YM instructor Rachael Sprot steps aboard to help. This month, Sophia Lagus and crew want to learn how to use their yacht radar for pilotage and navigation
To the uninitiated, radar can seem like a dark art. The splurge of blobs would look more at home in the Tate Modern than the nav station. Recent advances in yacht radar technology make it more affordable and easier to use than ever before, however it’s easy to misinterpret the display.
Radar has several applications on board: navigation, collision avoidance and even squall dodging in the tropics. It is easiest to become familiar with the radar by practising navigation, because there is only one moving object in this equation – you!
There are three main nav applications for the radar: ‘eyeball’ pilotage where radar takes the place of visual cues; to avoid isolated hazards by use of clearing ranges; and position fixing.
Sophia Lagus has owned her Malo 46 Wimsey for just under a year. She has thousands of miles of cruising experience as crew, but she’s relatively new to skippering and is happiest 12 miles offshore where there’s nothing to hit!
She’s a farmer by profession and her practical skills are excellent, but she’s less confident when it comes to technology. The boat is fitted with a modern radar, but when I jumped on board she hadn’t yet turned it on!
I joined her and crew members Bennie and Wendy in Dartmouth to help get her started.
Moored up the River Dart at Noss on Dart, there are a couple of miles of busy channel to navigate before reaching open water. Collision avoidance is largely close-quarters, with plenty of boats to dodge, but this isn’t the best setting for learning how to use radar for collision avoidance.
It is, however, an excellent setting to learn the basics of what radar can do and how to use it for pilotage.
While a chartplotter will show you what should be there based on GPS, radar will show you what’s really there. Think of it as a second pair of eyes to help you see what the human eyeball can’t when in fog, darkness or hidden hazards.
How radar works
The practical application of a yacht’s radar as a navigational device is straightforward for anyone with traditional navigation skills – it’s interpreting the radar picture that’s challenging.
Radar sees the world very differently from us. It stands for RAdio Detecting And Ranging, which is a pretty good description of what it does: it tells you what’s out there and how far away it is.
In a similar way to an echosounder, radar works by sending out a wave of energy and receiving reflections from objects that it hits. By timing how long
it takes for the echo to return, it calculates the distance to the object. It is both a transmitter and receiver.
Like us, radar can’t talk and listen at the same time, so an internal valve opens and closes to protect the sensitive receiver, making sure it doesn’t yell into its own ears.
Yacht radars operate within the ‘X band’ range of frequencies, which is around 9GHz with a 3cm wavelength. This allows the antenna to be relatively small and gives good resolution on the display.
Merchant ships usually carry an ‘S-Band’ radar too, operating at 3GHz with a longer wavelength and lower frequency. This is more powerful, has a greater range and is less affected by environmental factors such as sea-state but the antennas are huge and impractical on yachts.
Most modern radars have a nominal range of 24 or 48 miles. However, owing to the curvature of the earth this range is limited by how high they’re mounted. The radar horizon is slightly better than line of sight, and can be calculated using this formula: 2.2 x √height(m) above sea level
It isn’t just size and distance that determines how easy it is to detect targets, however: the material of the target is equally important. Steel or aluminium make better targets than GRP and wood.
What isn’t so well known is that the shape of the target matters too. Rough and uneven surfaces have a higher chance of reflecting some of the energy back to the antenna as the echo scatters in different directions. Smooth, flat surfaces will send all the radar waves off in the same direction, and not necessarily back to you.
Stealth ships with their sleek, slanting hulls are designed to do just this.
This means that rugged cliffs make good targets, but low-lying, sloping beaches make poor ones. Water itself produces a very strong reflection, which is inconvenient given where we’re operating.
Both waves and rain will return your radar pulse with the consistency of a well-trained Labrador; it’s just unfortunate that you didn’t want that particular stick back.
‘Sea- clutter’ and ‘rain-clutter’ mask the targets lying behind them. There are controls to reduce these effects, but they need to be used with care: they’re powerful but somewhat crude adjustments. Being too heavy handed with them can wipe out much of the detail on screen.
Any obstructions close to the radar create a blind spot. A mast-mounted radar such as Wimsey’s will have a blind spot astern. The beam will bend around the mast, slightly reducing this effect.
Practically speaking, this is the best place to have a blind spot as the closing speed of things approaching from astern will be less than that of those approaching from ahead.
A narrow blind spot of less than 5° isn’t a huge concern on a sailing yacht as the course often fluctuates enough to pick things up in the wobble.
