Radar and AIS on a Fast Powerboat

Prerequisites

This lesson belongs to the RYA Advanced Powerboat course. You should hold the RYA Intermediate Powerboat certificate or be at that standard, be confident with chart plotters, bearings and courses to steer, and know COLREG Rules 5 to 8 (lookout, safe speed, risk of collision and action to avoid collision) and Rule 19 (conduct in restricted visibility). It helps to have read the night pilotage lesson. The RYA one-day Radar course and the VHF Short Range Certificate (which covers AIS basics) are useful background but are not required.

Radar on a fast powerboat is the same technology as on a yacht or a ship, but the boat pitches, slams, yaws and covers ground quickly. That changes how you mount it, how you set it up and how you interpret it. AIS adds a second, very useful but incomplete picture. This lesson shows how to use both safely at speed.

Learning Objectives

By the end of this lesson you should be able to:

• Explain how small-craft radar works, and its limits of range, bearing accuracy and resolution.
• Describe good radar mounting on a powerboat, and how to check and align the heading line.
• Choose range scales for passage, pilotage and collision avoidance.
• Compare head-up, course-up and north-up displays and explain why stabilisation matters on a fast boat.
• Adjust gain, sea clutter and rain clutter correctly.
• Recognise blind arcs, shadow areas and false echoes.
• Use the electronic bearing line (EBL) and variable range marker (VRM) to assess risk of collision.
• Explain how AIS works, the difference between Class A and Class B, and its limits at speed.
• Plan and run a passage in restricted visibility, applying Rule 19.

How Radar Works

A radar sends out a short pulse (or, in modern broadband and solid-state sets, a continuous sweep or compressed pulse) of microwave energy from a rotating antenna. Anything that reflects the energy returns an echo. The set measures the time taken for the echo to return, which gives the range, and the direction the antenna was pointing, which gives the bearing. The echoes are drawn on a display called the PPI (plan position indicator), with your boat at the centre.

Leisure radars work in the X band (around 9 GHz, a wavelength of about 3 cm). This gives good detail and works with small antennas, but it is affected by rain and sea clutter more than the longer S band radars fitted to large ships.

Range: the radar horizon

Radar waves travel in nearly straight lines, bending a little more than light does. The radar horizon in nautical miles is roughly:

radar horizon = 2.2 x (square root of antenna height in metres + square root of target height in metres)

Worked example: antenna 3 metres above the water on a RIB's arch; target a cliff 64 metres high.

• Square root of 3 is about 1.7; square root of 64 is 8.
• 2.2 x (1.7 + 8) = 2.2 x 9.7 = about 21 nm.

For a small boat with a 1-metre-high hull the target term is 1, so detection range is about 2.2 x (1.7 + 1) = about 6 nm in perfect conditions; in practice much less, because a small GRP boat reflects poorly. A low, flat sandy beach may not appear until you are close, while a hill behind it shows from far off. That means the coastline on radar is often not the coastline on the chart. Identify the radar "coast" carefully before using it for ranges.

Bearing accuracy and resolution

The antenna beam is not a thin line. Its horizontal beamwidth is typically 4 to 6 degrees for an 18 or 24 inch radome, and 2 degrees or less for a 4-foot open array. Each echo is smeared sideways by the beamwidth, so:

• Two buoys close together at the same range may show as one echo.
• A harbour entrance may appear closed on radar until you are near it.
• Bearings to a point echo are taken to its centre; bearings to the edge of a headland need half a beamwidth allowance.

Range resolution depends on the pulse length; on short ranges a short pulse separates targets a few tens of metres apart. Range is much more accurate than bearing, which is why a fix from radar ranges is better than one from radar bearings.

Minimum range

Radar cannot see echoes very close to the boat, where the pulse is still being transmitted and the antenna's vertical beam passes over close objects. On small craft sets this is roughly 20 to 30 metres, often more in a sea. Do not expect radar to show a pot buoy beside you.

Mounting the Radar on a Powerboat

Height

A higher antenna sees further and has fewer blind arcs, but on a small boat:

• More height means more weight aloft and more motion, which smears the picture.
• More height means more sea clutter at short range.

On a typical RIB or sportsboat, an antenna 2.5 to 4 metres above the waterline on a radar arch, A-frame or short mast is a sensible compromise.

