Sometimes, rather than knowing where you are, it’s easier to know where you aren’t, says Justin Morton in his guide to yacht navigation

When you first start to learn about yacht navigation it can seem complex and pretty nuanced, but in reality what you are doing falls into two overarching styles. The first style of navigation is to know exactly where you are all the time, so you can manoeuvre to get to where you want to go and stay safe whilst you do it. This is the way commercial shipping, Naval warships, satnavs and, increasingly, your chartplotter, navigate. It’s therefore natural to want to do likewise.

The second, very old, style is rather than know exactly where you are, you instead know precisely where all the dangers are in relation to you. That is, you definitely know where you are not. You can then safely manoeuvre to your chosen location.

Chartplotter drawbacks

‘Knowing’ where you are is however very different from being told where you are. A chart plotter can tell you where you are but it is meaningless until you ask ‘So what?’, such as, ‘Do I have to worry about those rocks?’

It always requires some level of thinking, which has to be done there and then to help you orientate yourself. Consequently, because our position is always changing, ‘knowing’ where you are all of the time is time consuming and taxing.

Chart plotters tempt us into doing this type of navigation, and a symptom of relying totally on chart plotter navigation is that uneasy feeling you get when having zoomed in far enough to see the chart detail, the bit you are interested in is now off screen.

However, your position constantly shown on a chart plotter is a great comfort and convenience when you know there are no dangers nearby, and herein lies the clue to why the second style of navigation is of great benefit to a busy yachtsman.

If you know where all of the dangers are in relation to your boat, you can assess whether you are heading into ‘danger’. Helpfully the ‘dangers’, be that shallow water, isolated rocks, or any other feature, generally don’t move, so once you can work out where they are and how you can avoid them you don’t need to do any more recalculation.

Economy of time

I am a firm believer in economising on my effort when skippering because I always want to have some spare capacity to deal with any unfolding situations but primarily to relax and enjoy what I’m doing. This ‘knowing where you are not’ style of navigation is the least mentally taxing, allowing us to focus on the myriad other things going on around our boats. Also the less ‘challenged’ we are the lower our stress levels will be.

Here you are free to go either side of the green buoy. However, if you stay to the left of it, you need to know exactly where you are all the time

Know your limits & establish a boundary

The key to ‘knowing where you are not’ is being able to establish a boundary that is identifiable from the cockpit (and ideally anywhere you happen to be on the boat). In its simplest form, it is a line which on one side is safe, where you don’t need to think too hard about navigating, and on the other, you need to know exactly where you are.

A line of red or green buoys marking a channel is a good example, as you can see when you are complying with the rules of the buoyage system. Generally, as long as you don’t go outside the buoys that define a navigable area’s edges, you don’t need to know exactly where you are.

Go the other side of the ‘line,’ though, and you really need to start devoting time to knowing what’s underneath you. By choosing to ‘box’ yourself in this way you can devote less time to navigation but still remain safe.

Although there is safe water inside the buoys you’ll have to identify where the hazards are. It’s easier to stay outside them

Setting boundaries

This style of navigation is not a shortcut and requires a good amount of planning prior to departure. You will reap the benefits though, because the time spent planning is saved once on the water. If you are going for a day sail, drawing out a little sketch map of the area with your boundaries on will help you remember them and if it’s a passage, include the information in your passage plan.

And how much more do I need to learn to do all this you may be asking? Not much, I would suggest. You will most likely know the few techniques that are involved. All are taught as part of sailing courses and the rest is just expanding those techniques.

Some skippers never seem to navigate whilst others always seem to be bobbing up and down below or obsessing over the chart or plotter. Or perhaps you’ve noticed how relaxed your sailing instructor is while you are navigating your socks off. You have most likely seen both styles of navigation in practice.

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Both have their place and the ‘know where you definitely are not’ approach is only safe if you have a reasonable idea of where you are and always have the time to move back towards safety. If you want to give it a go, it’s best to employ just one or two of the following techniques at a time and build from there. And although I have referred to it as a ‘box’, it’s very rarely, if ever, a complete box.

Boundary depth contour

Depth contours often form the most convenient boundaries. They work day and night, and in fog. The only drawback is you need to know the height of tide, and your depth sounder needs to read depth correctly, so ensure you check it.

The basic principle is to look and find an obvious (and reasonably straight) depth contour that forms a boundary you can use. As long as the depth with the tide added is either more or less than that, you know which side of the line you are on.

Even though my plotter tells me the height of tide, I still manually write the relevant tidal heights out at hourly or half hourly intervals and have it close to hand. I also make sure I have my depth sounder offset to read depth from the surface, so all I need to do is add the contour depth to the tide height at the time.

