Welcome aboard, and welcome to Allmand ownership! I have the M16 (actually, it's the 5416, which is the predecessor to the M16), so I can't directly answer your question. However, a quick Internet search suggests that for similar boats (e.g., the Catalina 30) you'll use somewhere between .5 and .75 gallons per hour. What does that translate to in miles? Well, that depends. If you're motoring only, the Allmand 31 will make about 5 knots in my experience. If you can get some good wind, though, you can motor-sail and get that closer to 5.5, possibly even 6 knots on average. My blog has a summary of our trip and fuel usage when we moved our Allmand 31 last summer (Post-trip wrap-up |
). For planning purposes, I think I'd go with the assumption that I'd burn an average of 0.75 gallons per hour, and that I'd average about 5 knots, just so I'd be sure I wouldn't run out of fuel. So, that means I'd plan on getting about 6.5 nautical miles per gallon. With a 25 gallon tank, that's at least 160 miles per tank. Again, you'll probably do better than this (even an increase in average speed to 5.5 kts moves this to almost 180 miles per tank, and if you're closer to .5 gallons per hour at 5 knots, you're going to be closer to 250 miles per tank). As you point out, there are a lot of factors that can influence how far each tankful will get you, such as engine maintenance, bottom growth, prop fowling, waves, wind, and currents. That's why I suggest being conservative in your approach, especially until you figure out exactly how YOUR engine behaves.