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haulback
10-16-2007, 09:00 AM
Discussion somewhere about radar reflectors led me to thinking....

Ends up that the one I have put so much trust in for last 8 years has a RCS (radar cross section) of, at the very best - as stated by manufacturer - 4 sq metres. SHTP rules want one that has an RCS of 10 sq. metres.

Problem being, I can not find ANY that come close to this. Best I can see is one that has RCS of 7 sq metres. Now what???

This seems to be the most thorough report I have stumbled across on the matter, although I am sure there are others, and mostly what I am basing opinion on.

http://www.ussailing.org/safety/Studies/radar_reflector_test.htm#INDEX

Jim/Haulback

Alchera
10-16-2007, 10:46 PM
Interesting.... you seem to be right. The Davis reflector I've been using doesn't come close to 10 meters squared in RCS, nor do most of the others. I wonder where the original SSS requirement came from? I looked at the requirements back to 1996 in the archives and it was in there as well.

At any rate, it would seem that most reflectors on the market today do not meet the SSS requirement as currently written!

tiger beetle
10-17-2007, 08:26 AM
Interesting.... you seem to be right. The Davis reflector I've been using doesn't come close to 10 meters squared in RCS, nor do most of the others. I wonder where the original SSS requirement came from? I looked at the requirements back to 1996 in the archives and it was in there as well.

At any rate, it would seem that most reflectors on the market today do not meet the SSS requirement as currently written!

The Davis Echomaster meets the requirements of the SSS TransPac NOR by being an octahedral shape of 12" mininum diameter; the rules do not require an RCS of 10 square meters for this design. Note that Davis also documents their Echomaster as having an RCS of 13.2 square meters, which also meets the rules even if we don't believe the manufacturer.
http://www.davisnet.com/product_documents/marine/manuals/152_EchoRdrRfr_INS.PDF

The largest Mobri radar reflectors (S4, M4) have a documents RCS of 4 square meters - you would need three of these to meet the TransPac requirements.

So all is not lost.

The Sea-Me that Adrian distributes is a powered active radar responder (the manufacturer calls it a 'Target Enhancer') - when the unit is painted by an X band radar it amplifies and re-transmits that radar signal.
http://www.sea-me.co.uk/

- rob/beetle

Alchera
10-17-2007, 09:08 AM
The Davis Echomaster meets the requirements of the SSS TransPac NOR by being an octahedral shape of 12" mininum diameter; the rules do not require an RCS of 10 square meters for this design.

What I don't understand is where that arbitrary number of 10 square meters came from, since we are clearly OK with a design that doesn't meet that requirement. Why not pick a number that more closely reflects the performance of the Davis if that is what is acceptable?

- Mark

sleddog
10-18-2007, 09:30 AM
What I don't understand is where that arbitrary number of 10 square meters came from, since we are clearly OK with a design that doesn't meet that requirement. Why not pick a number that more closely reflects the performance of the Davis if that is what is acceptable?

- Mark

The "arbitrary number" comes from a long time ISAF Safety at Sea requirement for passive radar reflectors, now ISAF #4.10.1 that requires an RCS not less than 10 sq.meters and minimum digonal of 18", flown at least 13 feet off the water.

Curiously, US SAILING has modified and reduced this requirement, and refers to "minimum documented" 'equivalent echoing area' of 6 sq.meters and minimum diameter of 12". No height off the water is apparently mentioned by US SAILING in their ambiguous modification...

Somewhere along the line, SHTP rules writers chose to combine ISAF with US SAILING, leading us all to a certain level of misplaced confidence, that our little reflectors help us to be seen by big ship's radar, if we don't see them first.

The Pardey's were recently in town, and very confident in their all wooden boat's radar signature aboard TALIESIN. Larry had filled their 40 foot hollow spruce mast with aluminum foil. On subsequent testing here locally, they were disappointed to learn they could only be seen on radar to 2 miles, not as far as a local bell buoy and about the same as an Olson 30 signature.

~sleddog

haulback
10-19-2007, 08:29 AM
(quote)
The largest Mobri radar reflectors (S4, M4) have a documents RCS of 4 square meters - you would need three of these to meet the TransPac requirements.

