Electronic navigation, particularly with the advent of the Electronic Chart Display and Information System (ECDIS), has added more layers to the safe conduct of navigation. In order to deliver the promised enhanced safety of navigation, the Officer of the Watch (OOW) can be forgiven for abandoning many of the traditional chart work skills while adapting to the electronic chart. However, despite the move to electronic navigation, most of the traditional skills of yesterday remain just as valid today.
It is impractical to make constant reference to the ‘traditional’ paper chart and simply trust in historical position fixing to give an accurate position. Dead reckoning (DR) ahead, at intervals not less than twice the distance to danger, should be the primary tool for keeping the vessel safe from grounding. Additionally, monitoring visual bearings (on those vessels fitted with a centreline pelorus) or radar parallel indexes, will provide ‘real-time’ precise track control. However, in the absence of any visual bearings or radar parallel indexes, the charted fix and an up-to-date DR position will provide ‘near-real-time’ track control to keep the vessel safe from grounding.
Where not to be
It is fascinating that in the modern era, we know with absolute precision where a vessel grounded, courtesy of Global Navigation Satellite Systems. This means that, in the event of a grounding, the error was not so much failing to know where the vessel was, but failing to know where the vessel should not have been.
The task of navigation remains a vital component of situational awareness, which means ‘being able to identify, process and comprehend what is happening around you, where you are and where you are going’. In effect, you either have situational awareness or you do not. It is not something lost if you never had it in the first place!
Safe water and the no go line
In reality, the planned track is simply a ‘datum’ within the boundaries of the safe water limits defined by the ‘No-Go’ line that allows the navigator to keep the ship safe from grounding. This is like driving a vehicle along the road, where the kerb marks the ‘No-Go’ line and the driver maintains a safe distance and uses the white centre line or lane markers as reference points to maintain a safe position. (See Figure 1).
Another similarity with the road vehicle is the method used to drive smoothly, by looking ahead and scanning all round to predict where to place the vehicle on the road. The driver does not rely on looking down through the floor-pan (the shipboard equivalent would be real¬time position monitoring by visual bearing or parallel index) or simply using the rear-view mirror (calculating based on historical position fixing) to determine where the vehicle is heading, as this would result in erratic driving and be catastrophic. The driver looks ahead and uses constant feedback to determine where to point the vehicle. (See Figure 2.)
Modern electronic navigation systems provide this function by giving a continuous vector of a pre-determined length/time based on GNSS positioning. Additionally, ECDIS can provide a vessel-shaped predictor for monitoring the rate of turn when altering course. So to continue the analogy:
- Fixing is akin to looking through the rear view mirror;
- Parallel indexing is akin to looking downwards; and
- Fixing and dead reckoning ahead is akin to looking through the windscreen.
It is pretty obvious which method offers the best solution. So the navigator must always recognise that a fix is just historical information astern of the vessel in the wake. [See Figure 3.]
In Figure 4, we use the historical position to generate ‘near real- time’ positioning by constructing ahead a Dead Reckoning (DR) or Estimated Position (EP) (corrected for tide/current/wind) from every new fix for at least two fixing intervals, with the interval never less than twice the distance to the nearest danger.
The DR/EP position is then used to provide accurate track control, anticipate any hazards and give early warning of any alterations of course. Past maritime casualties remind us that many groundings were the result of poor chart work by ‘fixing the fix’ without any precise knowledge of the predicted position due to the lack of an accurate DR or EP.
It follows that proficient track control using the traditional paper chart means that ‘a fix is not a fix without an associated DR/EP position’ to keep the vessel safe from grounding. This is a key part of the information needed to maintain situational awareness.