In a recent post we discussed the importance of being able to estimate your position at sea if the vessel’s GPS fails. In this post we go a little further with a reminder of how we go about estimating and plotting our position using traditional methods, when out of sight of land.
An important thing to remember when navigating at sea is that an estimate of a vessel’s position should be kept on a regular basis throughout a passage. Some navigators do this every half hour, others hourly.
Dead reckoning (DR)
If a navigator uses the most basic method of recording the course steered and distance run, this is known as a dead reckoning or DR position. For example they might have steered a course of 090º for one hour at a speed of 5 knots and travelled 5 nautical miles, which they then mark on their chart and record the time. This should give them a rough indication of where they are based on their last estimated position which they marked on their chart one hour previously.
Estimated Position (EP)
While the basics of recording the distance run and course steered are easy enough to do, in order to estimate an accurate position other factors need to be taken into account, including leeway and tidal streams.
Leeway is the sideways movement that is caused by the wind. It affects different types of sail and power driven vessels in varying amounts. Skilled navigators learn how to estimate how far their vessel has been blown off course by the wind.
For example, shallow draught motor vessels have little resistance and slip sideways more easily than deep draught vessels. A sailing boat with a high angle of heel will slip sideways more easily than when upright, when its keel offers greater resistance to sideways movement. Also, another factor to consider is the amount of leeway in any given period depends on the strength and direction of the wind.
Tidal streams are the horizontal movement of water caused by the vertical rise and fall of the tide, which is in turn caused by the gravitational effects of the Moon and Sun on the Earth. The tide’s direction (known as set) changes approximately every six hours. To complicate things, the tide’s direction and speed (known as rate) varies from place to place and depends on the relative positions of the Moon, Sun and Earth at any one time time throughout the year.
At a spring tide the rise and fall is greatest, which results in faster flowing water, while at neap tides the rise and fall of the tide is less, resulting in slower flowing water. There are two neap tides and two spring tides per month. Fortunately, all the timings, direction and variation of speed are predictable and made available to the navigator in the form of tide tables and tidal atlas chartlets. Factoring in the tides is consequently important when estimating the position of a vessel at sea.
Compass variation and deviation
Remember that nautical charts are aligned to true north, while a vessel’s compass points to magnetic north. The angular difference between true and magnetic north is called variation. The amount of variation differs around the Earth and navigational charts include a compass rose which shows the amount of variation that should be allowed for when a navigator makes their calculation.
In addition to variation, compass deviation also needs to be factored in when using a vessel’s compass. A vessel’s compass can be affected by magnetic materials around the boat. This includes instruments, wiring, mobile phones, electric motors and loudspeakers. This is why it is important to site a compass well away from other instruments and metallic objects that could affect its accuracy. Every steering compass should be checked from time to time by a qualified compass adjuster who will prepare a deviation card that will show how much deviation error needs to be allowed for any compass heading.
How to estimate your position
There are five steps involved in estimating an hourly position on a chart as follows (see diagram):
- Make allowance for compass deviation and variation in order to get a true and accurate course steered. This will amount to a few degrees only but becomes significant over a large distance.
- Make allowance for leeway to give a true water track.
- Draw a line in pencil from the last known position on the chart and mark off the distance run in the previous hour in nautical miles, as recorded by the log.
- From this mark then plot the tidal set and rate in knots.
- The final mark D (see diagram) is the vessel’s new estimated position and a line can then be drawn between it and the last known position (A). This line is known as the course made good and shows the actual distance covered over the ground. This distance, divided by the time (in this example one hour) also gives the actual speed over the ground.