Radar Reflectors

[thinwater]

First, take the time to read this study:
http://offshore.ussailing.org/Assets...r+Test.pdf.pdf
It does a nice job of contrasting what is out there, readily available. Clearly, price isn't everything, though a few of the high end units are much better.

Then read this, about a home made design.
Duckworks Magazine - Passive Radar Reflectors
His approach seem logical. For boats that do not heel more than ~ 15 degrees (multis, power boats, and many others in moderate conditions) it seems to be a much better geometry than the octahedron style. More expensive to make, impossible to fold, and a bit heavy if hoisted.

Thoughts?

[David M]

I think the value of radar reflectors is over exaggerated.

Boats themselves make pretty good radar reflectors. Many yachts have a larger radar cross section than the best radar reflectors. The larger a boat, the better a reflector they make. Boats make better reflectors than what most pleasure boaters think, in my opinion. 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.

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...and nice trick electronics like real IMO approved ARPA...not MARPA. 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.

Putting a reflector at the masthead for over the horizon capabilities? It does not make a whole lot of difference at that distance. Collision avoidance calculations are not even started at that distance....the information is still too sketchy at that distance. Plus, ARPA will not auto acquire you unless your vessel is showing for a predetermined number of sweeps. It also takes very little time for a ship to make first contact to making consistent contacts. When the radar officer (correction- watch officer) starts getting consistent contacts is when the contact starts becoming "important". Before then, a radar return could very well have been the top of a large breaking wave near the radar horizon..and not a boat.

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.

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.

Yes I realize that ships are not the only thing to be concerned with as far as collisions go. Ships though are probably your greatest concern.

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.

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

[swagman]

Thanks David,
Great to get the practical merchant marine view on this.
JOHN

[thinwater]

Originally Posted by thinwater

First, take the time to read this study:
http://offshore.ussailing.org/Assets...r+Test.pdf.pdf
It does a nice job of contrasting what is out there, readily available. Clearly, price isn't everything, though a few of the high end units are much better.

Then read this, about a home made design.
Duckworks Magazine - Passive Radar Reflectors
His approach seem logical. For boats that do not heel more than ~ 15 degrees (multis, power boats, and many others in moderate conditions) it seems to be a much better geometry than the octahedron style. More expensive to make, impossible to fold, and a bit heavy if hoisted.

Thoughts?

Duckworks - Do It Yourself Lifelines and Bow Pulpit

And thanks for the input, Dave. Yes, it is clear that there is already a lot of metal in the typical FRP boat.

[Bash]

Originally Posted by David M

I think the value of radar reflectors is over exaggerated.

Consider my boat's metal reflectivity. The aluminum mast rises 20 meters above the water. Halfway between the first and second spreaders the radome is protected by a large stainless-steel mount that completely encircles it. Two large solar panels are mounted on a stainless arch on the stern. This arch also supports a bronze bell and a pair of stereo speakers. Also on the stern is a 7cm diameter aluminum mast that supports a wind generator 3 meters above the cockpit coaming. That mast has two aluminum struts. My dodger and bimini are supported by stainless frames. The top of the dodger is more than 3 meters above the waterline. An outboard engine is attached to the rail, as is a stainless-steel BBQ. Because of the boat's somewhat generous freeboard, this BBQ stands more than two meters above the water surface. My davits are constructed of 5cm stainless tubing. I've got a 25 kilo anchor on the bow, and a double-channel stainless roller that extends half a meter forward. There are six large winches on the coaming. In addition to the bow and stern pulpits there are six stainless stanchions on each rail. My boom vang, two meters long, is made of aluminum.

And now someone wants to sell me an eight-inch aluminum sphere to make me more reflective? I've got more reflectivity than that just in my turnbuckles and chainplates.

[paradix]

I don't disagree with anything David and Bash are saying (except with Bash's last sentence: that's not how a properly-constructed radar reflector works: its reflectance is many times greater than its size), but on the other hand a radar reflector is probably the cheapest item in your safety plan, particularly if you download a template, cut it out and fold and rivet it yourself (maybe a 30 minute job). And for many of the reasons in David's explanantion (poor radar equipment and use by smaller vessels for example) it would seem silly to overlook it on a small boat, especially with a wooden mast.

[idpnd]

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.

