Radar Reflectors

[CAVU]

Another post with a question about radar reflectors got me to thinking about this subject. Maybe a more in-depth discussion is in order. Many C-Dories don't have radar including me. If you don't have an active system to see others how well do the passive reflectors work so that others can better see you? I bought one of those funny looking three lobed reflectors but I have no idea if it really helps. My gut feeling is on the roof of a C-Dory it is so low to the water that you may disappear in the ground clutter? Any one with practical experience in picking these up on their radar?

[Valkyrie]

Ken,

Believe me, they really work! I had one of the Davis Instruments models on my sailboat, where it was mounted about ten feet off the deck, or thirteen feet over the waterline. I was in the Chesapeake heading south and had a Coast Guard friend of mine, who was XO on the Red Birch, a CG Buoy Tender, scan me with their radar. He said that I showed up with a return like that of a large tug! Other times, I radioed other boats and asked what kind of return they were receiving, and it was always very positive.

I'm sure that the height of the reflector over the water and the height of the radar are variables, but I'm not sure how much. Also, how to mount the reflectors, which are typically foil covered cardboard? (They are durable, though, and I only went through two in the eighteen years that I had Spirit.)

There are a few models out there that look like cylindrical fenders, which are the basic unit previously described, but are enclosed in plastic or fiberglass. I'm sure that any of them would be a big improvement over the basic C-Dory.

By the way, we take delivery in the Chesapeake next Thursday!

Nick
Valkyrie

[rogerbum]

Due to their design as corner cube reflectors that send signal back in the same direction from which it came, the reflectors made from a 12.5" circle provide a radar return signal that is approximately equivalent to 12 sq. meters of flat material. That's approx 129 sq. ft. or roughly equal to a 11.3 ft x 11.3 ft chunk of metal perpendicular to the line of site of the radar.

As far as the height goes....
The earth's curvature is only about 8"/mile - hence the reflector doesn't need to be very far up to provide many miles of visibility.

Roger on the SeaDNA

[dogon dory]

Sorry, Folks - Post Deleted By Author

[rogerbum]

[gljjr]

This is an interesting thread! I was looking at Radar Towers rather than arches and wondered if placing a vertical sheet of metal perpendicular to the shaft of the tower would suffice as a reflector? Since most towers are either Aluminum or Stainless it seems that it would be pretty easy to put an angle on the upright and end up with a mount for a small plate to act as a reflector.

Am I way off on my thinking?

[rogerbum]

[Sea Wolf]

Gary- The plate would only reflect radar waves when is was perpendicular (square) to the radar sender (source). The radar waves are reflected (kind of) like a BB projectile, only bouncing straight back when the reflecting surface is square to the source. Most of the time, your boat's and the plate's angles are at something other than totally perpendicular to the source due to direction of travel and pitching and rolling angles.

The real beauty of the 3-cornered reflector is that the geometry of the reflector will bounce the radar waves back to the source as it moves about on the boat, regardless of the boat's motion or that of the reflector. Has to do with incoming and reflected angles trapped in a perpendicular reflecting corner, in this case in three dimensional form, so you could mount a three cornered radar reflector below the radar tower instead of the plate, providing it will fit.

Joe.

[Bess-C]

I've been wanting to add a radar reflector, but I don't know where to mount it. I have a radome on a 17" tall radar arch on the back of the roof. I will have a dingy attached to the roof under the radar arch. I assume that I'll need to get the reflector up out of the way of the radar. My gps antenna, vhf antenna and anchor light are all mounted to the radar arch. Is there a standard mount that will work on the arch, but get the reflector up out of the way?
Lyle

[Sawdust]

Lyle,

Give Les a call (good luck) -- he has installed several.

Duster

[CAVU]

Valkyrie,
Thanks for the info. That was just what I wanted to hear. I have a model of reflector from West Marine called a Tri-Lens reflector. It got a good review from Practical Sailor magazine but I was concerned with its height above the water. On the roof of the C-Dory it is probably only 6' about the surface of the water. I like to fish out of Ucluelet off the west coast of Vancouver Island and I have been caught in the fog a few times. I slow way down and keep a good eye out but I get very nervous coming into the entrance buoy where all the fishing boats tend to concentrate. I have had a few boats come roaring by going much faster than I would deem safe in the fog. I guess they think since they have radar they don't need to slow down.

[Valkyrie]

CAVU,

Glad I could help, but my "It works, get it" answer seems to have been overpowered by the scientific discourse that followed my post!

I never cease to be amazed by the level of experience and knowledge found on this site.

Nick
Valkyrie

[dogon dory]

Sorry, Folks - Post Deleted By Author

[El and Bill]

Plan B made a reflector out of cardboard, covered with aluminum foil, a few years ago -- looked like the West Marine "corner cube" twin circle -- except his was twin semi-circles. He mounted it flush atop his radar dome and we tested it with our radar -- BOOM -- a marvelous reflection. For do-it-yourselfers or for those who enjoy experimenting, it might make an interesting afternoon and give a CD an inexpensive, easily mounted, adequate reflector.

[rogerbum]