Radar Reflectors

[Highlander40]

Fiberglass is somewhat transparent to radar? Well Kevlar is not. I found out when the kayaks on the hard top were mounted up on edge and the port side radar signal went blank.

The question is- does Kevlar reflect well?

My hull has a Kevlar layer and the kayaks are all kevlar. This thing should show up like christmas in Siberia!
(been there, the lights are the only joy they have and they stay on all winter!)

[Maine Sail]

Originally Posted by David M

I think the value of radar reflectors is over exaggerated.

As one who has spent thousands of hours in the fog as both a pleasure boater and commercial fisherman I simply can't agree with that premise.

Originally Posted by David M

Boats themselves make pretty good radar reflectors.

Some boats do to some radar but not all boats do. I have seen plenty of invisible "stealth" boats on many different radar units not just 2kw or 4kw sailboat domes.

Originally Posted by David M

Many yachts have a larger radar cross section than the best radar reflectors. The larger a boat, the better a reflector they make.

This is generally true but I have seen some fairly big sailboats not show up at all on certain points of sail.

Originally Posted by David M

Boats make better reflectors than what most pleasure boaters think, in my opinion.

Really? I might agree with that had I only ever used a top quality commercial type radar but with your average 2kw / 4kw units I find the opposite.

Here is an actual photo of two boats, one with a radar reflector, and one without. The bottom line is if any reflector is being used it can be better than none at all. More often than not when sailing on OPB's, in fog, the reflector is left as an after thought in the lazarette. No matter how good it is it won't do much good if it is not hoisted..

Originally Posted by David M

I have seen plenty of small reflectorless yachts lit up on radar from miles out....yes its true! I'm talking boats the size of a Cal 20 or less.

Yep me too but I have also seen many nearly invisible boats.

Like this one (look close and you'll see a sail boat in the fog heeled over.


Here's a radar shot of that boat when it was actually showing up. It's the red spec just above the 18 foot spot off my stbd stern quarter. The two targets ahead and to port and stbd were two J Boats traveling together both of which had reflectors when they went by. SOME BOATS JUST DO NOT SHOW UP ON RADAR!!! The guy behind me owns one!

Originally Posted by David M

Another factor is ships have much better radars than you find on yachts. Most yachtsmen have never used a ships radar. I have. I am a licensed third mate-unlimited with plenty of sea experience. Commercial ships radars are far superior in effective range, sensitivity, resolution and tuning abilities... Its really no wonder many yachtsmen think they need a radar reflector when they look at the capabilities of their own non-commercial boats radar. No offense.

No doubt commercial quality radar will see more targets clearer. The problem for most sailors/coastal cruisers, in foggy locals, is that the vast majority of boats you are coming into contact with are not big ships with 72nm radar they are usually boats with small 2kw & 4kw domes that often need the assistance of a reflector to pick up poor returning targets.

Originally Posted by David M

On the the other side of the story, radars must be tuned for the sea state and atmospheric conditions, otherwise your vessel can be lost on screen. Radars are not always tuned well, the watch officer is not watching the radar screen all the time nor is ARPA perfect.

Amen! Many boaters I know, with radar, have no clue how to really make use of it or how to effectively tune it. This can actually make having radar more dangerous from a false sense of security perspective.

Originally Posted by David M

Sure, radar reflectors might help in some situations, but don't put a whole lot of faith that they are adding much radar cross section (total reflectance back to the radar antenna) over what you are already sailing.

Funny I was sailing along with my buddy Tom in his Sabre 362 when the fog rolled in. I was picking him up just fine until we both tacked, then I lost him. He still saw me, I had two reflectors up, but I could no longer see him consistently, and even when I would pick him up it looked no better than sea clutter. I hailed him on the VHF and he raised his Davis Echomaster. Immediately, before he even hailed me to tell me it was up, I could see him large as life...

Originally Posted by David M

For large sailboats they are clearly a waste of money. Large sailboats are already a better radar reflector than any radar reflector you can buy. For really small sailboats, they might come in useful in some situations.

If you have a "large" sailboat what is the harm in an additional $45.00 to help you be seen by perhaps a marginally tuned 2kw or 4kw dome?

Originally Posted by David M

Putting an AIS transceiver on your boat will do far more good than any radar reflector ever will.