Woundn't this mean you just have 3 that cannot be (theoreticaly) seen, or at least not as well??? To my way of thinking, at least in this case, 4+4+4 would NOT equal 12 - they are just 3 smaller, individual targets.

Is there any other reflector - apart from the Davis - that meets the rules??? if not, why not just tell everyone not to bother to show up unless they have a Davis Echomaster????

Jim

tiger beetle
10-19-2007, 09:59 AM
(quote)
Woundn't this mean you just have 3 that cannot be (theoreticaly) seen, or at least not as well??? To my way of thinking, at least in this case, 4+4+4 would NOT equal 12 - they are just 3 smaller, individual targets.

Good point - I don't know if radar targets are additive.

- beetle

dogbark
10-19-2007, 10:58 AM
I think Jim is right that reflectors are not cumulative for the same reason that five handheld VHFs broadcasting at five watts doesn't give as much range as one 25 watt radio. But it is interesting reading the differences between the ISAF standard and the USSailing standard and the SHTP rule regarding reflectors. See http://www.ussailing.org/safety/ISAF/cat4_0607mono.asp
ISAF uses an 18" min w 10sqm, USSailing seems to allow 12" min and 6 sqm and SHTP a little of both. Even though the Davis site says the Echomaster has 12 sqm, the performance diagrams they have provided(courtesy of Admiralty surface weapons test) don't even show peaks of 10 sqm.
Given the current availability of passive reflectors I think Jim is correct, one needs to show up with an Echomaster or have some darn good documentation. Al

Alchera
10-19-2007, 11:54 AM
It appears this rule would be a good candidate for modification at some point, since the way it is written right now just forces skippers to buy the Davis Echomaster. The RCS requirement should be reduced to 6 sq m to bring it in line with the US Sailing requirements, and open up the race to other reflectors that work just as well as the Davis.

- Mark

tiger beetle
10-19-2007, 12:03 PM
It appears this rule would be a good candidate for modification at some point, since the way it is written right now just forces skippers to buy the Davis Echomaster. The RCS requirement should be reduced to 6 sq m to bring it in line with the US Sailing requirements, and open up the race to other reflectors that work just as well as the Davis.

- Mark
That would work.

Alternatively, drop the octahedral dimension and utilize the 10m2 requirement. I have not read through the radar studies to see how well the echmax reflectors worked, nor do I particularly want to mount something this large on the boat:
http://www.echomax.co.uk/Echomax_Products.htm

Would the TransPac allow an actively powered radar transponder/responder such as the Sea-Me in lieu of a passive radar reflector?

- rob/beetle

AlanH
10-19-2007, 03:16 PM
There is a commercially-sold passive radar reflector that is up there at 10m2. It's called th Tri-Lens, Luneberg lens reflector. The smaller model is nowhere near 10m2, but the larger model, which weighs 13 pounds, is.

Before looking at it, though, you might read this:

http://www.tri-lens.com/practical_sailor.txt

...here's an advertisement for it at Defender... cha-chinnngggg.. $500

http://www.defender.com/product.jsp?path=-1|118|107602|275622&id=303311

West Marine sells the little model, which is about 2.5 m2, for $150



http://www.westmarine.com/webapp/wcs/stores/servlet/producte/10001/-1/10001/64228/377%20710/0/Tri-Lens/Primary%20Search/mode%20matchallpartial/0/0?N=377%20710&Ne=0&Ntt=Tri-Lens&Ntk=Primary%20Search&Ntx=mode%20matchallpartial&Nao=0&Ns=0&keyword=Tri-Lens&isLTokenURL=true&storeNum=5003&subdeptNum=3&classNum=109

By all accounts the Mobri sucks, has all sorts of valleys in its response and only works even halfway well at very narrow angles of heel. Still and all, they're easy to get into the rigging, they don't have a lot of windage, and I've found an ebay seller that makes a knockoff for $15 instead of $70. That's cheap enough that you can put 3-4 of them around the boat, at assorted points, slung at different angles and hope that at least one of them will be "right" at any given time.

That's been my hope and prayer...three Mobri knockoffs in the rigging and an Echomaster on a stern mount about 10 feet off the water.

