I just returned from buying a second fuel can for Spartina, giving us a carrying capacity of about four gallons plus a quart of 50:1 gas/oil mixture for the two stroke outboard. I do not like carrying that much fuel on board. Usually for cruising a single 2 gallon 8 oz can will suffice. But there is a chance, if the wind is out of the south, that we may have to do a lot of motoring in very narrow, winding channels. Going inside the Chincoteague to Wachapreague, the only place on the seaside where we could refuel, could be as much as 35 nautical miles (40 statute miles) of motoring through the marshes.
Judging fuel consumption is not exact on a small boat. It changes with the current and the wind. On day three of the over the top we made a little over 10 nautical miles on the Chesapeake and Delaware Canal by starting with a full tank and refilling just once, still having fuel in the tank when we reached Delaware Bay. This was better than the usual mileage as we had a steady current going with us the entire time.
I found a brochure online for my outboard, the Nissan 3.5 two stroke, with the information in the screen shots above. The integral fuel tank holds 1.4 liters but I never completely fill the tank (filling the tank completely would allow fuel leak out the air vent in the gas cap when the outboard is kicked up into the raised position, where it is always when sailing). My estimate for what I consider to be a "full" tank is about a quart of gas.
The second screen shot shows fuel consumption of 1.7 liters per hour at full throttle. I have never run the outboard at fuel throttle, it is usually at half throttle or less with Spartina making between 3.5 and 4 knots. I estimate in typical conditions we get about 4 nautical miles per quart of fuel. With the second gas can we should have a range of 60+ miles.
Motoring is a backup plan. Hopefully we'll be sailing both on the ocean and through the marshes powered by a nice north wind. We'll see.