Bow-up trim at speed

A radome's vertical beam is quite wide, often about 20 to 25 degrees, spread equally above and below the horizontal. When a planing boat trims bow-up by 4 to 8 degrees, or more as it comes onto the plane, the beam tilts upwards. Part of the energy goes into the sky and the near-field coverage ahead weakens. Small targets ahead, such as another RIB or a buoy, may fade in and out. Some mountings include a wedge so the antenna sits level at cruising trim.

The diagram below shows the effect of trim on the beam and the minimum range zone close to the boat.

Click to zoom

Blind arcs and shadow sectors

Anything near the antenna in its horizontal plane blocks the beam: a mast, a spotlight, an aerial, a flybridge, a canopy frame, people standing on the arch. Behind each obstruction is a blind arc (nothing seen) or shadow sector (weak echoes). Mount the antenna above all of these if you can. If you cannot, find the blind arcs:

1. In open water with a clear picture, set a short range and watch the sea clutter near the centre; blind sectors show as dark wedges.
2. Or, with a small boat or buoy at about 0.5 nm, turn your boat slowly through 360 degrees and note the headings where the echo fades.

Write the blind arcs on a label beside the display. If a blind arc is ahead, you must "swing" the boat from time to time (alter 20 degrees one way then the other) to look into it, or post a lookout to cover it.

Heading line alignment

The heading line on the display shows where the bow points. If the antenna is fitted slightly crooked, every bearing is wrong by the same amount. To check:

1. Point the boat directly at a small, clearly identifiable fixed object (a beacon, a pier end) a mile or so away, preferably with no tidal stream setting you.
2. Look at the radar: the echo of that object should sit exactly on the heading line.
3. If it is off by, say, 3 degrees to starboard, enter a heading line offset in the set's installation menu to bring it onto the line.
4. Repeat a few times and on a second object to confirm.

A 3-degree error might seem small, but at 3 nm it puts a target 0.16 nm (about 290 metres) out sideways, enough to lead you onto the wrong side of a buoy.

Choosing Range Scales

The range scale sets how far from the centre to the edge of the screen. There is no single right scale, but these are good starting points for a fast boat:

Think in time, not distance. At 25 knots you cover 3 nm in 7.2 minutes. A ship at 15 knots approaching head-on closes at 40 knots, so a 3 nm range scale gives you about 4.5 minutes from first sighting on the edge of the screen to collision. That is not enough time to plot properly. In poor visibility, slow down, and use a longer range scale regularly to see what is coming.

A useful habit: every few minutes, switch up a scale to look ahead, then back down. Many sets offer a dual-range display that shows two scales side by side.

Display Orientations

Head-up

The heading line points straight up; the picture is drawn relative to your bow. This matches the view through the windscreen: something on the starboard bow appears up and right. It needs no compass input. Its drawbacks on a fast boat are serious:

• Every time the boat yaws, the whole picture rotates. Echoes smear into arcs and targets move around the screen.
• When you alter course, every echo moves at once, making it hard to see whether a target's bearing has changed.
• Bearings read off the screen are relative bearings and must be converted to true or magnetic using your heading at that moment.

North-up

North is at the top, like a chart. The heading line points in the direction of your heading. The picture stays still when you yaw or alter course, so you can compare it directly with the chart and plotter overlay. It needs a heading sensor (a fluxgate or solid-state heading sensor), not just a GPS COG input, which lags and becomes meaningless at slow speed.

Course-up

Like north-up, but the selected course is set at the top. The picture is stable and still roughly matches the view ahead. When you change course, you reset the orientation. This is often the best choice on passage.

The diagram below shows the same situation in head-up and north-up.

Click to zoom

Relative and true motion

In relative motion (the usual setting) your boat stays at the centre and every echo moves relative to you. A stationary buoy moves down the screen at your speed in the opposite direction to your heading. In true motion, your boat moves across the screen and stationary objects stay still. For collision avoidance relative motion is easier, because an echo that moves straight towards the centre is on a collision course. On many sets you can still show true or relative trails.

Stabilisation and MARPA

With a heading sensor, the set can run MARPA (mini automatic radar plotting aid), which tracks selected targets and calculates their CPA (closest point of approach) and TCPA (time to CPA). On a fast boat that pitches and yaws, MARPA tracks can be lost or wander; treat its numbers as a guide that needs checking against the EBL method below.

