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Instructions
Pick your propeller design. Some new designs are more efficient than others, but they may require a particular size of blade or configuration. The new "whale tail" design, for instance, offers more efficiency and therefore more more range, but it is larger than conventional propellers. Choosing it would therefore change the desirable size of your propeller. Likewise, a double-propeller system with one left-turning and one right-turning propeller may require you to decrease the size of each propeller to match your engine power.
Choose a propeller as wide as possible in diameter, with enough clearance between the propeller's edge and the underside of the boat to avoid creating flow across the hull. Each centimeter of a propeller's width increases its range, and so increases your efficiency. However, any flow against the hull reduces that efficiency.
Increase the size of the blades themselves as much as possible but decrease their angle as much as possible. The decrease in pitch will decrease speed, but this will itself increase efficiency. Furthermore, this lower speed will prevent the propellers from possible damage by forming gas bubbles in their movement through the water. That damage impairs efficiency and so decreases mileage and range.
Choose the propeller with the thinnest possible blades and the smallest and thinnest possible hub. These features will lower the weight of the propeller, and so the engine have to use less force and therefore less fuel to move it.
If the blades are too light, they may twist and disintegrate.