can assume that the keel is rotatable or has a large flap so that the hull may operate with substantially zero leeway these simplifying assumptions obviate mathematical complications but are not really needed to arrive at the general conclusion then the minimum operating angle of the relative wind here equal to the true wind angle since the water speed approaches zero may be called the wind pinch angle as follows wind pinch angle arctan sup c d c sub l aero arctan 2 lwl f re pi sup 0 5 keel vertical span hydro 6 where c d is the total wind drag coefficient of the entire vessel both parasitic and induced c l is the lift coefficient of the sails lwl is the load waterline length and f re is the dimensionless function of water reynolds number used to correlate hulls 39 parasitic water drag said drag being equal to f re lwl 2 water speed 2 water density 2g at zero leeway but not necessaily at zero heel keel vertical span is the measureable draft from hull to lower tip rating rules limit the keel span for instance to about 0 15 lwl f re averages about 0 0014 for hulls operated at low water speed hence the second.