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Old 08 May 2007, 18:09   #41
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Makes perfect sense to me Polwart.

In any event, these are of little use if the Watch Officer (think of Rusty Coaster) is as pissed as a fart!!!!!

When designing the ideal Radar reflector, think of a Stealth Fighter and make yours look like the opposite

And if all else fails, hang up a few pots and pans as high as one can. This last trick saved a few friends in South Ireland.

Basically the best Square on biggest metallic dish or Frying pan (non teflon coated) will do. Even a metal petrol can if you must, no smoking tho...
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Old 08 May 2007, 18:33   #42
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I have said before on this site that relying on a large vessel to see you and avoid collision by using either radar or visual means is not a method I would like to rely on. You need to keep out of their way.
On large vessels when I have been at sea there is a tendency to keep a poor visual watch at sea and use radar set to alarm when contacts are within a certain distance or to a collision avoidance setting. The problem is that to stop constant false alarms sensitivity tends to be gradually turned down till it would only detect the Queen Mary bearing down on them.
If you want a good example of just how good a lookout ships sometimes keep then look up the MCA report on the Jambo grounding of 2003 off Ullapool. This ship was over 50 miles off course with both watch members fast asleep and who only woke up when the ship ran into an island at full speed and eventually sank.
This was a modern well equipped cargo vessel with a full watch on duty......
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Old 08 May 2007, 18:38   #43
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I have said before on this site that relying on a large vessel to see you and avoid collision by using either radar or visual means is not a method I would like to rely on. ......
Bruce - all fair comment - but equally best to give them the best chance of spotting you in case you somehow miss them (or misunderstand their intentions). Or as JW highlights - the shit hits the fan and you become "disabled" in their path.
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Old 08 May 2007, 19:14   #44
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I have said before on this site that relying on a large vessel to see you and avoid collision by using either radar or visual means is not a method I would like to rely on. You need to keep out of their way.
On large vessels when I have been at sea there is a tendency to keep a poor visual watch at sea and use radar set to alarm when contacts are within a certain distance or to a collision avoidance setting. The problem is that to stop constant false alarms sensitivity tends to be gradually turned down till it would only detect the Queen Mary bearing down on them.
If you want a good example of just how good a lookout ships sometimes keep then look up the MCA report on the Jambo grounding of 2003 off Ullapool. This ship was over 50 miles off course with both watch members fast asleep and who only woke up when the ship ran into an island at full speed and eventually sank.
This was a modern well equipped cargo vessel with a full watch on duty......

The Jambo is a farce yet to be explained.

The summer Isles are a very tight nit set of Islands in front of Ullapool, but how a vessel the size of the Jambo got where it did in the middle of them is a mystery. You cant miss them. The ship did not sink totally, it was later dragged off an Island and scuppered, It was supposed to be heading in to Ullapool to refuel, but it was discovered that its tanks were full when it beached ?? And the Island it landed on was a basket weave case to get to from the minch .... in calm weather ? ,,,, hmm
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Old 08 May 2007, 19:24   #45
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The ship was not intending to refuel in Ullapool, it missed its waypoint around 50miles out to Clear Cape Wrath en route for Norway.
It also sank without aid or being pulled off the day after hitting the rocks and if you saw the size of the hole in the bow you would realise why!
It now lies in around 30m of water off the west side of the island on a direct route into the North end of Loch Broom, there are no "turns" required to hit the island coming in from the Minch as it is an outer island facing out to sea.
The suspicion at the time because it came from ireland was that it was landing drugs and the MCA searched it thoroughly after the sinking but unfortunately it was good old incompetence!
There is no real mystery, the report came to the conclusion that the crew were asleep and the man on the bridge admitted it while the second man supposed to be on duty admitted being away on rounds at night for over 2 hours, read into that what you will.
I have the official report in front of me, have searched for and dived the wreck and had a good poke around inside and out............
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Old 08 May 2007, 19:39   #46
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... But assuming that there is nothing "special" about this end of the spectrum - then the incident radiation will "see" anything much less than the scale of its own wavelength as "one homogeneous surface" - so dimples, scratches, bumps, or even mm scale retroreflective prisms on a surface will be treated pretty much as if it was just one flat surface.
Does it? That's interesting. Do you know the mechanism for this, Neil? I'll need to pay more attention to exactly what my radar is seeing because, from what you are suggesting, I won't be able to see a sandy beach, for example. It should behave as though it's a smooth angled plate.