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

Interesting perspective. I had been wondering why my 25 ton lump of steel wasn't equipped with one. This is probably why.

[thinwater]

http://www.seagrant.umaine.edu/files...al/05raref.pdf

I'm dead serious! Read the report.

[chala]

Originally Posted by Bash

Consider my boat's metal reflectivity. The aluminum mast rises 20 meters above the water. Halfway between the first and second spreaders the radome is protected by a large stainless-steel mount that completely encircles it. Two large solar panels are mounted on a stainless arch on the stern. This arch also supports a bronze bell and a pair of stereo speakers. Also on the stern is a 7cm diameter aluminum mast that supports a wind generator 3 meters above the cockpit coaming. That mast has two aluminum struts. My dodger and bimini are supported by stainless frames. The top of the dodger is more than 3 meters above the waterline. An outboard engine is attached to the rail, as is a stainless-steel BBQ. Because of the boat's somewhat generous freeboard, this BBQ stands more than two meters above the water surface. My davits are constructed of 5cm stainless tubing. I've got a 25 kilo anchor on the bow, and a double-channel stainless roller that extends half a meter forward. There are six large winches on the coaming. In addition to the bow and stern pulpits there are six stainless stanchions on each rail. My boom vang, two meters long, is made of aluminum.

And now someone wants to sell me an eight-inch aluminum sphere to make me more reflective? I've got more reflectivity than that just in my turnbuckles and chainplates.

Relative Reflection Intensity
1 m2 Sphere = 1.0
1 m2 Disk = 12 000
1 m2 Trihedral corner = 12 000
You must have a lot of turnbuckles.

[Paul Elliott]

Originally Posted by chala

Relative Reflection Intensity
1 m2 Sphere = 1.0
1 m2 Disk = 12 000
1 m2 Trihedral corner = 12 000
You must have a lot of turnbuckles.

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.

[forsailbyowner]

I have yet to run across a boat that doesn't show up on radar on the ranges I typically use. Once I was getting a blip from a pelican sitting on glassy water. I have a furuno that adjusts for seaclutter and rainclutter automatically.

[jackdale]

Originally Posted by thinwater

http://www.seagrant.umaine.edu/files...al/05raref.pdf

the winner was a tin hat!

I'm dead serious! Read the report.

If some of the other reflectors had been worn on the head of the paddler, they might have had better returns. I noticed that first picture has an octahedral mounted in the incorrect position.

Here is another study:

http://www.maib.gov.uk/cms_resources...s%20report.pdf

I am still reading it. It is part of a larger investigation - the sinking of the Ouzo.

Marine Accident Investigation: Ouzo

[Cormorant]

FWIW, I built one of those reflectors from the Duckworks article. Haven't tested it yet, though, but I plan to mount it on our boat in three weeks when we leave for the Bahamas.

It was very inexpensive -- $10 worth of aluminum flashing gave me enough material to make two reflectors. (But I only made one.) You can cut the flashing with good scissors or tin snips. Pop rivets are a couple of bucks. The actual folding and drilling and riveting is not a pleasant task, but can be finished in an afternoon.

We have a wooden boat with a wooden mast and not much metal besides screws in the wood and a box of fish hooks, so I figured best to err on the side of caution. . . .

Someday I would like to run a controlled experiment with another boat telling me how well we show up on their radar, with the reflector stowed below, then hoisted aloft, at various distances.

[barnakiel]

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

b.

[Bash]

Originally Posted by Paul Elliott

There are some junk reflectors being sold, but the good ones seem like reasonable insurance.

The last reflector I owned, on a previous boat, was a Firdell Blipper. During a passage one day it came crashing to deck, having ripped its pop rivets out of the mast. We discovered that the unit was completely full of water, and it weighed a ton. Imagine all that extra weight aloft! Curiously, we were unable to drain the water from the blipper, regardless of its orientation, until we finally drilled a hole in the bottom.

Of course, the next rage in passive reflectors, once we discovered what a pig the blipper turned out to be, were these hummingbird-feeder style reflectors that performed so poorly on test after test that even West Marine stopped selling them.

Check out the WM catalog. Right now they're recommending a tri-lens reflector. The one sized for my boat costs $839.00. Yes. For a passive radar reflector. But it's gotta work great if it's that expensive, right?

I'd sooner paint the mast with snake oil.