Radar Collision Avoidance, Part 2—Turning Plotting Into Action
10 CommentsReading Time: 11 minutes
More Articles From Online Book: Navigation and Marine Electronics:
- Knowing Where It’s At
- Electronic Chart Dangers
- Keeping Safe From Chart Inaccuracies
- A Useful, But Potentially Deadly, Feature of Navigation Apps and Plotters
- Chart Plotters And Autopilots, Never The Twain Should Meet
- Do You Still Need Paper Charts?
- Backup Systems
- 11 Tips for Safe Navigation With Phones and Tablets
- 12 Electronic Navigation Tips From a Cruise on Someone Else’s Boat
- Marine Electronics System Recommendations
- 6 Tips To Stop Marine Electronics From Ruining Your Cruise
- Marine Electronics Recommendations—Communications
- Marine Electronics Recommendations—Radar
- Which is Best For Navigation: Plotter, Computer or Tablet?
- Coastal Passages, Part 1—Making a Plan, 10 Tips
- Coastal Passages, Part 2—Rounding Headlands
- Coastal Passages, Part 3—Off We Go
- Costal Passages, Part 4—Keep On Plugging
- Coastal Passages, Part 5—On To The Finish
- Passage Anchorages
- Navigating in Fog—The Tools
- Navigation in Fog—Preparation
- Navigation in Fog—Underway
- Radar Collision Avoidance, Part 1—Plotting
- Radar Collision Avoidance, Part 2—Turning Plotting Into Action
- 8 Radar Use Tips
Thanks for an insightful article Eric, and additional commentary John. We are still deeply in the research phase pre purchase, refining our mission goals for the yacht and likely cruising grounds for a planned approximate decade of part time cruising. Most of our coast is blessed with incredible visibility most of the time. Radar is something I have wondered about as a tool. Useful or not? Necessary or not? These 2 articles have highlighted many potential benefits, especially for night and non AIS fleets. And also highlighted how much more I need to know before I can properly decide. I love the practical decision making and seamanship of pre decided parameters for different situations. Thanks heaps… off to read and watch more before returning to read this with more understanding of the content and context.
Hi Mike,
I am glad it is helpful. I find that radar usage in areas that rarely see fog can be even trickier sometimes because users do not have a lot of practice and have an even higher rate of poor seamanship. To me that means if you decide to equip the boat for those conditions and get underway, it becomes even more important to practice in good visibility as otherwise it is too overwhelming in bad visibility. I grew up dealing with poor visibility so it never occurred to me to try in good weather until I started training crew and realized how important that good visibility training is and how it must be done regularly if you don’t regularly practice in bad visibility.
Eric
Thanks Eric
Great advice! That makes heaps of sense for anything where the application will be most needed under pressure. Practice on a calm, clear day, well hydrated, with adequate sleep until you can reliably replicate results. Then increase the challenge until you can reliably repeat under pressure and worse conditions. Then there’s some hope of nailing it when you most need it. We do quite a bit of practice here blind navigating from down stairs, with various bits of kit switched off for the same reason. I’ve been in several situations where tech has failed at the moment you most need it. I love the tools available, and also love practicing for those times when it will fail.
I used to instruct vertical cave rescue, and one of our practices was get another instructor to set problems you would encounter while descending or ascending a rope. We’d then work those problems in pitch darkness, until we could do them as quickly and safely by feel as we could with light. Marvellous for equipment familiarity, confidence and competence. Best of all it keeps your head on the right side of what I need vs what I want (Easy to confuse) and having the skills/mental agility to improvise with whatever is to hand.
Eric and John, great article. Last night, I had to go around an oil rig and it’s oncoming traffic (probably a worker switch). I was surprised the stated perimeter on the plotter was only 500 meters for the rig. Too close to my liking.
I like at least 1 nm CPA offshore. Keeps the cortisol levels down.
John, I followed the link to the freestanding radar and I could not find out why you would rather have that unit freestanding rather than fully integrated with a multifunctional display. I’m curious what advantages you seek going freestanding.
As always, receive my best,
Marie
Hi Marie,
Fixed objects are a good point, this article is focused on other vessels which are underway but we often need to pay attention to things that don’t move. The good news there is that if you are really sure it can’t move, the risk of having the other vessel take the incorrect action is eliminated. I would be comfortable running lower CPA’s in this situation but there is no harm in keeping the same ones. We should also be watching for other small vessels that may not be distinct on radar such as an OSV or other tendering type vessels that could suddenly leave a fixed object.
