Radar on sailboats: making the most of it and using it safely

What is a RADAR

The word radar is an acronym coined by the US Navy at the beginning of World War II with the meaning of “Radio Detection And Ranging”. Its application for military purposes has spread over time, also finding diffusion in the civil field in aviation and in the nautical sector.

This tool provides two pieces of information, the distance and the bearing relative to our position of any solid object or surface in the range of our antenna. The principle of operation is not conceptually different from the mechanism that allows bats to fly and hunt in the dark. The bat emits ultrasounds and perceives its own sound waves reflected from its surroundings. It is thus able to mentally reconstruct the map of the cave in which it is flying and the location of the insects it is hunting.

The device, instead of emitting ultrasounds, emits radio waves through a rotating antenna. In addition to emitting, it receives the reflected waves of surrounding objects. Knowing the angle relative to the boat of the rotating antenna at the moment of transmission, the RADAR can calculate the distance and bearing to the target. This is by calculating the time it takes for the reflection to return. A computer then transforms this information into an image visible and understandable to the human eye which is presented on the display.

Radar on motor boats

What’s it for?

It is a precious navigation aid, especially in situations of reduced visibility. It can be used to know the position and distance of other boats, other obstructions such as rocks and buoys and the coast. For example, in fog, radar can be used to avoid collisions. Not only that, also to find the mouth of a port or the shape of a breakwater. Sound alarms can be activated on each device to warn us of the presence of ships or obstacles approaching relative to our position.

It is very effective for monitoring the movement of another ship and safely assessing whether we are at risk of collision. With the naked eye, even in broad daylight and with good visibility, it can be difficult to assess the true distance of a fishing boat, being able to understand in which direction it is moving and if you need to manoeuvre to avoid it. Radar can make these situations less stressful. At night and in case of fog it can become an essential aid to ensure safe navigation.

Marine radars

Equipment for civil use

The instrument for civil use, simplifying to the extreme, is composed of a transmitter, a rotating antenna, a receiver, a processor and a display. The rotating antenna is often only one and is used both for the transmission of impulses and for the reception of reflections. All devices have a rotating antenna. In dome radars typical of small fishing boats and other boats, the rotating antenna is hidden under the plastic cover of the dome.

Every wave transmitted by the radar that is reflected by a distant object returns as a weak signal, some surfaces and materials reflect better than others. The metal surfaces of a ferry reflect well, but the fiberglass of a boat does not reflect at all.

In addition to the properties of the various materials, it is necessary to simply consider the power of our instrument, a crucial factor that determines its range. Power also means electricity consumption and inevitably the instruments installed on sailboats are not very powerful. They have a typical rated range of up to 16 miles but perform best in the 3 to 6 mile range. It may not seem like much but it is perfectly adequate in order to assist during navigation in conditions of reduced visibility.

Elevation and radar range

Radar uses short waves that travel in a straight line, so it can only detect what is within its range and up to the horizon. The higher the target or the higher the antenna is installed on the water level, the greater the distance at which targets are visible.

With the antenna installed two meters above the water level, we can find the position of a buoy at a maximum distance of about 3 miles. This is due to the curvature of the earth’s surface. If, on the other hand, the target were a ferry that rises 25 meters above the water, this will be detectable at a distance of about 10 miles. A cliff 100 meters above sea level would be detectable at a distance of 20 miles or more. However, in this case, we will be limited by the power of our RADAR which may not reach such a distant range.

Furuno radars

Main Controls

Radars are rather complex instruments but for the practical purposes of use on a sailboat the controls that are used are few.


The RANGE determines the maximum distance that we are showing on the display. Most radars installed on sailboats have a RANGE that can be set from a quarter of a mile up to 16 miles. This maximum range is determined by the typical power of a small device suitable for a sailboat. It must understood that if we set our radar to the maximum range, not everything that is in the chosen RANGE will be visible.

We will deal later with the fundamental aspects relating to the visibility of other ships such as elevation and ability to reflect radio waves. A craft on a collision course with us could easily get as close as 1-2 miles before being visible on radar. When it is that close, however, it will be difficult to identify it if our radar is set to a very wide range. In this case it would be better to use a smaller range, for example 2 or 3 miles.

Conversely, the opposite situation must be considered. Let’s imagine a large commercial ship. If the visibility is very low and we are relying solely on the display to avoid trouble we will have to choose a higher range. This is to make sure you spot ships sailing at high speed in time.

Let’s do two calculations

If we travel at 6 knots and a ferry is coming towards us at 24 knots with a 3 mile display we would have only 6 minutes to react. This since the first echo of this ship appeared on the display. Imagine a night sail with fog and little wind, a ferry 3 miles away moving at 24 knots would be over us in 8 minutes. After having spotted it on the radar it would be really difficult to start the engine in such a short time and understand in which direction to escape. Or be able to call the ferry via radio, be able to give precise indications regarding our position and avoid the risk of collision. Even asking the ferry to change its route, which however has its own reaction times.

