About this site

Piers and Lin du Pré bought their new Fleming 55 / 129, Play d'eau, in 2003.

She was berthed in Beaucette Marina, Guernsey in the Channel Islands at N49° 30’.197 W002° 30’.350 until she was sold in October 2021.

This site charts the thrilling adventures they had in her.

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Recent Posts

Radar, MARPA and Radar Assisted Collisions

The Robert Avis radar course

Like many other boaters, we’ve been caught out by fog. We were in the Channel Islands anchored in Havre Gosselin bay on Sark’s west coast. It was a mid-summer hot day with only a gentle breeze from the north and clear blue skies. Soon after lunch, our eldest son pointed towards the Gouliot Passage and asked, ‘what’s that?’ In just a few minutes we knew. We were completely engulfed in thick fog.

The UK magazine Motor Boats Monthly (MBM)

In 2005 I was asked by Motor Boats Monthly (MBM), to attend a Radar Course and write an article on Radar and its use in Restricted Visibility. The course would be taken by (the late) Robert Avis who had command experience of eleven warships and six superyachts, and more than 350,000 miles at sea.

I have to admit to feeling very small after just a few moments of hearing Robert speak. It appeared that my knowledge of using radar at sea was wrong and very mis-informed. Furthermore, in talking about MARPA Robert showed how almost all leisure radars present potentially incorrect information to the extent that radar assisted collisions had been the result.

Anyone been caught out by fog?

Robert started with this simple yet direct question. After a moment of hesitation almost all of us put our hands up. Asking how we used our radar sets, most answered ‘Head Up’, and after even more hesitation, most admitted to using radar as a means to dodge potential traffic.

‘Well,’ concluded Robert. ‘The good news is you’re still alive,’ (muted laughter), ‘and the better news is that by the end of this course you will have learned how to use radar properly.

‘First, you’ll learn that the normal COLREGs change when you can’t see the opposing traffic and that Rule 19 comes into play instead. We’ll learn this rule so you can take the correct avoiding action.

‘Second, you’ll learn the best way to set your display to assist you in making the correct decisions.

‘Third, you’ll learn that MARPA be can a really dangerous feature unless you understand why the information it presents can be very misleading.’

To say Robert had out attention would be an understatement.

Radar, MARPA, and Radar Assisted Collisions

What do I need to know to avoid a collision?

To take the correct avoiding action, you need to know two things. Is there a risk of collision, and in which direction is the target pointing?

My radar has MARPA – what is it?

MARPA is an aid to assessing collision risk that you’ll find on many leisure radars. MARPA (Mini Automatic Radar Plotting Aid) is the less capable version of ARPA (Automatic Radar Plotting Aid ) that’s been on commercial shipping for many years.

Can I rely on MARPA?

No. It’s only an aid and the information it provides can be wildly inaccurate since it’s fully dependant on the type of speed and heading inputs your radar utilises which in turn defines if it’s Sea Stabilised or Ground Stabilised.

So what’s the difference between Sea and Ground Stabilisation?

Given the effects of tide and/or wind, your boat’s speed through the water (STW) can differ significantly from your GPS speed over ground (SOG). For the same reason, your boat’s heading (BH) can differ significantly from your GPS course over ground (COG). These differences amplify the slower you travel and/or the stronger the tide and wind.

For example, moving at 6kts into a tide of 4kts shows a STW of 6kts but a SOG of 2kts.

Unfortunately, most leisure radars utilise GPS SOG and COG making it ‘Ground Stabilised’. Calculating collision risk in this way has been attributed as a significant factor in ‘radar assisted collisions’ on a number of occasions.

For MARPA to be of use and not offer potentially misleading and inaccurate information, your radar must use boat heading (BH) and speed through the water (STW). This is called being ‘Sea Stabilised’.

What do the IMO, MAIB and MCA say about this?

The IMO (International Maritime Organisation), MAIB (Maritime Accident Investigation Branch), and MCA (Maritime Coastguard Agency) all recommend Sea Stabilisation for collision avoidance.

