These two coefficients permit reasonable extrapolation of the effect on shell resistance of variation in displacement and wetted surface with variation in boat, coxswain, and rower weight.
I have been unable to find block and prismatic coefficients for hulls other than the Eights. A request for information from many boat makers has produced no result whatever. ROWING's current assumption is that the smaller shells have forms geometrically sufficiently similar to the Eights to have very similar block and prismatic coefficients.
Shell Frictional Resistance, Water
For a calculated volume displacement (V, at density (d) for water at 60 F) and a defined saxboard beam (Bs) ROWING finds the wetted length (Lw) (and estimates length overall), the wetted beam (Bw) (based on a semi-circular hull cross section centered in the saxboard plane), the draft (D), and the wetted surface area (S).
Using the instantaneous shell speed ROWING calculates an instantaneous Lw Reynolds' number and finds the skin friction coefficient, Cf, for turbulent flow (Re > 1,000,000) according to the ITTC-63 line. ROWING then adds an allowance for wave resistance (Cf *0.0002), and an allowance for form resistance based on an assumed 1/2 bow angle. ROWING then converts Cf to a resistance coefficient (Kw = Cf d S/2g). Unfortunately values for Cb and Cp are not generally available for hulls in general and so:
Alternatively ROWING will accept a "hard" value of Kw, independent of shell speed or displacement. I have published values of Kw only for fully seated eights. It would be relatively easy to estimate Kw = R/V^2 by weighing the towing line resistance (R) of other seated shells, at the same time knowing V.
ROWING does not consider the effects of pitch and water depth.
Shell Frictional Resistance, Air
Work on the frictional resistance of seated shells to the flow of air has not come to my attention as yet and I can only guess at values to use in ROWING. I estimate an air drag factor, Ka = Ra /V^2, as roughly one tenth of the water resistance allowing for boat, rowers, oars, riggers, etc. in air. In the future seated and sea-anchored shells should undergo the simple weighing of their longitudinal resistance in various airs as I can find no published data in this regard.
ROWING produces a detailed table from which accurately to plot the speed and acceleration vs. time curves--closely resembling published curves produced by data from instrumented shells. Figure 4-1 illustrates a shell speed curve for a single closely matching that of an instrumented boat (with an added curve for the system center of mass).