radar reflectors and metal masts

[dylanwinter]

Lily M came with a dustbin half way up the mast

http://www.keepturningleft.co.uk/wp-content/uploads/S1880008-gubbins.jpg

http://www.keepturningleft.co.uk/wp-content/uploads/S1630006-main-3.jpg

I was going to buy a folding reflector

http://www.safety-marine.co.uk/dbimages/1828/Octahedral Folding Radar Reflector (RORC)-medium.jpg

and hang it from the cross trees when I think I need to be seen

my pontoon neighbour - a very nice ex sailor and now mobo man - has a radar on his boat and says that he would think that my double mast will make a massive bounce signal

far bigger than a folding jobbie

his radar picks up lobster pot markers

this is a pdf of a yachting monthly test of radar reflectors

http://keyassets.timeincuk.net/insp...20/filebank/reflector_performance_ym_june.pdf


they did their tests on a rib with a wooden mast - so it does not tell you much about how they improve the bounce from a yacht with a metal mast

D

 

[Robin]

Sure you can pickup pot markers, seagulls in flight too on occasion but what you need is a consistent return that can be distinguished at a decent range and doesn't vanish when the boat heels. Metal mass may well reflect the incoming radar beam but the question is where to, most likely not back to the sender. I was always a fan of the firdell/Gillie Firth cylinders even though mag tests were not ecstatic. We had one on the front of 3 masts (On 3 boats) and whenever I asked a friendly ship if we were clear was told 'yes'. Best response I had was Ushant traffic Control who I hadcalled for information on visibility in Chenal du four when we had none having just crossed the traffic lanes east of the Ushant TSS in very thick fog and headed in that way. They asked me for our lat/long position and said they had us, on their land based radar, very clearly at 20 miles range which was very reassuring.

Worst I found from the viewpoint of our own radar sets were the French fluorescent tube lookalike things and the silly collapsibles assembled but wrongly mounted, point upwards, as often seen on French backstays.

 

[James_Calvert]

I carry a Firdell from the eighties. Mostly I carry it in a near upright position in my wet locker. I normally hoist it to the crosstrees when in fog or on cross-channel passages.

But that report you linked to pretty much said it wasn't much good. In fact nothing much was of its type. But dangling it from the cross trees was probably better than a permanent fixing on the mast. So I haven't changed what I do.

Somewhere else I recall reading that for a grp or wooden yacht the concave interface the boat's hull made with the water actually provided the best radar return. The yacht's engine could also provide a good return. But a mast was much less good. That all obviously alters when the yacht is hull down in heavy seas much of the time. But hopefully there's not much fog around in those circumstances.

Very glad to hear about more up to date theories and practice.

 

[VicS]

Radar reflectors are desined to send a strong reflection back towards the source. A metal mast will not necessarily do that. It will tend to scatter the signal through almost 360°

A more recent and in depth report on radar reflector performance was that produced for the MAIB by QinetiQ, following the Ouzo incident, under carefully controlled conditions.

https://assets.digital.cabinet-offi...540f0b6024400008d/Radar_reflectors_report.pdf

You will see that the octahedral reflector is less effective than the cannister ( Echomax and Firdell ) types It also suffers from the drawback that it is difficult to hoist in the correct and effective "catch rain" attitude

 

[Mrnotming]

Sure you can pickup pot markers, seagulls in flight too on occasion but what you need is a consistent return that can be distinguished at a decent range and doesn't vanish when the boat heels. Metal mass may well reflect the incoming radar beam but the question is where to, most likely not back to the sender. I wasalways a fan of the firdell/Gillie Firth cylinders even though mag test were not ecstatic. We had one on the front of 3 masts (On 3 boats) and whenever Iasked a friendly ship if we were clear was toldyes. Best response I had was Ushant traffic Control who I called for information on visibility in Chenal du four when we had none having just crossed the traffic lanes east of the separation zone in very thick fog and headed in that way. They asked me for our lat/long position and said they had us on their land based radar very clearly at 20 miles range which was very reassuring.

Worst I found from the viewpoint of our own radar sets were the French fluorescent tube lookalike things and the silly collapsibles assembled but wrongly mounted, point upwards, often seen on French backstays.

