In Eq. (12), (∂F/∂l ) is the fluid force per unit length, U Rn and URt are the normal and tangential components of the fluid particle velocity relative to the element velocity, −→˙Un is the normal component of absolute fluid particle acceleration,
−→˙URn is the normal component ofthe fluid particle acceleration relative to the element acceleration. Also in Eq. (12),
w = 1025 kgm−3 is the water density, A is the external cross-sectional area (i.e. the same water-tight cross-sectional area as used for buoyancy calculations), Cn is the normal drag coefficient, Ct is the tangential drag coefficient and Cm is the added mass coefficient.