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I don't know if ships radar typically use polarisation or not.
I think it's horizontally polarised. I suspect there is very little return from waves because the angle of incidence is so small. I'd guess it only starts to happen when the wave begins to present a face square to the radar beam or when the ripples on a near vertical wave face begin to behave like the multiple faces on a radar reflector.
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Old 09 May 2007, 06:26   #47
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The ship was not intending to refuel in Ullapool, it missed its waypoint around 50miles out to Clear Cape Wrath en route for Norway.
It also sank without aid or being pulled off the day after hitting the rocks and if you saw the size of the hole in the bow you would realise why!
It now lies in around 30m of water off the west side of the island on a direct route into the North end of Loch Broom, there are no "turns" required to hit the island coming in from the Minch as it is an outer island facing out to sea.
The suspicion at the time because it came from ireland was that it was landing drugs and the MCA searched it thoroughly after the sinking but unfortunately it was good old incompetence!
There is no real mystery, the report came to the conclusion that the crew were asleep and the man on the bridge admitted it while the second man supposed to be on duty admitted being away on rounds at night for over 2 hours, read into that what you will.
I have the official report in front of me, have searched for and dived the wreck and had a good poke around inside and out............

Interesting - I didnt think about looking for a report, but found the one in question, ta Bruce I didnt know it sank of its own accord either. Having been at the site myself, yes there is open seaway to the west but it still seems strange how a vessel that size got where it did. To make such a deviation without notice from the crew is what surprises me. But I suppose, a lot of distance can be covered in "2" hours.

Sorry back on topic....
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Old 09 May 2007, 07:24   #48
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Does it? That's interesting. Do you know the mechanism for this, Neil? I'll need to pay more attention to exactly what my radar is seeing because, from what you are suggesting, I won't be able to see a sandy beach, for example. It should behave as though it's a smooth angled plate.

I think it's horizontally polarised. I suspect there is very little return from waves because the angle of incidence is so small. I'd guess it only starts to happen when the wave begins to present a face square to the radar beam or when the ripples on a near vertical wave face begin to behave like the multiple faces on a radar reflector.
Jeff, I'll try to find an intelligent description of the "sub wavelength" phenomenon - which is straightforward enough that you don't need 3 physics text books to translate it... ...my explanation would be hand wavy and wooley!

I don't know how sand behaves in a radar beam - but I imagine it is not a great "reflector". The sand also won't normally be perfectly smooth even at the cm scale. Add to that differences between wet sand and dry sand, loose sand and packed sand - and I suspect a real beach will give some sort of response back (rather than simply reflecting all of your beam skywards).

I will try to use an optical example - as you can see the effect more clearly...

If you shine a laser at polished glass - some of the light is transmited (and refracted on its way), some of it is reflected at the same angle to the normal as the incident beam. If you examine that glass under an electron microscope you would see that it is not actually perfectly smooth at all but has scratches, bumps and indents on it - but if these are significantly smaller than the wavelength of the light you won't see an effect from it. As an observed positioned at the laser source you will see virtually no evidence of back scattered light (unless the light is hitting the glass at 90 deg). If you took that piece of glass and roughened the surface your get scatter in random (or sort of random!) directions from the surface. It will immediately become obvious to an observor at the the laser source that the light is hitting a surface and some of that light coming back towards them. A similar effect would be observed with polished and brushed metals. A polished metal is not smooth at an atomic scale but is at the "scale" of light.

Reflective tape used on high visibility vests is in many ways maximising this effect. Similarly reflectors on a bike (which in someways are miniature version of a radar reflector).
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Old 09 May 2007, 09:53   #49
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I don't know if I would trust an electronic gadget as my only form of defence - also it is only suitable for 1 of the 2 bands in use. As an additional measure it would be fine.
I agree. The reason I wanted to put a radar reflector on the rib in the first place was to increase the likelihood of other larger boats / ships seeing me (in good and bad visibility) and hopefully avoiding a collision if I were immobilised, or to help someone like the lifeboat to pin-point me if I needed to be rescued.

If I were in either of these situations, the likelihood of me having lost power would be extremely high (otherwise I would revert to VHF quoting GPS co-ordinates etc.) hence why I didnít want to rely on an active system such as the Sea-me and decided to go with the Tri-Lens instead. The Tri-lens gives a good reflection regardless of the boatís angle (which is rarely constant on a rib!), and it installs very neatly on the A-Frame.

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Old 11 May 2007, 11:58   #50
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Unless something has changed in the past few years, you may not want to use this too often at head level due to the radiation. However in fog I am sure it would be useful.
My god! You mean your head radiates powerful RADAR waves that could damage the reflector
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