Morland, Saffman & Yuen's (1991) study of the stability of a semi-infinite, concave shear flow bounded above by a capillary–gravity wave, for which they obtained numerical solutions of Rayleigh's equation, is revisited. A variational formulation is used to construct an analytical description of the unstable modes for the exponential velocity profile U = U0 exp(y/d), −∞ < y [les ] 0. The assumption of slow waves ([mid ]c[mid ] [Lt ] U0) yields an approximation that agrees with the numerical results of Morland et al. The assumption of short waves (kd [Gt ] 1) yields Shrira's (1993) asymptotic approximation.