+1 Robin,
How the french approach these type of regulations:
1.Buy the latest Livre du Bord from Bloc Marine or in a French Chandlers.
2.Decide how far offshore you will be venturing normally.
3.Arm your boat with the cheapest possible tat to fulfil the regulations.

As long as the tat is certified CE or maybe NF or whatever other marking is needed, you are safe from being fined as a boat Matriculated in France, Their laminated red log book type document.
The quality of the gear is usually built down to a price, and sales are vast due to the regulation to carry the gear.
A visiting boat would I feel be Cat B usage if a European vessel or nearby non-EU.
Getting to France from other places might mean you traversed Cat A.
But your vessel is bound by its own countries regulations solely.
Hence the preponderance of silver paper tubes etc.
I assemble an Octohedral alloy one when needed only.
Ships tell me they have a good echo from the 11M alloy mast up to 4 miles away in say F4 Celtic sea conditions, ie 2m waves at most.
I remember the PBO bag of crumpled kitchen foil coming out quite well against the pukka brands when tested some years back!
What I know nothing about is the return afforded by these new radar sets operating on a different principle.Not up to speed here yet!
I have a Furuno 1623 stand alone set, nice to have, but I would like to add gyro stabilisation sometime as the swinging of the 10m vessel in monster seaas makes the blobs hard to track.
Hope this is broadly on topic?

 

[LittleSister]

Imagine the mast is chrome plated, and someone is shining a powerful torch at it from a distance. How much reflection will they actually see?

The theory is that a cylindrical section (or nearly cylindrical) mast will deflect most of the radio signal to one side or the other. Only that from a tiny central strip directly facing the 'sending' radar will reflect any signal back to the sender. Even that strip will not return the echo when the mast is not exactly vertical (and how often is that on a small boat at sea?) - it will be reflected down into the sea or up into the sky.

No doubt the situation is more complex that that in practice, but I doubt very much a metal pole makes a good reliable reflector.

 

[dylanwinter]

I understand about the roundness of masts - but even in your shiny example it is a long thing

- however, a Centaur is a mass of metal what with boom, guardrails, spinnaker pole, boathook, deckbrush, anchor, chain cleats, window frames and dinghy oars

I am just wondering if an additional square foot of metal will make the signal that much bigger

I am asssuming that a yacht is a lot of surfaces for the signal to bounce off

D

 

[johnalison]

You would do much better to instal a seawater pump to convey water to the top of the mast and spray it over the mainsail.

 

[Gwylan]

I understand about the roundness of masts - but even in your shiny example it is a long thing

- however, a Centaur is a mass of metal what with boom, guardrails, spinnaker pole, boathook, deckbrush, anchor, chain cleats, window frames and dinghy oars

I am just wondering if an additional square foot of metal will make the signal that much bigger

I am asssuming that a yacht is a lot of surfaces for the signal to bounce off

D

It's like horseshoes, whether they work depends on how much you believe in them. Then there are like life-rafts, there are not too many people who come back to complain about performance issues.

 

[Hoolie]

However, stealth aircraft are all flat surfaces that reflect radar signals away from the transmitter. Non-stealth aircraft have round surfaces that return some radar signals that make them detectable. Just like a mast and other metal objects on a boat.

 

[Robin]

+1 Robin,
french approach these type of regulations:
1.Buy the latest Livre du Bord from Bloc Marine or in a French Chandlers.
2.Decide how far offshore you will be venturing normally.
3.Arm your boat with the cheapest possible tat to fulfil the regulations.

As long as the tat is certified CE or maybe NF or whatever other marking is needed, you are safe from being fined as a boat Matriculated in France, Their laminated red log book type document.
The quality of the gear is usually built down to a price, and sales are vast due to the regulation to carry the gear.
A visiting boat would I feel be Cat B usage if a European vessel or nearby non-EU.
Getting to France from other places might mean you traversed Cat A.
But your vessel is bound by its own countries regulations solely.
Hence the preponderance of silver paper tubes etc.
I assemble an Octohedral alloy one when needed only.
Ships tell me they have a good echo from the 11M alloy mast up to 4 miles away in say F4 Celtic sea conditions, ie 2m waves at most.
I remember the PBO bag of crumpled kitchen foil coming out quite well against the pukka brands when tested some years back!
What I know nothing about is the return afforded by these new radar sets operating on a different principle.Not up to speed here yet!
I have a Furuno 1623 stand alone set, nice to have, but I would like to add gyro stabilisation sometime as the swinging of the 10m vessel in monster seaas makes the blobs hard to track.
Hope this is broadly on topic?

