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    Re: Fatal interaction betweeu yacht and ferry.
    From: Gary LaPook
    Date: 2007 May 07, 12:12 -0700

    Gary added:
    
    I also figured that it would be useful in a life raft.
    
    
    On May 7, 10:44 am, Gary LaPook  wrote:
    > Gary wrote:
    >
    > I read this entire report which was very interesting. What will put the
    > fear of god in you is the finding that even with their Davis radar
    > reflector mounted, that the probability of detection was still minimal.
    > Allot of sailors rely on those things.
    >
    > I looked at the issue of radar cross section in the past and built my
    > own radar reflector. The standard reflector, like the one in this
    > incident consists of three mutually perpendicular plates which form 8
    > corners so as to reflect radar energy back in the direction that it came
    > from, these corners are aligned to cover the horizon. Unfortunately,
    > there are deep nulls in the reflective pattern so there are many gaps in
    > the pattern.
    >
    > The Davis reflector on this boat is 12.5 inches in diameter meaning that
    > the edge where the plates meet to form each corner is only 6.25 inches.
    > Only one of the corners is effective for any particular radar beam so
    > the rest of the reflector has no value, the strength of the echo is
    > determined by the one corner or "bucket" facing the radar transmitter.
    > If you do the calculation for a bucket with a 6.25 inch edge circular
    > edge (such as the davis) for x-band radar (9.4 ghz, lambda or wavelength
    > of 3.2 cm) you find the radar cross section has a maximum value of  9.7
    > meters ^2. If you do the same calculation for an s-band radar (3 ghz,
    > 10cm wavelength), the kind used by large ships when off soundings,  you
    > find only 1.0 meters^2 since the effectiveness of a corner reflector
    > varies inversely with the square of the wavelength.
    >
    > The effectiveness also varies with the shape of the plates and with the
    > length of edge where the plates are joined. A corner reflector made of
    > plates that are square rather than round, as in the davis, returns an
    > echo 2.42 times as strong than the round plates. But the main
    > determinant of strength is the length of the edge. The strength of the
    > echo varies with the length of the edge to the fourth power. If you
    > double the length of the edge you increase the echo 16 times! So if you
    > combine these two factors and make a radar reflector with square plates
    > that is twice as big you increase the strength of the echo by almost 39
    > times. So if you make a reflector that has 12.5 inch buckets the RCS is
    > 379 meters^2 for an x-band radar and 38.4 at s-band.
    >
    > O.K. this would be too big to hoist aloft.
    >
    > So what I decided to do was to make a radar reflector with just one
    > bucket that I could aim at any ship that I thought might be a hazard.
    > This would be much more effective than shinning a flashlight on the
    > sails since it would set off the automatic alarm on the radar scope.  I
    > got 3 sheets of aluminum 12 inches square and joined them with two
    > hinges so that it can fold flat when not in use. It is a simple matter
    > to take it out and unfold it and aim it at the ship. Opened, it is
    > slightly smaller than the Davis radar reflector but returns an echo in
    > the direction of the radar of 318 meters^2 at x-band and 32.6 meters^2
    > at s-band, thirty three (33) times stronger than the standard radar
    > reflector.
    >
    > I haven't been run over yet.
    >
    > gl
    >
    > George Huxtable wrote:
    > >Fatal interaction between yacht and ferry.
    >
    > >There's quite a lot to ponder, in the highly workmanlike report of the UK's
    > >Marine Accident Investigation Branch, into the sinking last Summer of the
    > >yacht "Ouzo", off the Isle of Wight, resulting in the death of all three
    > >aboard her.
    >
    > >Go to-
    >
    > >http://www.maib.gov.uk/publications/investigation_reports/2007/ouzo.cfm
    >
    > >and from that download the fifty-odd page report Ouzo_.pdf at about 1.4 MB.
    > >Other options are a short synopsis, and a bulky technical appendix, mainly
    > >about photochromic spectacles and radar detection of small craft.
    >
    > > The MAIB brief is to find out what happened, and it seems to me they have
    > >bent over backward to avoid apportioning blame.
    >
    > >The second officer of the P & O ferry, "Pride of Bilbao", is due to stand
    > >trial for manslaughter, later this year, as a result.
    >
    > >My reaction, and I suspect that of many other sailors, is to think "There,
    > >but for the grace of God, go I". And no doubt many ship's officers will,
    > >from another viewpoint, think exactly the same, .
    >
    > >It surprises me that the master, though he was below at the time, appears
    > >not to be bearing any of the responsibilty.
    >
    > >I won't comment further, to avoid prejudicing those that prefer to make up
    > >their own minds. Read it yourself.
    >
    > >George.
    >
    > >contact George Huxtable at geo...---.u-net.com
    > >or at +44 1865 820222 (from UK, 01865 820222)
    > >or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    >
    > >contact George Huxtable at geo...---.u-net.com
    > >or at +44 1865 820222 (from UK, 01865 820222)
    > >or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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