Web Hosting

Free ADS




Our World 2.0






Web Hosting
Sailboat animated gif Sailboat animated gif

Using MARPA instead of AIS…

Returning to Brighton from Dieppe recently I realised that the AIS was not working, but had not time to fault trace or fix it so I thought I would revisit using the radar in earnest.

This is crucial given that the Traffic Separation Scheme that separates the Channel into two one way traffic streams for large vessels adds to the excitement for a yacht – as they say a “tortoise crossing the motorway” – gives you some idea of the experience.

DSC01864The rather shaky photo here – sorry but we were bouncing up and down a bit! – and I didn’t want to leave the helm too long just to take a photo! – shows the MARPA targets acquired on the Raymarine (C120 + Raymarine Radome 2kw) – click on the image for a larger view.


Just like the AIS you must take the heading, speed, and collision DSC01865avoidance data with a pinch of salt. I have seen a moored cross channel ferry registering 17kts on the AIS – and so it is here – this screen shot shows variable quality of data eg. was the second vessel in this MARPA list really doing 33kts ! – also the vessels were all clearly on a heading of about 250 degrees magnetic, but the heading data on the display shows the vessels going in all sorts of directions…

The bottom line is that standing in the cockpit with a pair of binoculars and taking bearings was my chosen primary source of data…and although we had bowled long from Dieppe for some 8 hours on a close reach doing 6.5 to 9kts with only the occasional sail trim adjustments… I was forced to reel in the genoa and come to a standstill to let an imposing cargo vessel from Iran thunder past.

Good decision as it turned out since it took a good 10 minutes to cross the wake after it had passed by!.

Track MARPAThis screen shot shows our track as we took avoiding action and then tried to get underway again before the next charging container vessel came barreling down the track.






Collision avoidance – ARPA , CPA or …what?

If you have a Radar it will probably be set to use ARPA to assist you in deciding whether you are on a collision course with another vessel . Many yachts have now added AIS to their nav electronics and can overlay the AIS targets on to the Radar and chart plotter display to add more information. So far the logic is that you calculate the Closest Point of approach (CPA) and Time to Closest Point of approach (TCPA) and make a judgement on which way to turn according to the Collision Regs.

SafePass is an idea from Fred Pot of SeaCAS. In this scheme the system calculates various CPA/TCPA results and plots “safe” and “dangerous” zones on your current heading to make it even easier to decide which new heading may be appropriate for your vessel. This depends on the other vessel not changing course and also on the circumstances of the impending collision with regard to the Collision Regs of course.

By the way one of the simplest, most practical and down to earth descriptions of how to use your radar can be found on Bill Dixon & Pat Watt’s personal web page


AIS Display

This is a shot of the AIS display as I crossed the English Channel (Sep 2006). It was quite busy as you can see…all the blue triangles are container vessels that were going considerably faster than my SOG which as you can see from the display at the top was around 6Kts at that time…



Here you can see that I was less than 1nm away from the approaching vessel on the starboard bow and the vector drawn out in front of the vessel represents 12 minutes sailing at the vessels current SOG – in fact the pop up says that I was 8 minutes from being run down…luckily that didnt happen…!



Electronic navigation – Old System

I have upgraded the electronics on Enterprise and I now have two almost separate navigation systems (backup before resorting to charts !)

Firstly there is the ‘old’ system – now 11 years old..
– Raymarine GPS
– Autohelm ST50 instruments (log, wind, depth, multi)
– Phillips navigator Mk8
– Autohelm autopilot 3000 (wheel based)
– All connected on a Seatalk bus

DSC00003The Philips navigator is really good. Its visible under the spray hood, and is always pretty much in agreement with the new system (all Raymarine based) that I run in parallel. Since the cockpit is small I had no room for a new Raymarine screen on the helm – never mind the cost! – so I put the new Raymarine C120 at the chart table and use my trusty old Phillips navigator in the cockpit.