Three-dimensional turbulent boundary layer equation is expanded according to the ordinary perturbation method with the flat plate flow as the zeroth order solution. The momentum thickness, the local skin friction coefficient and the cross flow angle at the wall including the first order term effects are compared with the measured values on a ship model in acceptable agreement. As applications of this first order solution, an approximate formula to the frictional form factor K_F is obtained and the scale effect of the velocity and vorticity distribution in the boundary layer over ship hulls is discussed.