Estimating ship's powering
L.K. Kupras' method
At the initial stage of a ship's design a propulsion power estimation is to be done to ascertain that the vessel can sail at the specified speed. As the ship design development progresses, models tests are usually performed in a hydrodynamics tank, which serve to optimize the hull form, adjust the propeller(s) design and ensure the speed/power prediction for a proper choice of the propelling engine(s).
But at the design beginning only estimates can be made to quantify the propulsive power, using one or several existing methods issued by ship hydrodynamics specialists, based on the results of model tests or actual ships records.
One of these methods was developed by L.K. Kupras (Ref.1 and Ref.2), which is the one included in this post, where also an approximate propeller(s) diameter(s) is calculated, without consideration to the blades number. It is a very simple method, with only 6 input parameters, but with a rather approximate results for an initial estimate.
This method applies to vessels of normal forms in laden draft condition, but not necessarily fully load, propelled either by one or two propellers and considering that the engine(s) can be directly coupled to the propeller(s) or through a gearbox, what is controlled by the gear ratio (Reduction gear Coef. in the program) that can vary between 1 and 4.
The result is the propulsive power, under ideal trial conditions, needed to achieve the specified speed. If what is especified is the service speed under average conditions, you should consider a trial speed 1 to 1.50 knots bigger to take into account resistance increase due to hull fouling, sea state and the reduced engine power in continuos service.
References
Ref.1 - Kupras, L.K. 1980: Optimization method and parametric study in precontacted ship design. Int. Shipbuilding Progress 23, 138–155
Ref.2 - El Proyecto Básico del Buque Mercante,(Basic design of merchant ships) by R. Alvariño, J.J. Azpíroz, M. Meizoso. Fondo Editorial de Ingeniería Naval. Colegio de Ingenieros Navales y Oceánicos. 2nd edition Madrid 2007
Estimating propulsion power
L.K.Kupras method
Estimation of required propulsion engine(s) power for the ship getting the especified speed, under ideal trial conditions and engine(s) running at its 100% nominal power.
Input decimals with dot not comma
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