18 hp Yanmar
We have a 29.5 on Lake Michigan. In general when cruising North the winds are from the dead North and when returning South the winds are from the dead south. So we end up motoring a lot when cruising. During an 8 hour motoring trip at about 5 mph (for some silly reason the Great Lakes use MPH instead of knots) we will burn about 4-6 gallons. The reason for the variation is wave action. If calm (1-2 footers we can normally go with slightly less than a half gallon per hour) the bigger the waves the more fuel you will use. For safety sake you really should go with the 1/3 rule. The 1/3 rule states:1/3 tank for travel1/3 tank for return1/3 tank for unforeseen circumstances (like a marina that doesn't carry diesel!)In addition, when cruising, I usually keep a 6 gallon jerry can of diesel - just in case, especially if we are crossing the lake. Long way to the otherside if the wind dies.