Radar Reflectors

[jackdale]

Originally Posted by cabo_sailor

There is also a Mobri reflector but I'm not to sure how well they work although they are quite small and usually mount on the shroud just above the spreader. They claim to be excellent.



Rich

I was doing a delivery last summer in which the boat had a shroud mounted Mobri. Tofino traffic could not pick us up on radar as we approached the entrance to Juan de Fuca.

[beetle]

Originally Posted by jyoung

i was just curious if someone could tell me a good radar reflector to buy. however, isn't it just rquiring flat metal surfaces for wave reflection? if so then couldn't you just make one for about $5? any thoughts

If you think that a flat surface is useful for reflecting energy back to the transmitting source from a bouncing, heeling sailboat, then you might benefit from reading through two papers that analyze and test radar reflectors specifically for use on boats:

Stan Honey, Jim Corenman tests
Safety Studies
(select 1995 radar reflector test)

Qinetiq radar reflector study
Marine Accidents Investigation: Download PDF document

The short answer is that no, a single flat surface is not a useful radar reflector unless the flat surface is oriented towards the radar source such that it will reflect the radar signal directly back to the source - this is almost never the case on a moving boat. What you do want is a trihedral corner that will reflect energy back in the direction it came from even when the energy is received at significantly off-axis angles (this goes back to high school physics class), and the size of the panels forming the trihedral relative to the frequency of energy you wish to reflect (bigger is better).

In general, the standard Davis aluminum octahedral reflector mounted in the catch-rain position and as high as your can above the waves is still the best compromise available for a passive reflector on small boat for cost, weight, windage, and radar reflection.

The active x-band transponder units are a whole different story.

- rob/beetle

[otherthan]

aint the mast plenty reflection for radars????

[beetle]

Originally Posted by jobi

aint the mast plenty reflection for radars????

Depends upon how much background noise there is (e.g., waves), and the radar's power, resolution, gain control setting, distance from target, etc. In flat calm conditions I can tune the radar away from its default/generic settings in order to pick up sailboats motoring along some miles out. In lumpy 6-8' seas that same sailboat doesn't show up - the return from the mast is indistinguishable from the radar return striking the wave (e.g., sea clutter). In both calm and lumpy conditions the same RF energy is striking the sailboat mast, but in lumpy conditions the radar gain is turned down to remove clutter from the display and the sailboat's mast return is lost.

A metal object will reflect some energy; if a metal mast is rounded in shape, it will tend to disperse the radar (RF) energy that strikes the mast, therefore only a small amount is reflected back towards the radar transmitter/receiver.

Upshot: a metal mast is a poor radar target relative to a radar reflector, as the reflect is designed to collect as much received RF energy as possible and reflect that energy directly back towards the radar receiver.

- rob/beetle

[David M]

Originally Posted by jyoung

i was just curious if someone could tell me a good radar reflector to buy. However, isn't it just requiring flat metal surfaces for wave reflection? if so then couldn't you just make one for about $5? any thoughts

There is a little more to it than having a flat metal surface.

The reason some of the three sided reflectors work so well is that after bouncing off two to three planes the wave is reflected directly back to the source. This is regardless of which angle the reflector is held to the source. This type of reflector must though be large enough with respect to the wavelength and the three planes must be exactly 90 degrees to each other.

If you were to do this with three mirrors you would always see your eye looking back at you regardless of the angle you are looking at the three mirrors.

[Bash]

Originally Posted by jackdale

Bash
How do you work with a BBQ mounted 4 meters above the waterline?
Lots of freeboard?

Cute emoticon. But you seem to be missing the point of the comparison. The BBQ, mounted on the pushpit, works better than the commercial reflectors.

It also does an equivalent job protecting my boat from lightning when compared with commercially available lightning arrestors.

And it also grills great steaks.

[Unicorn Dreams]

I've noticed around my marina, that the new HD radars seem to show everything very well.
Had my Davis relector off my boat and another sailor about 1/2 mile away called me on my phone and asked me if I went out the night before as he couldn't see my mast reflection. Plus I had promised to take him out the next time I went out..
We have covered docks for powerboats, so all he can see is my Davis. I told him I was replacing my rigging and the reflector was down right then. I replaced my Davis and there I was again. He took before and after shots and there was a bg difference evev with all the background clutter.
On Santana which was a steel hull 36' ketch, people could tell a big difference on whether my Davis was up or down. Plus Santanahad a flat sided pilothouse on her.
Any radar reflector should make a difference.

