Determining the EEDI ‘Weather Factor’ fw

F. C. Gerhardt, M. Kjellberg. SSPA Sweden AB

Influence of EEDI on Ship Design & Operation, 13 September 2017, London


The legislation on the Energy Efficiency Design Index (EEDI) requires determining a ‘weather factor’ fw that reflects how many percent of its calm water speed a ship can maintain in Beaufort 6 and corresponding waves. The higher the fw–value, the better the ship performs. In this paper we present and discuss a cost efficient way of experimentally finding the weather factor fw by means of wave tests in a seakeeping basin. To this end an evaluation software was developed to calculate fw from tests in both, regular and irregular waves in conjunction with calm water towing tank tests and wind tunnel experiments. Results show that the ‘fw standard curves’ from IMO Circular MEPC.1/Circ.796 are a useful tool to estimate an fw-value but are also very conservative i.e. the curves over-predict the speedloss in a seaway. Results from theoretical fw-calculations based on non-linear time domain seakeeping simulations are also presented and discussed.