Getting a Good Picture

Radar is only as good as its tuning. Modern sets have automatic modes that work well in average conditions, but learn the manual controls too.

Sea clutter on a small fast boat

Sea clutter is the bright, speckled area around the centre caused by echoes from waves. It is worst upwind, because wave faces facing you reflect more strongly. A small boat in a rough sea may be invisible inside the clutter. Turning the sea clutter control too high will remove it, but will also remove your target. Adjust gently, and look for echoes that persist over several sweeps: waves come and go; boats stay.

False echoes

• Side lobes: an arc of weak echoes either side of a strong close target.
• Multiple echoes: repeats of a large close target at two or three times its range, on the same bearing (often from a ship passing close).
• Indirect (reflected) echoes: an echo that appears on the bearing of an obstruction such as a mast or a nearby ship's hull.
• Second trace echoes: distant targets appearing at the wrong range in conditions of super-refraction (anomalous propagation).

Collision Avoidance by Radar

The principle

If the compass bearing of an approaching vessel does not appreciably change, risk of collision exists (Rule 7). Even if the bearing is changing, risk may still exist, especially with a large vessel, a tow or a vessel at close range. Rule 7 also warns against "assumptions made on the basis of scanty information, especially scanty radar information".

Using the EBL and VRM

1. When a target appears, place the EBL (electronic bearing line) on its centre and the VRM (variable range marker) on its leading edge.
2. Note the time, bearing and range.
3. Wait, keeping a steady course and speed. On a fast boat, two or three minutes is usually enough.
4. If the echo is still on the EBL and inside the VRM, the bearing is steady and the range closing: risk of collision.
5. If the echo has moved off the EBL, the bearing is changing. Which way it moves tells you which side it will pass.

This only works if your own course and speed are steady and the display is stabilised (north-up or course-up). In head-up, a small yaw moves the echo off the EBL and gives a false sense of safety.

The relative track

Mark the target's position every few minutes (most sets show echo trails, which do this for you). Join the marks and extend the line: that is the target's track relative to you. Its closest approach to the centre of the screen is the CPA. If the line passes through or close to the centre, you need to act.

The diagram below shows a plot of a target's echoes at 3-minute intervals, the relative track and the CPA.

Click to zoom

Speed changes the picture

Because a fast boat is usually faster than most other traffic, almost every target appears to move towards you from ahead. A slow fishing boat crossing your path will look almost like a stationary object moving straight down the screen. A ship ahead that you are overtaking will appear to come towards you even though she is moving away from you through the water. Be careful: on a relative motion display a vessel you are overtaking can look like one coming towards you. The plotter's AIS data, or true trails, help to resolve this.

Action in clear visibility

In sight of another vessel, Rules 11 to 18 apply as normal: crossing, head-on, overtaking and the responsibilities between vessels. Radar helps you assess risk earlier, but you act according to what you see.

Action in restricted visibility: Rule 19

When vessels are not in sight of one another in or near an area of restricted visibility, Rule 19 replaces the "who gives way" rules. Every vessel that detects another by radar alone, and finds a close-quarters situation developing or risk of collision, must take avoiding action in ample time. If the action is an alteration of course, so far as possible avoid:

• An alteration to port for a vessel forward of the beam, other than a vessel being overtaken.
• An alteration towards a vessel abeam or abaft the beam.

In practice, for a target ahead or on the starboard bow, alter to starboard; for a target on the port bow, an alteration to starboard is also normally right; for a target abaft the beam, alter away from it. Slowing down, or stopping, is often the best action of all, and on a fast boat it is easy.

Rule 19 also requires that, on hearing the fog signal of a vessel apparently forward of the beam, or when a close-quarters situation cannot be avoided with a vessel forward of the beam, you reduce speed to the minimum at which you can keep your course, and if necessary take all way off, navigating with extreme caution until the danger is over.

AIS

How it works

AIS (automatic identification system) is a VHF data system. Each fitted vessel broadcasts its identity (name, MMSI, call sign, type, size) and its dynamic data (GNSS position, course over ground, speed over ground, heading and rate of turn) at regular intervals. Your receiver plots these targets on the plotter or radar and can calculate CPA and TCPA.

Classes

Check your own set's specification, because report rates differ between Class B types.

