What is BHP, IHP, SHP, EHP, and Torque ?

Hydrodynamics for Ocean Engineers
Prof. A.H. Techet
Fall 2004

Marine Propellers
Today, conventional marine propellers remain the standard propulsion mechanism for
surface ships and underwater vehicles. Modifications of basic propeller geometries into
water jet propulsors and alternate style thrusters on underwater vehicles, has not
significantly changed how we determine and analyze propeller performance.
We still need propellers to generate adequate thrust to propel a vessel at some design
speed with some care taken in ensuring some “reasonable” propulsive efficiency.
Considerations are made to match the engine’s power and shaft speed, as well as the size
of the vessel and the ship’s operating speed, with an appropriately designed propeller.
Given that the above conditions are interdependent (ship speed depends on ship size,
power required depends on desired speed, etc.) we must at least know a priori our desired
operating speed for a given vessel. Following this we should understand the basic
relationship between ship power, shaft torque and fuel consumption.
Power:
Power is simply force times velocity, where 1 HP (horsepower, english units) is equal to
0.7457 kW (kilowatt, metric) and 1kW = 1000 Newtons*meters/second.

P = F*V

Effective Horsepower (EHP) is the power required to overcome a vessel’s total resistance
at a given speed, not including the power required to turn the propeller or operate any
machinery (this is close to the power required to tow a vessel).

Indicated Horsepower (IHP) is the power required to drive a ship at a given speed,
including the power required to turn the propeller and to overcome any additional friction
inherent in the system. Typically the ratio of EHP/IHP is about 1:2 (or EHP is 50% of
IHP).

Brake Horsepower (BHP) is the maximum power generated by an engine at a given RPM
as determined by the engine manufacturer.

Shaft Horsepower (SHP) is the power delivered along the shaft to the propeller at a given
RPM.

Regardless of how you think of engine power, as a general rule: the more power available
the faster the ship should go all other factors being equal. There is a tradeoff between
minimum required power, which would prevent the vessel operating at a fast enough
speed, and excessive power, which could be wasteful in terms of fuel, space, cost, etc.

Torque:
To use the power provided by the power plant (engine) to propel the vessel it must be
used to rotate the shaft connected between the engine and the propeller. Shaft horsepower
is converted to a rotary force (or moment) applied to the propeller. This rotary force
necessary to turn the shaft is simply torque.
Torque = Force * length (Newtons*meters)
When power is given in HP then torque can be found as

T = 5252.0 * HP / RPM

Where RPM is the revolutions per minute of the shaft and HP is the shaft horsepower.
You can see here that for the same power, a slower turning propeller will generate more
thrust.
Typically for engines and motors, power and available torque are provided as curves on
performance data sheets as a plot of BHP, Torque, and fuel consumption as a function of
RPM.