Alchera
10-25-2007, 05:35 PM
Are radar reflectors really not cumulative? I've been thinking about this, and it seems to me that adding radar reflectors *must* be cumulative in some way, perhaps just not in the way we think.

Take the following hypothetical case. Take a steel battleship. Glue Davis reflectors all over it so it is completely covered. Am I to believe the RCS is now only the same as a single reflector? I don't think so....

Or, pile up a bunch of Davis reflectors into a mound a mile high and a mile wide. Is the RCS that of a single reflector?

- Mark

seabird51
10-26-2007, 03:32 AM
The Davis chaffed through its halyard in 2002. I replaced it with a Mobri but since the advent of AIS I have asked every ship about my radar signature and have always been told that it was excellent with and without the Mobri. something tells me that most of the credit goes to the big wide and tall aluminum mast. Am I correct in this assumption? Or flirting with death.
Lou

dogbark
10-26-2007, 10:36 PM
Lou, I think you are definitely flirting with death, but so are we all, regardless of radar reflectors. Back to the real question though. I believe that an elliptical aluminum mast has a radar signature that is pretty good, particularly in larger sizes when in a certain orientation to the radar signal. However in other orientations it is near nothing. Look at the radar returns of the various reflectors that are designed to be optimal. There are still areas where the returns are very weak. The Pardey's with the wrinkled aluminum foil is another example. I am sure that in some instances they have a great reflector but in others they don't. In all the independent testing I am aware of only a very few commercially available reflectors seem to pass muster. Some of these are heavy and expensive or don't work well at more than a little heel. I will carry what the committee deems acceptable but I will not depend on it to guarantee I will be seen by another radar (whether there is an observer at the screen or not).

Mark is on the right track though about the cumulative effect though I think a better analogy is a small light bulb. A thousand of them would be seen better than one but still not a far or distinct as a focused fresnel type lens makes one. Al

dogbark
10-27-2007, 10:49 AM
Another study done in England, http://www.ybw.com/pbo/pdfs/radar_reflectors.pdf
Very interesting conclusions and recommendations at the end. Al

Ergo
10-27-2007, 11:30 AM
There's an interesting article in Cruising World this month about the boat that was sunk by a sperm whale at the same time some of us were returning in'06. The part that caught my attention was the writer's contention that the boat's kevlar main was a great radar reflector. Makes sense but I hadn't heard this claim before. If true, it would seem to satisfy the race requirement.

Bill Merrick

Alchera
11-12-2007, 12:23 AM
I find it pretty difficult to believe that kevlar can reflect radar. Anyone know of any other references to support this?

The other consideration is that even if kevlar *were* reflective, that still doesn't make it an effective safety device. Radar reflectors are designed to reflect a signal directly back at the transmitter. A flat sheet of steel can be invisible to radar if you angle it such that the radar signal reflects off in a direction away from the transmitter. That's why stealth aircraft have all those flat surfaces. So even if a sail were reflective, the odds of it reflecting a signal back to the radar transmitter would be slim.

-Mark

Paul Elliott
11-13-2007, 12:01 AM
As I recall, the sails were carbon fiber. Even so, carbon fiber shouldn't be a particularly good reflector, since I believe that it is quite lossy. Some forms of carbon are used in dissipative elements for radio-frequency anechoic rooms, although I admit that I don't know if CF has the same properties.

Even if the sails were reflective at radar frequencies, the orientation of the sails would have to be correct to send back a signal. I wouldn't trust that this would be the case.

So, how to explain the report of a strong radar return? I don't know, but it sounds iffy to me.

- Paul
- S/V VALIS

Chesapeake
11-15-2007, 05:39 PM
Are radar reflectors really not cumulative? I've been thinking about this, and it seems to me that adding radar reflectors *must* be cumulative in some way, perhaps just not in the way we think.

Take the following hypothetical case. Take a steel battleship. Glue Davis reflectors all over it so it is completely covered. Am I to believe the RCS is now only the same as a single reflector? I don't think so....

Or, pile up a bunch of Davis reflectors into a mound a mile high and a mile wide. Is the RCS that of a single reflector?