Depth Contour – As an example, Start Bay has a really prominent 10m contour about 350m from the shore, which makes an excellent boundary. It’s good practice to pick a contour some distance from any dangers to give you time to react if necessary.

Here I have superimposed an electronic boundary line (in green) on my pilotage app which will trigger an alarm if it is crossed.

Depths – As a back up, I always write down my depths and try to keep track of what the tide is at any given time. In this example, as part of a crew briefing I would mention that the boundary would be the 10m contour and what the height on the instrument would read with the tide added when we get there.

Depths sailing – We don’t all have chart plotters at the helm but fortunately most boats have depth instruments visible from most positions in the cockpit. Depths tend to decrease gradually so any crew member can monitor the depth and let the skipper know in enough time to do something about it.


Look at any chart and you will see helpful transits with identifiable features such as towers, headlands and masts already marked. They are a bit more obvious on paper or raster charts but most vector charts display established transit lines (sometimes called navigation lines) but they may need to be interrogated.

To the left of the transit, Start Point in the far distance is to the left of East Blackstone Rock (in shadow) indicating Nimble rock is to the right

Transits can be used in two ways: ones that cross isolated features to help pinpoint them, or more often they are used to provide a boundary line. Pick whatever works and the most obvious objects (as seen from the sea) are usually marked on paper charts but can include headlands and rocks as long as they are steep-sided.

Aerial photography can also help and you can use the divider measuring tool on your plotter to give you a bearing so you know its the correct object. Do make sure you read the bearing the right way around.

Pick your own

Picking your own unmarked transits is easy enough when you sail regularly in the same place and could even be something as strange as the line of an obvious street that runs perpendicular to the shore. Remember, whatever you choose needs to be easy to see and must work as a transit.

Nimble Rock is my favourite example of how ‘definitely knowing where you are not’ works. To quote from the Admiralty Chart, ‘Start Point Lighthouse open either side of East Blackstone [Rock] clears Nimble Rock’. Therefore, as long as the lighthouse and the rock don’t line up you can’t hit Nimble Rock (charted depth 1.9m). It is very rare for an additional transit to pinpoint anything exactly – in this case Scabbacombe Head lining up with the summit beyond does just that.

Transit in line, the rock is just ahead and to the right – you can see the turbulence. There was 4m of tide and about a knot of tidal stream

This navigational technique has been in use for far longer than depth sounders and electronic positioning systems. In reality, it’s far easier to just stay east of the 20m or 30m depth contour when heading north. You can also choose your own transits.

I am travelling south west here, heading into Start Bay, and because the 10m contour discussed earlier is a little close to some rocks in this area I have found a transit to provide an obvious boundary so I don’t get too close. I have decided to use the northern edge of the Mewstone with the red buoy I passed on the way out of the mouth of the river.

By keeping the buoy and rock ‘open’, I can judge my approach angle and stay a comfortable distance from the shore. Picking something with a steep side or that doesn’t dry out reduces the error when the tide falls and more of the rock is exposed.

Sighting over the binnacle compass can give you a quick visual reference

Clearing Bearings

The principle of a clearing bearing or safety bearing is that as long as an object bears ‘no more than’ or ‘no less than’ a bearing you have calculated, you will definitely be one side of that line or the other. Staying on the safe side ‘clears’ the object. Some helpful clearing bearings are marked on charts and confusingly can be named navigation lines or, as my app does, routes! If your chart plotter isn’t showing any you may need to turn them on in settings.

You can choose anything that you know you will be able to identify from the sea. You want to pick things that are quite close to mitigate errors that could be made by taking a poor bearing, but they can be useful in orientating you at any distance. You can use the same object multiple times to make a cone, and the fewer objects you need to identify, the less you have to think about.

Use a hand-bearing compass for clearing bearings

Hand-bearing compass

However, we’ve mostly neglected our compasses over recent years so you must make sure it doesn’t deviate wildly. Handheld compasses (without bubbles in) are the most accurate but sighting over the binnacle steering compass is an equally valid technique as long as the compass is directly between you and the object when you take the bearing.

As it usually gives bearings in 5-degree increments it is much less accurate than a hand-bearing compass, so give yourself a bigger safety margin i.e. draw your line to pass further from the danger. If your chart plotter is near your compass, definitely check your deviation as your compass might be quite a way out.

Here, Start Point Lighthouse in the distance bears about 225º. To clear Skerries bank with it on my left I have calculated it needs to bear ‘no-more-than’ 210º. In theory its not a difficult calculation to make but it can be.