Especially in case of fog, we will have to monitor the risk of collisions with ships already at a high range, 12-16 miles. At the same time we must periodically check that there is nothing near us by jumping to a lower range such as 2 or 3 miles. We may not have noticed a water buoy, fishing boat, or other sailing boat nearby. In case of dense fog this tool is the most effective method to avoid accidents but it must be used correctly.


Another fundamental control of each radar is the GAIN which controls the sensitivity of the receiving antenna. By increasing the GAIN the display will show progressively more and more of the received signals, even the weakest ones. This theoretically would guarantee not to lose anything of what surrounds us. But with the GAIN set too high, the risk is of not understanding anything and confusing the reflections of the waves with other ships. The GAIN should therefore be lowered only enough to have a clear picture of what is around us.

As the sea swells, the antenna will receive more and more reflections from the crests of the waves. In this situation we will be forced to further decrease the GAIN to eliminate these reflections. Especially if we want to keep an anti-collision alarm active. If the sea is bad, it will otherwise sound continuously due to the reflections of the waves. But be careful, ‘lowering the GAIN too much you will end up losing even the reflection of small boats.

Finding the right balance with rough seas requires experience. Most tools have recommended default settings. These are studied in relation to the state of the sea, calm, moderate or rough which correspond to a progressively lower GAIN. Remember to always check the GAIN setting. The radar normally “remember” the settings. If the last time we used the radar in rough seas and today we are sailing with flat calm and thick fog we will have to reset the GAIN. Otherwise, for example, we could risk not seeing a small fishing boat on our route.


There are two other controls that have a similar effect. The RAIN control, which is used to reduce the visualisation of the reflection produced by rain. It acts as a filter by eliminating the weakest reflections by imagining that these are the ones produced by the drops of water. Be careful to reset the RAIN filter setting to minimum when it stops raining. As this is a filter, using this function increases the risk of not seeing a small vessel so it must be used at the minimum acceptable level.


The same goes for the SEA function. The function is used to reduce the reflection of the waves near the boat, also for this setting you should use the minimum workable setting.

To summarise, choose the maximum usable GAIN and rely on the recommended settings for the sea conditions you are in. For RAIN and SEA filters, use the minimum usable filtering level. Carefully read the instructions for your instrument to find the corresponding functions and settings. Incorrect use of GAIN, RAIN or SEA can make it a totally useless instrument. This is even worse, it will give you a false sense of security by showing you open water in front of you when you are perhaps surrounded by a dozen boats.

Ships' Radars

Avoid collisions

Once we have identified a target on the display we can use the range and bearing data to determine if we are on a collision course. It should be remembered that the display always shows the position of the other targets relative to our boat. When we are stationary, interpreting the track of another boat on the radar display is simple and intuitive. When we too are on the move it takes an extra effort to understand what is happening and there is no substitute for experience.

Remember that a buoy, which is fixed on the bottom, moves on the display when we are in motion. However, we can refer to the same concept that we use to determine if we are in collision with a ship that we see with the naked eye. If the compass bearing of the other ship remains constant and is approaching, we are on a collision course. Using the radar cursor it is possible to measure these two pieces of information.

Range and VRM

The distance to a target, called RANGE, can be measured with a circle, usually called VRM on the display from the acronym Variable Range Marker. By drawing a circle that touches the closest limit of a target and observing it for a few minutes we can determine if it is approaching. Once it is established that the target is approaching, we will have to worry about understanding if we are on a collision course.


To do this we need to determine if the target bearing remains constant over time. On most instruments there is a feature called EBL which stands for Electronic Bearing Line. A bearing line that tells us the relative angle to our boat. By pointing this line towards the target it will be possible to determine if it remains on a constant bearing or not. But be careful: the bearing changes completely as we change our course.

So to determine if the bearing remains constant we will have to ask the helmsman or autopilot to go as straight as possible. This is to give us time to check if the bearing changes over time or not. This can sometimes be easier said than done in very bad seas. The autopilot helms straighter than an inexperienced helmsman so don’t hesitate to activate the autopilot. Below deck you can check the risk of collision on the radar display.

Radar display

RADAR, Chart plotter and AIS

Many of the modern instruments, even the cheapest ones dedicated to boaters, can be integrated with the on-board chart plotter. This allows the image produced by the radar to be overlaid on the nautical chart relating to the navigation area. In doing so, it will be easy to understand if a target identified by the radar refers to a fixed structure. For example, a rock, a lighthouse, a breakwater, a buoy or a mobile target such as a fishing boat.

The chart plotter can usually also be integrated with AIS information, another collision avoidance system which we will talk about in an other article. This allows us to determine even more quickly whether the trace produced by the radar on the display refers to, for example, to a ship at anchor. However, it is worth a warning, overlaying radar, charts and AIS helps us if we are used to interpreting all this information at once. Sometimes it can confuse us instead of helping us, so experiment and get used to using the tools available to you.

To see and to be seen

When we identify a large ship many miles away on the display, it is easy to make the mistake of imagining that they see us too. From personal experience, a sailboat without a hoisted or fixed radar reflector is not visible to a commercial ship more than 2 miles away. In rough seas it is possible not to be seen at all. The image that is reflected by the very limited surface of our mast is so small that it is confused with the reflection of the crests of the waves.


