English: The reaction ferry is crossing a river to the right with the same speed as the current (from above), held by a tether that goes to a traveller on another cable that is stretched across the river (not shown). The friction of the traveller is exagerated 3-5 times. The ferry is positioned such as to produce an angle of incidence to the apparent current (here 10°), that produces force R, that is exactly opposing the total force in the tether. R can be regarded as the vector sum of L (lift) and D (drag) of the ferry hull and also made up of components downriver drag and thrust in direction of crossing. Force T is then the propelling thrust and exactly opposite to the friction component of the traveller at the tether. This simplified diagram ignores the 3-dimensional situation and the sag of the cables and shows no control devices such as a rudder.