[barnakiel]

The simplest alloy octa works well, but it has to be the biggest one you can buy (or build).

The SeaMe and the likes are good stuff, but the need electricity and they cost ;-(.

b.

[otherthan]

are cargo ships better equiped to see you on radar then the average fellow cruiser??

[beetle]

Originally Posted by jobi

are cargo ships better equiped to see you on radar then the average fellow cruiser??

Not in my experience.

As regards equipment, a wider/longer antenna array will have better resolution than a shorter one. A more powerful antenna will punch through rain and provide a return better than a weaker antenna.

Both antennae will have the same problem when trying to pick out a weak return against a background of sea clutter - the weak return will look a lot like a wave's weak return. You want to do what you can to make your return stronger than a wave's return.

One disadvantage to a large ship's radar is that the antenna is mounted way up off the water, which places a sailboat below their horizon and in amongst the sea clutter.

- rob/beetle

[Zednotzee]

Originally Posted by jobi

are cargo ships better equiped to see you on radar then the average fellow cruiser??

Not really. They have more powerful radars, but they're mounted high up. By the time a small boat is within the radar horizon, it's effectively part of the sea clutter. If you check that link to the Qinetic radar study from the MAIB, you'll see it was commissioned due to the loss of the Ouzo.
From MS Pride of Bilbao - Wikipedia, the free encyclopedia

The MAIB report into the sinking of the Ouzo was released on 12 April 2007. It concluded that the sinking of the yacht was due to the Pride of Bilbao colliding with her, or passing so close that she had been swamped or capsized by the vessel's wash.

Those guys are mainly concerned with not hitting another large ship. Hitting you would only be a problem if your wreckage fouled their props. You will recall also Jessica Watson's collision-

Jessica Watson - Wikipedia, the free encyclopedia
The final report was released in June 2010. The report stated that both Watson and the Silver Yang's watchkeepers had failed to maintain an adequate lookout and that both had failed to properly employ the navigational aids. In addition, the report found that the watchkeeper on the Silver Yang had failed to offer assistance to Watson after the two vessels had collided.

[otherthan]

Originally Posted by beetle

Not in my experience.

As regards equipment, a wider/longer antenna array will have better resolution than a shorter one. A more powerful antenna will punch through rain and provide a return better than a weaker antenna.

Both antennae will have the same problem when trying to pick out a weak return against a background of sea clutter - the weak return will look a lot like a wave's weak return. You want to do what you can to make your return stronger than a wave's return.

One disadvantage to a large ship's radar is that the antenna is mounted way up off the water, which places a sailboat below their horizon and in amongst the sea clutter.

- rob/beetle

so to rely on there radars is not a good idea!!!
Thanks for your post
rgds

[beetle]

Originally Posted by jobi

so to rely on there radars is not a good idea!!!
Thanks for your post
rgds

'rely on there radars' for what purpose? Depending upon the purpose, it may be a very good idea.

I rather do rely on their radars for them to see me, as there's little alternative at the moment other than the eyeball - that's what a radar reflector is good for, as it helps them to see me in the dark (during daylight hours as well, I find). At the same time, a reflector is only helpful if their radar is turned on and someone is looking at it.

I also rely on my radar to see them. Big ships are darn good radar reflectors, I'm pretty good at finding them.

An AIS transponder that I operate is also helpful at telling the ship where I am, but that only works if the ship is operating their receiver and someone happens to be looking at it.

- rob/beetle

[otherthan]

Originally Posted by beetle

'rely on there radars' for what purpose? Depending upon the purpose, it may be a very good idea.

I rather do rely on their radars for them to see me, as there's little alternative at the moment other than the eyeball - that's what a radar reflector is good for, as it helps them to see me in the dark (during daylight hours as well, I find). At the same time, a reflector is only helpful if their radar is turned on and someone is looking at it.

I also rely on my radar to see them. Big ships are darn good radar reflectors, I'm pretty good at finding them.

An AIS transponder that I operate is also helpful at telling the ship where I am, but that only works if the ship is operating their receiver and someone happens to be looking at it.

- rob/beetle

oh I never thot about having a radar onboard...now I understand the benifits....cheers

[Coachbolt61]

What about two reflectors one above the other, one mounted in the normal flat position and the other in raincatcher position?
Coachbolt.