- Mark
I would be careful about assuming the additive nature of separate radar reflectors. If the reflected signals are in phase they would add together, e.g. 1+1=2. However, if one radar reflector is 1/2 wavelength further away than the other it will be completely out of phase, so the reflected signal would be 1-1=0, or no measurable radar reflection. The 1/2 wavelength of 2.45 GHz microwave radiation is about 2 1/2 inches. You cannot control the relative position of the reflector on a boat to that accuracy. The constructive and destructive interference also occurs with light. That's where the antireflective coating on your camera lens comes from. I could go into this ad nauseum, but the bottom line is that you can not assume that individual reflectors are additive. If my analysis is incorrect, please let me know.

Alchera
11-16-2007, 09:53 AM
I would be careful about assuming the additive nature of separate radar reflectors. If the reflected signals are in phase they would add together, e.g. 1+1=2. However, if one radar reflector is 1/2 wavelength further away than the other it will be completely out of phase, so the reflected signal would be 1-1=0, or no measurable radar reflection. The 1/2 wavelength of 2.45 GHz microwave radiation is about 2 1/2 inches. You cannot control the relative position of the reflector on a boat to that accuracy. The constructive and destructive interference also occurs with light. That's where the antireflective coating on your camera lens comes from. I could go into this ad nauseum, but the bottom line is that you can not assume that individual reflectors are additive. If my analysis is incorrect, please let me know.

While this may be true in a carefully controlled laboratory environment using perfectly coherent radar pulses and perfectly designed and arranged radar reflectors, I don't think this effect is significant in the real world. Radar pulses are not perfectly coherent frequencies, and reflections distort and modify the waveform and polarization, even from a radar reflector. If the destructive interference effect were a serious factor, then it would be the same problem with large targets even without radar reflectors, since you have many surfaces all reflecting back to the transmitter in that case as well. But we all know that large radar targets in general have larger returns than small radar targets.

- Mark

sleddog
11-17-2007, 08:08 AM
Dear Skip,

In answer to your question, radar reflectors are incoherent reflectors and so they do add.

Some folks argue that two small reflectors are better than one big one,
because two reflectors will tend to fill in each-other's nulls. That is the
theory behind the Firdell Blipper that has multiple small corner cube reflectors inside.

Others argue that you are better off with one big octahedral-type reflector
because the peaks are large and an operator on a ship is more likely to
notice one of the peaks. Unfortunately the ARPA (Advanced Radar Plotting Aids) loses track of small boats in the nulls.

Sea-Me active radar target enhancers are not available in the US, but are
available from the UK via the web, and work great at X-band. No reflector
helps much with those ships that only use S-band radar. No reasonably-sized
reflector is large enough to make much difference at S-band.

As a practical matter, it is fine to use a radar reflector, and the Sea-Me
is a good choice, but the sailor on a small boat should assume that he/she
is invisible and take the responsibility to detect and avoid ships. The
small boat sailor is far more able (and motivated) to detect ships than vice
versa.

Stan Honey

Adrian
11-21-2007, 06:42 PM
Hi All,

A small correction to Stan's post about the availability of the Sea-Me. North American Mini Transat Ltd does carry the Sea-Me & can deliver to the US by mail or courier from Victoria, BC.

We are currently offering a discount on the first shipment we have in stock to promote the "visibility" of the Sea-Me. Please take a look at the website for details: www.northamericanminitransat.com

Adrian

BobJ
11-23-2007, 10:23 AM
Dear Skip,

As a practical matter, it is fine to use a radar reflector, and the Sea-Me
is a good choice, but the sailor on a small boat should assume that he/she
is invisible and take the responsibility to detect and avoid ships. The
small boat sailor is far more able (and motivated) to detect ships than vice
versa.

Stan Honey
This reminded me that when I sailed my qualifier a couple of years ago, I had a C.A.R.D. radar detector running the whole time (I hadn't bought an AIS receiver yet). During the day, 3 container ships passed me and none of them set off the CARD, apparently because their radar was turned off. One of them set it off after he was nearly past me, I assume because he turned on his radar when he saw me. It made me wonder if any ships passed by at night which I never saw.

