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    Re: Level of observation accuracy in medium seas
    From: Robert Gainer
    Date: 2004 Jul 23, 13:57 +0000

    David,
    This is just an opinion because I don�t have any research to back it up with
    and the few times that I have been in these conditions there was no way to
    see how accurate the results of my sights were. When I was at the top of a
    large wave (50 to 60 feet) it was possible to see a horizon that was the top
    of a higher wave that was only 1 or two miles away. You cannot tell how far
    or high the wave is that makes the false horizon because a white layer of
    foam or spray covers the surface of the ocean. Some times it�s very hard to
    tell what is the true horizon. But I have taken two sights in rapid
    succession and found the sun was going backwards. No mater how bad the
    weather gets I think it�s unlikely that the sun will try to run the other
    way and hide. What does this tell you about the practice of navigation? Even
    with lots of experience it�s not easy on the bad days and the corrections
    for temperature pressure and dip don�t really mean much. You are just
    getting an estimate of position.
    All the best,
    Robert Gainer
    
    
    >From: David Weilacher 
    >Reply-To: Navigation Mailing List 
    >To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    >Subject: Re: Level of observation accuracy in medium seas
    >Date: Fri, 23 Jul 2004 08:13:03 -0500
    >
    >Yep.  I exagerated wave height to make the point that it didn't matter how
    >big the waves were at the horizon, I didn't think it would make a
    >measurable distance to what your eye could see.
    >
    >In realistic terms, it is my intention to always be at home watching TV in
    >those conditions.
    >
    >Keeping with the same exaggeration, I also can't see how, if my boat is at
    >the top of a 50 ft wave and all I can see is the tops of 50 ft waves 8
    >miles away, that will make any difference at all in my height of eye,
    >compared to flat seas.
    >
    >Seems to me that what is happening is that we are adding 50 feet to the
    >radius of the earth and that difference isn't measureable in any practical
    >sense.
    >
    >
    >-----Original Message-----
    >From: Robert Gainer 
    >Sent: Jul 23, 2004 7:44 AM
    >To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    >Subject: Re: Level of observation accuracy in medium seas
    >
    >Dave
    >A 50-foot wave by the Beaufort Scale means that you are in �exceptionally
    >high waves. The air is filled with foam and the sea completely white with
    >driving spray. Visibility greatly reduced�. The short description is
    >�hurricane�. You might want to check out
    >http://facs.scripps.edu/surf/luds.html, it has a very good and readable
    >description of predicting wind speed and wave height. In practice you will
    >not get a shot at anything because the wind and spray make it all but
    >imposable to hold the sextant still. I have been in these conditions and
    >speak from experience. By the way I think the height of a wave is measured
    >from the trough to the crest.
    >Robert Gainer
    >
    >
    > >From: David Weilacher 
    > >Reply-To: Navigation Mailing List 
    > >To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    > >Subject: Re: Level of observation accuracy in medium seas
    > >Date: Fri, 23 Jul 2004 06:42:03 -0500
    > >
    > >Hi Jarad;
    > >
    > >Can you point me to your source for Noaa wave height definition?
    > >
    > >Dave W
    > >
    > >
    > >-----Original Message-----
    > >From: Jared Sherman 
    > >Sent: Jul 22, 2004 9:34 PM
    > >To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    > >Subject: Re: Level of observation accuracy in medium seas
    > >
    > >Dave-
    > >  <50 foot waves with a mile between peaks.  I take my shot when my boat
    >is
    > >at the top of a wave.  This is easy to tell because I can actually see a
    > >horizon.  The horizon I see is 8 miles away.>
    > >  Seems like short horizon. NOAA says that waves are measured from the
    >sea
    > >level, not from the trough to peak, so are you talking about real fifty
    > >foot
    > >waves, or "real" 50 foot waves, which most sailors would call hundred
    > >footers?
    > >
    > >If the former, you're observing from 25' above sea level, figure ten more
    > >for your deck and standing eye height, since you've got a good enough
    >grip
    > >to rider those doggies. That's 35' asf now, about your eight miles.
    > >(7.9+)
    > >
    > >Nah, you're only in 25' waves, that's the problem. Wait for rougher
    > >weather,
    > >you'll get a better horizon.
    > >
    > >But you could certainly figure the math. A sphere (close enough)
    >25,000
    > >miles in circumference, two points 8 miles apart on that. Change the
    >radius
    > >of one by the 25' your far wave is blocking you...run some tangents and
    > >angles..."A simple exercise left to the reader."
    > >
    > >Just remember, you're only in 25' waves.
    > >
    > >
    > >Dave Weilacher
    > >.US Coast Guard licensed captain
    > >.    #889968
    > >.ASA instructor evaluator and celestial
    > >.    navigation instructor #990800
    > >.IBM AS400 RPG contract programmer
    >
    >_________________________________________________________________
    >Is your PC infected? Get a FREE online computer virus scan from McAfee�
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    >
    >
    >Dave Weilacher
    >.US Coast Guard licensed captain
    >.    #889968
    >.ASA instructor evaluator and celestial
    >.    navigation instructor #990800
    >.IBM AS400 RPG contract programmer
    
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