Horsepower

• Horsepower is a unit used to describe power in many different engines. It is a random unit of power, defined by James Watt, who worked on steam engines. He defined 1 horsepower as being how much work a pony can do in one minute -- lift 30,000 pounds 1 foot. One horsepower is also equal to 746 watts in electricity. This unit of measure gives a way to compare engine sizes and power.
  
  

Thrust

• Thrust is the force that moves a boat forward in the water. The formula for thrust is exit mass flow rate times the exit velocity minus the free stream mass flow rate times the free stream velocity plus the pressure differences across the engine times the engine area. The horsepower of an engine helps control the velocity and the flow rate of the water moved by the engine, but there are clearly many other factors at work in the process besides the horsepower of the engine. Thrust is indirectly related to horsepower, since power equals force times velocity, and force is equated to thrust in moving a boat.
  
  

Efficiency

• A high horsepower rating does not necessarily translate into power or more thrust in practical application. The horsepower and expected thrust ratings are calculated at maximum power. In an electric motor, that may mean an engine can only operate at maximum thrust for an hour, depending on the batteries and the modulation system. The efficiency of a gas or an electric motor is one of the determining factors in the actual thrust a particular motor gives out. The efficiency in the shaft bearings and the transmission affect the thrust as well.
  
  

Other Factors

• The propeller, including the surface area of the propeller and how it functions, also affect the amount of thrust an engine with a particular horsepower will produce. The weight of the boat also affects the actual thrust an engine gives, regardless of the horsepower rating. Additionally, the torque of the propeller and the drag of the blades through the water affect the actual thrust.