I thought for a minute we were talking lifeboats!

 

[dylanwinter]

However, stealth aircraft are all flat surfaces that reflect radar signals away from the transmitter. Non-stealth aircraft have round surfaces that return some radar signals that make them detectable. Just like a mast and other metal objects on a boat.

you make an interesting point

I wonder why Yachting Monthly decided to do their tests without at some point trying them out on a yacht with a metal mast

D

PS - I missed spreaders, stays and turnbuckles off the list

 

[LittleSister]

- however, a Centaur is a mass of metal what with boom, guardrails, spinnaker pole, boathook, deckbrush, anchor, chain cleats, window frames and dinghy oars

I am just wondering if an additional square foot of metal will make the signal that much bigger

I am asssuming that a yacht is a lot of surfaces for the signal to bounce off

D

A yacht is indeed a lot of surfaces for the signal to bounce off. (I understand the fibreglass hull will also give an echo.) But bounce off where? Few of then will give much reflection reliably back towards the sending vessel, and most are low down (and hence only visible at relatively close quarters).

Your radar reflector may only be a square foot, but it's a square foot designed to return most of the signal reaching that square foot back to where it came from, even when the vessel is heeled, and to be easily accommodated relatively high up.

In practice, I'm told, most yachts will give a reasonable echo most of the time, but why not up your chances?

 

[Neeves]

I asked a passenger ship, ferry (Spirit of Tasmania) that then ran a route out of Sydney to Tasmania, if and when he picked me up on their radar. We are a 38' fibreglass cast, 2 diesel inboards and a 15m mast sitting about 1m off seawater level. Seas were maybe 1m.

He picked us up at 15nm.

Large commercial vessels have a range of radar systems - I'm not sure on which they picked us up. But a professionally operated ship will have all systems running and monitored.

However I would not rely on a commercial vessel actually identifying were were there - the fact we might be detected by radar, as a result of a mast or some sophisticated and expensive radar reflector is totally irrelevant if the person on watch does not register the target and then do something about it. A good watchkeeper on a modern commercial vessel will notice you whatever - if the watchkeeper is making a cup of tea your sophisticated radar reflector is simply offering you false confidence. A 200,000t bulk carrier moves a fair distance in 20 minutes. If they run you down they might not even notice.

Jonathan

 

[EdWingfield]

A friend recently followed all the research and theory before buying the 'best' radar reflector.

He tested it by asking the OOW of a cruise ship 1M away, smooth sea and good viz, if he was visible on radar? OOW came back and said No. OOW said to stand by whilst he made adjustments, then, again No.

At the recent RYA conference we were told to rely on none of them.

 

[Birdseye]

Lily M came with a dustbin half way up the mast

http://www.keepturningleft.co.uk/wp-content/uploads/S1880008-gubbins.jpg

http://www.keepturningleft.co.uk/wp-content/uploads/S1630006-main-3.jpg

I was going to buy a folding reflector

http://www.safety-marine.co.uk/dbimages/1828/Octahedral Folding Radar Reflector (RORC)-medium.jpg

and hang it from the cross trees when I think I need to be seen

my pontoon neighbour - a very nice ex sailor and now mobo man - has a radar on his boat and says that he would think that my double mast will make a massive bounce signal

far bigger than a folding jobbie

his radar picks up lobster pot markers

this is a pdf of a yachting monthly test of radar reflectors

http://keyassets.timeincuk.net/insp...20/filebank/reflector_performance_ym_june.pdf


they did their tests on a rib with a wooden mast - so it does not tell you much about how they improve the bounce from a yacht with a metal mast

D

Its simple Dylan. Just imagine your metal mast as if it were chromed and polished, and the radar set as a powerful torch. Then ask yourself how much reflection there would be at a couple of miles. There is only a thin vertical line on the mast which is at the right angle to reflect back in your direction. You might get a much better reflection from your shaving mirror if it was at the right angle to reflect back to you. You often see that with the sun shining off a broken bottle or abandoned metal. And thats the real issue.

