USCG Patrol Craft Hydrodynamic Fuel Efficiency Improvements

A computational analysis program, conducted at full scale, has been completed on the USCG Fast Response Cutter (FRC) to evaluate how appendage and propeller redesign affects calm water powering performance and erosive cavitation onset. Aft working forward, the geometric variations considered included: addition of a stern flap, wake adapted rudder redesign, propeller design refinement, wake adapted skeg redesign, and redesign of the spray rail system. The first activity was to use a wake alignment procedure to redesign the current rudder in an effort to improve the rudder drag characteristics and minimize or eliminate rudder cavitation. The wake aligned redesign eliminated rudder cavitation over the entire speed range, and decreased total drag by 6% at flank speed. Replacement of only the current rudders with the wake aligned redesign is predicted to increase flank speed from 28.9 knots to 29.4 knots. The stern flap and spray rail efforts followed the historical guidance of similar work performed on the USCG Island Class patrol boats, as documented in Cusanelli and Barry (2002). Following the geometric guidance of the Island Class stern flaps the final recommended stern flap for the FRC results in a brake power demand reduction of 15%. The Island Class achieved a 12% reduction in required brake power at similar speeds. The combination of the new stern flap, wake adapted rudders, current propeller, current skegs, and new spray rail yielded an increase in flank speed from 28.9 to 31.4 knots. Skeg redesign resulted in a 30% decrease in the magnitude of the radiated pressure pulse amplitudes experienced in the propeller tunnel above the propeller. The redesign of the skeg did not affect the propeller behind efficiency. Modification of the current propeller geometry was the final redesign task. The final system, which included the new stern flap, wake aligned rudders, redesigned propeller, wake aligned skegs, and new spray rail system decreased power by 18.6% at the prior 28.9 knot flank speed of FRC. The new flank speed, with the final system, has increased to 32.9 knots in the full load, end of service life condition. The combined effect of all redesign activities reveals an annual fuel consumption savings of 24,000 gallons per vessel per year, which corresponds to a 13.6% savings when compared to the original as-built system. Assuming a fuel cost of $4 per gallon, the annual cost savings per vessel per year equals $96,000. This savings extrapolated over a 58 ship fleet equates to $5.5 million in savings per year for the class. This saving per year yields a savings of $110 million for the 20-year operating life of the 58-ship class.