Eric
Thanks Eric. People coming to and from the rig was my worry. There was a ship with AIS going to the platform. At least I had something to do!
Hi Marie,
On free standing radar be benefits: https://www.morganscloud.com/2015/02/24/marine-electronics-recommendations-radar/
Thanks John! You are right that MFD can play games with you, be glitchy. Ours sure does and it’s rarely the right time! Food for thought!
Hi Eric,
Thoughtful piece with many excellent points, thanks.
From a big ship perspective, CPAs in my experience were situational as you describe, but I never remember any specified CPA targets. It is interesting also that the Collision Regs are not prescriptive in this regard, simply stating a “safe distance”.
The biggest variable I do remember was visibility, and again the Collision Regs don’t distinguish between actions in haze, mist, fog or heavy fog.
In haze or mist as is frequently experienced in say the North Sea, it would be common place for shipping to remain at “cruise” speed burning heavy oil (which could be between 12 and 20 knots). This means they have no immediate ability to slow down further (requiring a switch to diesel), and alterations of course would be the only option for taking avoiding action.
It’s this intermediate visibility (mist) that is often hardest, with vessels appearing out of the mist still at high speed, taking actions having come in sight of each other. So in mist with visibility of around one nautical mile, I would be taking avoiding actions based solely on radar plotting, at about 7.5 NM for other shipping, providing there was sea room.
In open waters, I would be aiming for a 2 nautical mile CPA, ship-on-ship whatever the visibility. This was also our required minimum distance off for hazards like shoals or headlands.
The keys here in my mind are Rule 19d which encourages early action to avoid collision, AND Rule 8 which also encourages such action, but applies in any visibility. With sufficient sea room, for me this meant (and still means today) a minimum 45 degree alteration of course or slowing to half or less my previous speed.
Small alterations of course or speed are just not obvious enough and any aspect changes not readily apparent visually or on radar. This applies as much for big ships as small vessels like ours, I believe.
And once the danger of collision clearly passes, you can come back on course / speed to reduce distance lost to the next waypoint.
Hi Rob,
Interesting that you never had CPA targets but it also sounds like the 2Nm you were typically using was nice and conservative. Professional mariners do this stuff all day long and are probably much better at making judgement calls on CPA than cruising sailors who don’t run radar often and have comparatively very maneuverable vessels. I know a handful of unlimited masters and I think I have heard all of them mention CPA to other vessels as part of standing orders. I always appreciate hearing what your professional navigator practices were as it is very useful for people on cruising boats to know what to expect.
My own experience was that I was having a lot of problems with crew seeming fixated on maintaining course and speed and they would keep pushing it until they had to make a huge course correction which was often very disruptive and still left us with a CPA I wasn’t happy with. Once I tried giving the CPA as part of standing orders, things got so much better. I think that in their minds, suddenly they had a direct actionable order which took priority over the course orders they had whereas before one was clear and the other (don’t hit anything) vague. In a strange way, it really empowered the crew to take action whereas before it was like they were afraid of waking me up by changing course which actually kept me awake as I didn’t trust that they were being sufficiently conservative.
Good point on the limitations of speed changes for large ships. On the other hand, a cruising vessel can often roll up the jib for a few minutes or intentionally overtrim and slow right down before getting back up to speed.
On the size of the course change, I think it really depends on the tCPA and you have to really pay attention to rule 8. When tCPA is high, the chance of a collision is not significant yet, afterall we only pay attention to the vessels in our immediate area and not ones 10’s or 100’s of miles away. If we make our course change then and in accordance with COLREGS, it can be small (I never do less than about 15° even when showing a different light) but the other vessel has time to understand our intentions and the risk of collision always stays low. However, if we wait until the tCPA is low, then we need to make big (often 60°+) and obvious course changes. Of course our plots will also force us into this too as they will show large corrections needed if we have waited. Plotting will also make sure we don’t do a series of small course corrections which are specifically prohibited.
It takes a little while for the other vessel to realize our course change so there needs to be plenty of time in case one vessel does not act as expected. Some people like to make a big change and then turn back earlier to maintain CPA but I find that quite nerve wracking when on the vessel being turned towards unless they have explained intentions over VHF. One other factor here is that we transmit AIS and large ships (I think it is 10,000 GRT and up by regulation if I remember right) are required to have ARPA so smaller changes are much easier to perceive than in the days of hand plotting where you need a large course change and a full plot afterwards for the other vessel to really understand the change which can easily mean you are 2+ Nm closer. But to your point, we should always be thinking about whether the other vessel will understand our actions and it is always better to do more than less.
Eric