A commercial ship relies on automatic identification systems for other ships on a possible collision course. A small vessel’s anti-collision system is called MARPA. It analyses all the reflections received and identifies as a target only the reflections that it can detect with sufficient confidence. It is calibrated to avoid false alarms due to rough seas. Sailboats built mostly of fiberglass or wood are practically invisible. This of course also applies to other sailboats in relation to our onboard radar.

More than once with bad sea I found myself a few hundred meters from another sailboat. Although I was keeping a watch and had the alarm on the active instrument. Even small fishing boats, although constructed of metal, are sometimes only visible within a mile or two. This is because in rough seas they are often hidden by the waves and we will notice them only when they are very close. Also only if we are paying close attention to the radar screen.

Passive RADAR reflector

Passive RADAR reflectors

To increase our visibility to the instruments of others, we need to increase the image we reflect of the waves emitted by the radars of other boats. There are two solutions: Passive radar reflectors and active radar reflectors. The cheapest choice is to have a passive radar reflector on board. This is nothing more than a metal surface that reflects radio waves well. Surely you have already seen this type of tetrahedron-shaped reflector made up of two square metal plates wedged one inside the other. It should be hoisted in case of bad visibility. Having one on board stowed below deck is obviously useless. Although it is among the list of mandatory equipment, it is almost useless for practical purposes for the simple fact that it is not used.

There are passive radar reflectors to be installed permanently on the mast, usually cylindrical in shape closed inside a plastic shell. These reflectors make us clearly visible already a couple of miles or more away and are suitable for cruising boats. On racing boats these reflectors are not very popular not only for the additional weight that goes onto the mast. Also for the continuous problems of halyards that get entangled in manoeuvring. Also beware of the small cylindrical passive reflectors that are sold for racing boats to be installed on the rigging. Their surface is so small that they contribute little or nothing to being seen.

Mini radar reflector

Active radar reflectors

Let’s move on to active radar reflectors, which are undoubtedly the most effective solution. An active reflector, called Radar Target Enhancer, is a device that performs the same function as a passive reflector. But, instead of passively reflecting the received wave, it actively transmits an amplified radio wave. Doing so makes us more visible, as if we were a large ship, on the display of other boats. RTEs are mandatory in many races and if you have one on board, always leave it on. At times, if you also have a radar on board you might need to turn off the active reflector when using the radar, check with the manufacturers of both instruments. This is to avoid damaging it with the strong radio waves coming from your antenna, especially if installed close to each other.

Active radar reflectors have lost some of their importance with the diffusion of AIS. However, their are integral and to safety and in many areas only large ships are required or decide to have an AIS.

Activ Echo

The radio frequencies on which the radars operate

Before concluding, some brief points on radio frequencies. Marine devices operate on two frequencies, the S-band and the X-band. Those installed on sailing and fishing boats are usually operating on the X-band, on commercial ships both radars are usually found. Most active radar reflectors operate exclusively on the X-band. This would seem at first glance to make sense as both recreational boats and commercial boats use radars on this frequency.

For example, all Mini transats were equipped by regulation with an active radar reflector called Active Echo which operates on the X band. Now the requirement has been replaced by the obligation of an AIS Class B. This, however, presents a fundamental problem that can give rise to a false sense of security. The x-band is very widespread, but you must keep in mind that due to its precision it also detects rain.

Commercial ships, which have the obligation to keep a watch with automatic alarms, often rely on radar on the S-band. This instead does not detect rain: precisely in case of bad weather when visibility is low and the risk of not being seen even greater. There are dual band radar reflectors on the market and it is recommended to install this type of active reflector in case of new purchase. From the point of view of our on-board device, which operates on the x-band, we should keep in mind the difficulty of use in case of rain.

Identify squalls and rainfall

During the Global Ocean Race we used this limitation to our advantage. In fact, with a radar operating on the x-band it is possible to see squalls and clouds. Rain showers associated with a front even many miles away are also seen. During navigation in the trade winds and in the doldrums the instrument therefore becomes a very precious aid, warning us in time of the arrival of a gust of wind. Especially at night when it is not easy to see the clouds. Having said that, the fact remains that when we encounter a rainy downpour, just when the visibility is reduced to nothing, the radar will be very difficult to interpret.

Even a large merchant ship could be hidden within a rain zone. It is therefore necessary to monitor and study the evolution of the image on the display and never come to rushed conclusions by only taking a quick look at the image on the radar screen.


The advent and diffusion of radar on the market for boaters has certainly provided a very powerful tool to increase safety on board. Whether it is used alone or in conjunction with an AIS. It is necessary to know how to use the instrument well in order to fully exploit its potential. We recommend that you read the instructions for your radar and train yourself to use it. You need to reconcile what the display is showing you with the visible world around you in preparation for real life situations.

During my round-the-world tour at the Global Ocean Race, our device was switched on for almost the entire race. This partly to compensate for the fact that there was not always someone in the cockpit. This is clearly an extreme situation that the cruiser does not have to face. A careful watch in the cockpit is always required but in case of reduced visibility the radar can really become your only pair of eyes.