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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #21  
Old 02-19-2014
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Re: Radar signature

Quote:
Originally Posted by CaptDave View Post
My random thoughts...

The corner reflector (the spherical type with intersecting disks) is designed to work at all angles of heel since the idea of the "corner" is to create a 180 degree reflection using the two or three surfaces of the corner as long as the radar enters the corner +/-45 (90 degrees of returnable angle) degrees. In the spherical design once that corner rotates out of it's returnable path the next one rotates in so it works at all angles. The strongest return is when you have a whole disk aligned perpindicular with the source since you basically have two corners reflecting at that point, but on a bobbing rocking boat (The source is also rocking and bobbing) it's impossible to keep it aligned without some active alignment. The bigger the cross section reflector the bigger the return. Unfortunately the size of the disk that can be reasonably placed in a sailboat's rigging is somewhat small.

Putting metal in a metal mast won't accomplish anything. The radar is reflected by the mast itself, but the circular shape of the mast only returns a small portion to the source as the rest is scattered radially, and a boxed mast (square tubing) would only reflect, although very well, when it's aligned perpindicular to the source (vertically and rotationally). A wooden mast is basically transparent to radar so metal in a wooden mast would be of benefit.

Also, adding to it is the radar operator. If he has his gain turned down to eliminate sea clutter and only see the signature of a 500 foot steel hull so he doesn't have to stare at the screen except once in a while when there is a return, you won't be seen no matter what kind of dinky reflector you have.

AIS is cool but not everyone has them yet. And you can still run into another smaller pleasure boat not required to carry it.

So best I can offer is: Do what you can, keep a good watch when you can, keep some rum close at hand for the close shaves, and pray to the gods for safe passage.
Excellent post!

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Originally Posted by knuterikt View Post
Here is a test from 2005 - don't think much have happened since then
http://www.yachtingmonthly.com/fileB...ce_ym_june.pdf
One more
http://www.maib.gov.uk/cms_resources...s%20report.pdf
That YM article is fabulous. Nice to see a print pub offer hard data and honest appraisal. The tri-lobe polars are weak but consistent; folding reflectors return strong signal but only when perfectly oriented. The Sea-Me signal is awesome -- literally off the scale at nearly all orientations and angles of heel. >100m^2 effective signal. If anything is gonna set off klaxons on a merchant ship's bridge with the gain turned low, that's it. (One update: both the Sea-Me and EchoMax active units are available in dual band, but they are very expensive. AIS becomes the better value then, but AIS only works for ships with working transmitters.)

I love the idea of the active reflectors, but where we are going they are unlikely to earn their keep; and by the time we head for more congested areas, hopefully AIS will have better coverage & universality.
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  #22  
Old 02-19-2014
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Re: Radar signature

Fry-
Packing your mast with aluminum foil, carefully crinkled, is recommended by the Pardey's in at least one of their books. However, they specify that is for a wooden mast. Won't matter on a metal mast, which already is diffracting radar rather than reflecting it. Of course, they also think that wood is a superior material for building masts, and AFAIK they didn't do any objective testing on the tin foil.
Other tests over the years have indicated that randomly, or even neatly, folded foil isn't really going to help much.
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Old 02-19-2014
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Re: Radar signature

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Originally Posted by bobmcgov View Post
Excellent post!



That YM article is fabulous. Nice to see a print pub offer hard data and honest appraisal. The tri-lobe polars are weak but consistent; folding reflectors return strong signal but only when perfectly oriented. The Sea-Me signal is awesome -- literally off the scale at nearly all orientations and angles of heel. >100m^2 effective signal. If anything is gonna set off klaxons on a merchant ship's bridge with the gain turned low, that's it. (One update: both the Sea-Me and EchoMax active units are available in dual band, but they are very expensive. AIS becomes the better value then, but AIS only works for ships with working transmitters.)

I love the idea of the active reflectors, but where we are going they are unlikely to earn their keep; and by the time we head for more congested areas, hopefully AIS will have better coverage & universality.
European magazines and British especially test all kinds of equipment all the time. Seems US magazines don't do this to the same extent.
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Old 02-19-2014
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Re: Radar signature

I haven't read the Pardy's book. How would they have known that their foil idea worked? Did they see their returns aboard another ship both with and without?
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  #25  
Old 02-19-2014
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Re: Radar signature

Minnie- great point. That's why every once in a while go out with a buddy and check how he/I show up on AIS/Radar and we chat on each radio on the two boats. When one of is off cruising also pick a time to chat on SSB. He has Ham at his house. Also wonder how many folks have AIS but don't know they are not sending.
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Re: Radar signature

Funny. I have a buddy with AIS and he asks me to check all the time. He's never transmitting!

I wish he was, as everytime I ask where he is, he gives me some erroneous coordinates. Usually reading the location of the cursor, rather than his vessel, or something like it. Cracks me up. He once told me he was essentially 5 miles inland.
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Old 02-19-2014
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Re: Radar signature

Quote:
Originally Posted by knuterikt View Post
Here is a test from 2005 - don't think much have happened since then
http://www.yachtingmonthly.com/fileB...ce_ym_june.pdf
One more
http://www.maib.gov.uk/cms_resources...s%20report.pdf
This is exactly the article that I was thinking of. Thank you! I though I had read it in one of my print mags, but someone must have linked to it. There was a similar discussion on CF a few months ago, someone must have linked it there.

Oh, yeah, and Yachting Monthly is fantastic. I picked up a copy when I was in England a few years ago. I wish there was a US publication like that.
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Old 02-19-2014
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Re: Radar signature

Minne-
I don't know how or if they tested it, just that there were no objective tests comparing the idea to other options. And, despite their years of experience and wisdom, personally I wouldn't want a wood mast on any boat I owned or sailed, in any case. Unless it came with a full crew and maintenance staff to deal with the brightwork and rot for me.
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Old 02-19-2014
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Re: Radar signature

My boat has raymarine RL70RC and I have only noticed It not picking up Inflatables and Plastic swimming islands. Sailboats and powerboats and ships are picked up great. I find if your not watching or fail to turn it on it's not much good. This is old radar , lots of newer and better stuff out there. I would worry about Inattentive captains, as it takes two in most cases......Dale
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Old 02-20-2014
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Re: Radar signature

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Originally Posted by Lake Superior Sailor View Post
My boat has raymarine RL70RC and I have only noticed It not picking up Inflatables and Plastic swimming islands. Sailboats and powerboats and ships are picked up great. I find if your not watching or fail to turn it on it's not much good. This is old radar , lots of newer and better stuff out there. I would worry about Inattentive captains, as it takes two in most cases......Dale
I've got Raymarine 4kw and find it not very reliable. In good conditions I can see little stuff, but in any sort of sea the waves clutter hides boats pretty well.

It's indicated as a 40 mile radar, but on a good day I can pick up the broadside of a freighter at 50 miles and, just offshore Wisconsin, I can pick up cliffs on the Michigan shore around 60 miles out. Never the less, it does not see 18' boats in 3-4' waves no matter how it's tuned.

GTJ
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