Re: offshore capable boat?
well, my numbers in my post above are off the chart when Yamaha tested my same boat model with my same engine. Their prop was close but not exact. My bottom is painted; they had a T-top. (21' deep V center console, 80 gallon tank, 150 2 stroke carb). I don't know how my MPG and GPH compares at all; no true way to measure, so I use this info as "close enough." The RPM/MPH tested by tach and GPS is pretty close even though my prop was a little different from the test prop. In the last few weeks I have been trying props and charting the results, and just settled on one that give me good numbers but I can't speak to consumption. Since I go all over the bay and rivers, I have fuel supplies around if needed, unlike going offshore.
My guess is that an improper prop, over or under, hurts your fuel efficiency. If you are under propped, it's like spinning your wheels or like a low gear ratio (why my old landcruiser got bad mileage). If you are over propped you are straining, like pulling a trailer or a load of firewood in a pick-up. But there the analogy between boats and cars dies, b/c cars don't plane. Maybe "overdrive" is the closest equivalent.
