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    Re: Lights etc.
    From: George Huxtable
    Date: 2003 Oct 10, 18:24 +0100

    There's some interesting stuff coming up here about lights, VHF
    communication, and other matters. Some comments and personal opinions
    follow.
    
    I agree with Keith Williams (and not with Steven Wepster). Steven's boat,
    just like mine, is 8 metres long. When under sail, if 7 metres or less, he
    would be allowed to use a single white light, when out at sea anywhere in
    the world. Whether it would be prudent to do so is another matter. Steven
    excuses his use of that single white light on the dubious grounds that his
    boat is "only a metre" above the requirement.
    
    The problem is that another vessel, seeing a single white light out at sea,
    will interpret it as a stern light (what else can she do?) and will shape a
    path accordingly. I don't think such confusion can help anyone. Isn't it
    safer to give some notion of your vessel's heading? I presume that Steven's
    argument is that a white light is visible at a greater distance than a red
    or green (which is certainly the case). But is the game worth the
    candlepower? I think we should show the lights that others expect us to
    show, even if it sounds a bit holier-than-thou to say so.
    
    ====================
    
    I'm keen to learn more about the LED-array alternative. Can anyone provide
    more information, please? These are questions to which I would like
    answers.
    
    1. How much current reduction for the same brightness, compared with
    incandescent??
    
    2. How uniform is that brightness when seen over a range of 135deg, from
    straight-ahead to the port quarter, and at heel angles up to (say) 30deg?
    
    3. How sharp is the cutoff on the starboard side of the bow, and further
    aft than 135deg port?
    
    4. Does a comparable green array, of similar brightness, exist for the
    starboard side?
    
    5. Does the transparent plastic encapsulation degrade after long-term
    exposure to bright sunlight (as has the casing of my tricolour, which has
    become crazed)
    
    6. How much do they cost?
    
    ===================
    
    Doug tells a real horror-story about events off San Diego. Isn't it the
    case that US pleasure-boat sailors require some sort of certificate of
    competence? I agree that such a certificate would not necesserily mean that
    they WERE competent. In the UK, no such test (or even age-limit) is
    required, though some other European nations demand it.
    
    In the tow circumstances that Doug graphically describes, "barely
    underway", I would expect the towline perhaps to be flicking between taut
    and slack, dipping into the sea. If so, any boat passing between tug and
    tow was indeed fortunate to pass clear under, and ran a risk of being
    lashed by the line or even tweaked out of the water (and would deserve
    little sympathy).
    
    Doug's account of all the communications on VHF is enlightening, though it
    sounds rather strange to a European. I understand that in US waters use of
    VHF in agreeing manoeuvres is common and indeed expected, particularly in
    inland waterways.
    
    Here, such use of VHF is definitely discouraged. It's used, occasionally,
    for alerting others to your presence, or discovering their intentions,
    though I've never done either. Agreeing at sea to a crossing-plan that
    doesn't accord with colregs is a definite no-no (though Doug didn't imply
    that was happening). This derives partly from a collision off Odessa about
    15 years ago, in which two vessels came to such an agreement by VHF, but
    then somehow failed to implement it, and collided with the loss of more
    than 400 lives. The masters each got 15 years in jail.
    
    Of course, one reason for a different attitude to VHF in our waters is that
    language-confusion is a bigger problem here. English is commonly used, but
    the potential for misunderstanding is that much greater when it's an
    unfamiliar language.
    
    ================
    
    Radar.
    
    Doug said-
    
    >There are small radar reflectors
    >one can attach to the upper mast that increase the radar return of your
    >vessel by a factor of 2.
    
    Yes and no. I have had one of these for 30 years. It acts as a sort of
    talisman to keep the big-ships away, a bit like a rabbit's foot in the
    pocket. It works, to some extent. But whenever I discuss these things with
    a merchant mariner, they regularly tell me of encounters with small craft,
    even those carrying reflectors, which didn't show on radar until
    surprisingly close up.
    
    The UK yachting mag., Practical Boat Owner, has made a useful study of this
    question, finding that any combination of (boat + reflector) can show, at
    certain aspect directions, a null in the radar return, with significantly
    less reflection than for the boat on its own, or the reflector on its own.
    
    How can reflection from the two, combined, be less than it is from either
    component separately? Simple, really. It depends on the relative phase of
    the two reflections. If they are in phase, the signals will add. If they
    are out of phase, they will cancel. At the short wavelengths involved in
    radar, a small shift in the relative positions by a few centimetres is
    sufficient to change from adding to cancelling. This can happen due to a
    small shift in the heading of the craft doing the reflecting. There may be
    many such "nulls" in the polar diagram of the reflection.
    
    So, what's the conclusion from this? Even in the best conditions of no
    wave-clutter and a good radar reflector, a small craft may be quite
    invisible by radar for some time, until its aspect changes. The radar
    screen may be clear of images, but that doesn't mean that there is not a
    vulnerable small craft, properly fitted with a reflector, somewhere in
    danger near the bow. The bridge windows serve a real purpose.
    
    George.
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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