A radar detector or reflector, no matter how effective, is only good if ships' radar is running and apparently that is not always the case. Larger commercial ships are required to run their AIS however. My approach has been to use the CARD to pick up any smaller boats running radar, and the AIS for the larger ships.

AlanH
11-29-2007, 05:02 PM
OK, so what do we (the SHTP RC) do about the existing radar reflector rule, now that we know that nothing generally available on the market even comes close to our written rule?

BobJ
11-30-2007, 11:17 AM
Make the minimum return match what most of us have, as accepted for prior races. See page 2:

Davis Echomaster (http://www.davisnet.com/product_documents/marine/manuals/152_EchoRdrRfr_INS.PDF)

Adrian
12-06-2007, 07:10 PM
Hi Bob,

The independent British study makes the following statement about the Davis Echomaster Reflector - I have cut & pasted the report's recommendations too:

"· The Davis Echomaster failed to get close to ISO8729 during this testing. Its peak
RCS is too low at 7.5m2 and its average performance is only 1.75m2. This
reflector is priced at 60 [pounds sterling] and is lightweight; it can be mounted on a rod as well
as by suspension (in the correct catch-rain position).
· The 4” tube reflector performed very poorly.
· It is concluded that either the active Sea-Me, POLARef and the Standard or
Large Tri-Lens radar reflectors are the best reflectors at heel and elevation
angles of over 10˚.
QINETIQ/D&TS/SEA/CR0704527/2.0 Page 31

6 Recommendations
· Based on the results of this report it is recommended that yachtsmen always
fit a radar reflector that offers the largest RCS practicable for their vessel.
· The RCS of the radar reflector should have a minimum consistent RCS of 2m2.
· The Sea-Me is the recommended product if power is available
· If power is not available then the passive Large Tri-Lens reflector is
recommended
· The 4” tube reflector is not considered suitable due to its poor performance. It
is also recommended that the 2” tube reflector is not suitable since the
performance of this target will be even lower.
· It is recommended that poorly performing radar reflectors are not fitted as it
is possible that the user could be lulled into a false sense of security believing
that their chances of detection has been enhanced.
QINETIQ/D&TS/SEA/CR0704527/2.0 Page 32"

In our highly litigious world RCs need to be careful about safety recommendations and not recommend something that independant safety authorities say do not work.

Adrian

BobJ
12-06-2007, 08:37 PM
In our highly litigious world RCs need to be careful about safety recommendations and not recommend something that independant safety authorities say do not work.

1) The SHTP R/C's have never been in the business of making "safety recommendations." As always, each skipper is solely responsible for the safety of his vessel and crew, including the selection of equipment adequate for that safety. The SHTP08 Rules and Conditions are intended to be a minimum list of items required to participate in the race, not a list of "safety recommendations."

While your quoting of the study is helpful, your comment at the end of your post appears to be worded to put pressure on the R/C. This is what you attempted to do in the SSB thread on the old board. Moving on,

2) "Independent safety authorities" did not say the Davis Echomaster doesn't work, they said it didn't "get close to ISO8729 during this testing."

3) The Davis Echomaster isn't a 4" tube, just to clarify your incomplete cut/paste.

4) Would you put the Large Tri-Lens reflector on the mast of a Mini? (It's too big for my boat, which is 30'.) If not then (surprise) the only thing left, according to the particular study you quote, is your "Sea-Me."

5) Am I right that your "Sea-Me" is over $900 USD, even after the discounts?

I'm happy with my $55 Davis Echomaster. During the '06 race, two Matson ships said their radar picked me up fine - one even asked what kind of reflector I had because the return was so good.

sleddog
12-12-2007, 06:43 AM
This group is well aware of the fallability of radar and radar reflectors. Nevertheless, it is a good reminder to re-read the below note from the 60 foot HUGO BOSS, mid-South Atlantic in the Barcelona World Race. Even an active reflector didn't prevent their almost being run over. I will say that between the West Coast and Hono, Matson does a good job on keeping bridge lookout. Other shipping you take your chances.

