2.3 Air resistance component

The air resistance with no wind present may be expressed as

(2.99)

where ? _a is the mass density of the air and A is the area of the above-water hull form projected onto a transverse plane of the vessel. We note the different form between eqs. (2.3) and (2.99); we used a surface area S in eq. (2.3) and a projected area A in eq. (2.99). It is logical to use S when frictional forces dominate, whereas it is logical to use A when pressure forces dominate. Efforts are made to design streamlined superstructures for high-speed vessels in order to minimize C _D . Wind tunnel tests are commonly used to determine C _D . Typical values of C _D are between 0.5 and 0.7. Because ? _a is only 1 .25 kgm ^{? 3} for dry air at 10 C whereas ? for salt water at 10 C is 1026 .9 kgm ^{? 3}, the air resistance makes a small contribution; however, it should not be neglected a priori. For instance, an SES on cushion has a small water resistance relative to a similarly sized catamaran. The air resistance will then be more significant for the SES than for the catamaran. The airflow can also influence trim and sinkage, which again affect the resistance. If a model is made with complete superstructure and is mounted below the...