The Propulsion Efficiency Design Index (PEDI) is designed to give a clear and concise description of a ships energy efficiency in terms of how well designed it is for self-propulsion. The PEDI can be used to compare ships at different speeds and thus investigate the energy efficiency at speeds the ship will operate at. A database containing more than 700 ships allows any conventional ship-type to be ranked in a group of comparable vessels.
PEDI ranks vessels at the same speed. This provides a clear image of how well the ship is designed to perform at different speeds compared to similar ships. By doing so, a ship is only evaluated on its actual power requirement in a given condition. Fuel type and engine size cannot affect the ranking of a vessel because it is not included.
PEDI for tankers as a function of DWT, the design speed for this segment is between 14 and 16 knots
Example of PEDI ranking for a tanker segmet, the segment has a deadweight variation of 7%.
The PEDI is based on the required propulsion power at a given loading condition and a given speed. Each vessel is therefore associated with a number of PEDI-values corresponding to different speeds. The loading condition is typically given at scantling draught for tankers and bulk carriers and at 70% scantling draft for container vessels. By using these loading conditions a vessel is properly rewarded for it's design loading capacity. The index can be developed at other loading conditions if required.
The power requirement for the different speeds is found using the statistically validated power prediction tool, Ship-DESMO (1). The power prediction method finds the resistance of the hull in calm water and accounts for hull and propeller interaction as well as shafting efficiency.
The predicted powers are calibrated against the vessels design point. The design points are specific to each vessel in the database and consists of interrelated values for a particular loading condition, speed and power requirement, often contract specified values is used. This calibration ensures that innovative hull design is reflected in the PEDI-value.
Once the PEDI-values have been found, the ship is benchmarked against a group of ships of the same type and comparable size. Ships will always be compared at the same speed in order to achieve a fair ranking.
(1) Kristensen, Hans Otto, Determination of Regression Formulas for Main Dimensions of Tankers and Bulk Carriers based on IHS Fairplay data. Project no. 2010-56 Report no. 02-04
The benchmarking method is based on the hydrodynamics of the hull. Knowledge about the dimensions and cargo carrying capacity is therefore essential for the calculation of PEDI.
In order to benchmark a vessel, the following is required:
Lenght between perpendiculars
scantling draught and deadweight
Service allowance (sea marging)
For container ships:
Norminel TEU capacity
Design draught and deadweight