Updated : 3/12/2007 15:05 GMT
"Last night Capey noticed a ship on the horizon that was coming towards us. I made sure the active echo was on and switched on the AIS so he could see us clearly, but the ship kept coming on a collision course. About five miles away I went below and called them on the VHF. I kept calling and calling but no joy, so I grabbed the search light to grab their attention and light up the sails.

We had our big furling gennaker up called the 'big fella' and Capey had tried sailing high and low but the ship seemed to alter course to keep us in their sights. At this stage it was getting critical so I grabbed a white hand flare and set it off but still it kept on coming. We had no choice but to put HUGO BOSS head to wind and over we went, sails flogging and mast nearly in the water. The ship passed by so close I could nearly read its name on the transom. Once upright and on our way again I went below and called again – finally an answer. I asked his name and that was the last I heard. Bloody rude and dangerous and very obvious that a proper look out was not being maintained."

moonduster
12-16-2007, 06:55 PM
The full text of the British Inquiry can be found here - http://www.maib.gov.uk/publications/invest...s/2007/ouzo.cfm

It's well worth the read for a number of reasons, all of them alarming.

I have a Davis Echomaster that is assembled directly on my backstay about 1/2 way up. It used to be higher, but would foul the main halyard when attaching or removing it from the sail in a stiff breeze if too much slack was in the halyard. Commercial ships see me just fine at 10-12 nm in a variety of conditions. That is, if they have their radars running, which is rare.

Regarding the Hugo Boss report, it just doesn't add up. Did the commercial vessel not have a watch, or did they keep altering course? It can't be both. Bottom line, HB should have changed course 10 degrees behind the vessel and there wouldn't have been a story.

Warriors Wish
02-20-2008, 08:31 AM
I apologize for revisiting this subject however I am struggling for a good solution and hope perhaps one of you has a suggestion.

I believe that unless I go to an active system my alternative is a Davis Reflector mounted 13' off the water.

I have looked a couple of different solutions for mounting with my biggest concern being chafe (sail primarily). I am leaning to mounting a unit on a pole from the transom. Has anyone done this? what type of pole did you use? Did you have to support the pole with guy wires? Have you had any problems?

I have read that someone has mounted this unit to the backstay. My mainsail would not clear a unit mounted this way however if the unit was mounted so that it rolled when the mainsail passed by it might be work. I would also need to include a chafe patch on the main. Would anyone care to comment about a unit mounted this way.

Thanks,

Don
Warriors Wish

Alchera
02-20-2008, 09:41 AM
I have looked a couple of different solutions for mounting with my biggest concern being chafe (sail primarily). I am leaning to mounting a unit on a pole from the transom.

Why can't you hang it from a spreader using a flag halyard? That's a pretty simple approach, and there are ways to rig it up so that it doesn't flop around.

- Mark

Warriors Wish
02-20-2008, 10:56 AM
Simple approach however when eased the mainsail chafes against it. I thought about keeping the reflector on the windward side and simply gybing when neccessary. Am I over reacting to the chafe?

BobJ
02-20-2008, 10:59 AM
Don, my reflector is on a flag halyard as Mark suggests. I was worried about chafe too - the edges of the Davis Echomaster are bare aluminum. My first attempt was to toss a light line over the spreader and hoist the reflector up. Sure enough, before long the reflector cut through the line.

Then we screwed two small eye straps to the underside of the spreader, spaced well apart. The reflector goes up on the outboard end and clears the return (down) end of the halyard. On my boat, the main doesn't quite reach the reflector so it doesn't chafe on that either. This is a function of vang tension and how far you ease the main down wind - mine's a sprit boat so the main never gets eased that far. (But it's close - if the reflector went up on the inboard end of the halyard it would chafe the main.)

So I would try it and see if with the right spacing you can avoid the chafe.

The other benefit of the flag halyard is that you can fly the SHTP battle flag! (R/C, we ARE getting battle flags again, right?)

Alchera
02-20-2008, 11:19 AM
Another way I deal with chafe prevention - I wrap all the edges of the Davis with a couple of layers of electrical tape. If you stretch the tape as you apply it, it conforms and shapes itself to the edge of the reflector. Uses up a bit of electrical tape, but it really stops the chafe. Also, you need to redo the tape every couple of years because it gets brittle and starts coming off.

- Mark