The MAIB’s report on the yacht Wahkuna collision in 2003 makes chilling reading (see below). Their comment is “Yachtsmen should be aware of the characteristics and limitations of the radar set they are using, and … Sea Stabilisation should be selected for anti-collision use.”

How do I check if my radar is Sea Stabilised?

Does your radar manual have any mention of Sea or Ground Stabilisation? If not, contact the manufacturer or supplying installer to clarify.

You can also run your own test. On a day when there is no wind,

    Step 1 – Choose somewhere where you know what the tide is doing
    Step 2 – Target a fixed object such as a buoy
    Step 3 – Stop the boat and become stationary in the water (i.e., not making way)
    Step 4 – Locate the buoy and choose a range scale to fit
    Step 5 – Target MARPA, and wait….

Your radar is Sea Stabilised if MARPA shows the buoy moving at the speed of the tide, but in the opposite direction from the tide.

If MARPA shows the buoy is stationary but the radar has an apparent speed for your vessel, the radar is Ground Stabilised.

What’s needed to make my radar Sea Stabilised?

Provided your radar can handle Sea Stabilisation, it needs a STW data source usually taken from your boat’s paddle wheel log, and a boat heading source with a fast refresh rate of some 40Hz or better, usually taken from a flux gate, or gyro-compass.

Why does the compass need such a fast refresh rate?

Simply because when turning, or in an emotional sea, your boat can alter heading faster than most compasses can cope with, causing MARPA calculations to lag and become confused. Bear in mind that few radars have fast processors.

Applying COLREGs Rule 19

My radar has features called relative and true motion. What’s that about?

To cut to the chase, relative motion shows the moving relationship between you and other vessels. If the target’s relative trail (synthetic after-glow) or vector, points towards the centre of the screen (you) and its range is decreasing, a risk of collision exists.

When you know you have a potential risk of collision, you need to decide what avoiding action to take. For this, you need to know the target’s aspect – the direction it’s pointing if you could see it. This can be very different from the direction it’s travelling.

For this, you need to know its ‘true vector’ which gives an indication of the direction the target is pointing (its aspect) which is exactly what you need to know for the correct avoidance action (rule 19d – see later).

Therefore, for collision avoidance, it’s best to set your radar to relative motion, and only use true vectors to check a vessel’s aspect – the direction it’s pointing.

What action do I take if my radar suggests I’m on a collision course in restricted visibility?

Remember that the COLREGs for restricted visibility are very different from those when you are sight of other vessels. Rule 19 takes over.

What should I use if MARPA can’t be relied on?

The good old fashioned Electronic Bearing Line (EBL) is by far the best indicator of a risk of collision. It’s the electronic equivalent of the constant bearing line technique you use in good visibility when watching a vessel bear down on you with your mark one eyeball.

Even if properly set up, what other factors affect MARPA?

MARPA takes time to calculate the selected target’s details. Allow it to settle. It is not helped if,

  • your heading and speed change.
  • if the target starts changing its heading or speed.

Errors increase significantly if,

  • either one of you is travelling slowly or the tide or wind is strong.
  • the sea state is emotional – anything but smooth.

What about AIS?

High caution is also required with AIS data. The information provided is only Ground Stabilised and subject to the accuracy (or lack of) of the transmitting vessel.

Two pics which may help…

Assume your course is Northerly, at 8kts and that there’s an Easterly tide running at 2kts.

The first image is Ground Stabilised. Which vessel poses the threat? What is MARPA telling you? The second image is Sea Stabilised. Which vessel poses the threat?

Ground Stabilised
In this Ground Stabilised display which target is the threat?
click to enlarge

Sea Stabilised
In this Sea Stabilised display which target is the real threat?
click to enlarge

Yacht Wahkuna and P&O Netlloyd Vespucci

In poor visibility, the vessels collided in the English Channel. Both had detected each other by radar at 6 miles. Both skippers misinterpreted the information their radars gave them and both took the wrong action. Minutes later, the vessels collided. The first 3m of the Wahkuna’s hull were demolished and she was dismasted, yet the master of the container ship was unaware a collision had occurred. The yacht’s crew abandoned to a liferaft for 5½ hours before being rescued.

