Appendix: vortex theory of the propeller
Marine propellers

The vortex theory of the propeller [2][5][7][8] is the extrapolation of the theory of the limited-span hydrofoil to the case where, in addition to its translational motion, the wing is driven by a rotational motion around an axis parallel to the speed. In the case of a propeller, the vortices linked to the blade surface enter or escape the blade (figure 55 ) to form free vortices extending to infinity downstream along a helical trajectory with a pitch of 2πr tan β _i .

Figure