 kts. How can you possibly get the same mileage doing 15 kts at WOT – 6000 RPM!!! I don’t believe you! At WOT you would be drinking that fuel faster than I could drink water. Try again!
PS on my new Yamaha 70 the same rpm would give me 8 kts so I could likely make the round trip with one tank. The 14X11 prop really gives me more distance for the litre on the same time frame due to the greater speed, but if I wound her out to 6200rpm I would get well over 15 kts but the gas would be gone just as fast! I might make it one way on a 25 litre tank and get there in one hour but I would need a second tank to get home on at that speed. I keep my speed down to conserve on gas. 4000rpm seems to be the max otherwise she is a heavy drinker.