If you are talking about while the boat is on the water, your fuel economy will probably be less than 4 mpg. The difference in frontal area (aerodynamic drag) between an open bimini and a stowed bimini would be minimal. If you could accurately measure fuel economy difference it would probably be no more than a few hundredths of a mile per gallon. This would not be enough for me to leave the bimini stowed on a warm summer day. I would stick with personal comfort to determine whether to open the bimini and let it go at that. If you were to try to measure it, you would need a calibrated digital tach to get the same RPM and a fuel flow with digital readout connected to a GPS.

Your replies mostly go along with my thinking.... I'm not ambitious enough to go to all the trouble to nail it down to the last tenth of a mpg. As it is, a boat has many drag producing elements as it is that can sometimes be mitigated but not eliminated.

I realize that boating and fuel economy do not go hand in hand..... aircraft tend to use a lot of fuel per hour too, only they do a lot better in the speed department. I have a lot to learn about my Sundowner and plan to experiment with loading, trim and power settings to see what will give me a decent speed and reasonable economy. I'm not a kid who has to be at full throttle all the time.... rather, a prudent sailor who prefers to know his vessel and how to get decent performance from it. For that reason, I appreciate you sharing your knowledge and experience with me.

In the FWIW department, my last two trips this fall showed an interesting contrast in fuel consumption..... The first, a 35 mile trip on the St.Clair and Black Rivers by Port Huron, MI, consumed 11 gallons of fuel. Thirty of the miles were on plane running down and up the St. Clair while 5 were at no-wake speeds on the Black. The St.Clair was pretty smooth (except where the river begins at the foot of Lake Huron) but not glassy. The second trip of 24 miles was on Lake Huron, from Port Sanilac to Lexington and back. Waves were running 1 foot or so and I was on plane at 22-24 mph. Fuel consumed, 10.8 gallons...... Loading in both cases was almost identical. Wind and wave action were the main difference, but it surprised me that it would have that much of a effect. Like I said, I have lots to learn which is part of the fun.