Stemming the tide
In Fig 8.0 we have two boats traveling northwards. The blue boat has turned slightly towards the direction of the tide, stemming it, to travel in a straight course. The boat to the left which has not adjusted its heading to compensate for the westerly tidal flow has drifted off course. When the blue boat is stemming the tide, it therefore travels along a straight ground track but slows down as it is impeded by the tide a little. We have mentioned in a previous module how we can use this effect to slow the boat down but maintain steerage when we manoeuvre alongside a quay.
However, when calculating a Course To Steer (CTS) we are primarily concerned with how much we change the heading of the vessel to counteract the effect of the tide.