The
purpose of the propulsion system on a ship is to convert fuel energy into
useful thrust to propel the ship. Figure below shows a simplified picture of a
ship’s drive train.

BHP “Brake Horsepower”
” is the power output of the engine. It is called “brake” because engines are
tested by applying a mechanical load to the shaft using a brake. The power of a
rotating engine is the product of the torque and the rotational speed.
SHP – “Shaft Horsepower”
is equal to the Brake Horsepower minus any mechanical losses in the reduction
gear. The reduction gear reduces the RPM (revolutions per minute) of the engine
to an efficient propeller speed, such as reducing from a few thousand RPM for
gas turbines to a few hundred RPM for a warship.
SHP=
BHP losses in reduction gear
DHP – “Delivered
Horsepower” is the power delivered to the propeller, which includes the losses
due to the gearbox, the bearings and the stern tube seal. The delivered
horsepower is usually 95%98% of the Brake Horsepower, depending on the
propulsion system.
DHP=
SHP losses in shafting, shaft bearings & seals
DHP = torque x 2πn
THP – “Thrust
Horsepower” is the power from the propeller thrust, equal to the product of the
speed of advance and the thrust generated by the propeller (with suitable unit
conversions). This power includes the losses of the gearbox, shafting, and
propeller.
THP=
DHP – Propeller losses
EHP – “Effective
Horsepower” is the power required to move the ship’s hull at a given speed in
the absence of propeller action. It is equal to the product of the resistance
of a ship and the speed of the ship. This power is equal to the Brake
Horsepower minus losses due to the gearbox, shafting and propeller, as well as
interaction between the propeller and the hull.
Basic Idea of Efficiencies
Gear
Efficiency:
Highlights
the loss of horsepower from the engine to the shaft as a result of the
reduction gears
SHP
is always less than BHP
Shaft
Transmission Efficiency:
The
loss of horsepower from the reduction gears to the propeller due to the
bearings and seals that support and seal the drive shaft
The
loss of power is converted to heat and sound due to friction
Propeller
Efficiency:
The
losses incurred in the propeller reduces delivered horse power to thrust horse
power. The propeller has an efficiency of 60% to 70% and hence the thrust power tp is given by 
tp = dp x propeller efficiency
Hull
efficiency:
Hull
efficiency is usually defined as the ratio of Effective power and thrust
power.
It
is also given by the formula:
Hull Eff. = (1  t )/( 1  w )
Where:
t = thrust deduction & w = wake fraction
The thrust power will therefore differ from the effective power. The ration of ep to tp is a little more than unit for single screw ships and about unity for twin screw ships.
ep = tp x hull efficiency
ep = dp x QPC
Mechanical Efficiency:
The power produced by the engine is the indicated power ip. The mechanical efficiency of the engine is usually between about 80%  90% and therefore only this percentage of the ip is transmitted tot he shaft, giving the shaft power sp or brake power bp.
Mechanical efficiency x ip = sp or bp
Transmission efficiency:
Shaft losses vary between about 3% and 5% and therefore the power delivered to the propeller, the delivered power dp, is almost 95% of the sp.
transmission efficiency x sp = dp