Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Level of observation accuracy in medium seas
    From: Jared Sherman
    Date: 2004 Jul 23, 13:02 -0400

    ----- