On a recent sail from Point Roberts to Vancouver we made some interesting observations. We were delivering three boats from Seattle to Vancouver.
The two other sail boats were travelling close to us, yet they do not appear on radar. Both had a radar reflector. However those were Mobri reflectors (cylinder). These types of radar reflectors are not effective at all. The manufacturer suggest putting one on each cap shroud, just above the spreaders. Even so, they prove very ineffective.
The tubular (Mobry type) reflector has a maximum of 2 sq. meters of effective radar cross section while the Davies reflector has a maximum of 12 sq. meters of effective radar cross section. In other words: The Octahederal (Davis) radar reflector has 6x more active reflective area than the tubular (Mobri) type.
Which one would you want to have on your boat when in fog or at night in a high traffice area such as Vancouver or the Strait of Georgia?
If you want to read more about the types of radar reflectors mentioned above read the West Advisor paper.
An interesting fact (from the 'West Advisor series'): "The RCS (Radar Cross Section) of a given reflector goes up by the fourth power of the radius, resulting in this dramatic increase in effectiveness. For example: a reflector of twice the size of a similar but smaller model has a RCS that is 16-times larger. Moral: buy the largest RTE (Radar Target Enhancer - radar reflector) you can."
Radar operation - zoom level
Another interesting observation is with regards to zoom levels. We were sailing past the BC Ferries terminal in Tsawassen when ferry left the dock and crossed only a few hundred metres in front of us. On a typical zoom level of 2 NM (often this is even set to 6 NM) one could barely make up the ferry as a small spec on the screen that one might easily oversee. At a zoom level of 6 NM, the ferry all but disappeared on our screen.
Zoomed in to 1 NM the radar echo becomes more visible and at 2,000' it is hard to overlook the ferry. Zooming in further to 1,000' the echo shows up as a big blob, however at this zoom level perspective gets lost.
It is not uncommon for sailors to be zoomed out to 6 or more NM because that shows a large area of where we are sailing in and where we want to go. Radar echoes of even large ships will easily be lost in the clutter of other symbols on the screen.
The ferry barely shows up and will be easily overlooked. At 6 NM zoom level (no picture) the ferry could not be seen.
At 1 NM zoom the ferry can now clearly be made out, but it is still not a very big echo and one has to pay attention to the screen.
At 2,000' the ferry clearly shows up.
At 1,000' zoom level it's impossibly not to see the ferry, but perspective on the screen with regards to navigation gets lost. Note, even at this zoom level the two sail boats do not show on the screen.
Christof is the owner and operator of Simply Sailing. An enthusiastic sailor he loves to share his knowledge with other sailors.
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