AIS at speed: limitations

• Not everyone has it. Small fishing boats, many leisure craft, kayaks, swimmers and most buoys are not on AIS. Radar and lookout still do the work.
• It is not your own sensor. The data comes from the other vessel's GNSS and is only as good as their equipment and settings. Wrong MMSIs, wrong ship size or a vessel still showing "at anchor" are common.
• Delay. A Class B (CS) target at 30 knots moves a quarter of a mile between reports. Your plotter extrapolates, but turns are missed until the next report.
• COG is not heading. A vessel set by a strong tidal stream, or a fast boat yawing in a sea, has a COG different from her heading. CPA calculations use COG.
• Your own signal may be filtered. In busy areas some ships' displays filter out Class B targets to reduce clutter, so your boat may not appear on their screen.
• Reduced range. A low AIS aerial on a small boat may give only a few miles' range.

Using AIS well

• Set a CPA and TCPA alarm suited to your speed, for example CPA 0.5 nm and TCPA 10 minutes, and reduce the alarm range in crowded water to avoid alarm fatigue.
• Use the target's name to call it on VHF if you need to agree action. But remember that agreeing action by radio does not change the COLREGs, and misidentification of who you are talking to has caused collisions.
• Overlay AIS on radar. If an AIS target and a radar echo coincide, you have confirmed both; if there is a radar echo with no AIS, it is a non-AIS vessel or object.

Tactics in Restricted Visibility

Fog is one of the most dangerous situations for a small fast boat: you are hard to see, hard to detect on radar and travelling quickly.

Before you go

• Check the forecast for visibility, not just wind. Advection fog in spring and early summer often forms over cool water and lies along the coast.
• If fog is expected, consider not going, or plan a route that stays out of shipping lanes and close to safe shallow water where large ships cannot go.
• Make sure the radar works and the crew know how to use it, the radar reflector or active radar target enhancer is fitted, and the AIS is transmitting.

When visibility drops

1. Slow down to a safe speed under Rule 6. On most fast boats this means coming off the plane. You need to be able to stop within the distance you can see.
2. Lights and sound signals. Switch on navigation lights. Sound one prolonged blast at intervals of not more than two minutes (power-driven vessel making way), or two prolonged blasts if under way but stopped. Boats under 12 metres must have some means of making an efficient sound signal.
3. Lifejackets on, crew briefed. Everyone on deck in lifejackets, kill cord on, liferaft and grab bag ready.
4. Fix your position and note it, then plot positions at regular intervals.
5. Dedicated radar watch. One person watches the radar continuously; the helm steers and the lookout listens and looks.
6. Listen. Turn engines down from time to time to listen for fog signals. Open boats have an advantage here.
7. Avoid shipping lanes. If you must cross a traffic separation scheme, do so at right angles (Rule 10), quickly but at safe speed, after a careful radar and AIS check.
8. Consider stopping or anchoring in shallow water out of the traffic until visibility improves.

Worked Example

You are crossing a busy estuary approach in a 7-metre sportsboat in visibility of about 0.5 nm. Heading 090 degrees true at 12 knots, radar north-up on the 3 nm scale, heading sensor fitted.

At 1400 an echo appears at 060 degrees, 3.0 nm. There is no AIS target on it. You put the EBL on 060 and the VRM at 3.0 nm.

At 1403 the echo is at 060 degrees, 2.2 nm. Bearing steady, range closing 0.8 nm in 3 minutes, so the closing speed is 16 knots. TCPA at this rate is 2.2 / 16 x 60 = about 8 minutes. Risk of collision exists.

Decision. The target is forward of the beam, on the port bow. Rule 19 says avoid altering to port for a vessel forward of the beam. You reduce speed to 6 knots and alter 40 degrees to starboard to 130 degrees, a bold alteration that will show on the other vessel's radar.

At 1406 the echo is at 045 degrees, 1.5 nm. The bearing is drawing left (anticlockwise), so the target is now passing down your port side, and the echo trail passes well clear of the centre. Extending the trail gives a CPA of about 1 nm. You keep watching until the target is past and clear, then resume your course, still at safe speed.

