1. Cleaned and waxed hull: this is self explanatory. Everything below the waterline affects your boat speed and performance more than anything above. If the hull is not smooth there will be resistance between it and the water resulting in increased fuel consumption to attain a given speed.
2. Clean propeller with correct pitch: as above, but with the addition that fouling will impede the prop’s hydrodynamics. The pitch (angle of the blades) also needs to be set for the strength of the engine and the type of boat to be efficient.
3. Optimized engine rpm: Every engine/boat combination has an optimal engine rpm. Making the engine run n% faster does not necessarily result in an equivalent increase in speed. Creating a fuel consumption chart across the range of rpms your boat runs at will help you identify the most efficient engine speed for your boat. As a rule of thumb, you should never try to go as fast as possible. With a displacement hull, keep your speed well below the maximum hull speed: Maximum Hull Speed = 1.34 * LWL1/2. As you get closer to this figure, significantly more energy is required for every incremental increase in speed. It is a worthwhile exercise to create a fuel consumption chart for your boat.
Note: With a sailboat, the LWL increases as the boat starts to heel, which increases its speed by reducing the drag with the same energy (wind) input.
With a planing hull, bring the vessel to a plane quickly and then reduce speed. Planing reduces your wetted area and thus reduces your drag requiring less energy to propel your boat. As above, increasing speed after your boat is planing, exponentially increases fuel consumption.
4. Charged with solar power: an excellent way to augment your energy needs and reduce engine idling to charge batteries. This can be taken to a higher level in an all-solar-powered electric boat.
5. Tapped into wind power: another viable alternative energy source to keep batteries topped off and refrigerators and AC running.