Energy saving Flow Cap(ESFC) is a propeller boss cap with a few leaves. As a simple and effective energy-saving device, it has some advantages such as simple structure, easily installation, low probability of failure and low cost. An investment for an ESFC, calculated by CNY 5000/ton fuel price, period of ROI would be 5 months to 10 months. ESFC is effective for both new and old vessels, especially which is with large hub vortex and pitch ratio. This energy saving concept archived into the vision of the public in 1987~1989, but has not been in a wide application in China at that time. 
The fins of ESFC change the flow velocity distribution around the boss, and the direction of the flow, so that the flow direction around propeller hub changes also. 
The fins ESFC make the vortex disappeared, to prevent its aggregation.
The fins ESFC optimize the propeller flow field, improve the propeller thrust and offer a positive torque.
Yuanhang Propellers began to research ESFC and cooperate with ANDA Shipping Company about the ESFC project in March 2011. In September 2011, Yuanhang Propellers designed first ESFC for two 20500DWT bulk carriers “EN YAO” and “EN HUI” which are belong to Fujian ANDA Shipping Co., Ltd. The vessels took sea trial in November and get a significant result that oil consumption about 5%.
So far, Yuanhang Had design 25 ESFCs for ship owners from China, according to the owners’ feedback, energy saving effect is form 2% to 5%, the energy saving effect during on-going operation is obvious.