Reading radar displays
Now that we know how radar works, we need to understand how it displays the information it’s gathered. A radar display is a circular one and we are at the centre. The antenna rotates once every 2-3 seconds and the picture refreshes. It displays a set range which we need to select.
On older radars, the range could only be set to multiples (or fractions) of three (3/4, 1.5, 3, 6, 12 miles and so on). Modern displays have been unshackled from these metrics and it’s common to be offered 1, 2, 4 and 8-mile ranges too, but it might slightly distort the picture.
The biggest full ring is the overall range that you’ve set, which is shown at the top of the screen. Since most displays are rectangular you’ll see a bit further out to the sides. There will also be a series of smaller rings at regular intervals, giving a quick ready reckoner of a target’s range.
To complete the set-up the antenna needs tuning. On most modern radar sets this is done automatically but can be overridden in the settings. Another adjustment which is now mostly automatic is the gain, or sensitivity. If it is too low the picture will be incomplete. Turn it up too high and the whole display will be filled with ‘noise’.
As the antenna sweeps around it builds up a picture of its surroundings. This picture is colour-coded to help differentiate between weak targets and strong targets. The more intense the colour, the stronger the returning signal and the more confident you can be that something is out there.
On Wimsey’s set red indicates a strong target, yellow is a weaker one and blue is weaker still. A rock awash would be an example of something that might only come up as blue or yellow initially so it’s important not to dismiss these minor targets but to keep an eye on them.
The display can be oriented three different ways: Head-Up, Course-Up and North-Up. Head-Up shows the picture as it appears looking forwards. This is an un-stabilised picture, meaning that if the direction you’re facing changes, so too does the picture on screen.
The helm fluctuations which are inevitable on a sailing yacht mean that the picture constantly shifts, which is very confusing. Course-Up sets the picture to the direction you’re heading in at that moment, but you must remember to re-set it if you change course.
North-Up is the gold standard: it’s compass-stabilised and means that whatever happens on the helm, targets will still be shown in the same place.
Radar displays have two modes: relative motion or true motion. Relative motion takes the raw data of the echoes received and displays them in a very pure way.
In relative motion we are the centre of the universe still and everything revolves around us. The radar doesn’t see the fact that we’re moving, it just sees the end result of our motion as their motion.
If we’re sailing past a stationary object like an island, the island will appear to move.
True motion is the radar’s attempt to show what is actually happening instead. The island isn’t really moving, we are. It processes the raw data, takes into account our own speed and heading, and shows us the world objectively.
The problem with true motion is that it relies on our instruments to deliver an accurate report of what we’re doing, so that the radar can calculate what everyone else is doing. If our instruments aren’t accurate, or our speed and heading fluctuate, then the radar will struggle.
I’ve never had much success getting true motion to work on a small yacht’s radar. It’s also the wrong mode to use for collision avoidance, but we’ll cover that in a future article. Unless you’re very proficient in the use of radar, set it to Relative Motion and North-Up. This is the best way to get meaningful information from it.
We set off from Noss Marina a mile up the River Dart. We were using a standard Raymarine Radome which allows the user to choose from Harbour, Coastal and Offshore modes. These are preset for optimum performance within those environments.
We chose Harbour Mode. This reduces the effects of certain echoes so that strong targets close-by don’t overwhelm the picture. You can learn a huge amount about how radar works just by closely observing a single scenario.
Stemming the tide in the river we scrutinised the display. It clearly showed the finger pontoons of Noss Marina and the empty space of Lower Noss creek behind.
The steep edge of the western shore with its wooded slopes made a very distinct target, but the low-lying mud flats which were exposed at low water were invisible, probably because the radar wave struck them with a glancing blow and bounced away from the antenna.
A low-profile barge made a much better target than a fleet of small motorboats, probably due to the fact that it was steel and the motorboats were GRP.
The mooring buoys showed up surprisingly well given how small they are, probably due to their shape rather than their material: they’re round so they have a better chance of returning at least some of the pulse in the right direction.
Harbour mode also helped to prioritise these small features, so they showed up very well compared to the pontoons which were probably de-prioritised because they were much closer to our position.
Once everyone understood the radar picture, it was time to head down-river using the radar to stand in for the naked eye.
Bennie stayed below monitoring the radar, telling us what she could see on the radar. She soon identified the chain ferry departing the eastern bank. She was able to say that we should hold station and let it through.