Really, and this clown was sending an AIS signal while doing 30 knots in pea soup? AIS is only as good as the boats broadcasting it of which very, very few coastal pleasure boats are these days, though it is growing...slowly. AIS is great but it can not be depended upon, yet, except at sea with large ships then it is very useful. Actually I would rather have an active reflector for coastal use at this point than AIS though I do now have AIS...



I have witnessed the effectiveness of reflectors on many occasions for small radomes, and they do work. Not all boats are bad targets, this is true and some are actually excellent, but many are not, and for those of us with pleasure quality, wide beam radar, this visibility can change from tack to tack.

[senormechanico]

We are experienced radar users. On our previous boat we had a Vigil RX which could pick out seagulls and dinghies. However, on a return trip from Coming back from Barkley Sound in pea soup fog, we almost sailed into the side of a 60' foot wood commercial fishing boat. He had a big Furuno radar dome on the stern, but NO reflector. I told him he was invisible on my radar, but he didn't seem to care. He said, "What's the problem? I have RADAR".

How he could come to the conclusion that it would keep him from being hit is beyond comprehension. In fact, we came within about 3 boatlengths under sail before we saw him directly ahead! I looked again at the radar and he just was NOT there.
I changed scales and the coastline showed up fine. Just to make sure it wasn't incorrect settings, I left the settings and after several minutes passed, other targets showed up normally. I can only conclude that boat was a ghost with a talking captain. <sarcasm>

[barnakiel]

Hi,

There are radars and radars. When I hear a "pro" mariner say I can't see you on my radar it is BS to me. Been on big ships, seen what the modern radar can do.

On a small boat with a small radar it is all different. The units are cream in fine/moderate weather but do not perform too well in chop/rain. Still, they are GREAT aid.

Yesterday sailing towards Las Palmas we could not see a huge cargo until it was within 4 Nm (sic!) - there were too many city lights in the background and the ship was coming head on. However, on the radar I could see it coming as soon as it left the harbour. Great help.

Re the comment on many users not understanding how to use the radar - yes, it is the case.

Re the AIS/radar - well, try to have both. If impossible, pick up the system that better serves your sailing needs.

b.

[s/v Jedi]

I observe that Barnie's comment about the SeaMe is conveniently ignored ;-)

So, let me write another one of my little stories: it doesn't matter if you can be detected on a ship radar or not because there is nobody trying to determine if that little blib is you or just a wave or a ghost echo or whatever. What matters is that an alarm sounds on their bridge if there is a risk of collision with you.

There are two ways to get that alarm to sound:

1. AIS transponder. Too simple to go into details for this one.

2. ARPA. Don't mix this up with MARPA. These ships have ARPA and use it. This means that their radars do automatic target acquisition. That process works something like this: in at least 70% of the radar's scanner revolutions a clear and stable target is determined... it gets designated a target. Now, in at least 50% of the scans that target must be determined and as long as it is, the radar will calculate possible collision scenario's and sound the alarm if needed.

Compare that to you looking at the screen. You see a blib in 6 out of 10 scans, what do you do? Yes, you go look out if something is there. The radar on that ship ignores you in that situation.

The SeaMe (and other similar products) are designed to make sure you are designated a target by ARPA and that these radars keep tracking you automatically. In addition to that, it can alert you of a radar painting you and you can choose to turn it off (Somalia etc.)

The tri-lens (not snake oil, it is based on an old and proven principle) is actually a very good reflector for multi hulls but it's price is close to the SeaMe.

So I wouldn't dismiss the SeaMe that quickly... I actually installed one after researching the whole radar reflector thing and got rid of that big ugly high-windage thing up there.

cheers,
Nick.

[Kefaa]

For those with a hollow wooden mast, it has been suggested to loosely pack it with aluminum foil. It has been shown to increase the radar profile by 300% Self Sufficient Sailor - Google Books

At 31' I am also looking at the trilens based on http://www.tri-lens.com/practical_sailor.txt and http://www.theradarreflectorsite.org...PassiveRTE.pdf Mounting height is always an issue. The higher you go, the further you can be seen, until you heel. Then it moves across the angle (relative to the water and horizon) faster, but should always be higher than the rest of the boat. But you need to support the mass.

All of which brings me back to going to active. http://www.theradarreflectorsite.org...gActiveRTE.pdf which has an issue of saturation. When the weather is its worst, the closer you are to the target the less likely you are to be seen.

I guess if this were simple, everyone would have "the" answer...

[thinwater]

Originally Posted by Paul Elliott

By the way, that 1 m2 disk is only that good when it is "pointing" directly at the radar. In any other direction it is a stealth shape.