Only a sphere will always have some small part at the right angle to give a reflection in the right direction both horizontally and vertically all of the time. Thats why the efficiency of radar reflectors is compared to a sphere as the datum. The mast should give some reflection in the right horizontal arc but depending on the attitude of the boat it could be reflecting towards the moon or towards the bottom of the sea.

There are two things most important in radar reflector design. The most important is size, the bigger the better. The second is lots of facets to refelect in lots of directions in the hope that this combined with boat movement will hit the target. Which is a long way of saying dont count on the masts.

.

 

[yachtorion]

I realise it's unlikely anyone is going to enforce it, but one reason to carry the reflector is legal:

"Regulation 19 para.2.1.7 requires radar reflectors to be carried, where practicable, by ships under 150 GT. For UK-flagged this includes pleasure vessels."

https://mcanet.mcga.gov.uk/public/c4/solasv/annexes/Annex15.htm

Hard to argue it isn't practical to carry one on a floating storage container like a Centaur.

 

[AntarcticPilot]

I understand about the roundness of masts - but even in your shiny example it is a long thing

- however, a Centaur is a mass of metal what with boom, guardrails, spinnaker pole, boathook, deckbrush, anchor, chain cleats, window frames and dinghy oars

I am just wondering if an additional square foot of metal will make the signal that much bigger

I am asssuming that a yacht is a lot of surfaces for the signal to bounce off

D

The analogy isn't really accurate, though it is a useful way to visualize the problem - but the real situation is even worse. The point is that a piece of metal like a mast actually absorbs the signal and then retransmits it (the proper technical term is that it resonates) - and it isn't even that simple, as Jumbleduck might well explain in more detail than I dare! How well it will resonate depends on the combination of the mast's dimensions (both length and diameter) and the wavelength and bandwidth of the radar. Because a mast is long compared with the wavelength and also of circumference comparable with the radar wavelength, it will usually resonate quite well at the range of wavelengths considered. However, the radar signal arriving from one direction will be retransmitted equally all round the horizon, making the reflected power in a particular direction MUCH weaker than the reflection from an equivalent flat plane perpendicular to the radar. The retransmitted power will also have a maximum in a plane perpendicular to the mast - so heeling will cut down the power visible to a ship's radar even more. Rigging wires, although individually they won't be good reflectors because they are thin compared to the wavelength, will certainly complicate the picture, modifying the strength of the reflection in various directions.

A corner reflector (i.e. any "passive" radar reflector) operates by maximizing the area of "flat" surface perpendicular to the incoming radar waves. The principle is easily seen if you go to a place where there are two mirrors in a corner! However, they depend on the corner being accurate (even small errors will reduce their efficiency considerably), and also this is a matter where size matters - a radar reflector can only return as much signal as it actually intercepts! So, small reflectors are less effective than big ones, and well made ones are better than badly made ones. A secondary point concerning the angles is that they also have to be robust enough to maintain the right angles accurately - a folding octahedron that is stowed at the bottom of the cockpit locker is unlikely to remain accurate enough to be effective, and corrosion and/or shaking around will certainly cause problems for other types. The required accuracy is much higher than people think - I once worked with people operating a corner reflector for a radar at much longer wavelengths, and for a reflector several metres across, the required accuracy was a centimetre or less to attain the performance needed. Admittedly that was for a radar operating from a satellite 700 km up!

Anyway, the upshot of all this is that it is probably true that the "hole in the water" represented by your hull is probably at least as good a reflector as your mast; the only thing is that the "hole in the water" will disappear into surface scattering sooner than the mast reflection.

 

[GrahamM376]

I realise it's unlikely anyone is going to enforce it, but one reason to carry the reflector is legal:

As you say, enforcement in UK waters is unlikely but not so elsewhere. I don't understand people's reasoning such as "I don't cross shipping lanes so don't need one" as there's more traffic closer to the coast and fishing boats have caused us more anxious moments than big ships in lanes whose movements are predictable. Fog can form very quickly sometimes so I'd rather have the reflector permanently rigged.

 

[st599]

The angle of incidence = the angle of reflection

How much of the mast will be correctly oriented at the exact time that the radar impulse hit it?