What made this exercise difficult was the speed with which we were moving. With an ebb tide beneath us we couldn’t do less than 3 knots over the ground.
In confined areas it’s necessary to zoom in, resulting in highly reduced visibility ahead. Travelling at 3 knots and using the quarter-mile range setting Bennie only had five minutes to interpret what was appearing at the periphery before we reached it, and that assumes the target is stationary or crossing.
Another vessel coming towards us would reduce the time even more. The channel was also only 120m wide so it would have taken less than a minute from being central to bumping into the trots.
For this kind of pilotage intense focus is required and in a true pea-souper it would have been foolhardy to attempt. Timing the pilotage for slack water would make it easier.
When choosing an area to practise radar pilotage, try to find somewhere that has plenty of interesting land features, but not too many vessels.
Once outside the River Dart we moved onto 3-point fixes. The same rules apply for a radar fix as they do for a compass fix: make sure the landmarks you’re using are obvious on both the chart and the screen; take your readings in quick succession to minimise the distance travelled between them, and try to achieve a good angle of cut.
The difference though is that we use distance, rather than bearing, as our measure. This is because radar is an excellent judge of distance, but a poor judge of angle.
The radar beam is usually 5-6º wide on a leisure set. As the antenna sweeps around it picks things up on the leading edge of the beam and keeps registering them right through to the trailing edge of the beam.
You can produce a more focused beam with a longer antenna, but they’re far too cumbersome for the average cruising yacht. This means bearings can be several degrees out.
VRM and EBL
You can take a position fix on a single object using range and bearing, but you’ll get a more accurate fix with range measurements to three separate targets instead.
Distances on screen can be measured by setting up the Variable Range Marker (VRM) and Electronic Bearing Line (EBL). These are usually within the ‘Target Tracking’ part of the menu, and intended mainly for collision avoidance, but they work just as well on land features.
Having consulted the radar picture and the chart, Wendy chose Western Blackstone, Dartmouth Castle and Wash Point as three prominent features.
She then measured their ranges with the VRM:
- Western Blackstone 0.24 miles
- Dartmouth Castle (Battery Pt) 0.3 miles
- Wash Point 0.25 miles
Next, set a pair of compasses or dividers to the right range and draw an arc around the object on the chart. Your position is where the three arcs intersect.
As with any other three-point fix, the bigger the area of intersection, the less accurate it is and the more suspicious you should be of the results. Once plotted, they gave a position accurate to about 50m.
The final tool for the toolbox is clearing ranges. These work just like a clearing bearing by delineating a danger zone. Dartmouth Harbour is well marked and steep-to, so there are few isolated hazards, but we can pretend that Western Blackstone is hard to identify on radar.
If it was invisible to radar we would need to create a relationship between it and something that will be visible, like the cliffs. We measure the distance between them and find that it is just less than a cable.
If we stay 2 cables off the cliff we will remain in safe water. We can monitor this by setting up a 0.2nM VRM on the radar display. If we keep Blackstone
Point outside the 0.2nM circle, we avoid Western Blackstone.
Using a safe distance off an identifiable feature allows you to avoid hazards. Like a clearing bearing, this is a really simple, powerful tool for pilotage.
One nice feature of Wimsey’s setup was the Raymarine display. It allowed those outside to instantly match what was in front of them with the radar picture. As we piloted down the river the channel was well-defined on the radar, and the image on screen started to make sense.
As we were heading back up the river Sophia said that the day had been a good introduction to radar, but that she still didn’t feel completely confident with
Sophia is a naturally cautious skipper and her instinct to be wary of using technology which she isn’t completely comfortable with is the correct approach.
As she identified, it takes practice to develop good radar skills. But the nice thing about Wimsey’s set-up is that on a quiet day with good visibility she can watch the radar from the helm.
This will allow her to build up a bank of experience, getting a feel for what it can do, and also what it can’t, before she needs it in earnest. She will also need to learn how to use the radar for collision avoidance, to complete her skillset.
The radar is often overlooked in a skipper’s skillset, but it has become an important element in Yachtmaster Offshore exams, and is an essential skill if the visibility closes in. It will be very tempting to turn it on if the fog rolls in, but by that point it is too late to learn.
Unlike berthing or changing a fuel filter where most mistakes are easily resolved, getting radar wrong can be disastrous. Make time for training and familiarisation with your equipment before you actually need it.
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Source: Yachting Monthly