The Ship Drive train - different powers required for a ship

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 


Previous Post Next Post