Common Mistakes

• Head-up on a yawing boat. The picture smears and EBL checks become meaningless. Use a heading sensor and north-up or course-up.
• Too short a range scale at speed. You see targets too late. Look ahead on a longer scale regularly.
• Too much sea clutter suppression. Wave echoes disappear, and so does the small boat you need to see.
• Not knowing your blind arcs. Mark them on the display and cover them with lookout or swings.
• Uncorrected heading line. Every bearing is wrong. Check alignment when the radar is fitted and every season.
• Trusting AIS alone. Many vessels and all buoys do not show on AIS.
• Small alterations in fog. A 10-degree change is invisible on another vessel's radar. Make large, early alterations or slow down.
• Treating MARPA numbers as exact. On a fast boat tracks wander. Confirm with EBL and echo trails.

Summary

• Leisure radar uses X band; detection range depends on antenna and target height, and on how well the target reflects.
• Bearing resolution is limited by beamwidth; range is more accurate than bearing.
• Mount the antenna high enough to clear obstructions but not so high it adds motion and clutter; allow for bow-up trim at planing speeds.
• Find and label blind arcs, and check heading line alignment against a fixed object dead ahead.
• Think in minutes, not miles. Use short ranges for pilotage and longer ranges regularly for early warning.
• North-up or course-up with a heading sensor gives a stable picture on a fast boat. Head-up smears.
• Use gain, sea and rain clutter carefully; persistent echoes are real.
• A steady bearing with closing range means risk of collision. Use EBL, VRM and echo trails.
• In restricted visibility Rule 19 applies: avoid altering to port for vessels forward of the beam and towards vessels abeam or abaft; slow down early.
• AIS is valuable but incomplete and delayed. Use it with radar and lookout.

Check Your Understanding

1. A radar antenna is 4 metres above the water. Roughly how far away will it detect a 25-metre high headland?

Answer: 2.2 x (square root of 4 + square root of 25) = 2.2 x (2 + 5) = 15.4 nm, in good conditions. The actual range also depends on the target's reflectivity and the set's power.

1. Why might small targets ahead fade in and out when a powerboat comes onto the plane?

Answer: The bow-up trim tilts the antenna's vertical beam upwards, so less energy goes towards low targets just ahead. Sea clutter and the boat's motion add to the effect.

1. How do you check radar heading line alignment?

Answer: Point the boat directly at a small fixed object about a mile away and see whether its echo lies on the heading line. If it is offset, enter a heading line correction in the installation menu, then repeat the check.

1. Why is head-up display poor for collision avoidance on a fast boat?

Answer: The picture rotates every time the boat yaws or alters course, smearing echoes and moving them off the EBL, so you cannot reliably see whether a target's bearing is steady.

1. An echo stays on your EBL while the range closes from 2.5 nm to 1.8 nm in 3 minutes. What does this mean and how long before it reaches you?

Answer: Steady bearing and closing range means risk of collision. The closing speed is 0.7 nm in 3 minutes, which is 14 knots, so about 1.8 / 14 x 60 = about 7.7 minutes to collision if nothing changes.

1. In fog you detect a vessel by radar on your port bow with risk of collision. Which alterations does Rule 19 tell you to avoid, and what would you do?

Answer: Avoid an alteration to port for a vessel forward of the beam (unless being overtaken). A bold alteration to starboard and/or a big reduction of speed would be appropriate, with continued monitoring.

1. Give three limitations of AIS for a small fast boat.

Answer: Any three of: many vessels do not carry it; data depends on the other vessel's sensors and settings; Class B reports can be 30 seconds apart; COG is used, not heading; ships may filter out Class B targets; low aerial gives limited range.

1. Name four actions to take when visibility drops on passage.

Answer: Any four of: slow to a safe speed, switch on navigation lights, sound fog signals, lifejackets on and kill cord worn, fix and log position, keep a dedicated radar watch, listen with engines throttled back, avoid shipping lanes, consider stopping or anchoring in shallow water.

Further Reading

• RYA Advanced Powerboat Handbook G108
• RYA Navigation Handbook G6 (Sara Hopkinson) and RYA Radar Handbook G34 (Tim Bartlett)
• MGN 369 (M+F) Amendment 1, navigation practices in restricted visibility
• MSN 1781 (M+F) Amendment 3, COLREGs as UK law
• A.G. Bole, A. Wall and A. Norris, Radar and ARPA Manual
• A.N. Cockcroft and J.N.F. Lameijer, A Guide to the Collision Avoidance Rules
• Dag Pike, Fast Boat Navigation (Adlard Coles)