A river of uniform width, w, flows straight in the direction from west to east at a steady speed u. P is a pier on the south bank of the river. Q is a pier on the north bank of the river directly across. R is another pier on the north bank of the river a distance of w/2 east of Q (cf. figure 1). A boat starts at P and travels with a constant speed v relative to the flowing water, but always pointing directly towards Q.