From model tests or from calculations, boat designers and builders can estimate how much horsepower is needed to propel a boat of a given weight at a given speed. In pure theoretical terms, this is the effective horsepower (EHP). Now, because not all of the horsepower that is sent down the shaft is effectively transferred into propelling the boat, we have to figure in a factor known as the overall propulsive coefficient (OPC), which takes into account the efficiency of the propeller and a few other smaller effects. For many stock propellers, the OPC is about 50 percent, meaning that half your power, and half your fuel and money, are lost in the transition. Improved, modern, custom propellers, often high-pitch, coupled to high-ratio (deep reduction) gears, can deliver OPCs of 60 percent to 70 percent. If you've ever wondered why anyone would buy a $5,000 custom prop when a $2,000 stock one will do, the answer lies in that potential 20-percent to 40-percent improvement in fuel efficiency and range.