Navigation using digital or paper charts is key to good seamanship, but understanding the seven basic types of coastline and their challenges, and keeping your eyes out of the boat is just as important, argues Rachael Sprot
There’s something very comforting about seeing your GPS position overlaid on top of a chart on your smart phone, tablet or chart plotter, but this is often a simplified picture of our surroundings.
There’s more to navigation than just knowing your position; it’s as much about understanding the environment you’re in and having a good overview of an area so that you can quickly respond to changes of conditions and unexpected occurrences.
Digital navigation, with its small screens and constant zooming in and out, requires us to have a better command of our situational awareness, not less.
It’s simply impossible for a 9-inch screen on a chart plotter to show the whole of the English Channel to the same level of detail as the equivalent paper chart.
One day perhaps we’ll don a pair of red and green glasses and look at 3D charts of the ocean floor on high definition screens in our digital nav stations.
But until then, understanding the shape of the bottom is a key stage of the passage planning process, which will determine which instruments can be relied upon, what techniques to use, and what safety margins to employ.
Plus being more knowledgeable about your coastlines will help you better plan your navigation. Sailing somewhere new can be intimidating – each type of coastline brings new challenges and requires its own set of techniques.
Many of us fall back on our digital navigation tools when we are cruising somewhere new, but using these alongside some more traditional skills will leave you armed to tackle almost any eventuality.
If you can identify the type of coastline you’re sailing in you can quickly adapt to your new surroundings, giving you the confidence to venture further afield.
In fact, you could sail all the way to New England, only to discover that the navigation is similar to that of the West Country.
Understanding the type of coastline you’re sailing in might sound old-fashioned, but as navigation occurs more and more in the digital realm it is critical that we can still verify our calculations against the real world around us.
Profile of the seabed
Whenever you’re sailing somewhere new you need to build yourself a mental map of the area and identify significant features. It’s tempting to plan an exact route but in reality few sailors stick precisely to their intended course – what you actually want is a sailing ground or broad area to work within.
If you didn’t have a GPS you wouldn’t expect to stick precisely to your ground track anyway, so you’d have a much wider view of where you might end up. Annotating electronic charts can be difficult at best, so it is worth investing in a few key paper charts for coastal passages; that way you can keep notes of potential hazards for future trips.
Depth gives a powerful indicator of where you are, and you should always have a minimum depth in mind and an eye on the echo sounder in coastal waters.
You may need to zoom in and out on an electronic chart to display the relevant contour lines but once you’ve identified them, the benefit of electronic charts is that you can highlight the ones you want.
Beware of uneven bottoms and steep contours though, as they make the use of the echo sounder problematic – if there’s only a cable (200 yards/182m) between the 50m contour line and drying rock, would you notice the depth change in time to turn away?
At 6 knots it takes a minute to travel that tenth of a mile, which isn’t much warning if you’re not paying attention.
The shape of the seabed is only half the story, what it’s made from is equally important.
Rocky coastlines shelve unpredictably with abrupt changes in depth whilst sand or mud tends to shelve in a more reliable fashion, although there are plenty of exceptions to this rule!
If you can’t find any information about the seabed, have a good look around you: it is often a reflection of the coastline.
It’s much easier to find the source data on a paper chart than a digital one, but an electronic chart is no more reliable – just because your Navionics charts are ‘up to date’ doesn’t mean they’re using up-to-date data.
Some areas of Arctic Norway that we visit on our Lofoten expeditions were surveyed over 100 years ago by lead line!
It might still be perfectly accurate, but there’s more chance of something being missed than in an areas frequently surveyed by modern methods.
If you can’t find the source data in your chart plotter then think carefully about the area: is it likely to change? Is it a major shipping route or are you off the beaten track?
We’re all taught to watch out for the difference between the chart datum on a paper chart, and that of the GPS set, but we tend to forget this when working from digital charts.
On one of our expeditions to Greenland our plotter showed us driving straight across a headland, even though we had safely eyeballed our way around the coast not through it!
Often the difference between the two datums will be small enough for you to overlook, but big enough to put you aground in pilotage waters.
Familiarise yourself with the chart settings in your plotter and read below for lessons on eyeball navigation techniques to use instead.
Owing to the uncertainty surrounding their position traditional navigators tend to be better at identifying hazards and giving them a wide berth.
Too often I’ve come on deck to find that what looks like a safe distance off on the chart plotter feels too close for comfort in real life.
It’s easy to get lost in the scale on a digital chart and because we have high confidence in our position we cut corners that the traditional navigator wouldn’t.
Abrupt changes in depth and uneven bottoms deserve much wider safety margins than gently shelving coastlines.
Five points for better coastal cruising
1. Coast type
Think about where you are planning on cruising and what sort of coastline you are likely to be sailing near. It’s good to be mindful of the likely challenges you might face before you get there.
Is finding a spot to anchor going to be difficult? Will it be helpful to brush up on your eyeball navigation skills?
Always look at the coastline and consider what that means for the seabed beneath. Usually a steep-sided coastline will mean deep water even up close.
A long shallow slope towards the sea will mean the same underwater. Check if this working hypothesis is backed up by your charts.
3. Consider possible changes
Charts are only as good as the information upon which they are based. If you are planning to sail in an area dominated by sand banks then the chances are these will frequently move, particularly in stormy weather.
If you are far off the beaten track then the charts may be based on old data so should be taken as a guide.
4. Be aware of zoom
The ability to zoom in for more detail on a digital chart is great for looking in detail at specific hazards, but often what looks like a safe distance on the screen can be uncomfortably close when you are up on deck in the real world.
The great thing about paper charts is that they allow for easy annotation. Marking up known hazards or shifting sandbanks allows you to personalise your data.
This will help keep it in mind when you glance at the charts and for future cruising.