For example, ‘lighthouse bears 225º, it shouldn’t bear more than 210º, so I need it to reduce.’ When you are tired, you can’t think which way you need to go! It’s much quicker and more orientating to look where 210º points on your compass and imagine a parallel line back from the object rather than taking a bearing to the object.

In this example, you can see I need to be much further over to the right. When accuracy is important you should use a hand-bearing compass but again it’s easier to align yourself with your bearing and then imagine the parallel line back from the object.

Justin sets a boundary line on the plotter as a quick reference

Boundaries, routes & waypoints

Modern chart plotters usually have a boundary function and you could even use just a route line to ‘box’ your sailing area in, but whatever you use, to be useful, the question the plotter needs to easily answer is ‘how far away from the boundary am I?’

Unfortunately, as you zoom in and out, the physical distance on the screen changes, requiring some additional assessment. It is therefore much easier to use an electronic boundary line as a handrail as it is not imperative to know how far away you are, just that you are definitely the correct side of the line.

Chart Plotter boundary lines work really well when there isn’t much to look at to orientate yourself. Here, from my lounging position in the saloon, I can see my compass course and using the chart plotter can also monitor if I am being swept to the right towards the shallow bank.

To make absolutely sure you have identified the correct object, it takes seconds to take a bearing on a chart plotter to confirm. Just make sure the bearing is the right way around. This house I was passing was going to become important later in my sail

The electronic boundary line takes any ambiguity out of whether I am drifting left or right but it’s not the distance to the line that’s important here, just that I am staying to the left of the line.

Where modern chart plotters are really useful is if they have a Look Ahead / Perspective 3D or Augmented Reality function. You can set any object, be that on land or at sea, as a waypoint or mark. This can be useful if a particular object is difficult to find initially as the plotter can point it out for you.

You can even ask some advanced cameras to track whatever you are looking for. For the chart plotter to be accurate any cameras need to be set up properly and your electronic compass definitely needs to be calibrated correctly. If you don’t have a camera or any 3D-type functions, it will be capable of giving a bearing from your boat to any object you select. You can then identify it by compass.

Sectored lights are some of the most helpful but least used daytime objects

Use whatever’s there (as long as it doesn’t move)

The final technique is to just use whatever is there. Remember we are trying to draw a boundary line that can be identified when on the water without too much effort, preferably by just looking. This could be something as simple as not going north of a line drawn between a particular cardinal mark and a green buoy. It could be staying east of the last boat on a line of trots and a marina outer pontoon.

You can even use sectored lights in daylight if you can see them, the crossover point between white (centre) and red or green is particularly useful.

Judging distance is hard at sea so unless you have practised and really know your distances don’t rely on judging a distance off a single object to keep you safe.

Having looked at the chart, I can use the moored yachts on the left and the industrial moorings on the right as my boundaries (about 200m of width). This is a much more flexible approach than hugging a chart plotter route. I should, of course, keep to the right in a channel

Putting it all together

You may be forgiven for thinking I don’t like chart plotters, but that couldn’t be further from the truth. If your only job onboard is the navigator, you have the time to draw out the information and orientate it to what is around you.

Most of us sail short-handed or with our families, so we skippers have to juggle childcare, deck work and helming whilst still trying to work out where we are and where we need to go next. That’s why I think both on-deck navigation skills and modern electronics used together are a really powerful combination that makes things safer and our lives easier. This final example I think highlights this.

With Start Point astern it’s a clear run for home using my old 210º clearing bearing to starboard

Changing tack

Here is a common scenario, namely, having relied on hugging your route to navigate, you now can’t follow it. In my example I am rounding Start Point with the intention to cut through the inner channel into the bay to get into sheltered water and get to Dartmouth sooner – it’s been a long day.

Unfortunately the wind is such that I now need to tack up through the channel. Whether I’d have planned for this or not, I now have a lot to do and sort out. I’ve got to get the boat ready to tack a good few times, possibly reef, make sure things are stowed below, brief the family, and work out the new route.

If you just rely on the chart plotter for navigation it will be unnerving when you can’t see it or worse, hold you to it. This fixes you, and as arguably the most competent person onboard, reduces your effectiveness should your skills be needed elsewhere. Ideally, what you need is a way to be able to navigate whilst up on deck and looking out. In this example, a clearing bearing and a transit can help you do this.

Having worked out exactly where you don’t want to be, it is comforting to glance down at the chart plotter when you need to and have it confirm you are exactly where you thought you were.

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