Planning to go the Caribbean route with the longest required leg being just under 700sm. The plane is an experimental 180 hp Super Cub with stock tanks, (no headers) a cargo pod and 35s... I figured that to have climb fuel and 2 hour reserve I'll need around 90 gals usable to make 700 miles.
Just thinking out loud. This is what I would plan on doing. But then I wouldn't even think of that distance over water without floats under the plane or two engines and lots of fuel.
700 miles between possible landing locations.
A carburetor 180 Lyc at 75% burns roughly 10 gph. Yes you can pull the throttle back for less fuel burn with the accompanying loss of speed.
What is the cruise speed of this Cub, 80-85 mph? Let's call it 70 mph to account for unplanned head winds. It doesn't take much of a head wind to drop a Cub's ground speed a lot.
700 miles @ 70 mph = 10 hours plus 2 hours reserve = 12 hours X 10 gph = 120 gallons.
120 - 36 = 84 gallons extra. 84 gallons = 11.2 cubic feet. Or 504 extra pounds of fuel.
You need a 11.2 cubic foot tank in the back seat area. Build it so that the front of the tank is against the back of the front seat. Pay attention to the CG location expecting an over weight situation. The Cub will handle the weight as long as the CG is within reason. A custom built aluminum tank for this purpose is common for long distance ferry flights.
Your tolerance may be different than mine. Having been in situations where there was nowhere to go for more gas, with unexpected headwinds, flying for nearly an hour with the gauges on
E, I consider the above a minimum. If you have an accurate fuel flow meter on board you may be able to tighten the requirements a bit.
