Understanding Ship Drive Train and Effective Horsepower
Ship drive train system components, including the engine, reduction gear, shaft horsepower, delivered horsepower, and thrust horsepower, are crucial for understanding the effective horsepower needed to propel a ship. Effective horsepower (EHP) calculations are essential for determining the power required to move a ship hull at various speeds. The relationship between EHP and the power train system can be explored through towing tank experiments and Froude's Law for scale conversion. Calculation methods for EHP are discussed with a focus on the power measurement units and the impact of vessel speed on the required EHP.
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Presentation Transcript
Ship Drive Train & EHP Sections 7.1-7.4
Ship Drive Train System EHP Engine Reduction Gear Screw Strut Bearing Seals THP BHP SHP DHP
Ship Drive Train and Power Engine Reduction Gear BHP Brake Horsepower (BHP) Power output at the shaft coming out of the engine before the reduction gears
Ship Drive Train and Power Engine Reduction Gear Bearing BHP SHP Shaft Horsepower (SHP) Power output at the shaft coming out of the reduction gears
Ship Drive Train and Power Engine Reduction Gear Strut Bearing Seals BHP SHP DHP Delivered Horsepower (DHP) Power delivered to the propeller DHP=SHP losses in shafting, shaft bearings and seals
Ship Drive Train and Power Engine Reduction Gear Strut Bearing Seals Screw BHP SHP DHP THP Thrust Horsepower (THP) Power created by the screw/propeller THP= DHP Propeller losses THP is the end result of all HP losses along the drive train
Effective Horsepower (EHP) EHP Engine Reduction Gear Strut Bearing Seals Screw BHP SHP DHP THP
Effective Horsepower (EHP) The power required to move the ship hull at a given speed in the absence of propeller action *** EHP is independent of the Power Train System *** EHP can be determined from the towing tank experiments at the various speeds of the model ship EHP of the model ship is converted into EHP of the full scale ship by Froude s Law. Measured EHP V Towing carriage Towing Tank
Effective Horsepower (EHP) EHP Calculation On Equation Sheet (Ch. 7) ft s ( ) lb Power which is a measure of R V T S lb ft s = 550 1 HP
Effective Horsepower (EHP) The required EHP varies depending on the vessel s speed. Resistance increases as speed increases, requires more HP Typical EHP Curve of YP TOTAL RESISTANCE CURVE YARD PATROL CRAFT 20000 Total Resistance, Rt (lb) 15000 10000 5000 0 0 2 4 6 8 10 12 14 16 Ship Speed, Vs (knots)
Ship Drive Train and Power EHP BHP SHP DHP THP Shaft/ Bearing Hull E/G R/G Prop. Relative Magnitudes BHP> SHP > DHP > THP > EHP The reverse relationship can NEVER be true because there is ALWAYS some loss of power due to heat, friction, and sound
Ship Drive Train and Power EHP BHP SHP DHP THP Shaft/ Bearing Hull E/G R/G Prop. Every Role Supports Positive Heroes Be Strong, Determined, Tenacious Everyday
Propulsion Efficiency Gear Efficiency The loss in HP along the drive train can be related in terms of Efficiency, or 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
Propulsion Efficiency Shaft Transmission 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
Propulsion Efficiency Hull Efficiency Hull Efficiency ??? ???= ????????? ?????????? ? ???? ?????????? ? ???= Hull efficiency changes due to hull-propeller interactions. Well-Designed ship : ? ???> 1 Poorly-Designed ship : ? ???< 1 Flow is not smooth. THP is reduced. High THP is needed to get designed speed Well-Designed Poorly-Designed
Propulsion Efficiency Propeller Efficiency Propeller Efficiency ??? ???= ? ???? ?????????? ????????? ?????????? ??????????=
Propulsion Efficiency ** Propulsion Efficiency ** Propulsive Efficiency (Coefficient (PC)) ??? ???= ????????? ?????????? ? ??? ?????????? ??????????? = ?? = On Equation Sheet (Ch. 7) Combines the losses due to the bearings, guides, and the propeller efficiency Compares the output from the reduction gears to the required towing HP Commonly ranges from 55 - 75% Once ??????????? is found, can try different power plants, gearing, and fuel efficiencies