The MAIB concluded some contributory factors to the accident included:

  • Misunderstanding by the yacht’s skipper of which COLREGs apply in fog
  • The inability of the yacht’s skipper to use radar effectively
  • Over-confidence in the accuracy of the Vespucci’s ARPA
  • Both skippers taking the wrong action

The report stated the Vespucci’s radar was Ground Stabilised, the incorrect format for collision avoidance. It should have been Sea Stabilised in accordance with IMO guidance.

The report emphasised that whenever radar equipment is fitted on board any type of vessel, that watchkeepers are fully versed and trained in its use and capabilities. The fitting of radar and (M)ARPA, without knowing its limitations or how to use it, can contribute to accidents.

Rule 19 of the COLREGs applies when in restricted visibility

What is Restricted Visibility?

Simple – “when vessels are not in sight of one another.”

What action should I take?

We have learned that in restricted visibility, using radar as a means to ‘dodge’ targets is wrong and in itself has led to accidents. Instead, we apply Rule 19.

So what does Rule 19 tell me to do?

There are two main differences. First, Rule 19 shows there’s no such thing as a stand-on vessel in restricted visibility and that the onus falls on both vessels to take avoiding action “in ample time”. Second, this applies even if you are being overtaken where it remains your responsibility to take avoiding action.

What is “ample time?”

Robert advised us that in open seas a commercial vessel would expect to come no closer to other traffic than 2 miles and that 2 miles would be a good guideline to use. Other commercial operators may dictate a greater distance.

So what does Rule 19 say?

First, if you detect another vessel by radar alone, take avoiding action in ample time provided that if you are going to change course, avoid the following as far as possible:

    a) an alteration of course to port for a vessel forward of the beam, other than for a vessel being overtaken;
    b) an alteration of course towards a vessel abeam or abaft the beam.

Rule 19 of the COLREGs
click to enlarge

Second, except where it has been determined that a risk of collision does not exist, every vessel which hears the fog signal of another vessel apparently forward of her beam, or which cannot avoid a close-quarters situation with another vessel forward of her beam, shall reduce her speed to the minimum at which she can be kept on her course, taking all her way off if necessary and in any event navigate with extreme caution until danger of collision is over.

What sound signals should I make?

Rule 35 defines which sound signals should be made in restricted visibility. For leisure vessels, these are mainly,

One prolonged blast Power driven making way At least every 2 minutes
Two prolonged blasts Power driven, under way but not making way (2 secs between blasts) At least every 2 minutes
One prolonged and two short blasts Sailing of Fishing vessel, not at anchor At least every 2 minutes

A prolonged blast is 4-6 seconds, and a short blast is about 1 second.

What’s the moral of this story?

Using Radar authored by Robert Avis
click to enlarge

Unless you know without doubt that your radar is Sea Stabilised, use your EBL, know COLREGs rule 19 and how to apply it, don’t trust MARPA, and remember that even at best, MARPA is only an aid and not a get out of jail free card.

‘Using Radar’, author Robert Avis

This is an excellent reference book and written in typical Robert Avis style. I’d recommend ‘Using Radar’ as a ‘must have’ for anyone who’s ever likely to be caught out by fog.

ISBN-10: 1574091050; ISBN-13: 978-1574091052

© Piers du Pré 2013. No reproduction without permission

from the Nav Table of
Play d’eau
Fleming 55

2 comments to Radar, MARPA and Radar Assisted Collisions

  • Robbie

    Happy birthday Piers.
    You posted this dissertation the day after your birthday celebrations!
    How sad!
    It doesn’t mean much to me but I’m sure your nautical friends will put it into practice.
    Love reading about your adventures.
    Have fun. Love Robbie

  • Hi Robbie. Good to hear from you. I’ve had this published three times now and felt its right place was on our website.

    The birthday celebrations have been full-on. Such fun, and the weather has been so kind to us.

    The children will be giving me their photos before they start leaving some time later today which will allow me the time to start adding posts.

    Virtual hugs to you and yours.


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