Skin Friction Reduction Effect on Operational Costs

The following analysis is based on the operational profile for a naval catamaran.  Recent ship designs that require high speed are evolving from vessels that generate tremendous wave and form drag (viscous drag) and have very little skin friction into long slender hull forms, i.e. catamarans and trimarans that have overall less viscous drag, but their frictional drag component is now much larger as a percentage of the total drag.  

Figure 1 shows the skin friction component as a percentage (%) of the total drag.  As the ship travels faster the percentage (%) of skin friction approaches 70%.  

Figure 1: Speed vs % of Total Drag due to Friction

Figure 2 shows the effect of skin friction reduction on the aforementioned catamaran.  The bare hull ship with no skin friction reduction is shown on the orange line.  At maximum power, 16.6 MW, the maximum speed is 46.6 kts.  With 60% skin friction reduction (SFR), the aforementioned maximum speed is achieved with only 10 MW.  If the full 16.6 MW of power is used with the 60% SFR, then 55.8 kts is possible.

Figure 2: Speed vs Propulsion Load

Figure 3 is based on the consumption rates per hour of the additive and fuel for the subject vessel (for fuel at $2.50/gallon).  The fuel rates vary as a function of propulsive power and the additive rates vary as a function of speed. At higher speeds, the benefit is more pronounced since fuel expenditure is related to the cube of speed while additive expenditure, with the Cortana ejector system, is only linearly related to speed.  Note, that for the same dollar expenditure rate, additive drag reduction can provide as much as a nine-knot (9 kts) increase in speed with this hull form.  

Figure 3: Speed vs Operational Costs

  Figure 4 displays the effect of 60 percent friction reduction on endurance assuming a fuel load of 42 metric tons.  The gray dashed line denotes the base vessel with in-service drag, i.e. the hull has been fouled over time with barnacles, etc.  Without skin friction reduction (the orange line), the most economical operating speed is about 32.5 knots with a range of just over 600 nautical miles (nm).  With skin friction reduction (the blue line), the most economical operating speed increases to approximately 37 knots and the range increases to approximately 790 nm.  Note, with skin friction reduction, one can achieve the same range of just over 600 nm, but instead of operating at 32.5 knots, one can now operate at 55 kts, or a 69% increase in speed. 

Figure 4: Speed vs Range

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