I would guess that the Radar Flag mentioned in the reflector study will have similar characteristics. The "foil hat" was actually a clever multiple reflector design (and not exactly a practical piece of headgear), and it's main claim to fame was that it sat higher above the water then did the kayak-deck-mounted reflectors under test. It sounded like the low test-height was a big issue in the tests.

There are some junk reflectors being sold, but the good ones seem like reasonable insurance. A radar reflector is also required in many ocean races, but that doesn't mean much, since even the junk reflectors meet that requirement.

There are some things that are truly useless -- if I hang mistletoe from the spreaders while solo-sailing I'm still not going to get kissed (and when crewed this could be a dangerous thing to do!), but a good quadrahedral reflector, or one of the "Tri-Lens" style luenberg reflectors should provide a more consistant return than will the random rigging on my fiberglass boat. Note that a rounded aluminum mast isn't a particularly good reflector design.

I reasoned my comment might make people read the link.

[Catalysis]

We have been preparing our catamaran for our Baha Bash this summer and have decided to take the "belt and suspenders" approach with a Raymarine AIS 500 "B" class (transmit and receive device) plus a tri-lens Luenberg passive reflector. Logic is to be protected in the event of an electronics system failure in the "June Gloom" that often pervades the California coast at that time of the year.
Paul
SV "Indigo"

[Martinini]

Buy a round Davis reflector and paint it black and you have the reflector and a daytime at anchor ball. Two birds with one item.

[Martinini]

Originally Posted by barnakiel

Why a passive one, if things like Sea-Me are believed (and I think proven) to be x-times more efficient?

b.

Passive, about $60 ---- Sea Me $650.00 USD, for some money can be an issue.

[Paul Elliott]

Originally Posted by s/v Jedi

The tri-lens (not snake oil, it is based on an old and proven principle) is actually a very good reflector for multi hulls but it's price is close to the SeaMe.

If I'm not mistaken, the Tri-Lens should have good performance on a monohull, too. The "LensRef" Luenberg reflector has a design that only works over a narrow angle of heel, and it would not be appropriate for a monohull.
The Tri-Lens, also based on the Luenberg lens principle, uses a different reflector geometry that gives strong returns at least up to 25 deg of heel (I can't find the plots, but they are impressive). What the Tri-Lens reflector gains in heel angle, it loses elsewhere, which is why there are three lenses in the design.

(I don't have one, but do admire the design. Heavy, though.)

[Paul Elliott]

As for the optimal radar reflector height, take a look at this thread from 2005:
http://www.cruisersforum.com/forums/...tors-2092.html

About halfway through it I post some (I think) interesting plots of peaks and nulls as a function of distance and reflector height: http://www.cruisersforum.com/forums/...tml#post155242 -- but there are plenty of other good contributions. Do read the whole thing.

The upshot is this: There is an "optimum" height for your radar reflector, and putting it higher may very well cause you to not be seen when you most need it.

[BubbleHeadMd]

I'm not advocating that sailors should upholster their boats in tinfoil, but not all radar are created equal. My sub was transporting a load of Royal Academy midshipmen in the English Channel, in the fog. A submarine's surface search radar in a word, "blows". I even had the "H" model, with fancy touch screen. Too bad they didn't give it any better sensitivity.

We don't submerge unless we have 1000 (that's right people, 1000 fathoms) beneath us unless it's a special circumstance. We wait until we're past the continental shelf.

I was the radar operator, the sole guy who could give enough warning to avoid a collision and sailboats were the bane of my existence. I don't care if you hoist a beer can, just put something up there.

Or you can just say I was a crappy radar operator, whichever.

[Maine Sail]

Originally Posted by Martinini

Buy a round Davis reflector and paint it black and you have the reflector and a daytime at anchor ball. Two birds with one item.

So you could be moving with an anchor ball displayed??

[senormechanico]

Originally Posted by Maine Sail

So you could be moving with an anchor ball displayed??

I'm glad I read to the end of the thread before posting.
Exactly what I was going to say.
Uhh, I guess I said it anyway!

BTW , Thanks again for that fantastic LED light review.
I installed six Sailors Solutions lights in the saloon.
Only ten percent of the amp draw of my old 10 watt halogens.
They produce more even distribution with slightly more intensity than before. The light they produce is hard to tell from incandescent.

http://www.cruisersforum.com/forums/...son-21839.html