I was sailing with an owner aboard his trimaran a couple of years ago, tacking across Galveston Bay in a brisk breeze when we heard a crack like a rifle shot. Anxiously we looked around but could see nothing obvious and so, with a little tension remaining , we carried on. The next gust produced another bang and this time we noticed a steel cable trailing in the water behind the boat; one of the ama stays below the deck had parted. As best we could, we leaned out over the side of the hull and checked the remaining stays only to find a couple more that looked on the verge of parting! Sensibly we decided to put in to Smith Point, our closest destination and a place known well to the owner. Cautiously we approached the entrance but as we did so with the bows clearly pointing down the channel , I realized that we were not going to make it as the cross current was taking us sideways. But the vessel's owner was clearly not used to such things and believed that we would end up exactly where the bows were pointing. This inevitably led to a last minute panic as we found ourselves heading for the rocks on the north side of the entrance and it required a fairly quick gibe (with considerable stress to the remaining ama stays) to extricate ourselves. Our second attempt was better and we did eventually make it to the small fishing port there where we tied up and began planning the necessary repairs.
One of the difficulties in navigating at sea is that the sea itself may also be moving over the ground so that at times you may be going faster than you think, or in a different direction to your heading. And at times you can be sailing through the water yet getting absolutely nowhere over the ground.
The concept is sometimes a little hard to grasp. Imagine you are in an airport where they have moving walkways between the terminals. You and your partner walk side by side at the same speed until you reach one of these pedestrian conveyor belts. You decide to take the conveyor while you partner continues walking alongside the track. Despite the fact that you both continue walking at the same speed, you reach the end of the walkway a considerable distance ahead of your partner and then have to wait for them to catch up. While waiting you watch a couple of kids get on the conveyor in the wrong direction walking fast yet remaining in the same spot. The conveyor is like the ocean. Sometimes it helps you on your way, sometimes it is going in the wrong direction and sometimes it is stopped and not moving at all.
Even with a GPS to obtain your position you cannot find out where the current is taking you without the use of a reliable compass and knot log. The compass needs to be free from deviation and the knot log accurately calibrated to measure the distance you travel through the water. With this information you can compare data from the GPS with that from the knot log and compass and calculate the effects of current and leeway. But to do this you need to know how to plot a D.R. (Dead Reckoning) position on a chart.
Dead Reckoning is a way of showing on a chart, an approximate position for where you are. It usually starts with a known position (a fix) such as being close to the South Jetty Light, or a position from the GPS, or perhaps a Celestial fix. From this position a line is drawn representing the course travelled as shown by the compass (after allowing for Magnetic Variation) and then the distance travelled since the time of the fix is marked off along the line. For example, if you were at position "A" in Fig 1 at 1200 hrs and then travelled on a true course of 200 for 5nM you would show your D.R. position to be at "B".
Using a comparison between a D.R. position and a fix will show you where the current (and leeway of the vessel if applicable) is taking you. To see how this can be done, imagine you are taking a cruise along the Texas coast from Galveston towards Freeport. Because there is no wind, you motor out from the Galveston Yacht Basin, reaching the end of the south jetty at exactly 1300 hrs (1pm). From there you head on a course of 230 degrees toward Freeport. Each hour you make an entry in your log book, recording time, course, distance travelled and GPS position as well as other information on the weather, sea state etc. so that after several hours your log looks like Table 1.
By plotting your GPS positions on the chart you can see where you are. Fig 1 shows that you are drifting off to the south of your chosen course. But your progress nevertheless is a little better than anticipated. As you are motoring in calm conditions you can assume that this is as the result of a current which usually sets southwest along this portion of the Texas coast. If D.R. positions are also plotted on the chart then the set (direction) and drift (speed) of the current can be ascertained. Once this has been done, and the effect of the current is known it can be it can be allowed for by making a slight course alteration.