Philosophically I agree with
smckean that it is highly impractical to use a plotting board on a C Dory, and for the reasons he stated. I also have many tens of thousands of miles as skipper of my vessels crossing oceans, and used basic non electronic navigation, as well as being trained in the use of radar plotting.
But as to MARPA, I'll quote from a
Panbo article of Dec. 3, 2015
" When a yachtsman says that MARPA is critical to safe navigation, I tend to wonder if they've been overly influenced by the commercial world where radar beam widths are much narrower and pulses much shorter."
Although this refers to a Navco unit--in many ways it refers to all small boat radars, and their ability. If you want the whole story, read it, the comments, and then various articles dealing with MARPA, the heading sensors etc.
I have the Garmin 24XHD radar, with a 1040xs--the combination supports MARPA --but to really get it to work, you need the extra heading sensor, with a second 10hz GPS receiver and a 9 axis sensor--"gyro rated" compass. (over $700). Retail price of all of this today is close to $4,000, including the cables and set up for the MNEA 2000 backbone. I am doing overlay at times--and a simulated MARPA without the special heading--but in reality I am only manually tracing one object. If I really want accurate information and I am lucky, I can use AIS do the projection in my head or maybe have to go back to a plotting board.. I lugged a pad of radar plotting sheets all over half of the World, but even in the stability of a 62 footer, with a real chart table, and one of the best small boat radars available in mid 1980's, things just happen too fast even at 6 knots with many targets to plot the objects! For an extreme example--we had over a dozen fishing boats, criss crossing pulling nets at one time--dense fog middle of the night within a 2 mile range You cannot plot that in real time.
The reality is that the C Dory is not a platform where you can manually even plot one vessel and still keep eyes on the radar, chart plotter (or overlay), depth sounder and keep a lookout--or respond to the person who is keeping a lookout, steer etc...
As to radar plotting--I do it in my head. Basically constant bearing is danger, just like visual. How accurate is your radar discrimination? Are there two boats at 5 miles or one? Without AIS, can you rapidly calculate how fast that boat is moving ( without MARPA)? Some of the latest digital radars are better. But as I understand today's technology, we are not quite there yet, without the very accurate heading sensor. (Some say that they can do it with the $200 point one), I have not tried that--and don't spend that much time in busy shipping lanes with fog--if I was in some areas--like Boston, I would have all of the latest equipment--if I wanted to run in those conditions. Don't forget one of those ferry's hit a breakwater not too long ago--what chance does a C Dory have?
A quote from a professional mariner on
Cruising Anarchy
MARPA can give you the general information that can be used to determine those targets that aren't really a threat and to identify those that may be a threat...... However, anyone who uses electronics to plot a technically correct close quarters route is foolish. There is a reason that my Night Orders book required a minimum CPA of 2 or 3 miles (at sea) and why other margins were usually considered in terms of miles or maybe half miles. You must keep a margin of safety far greater than what the decimal points tell you. The consequences of error are too big to consider.
A frequent failing of the average pleasure boater is that he thinks in terms of his own relative idea of close. To him a quarter mile is 1200 feet or twenty to thirty times his own boat length and between yachts that's a huge separation. But get to commercial shipping and it's not even half of their turning circle.
Use your AIDs to navigation but recognize their limitations. MARPA is just another aid like your compass for instance. Do you steer to the nearest degree or to the nearest 5, 10 or even 20 degrees?
How many C Dory's really use MARPA?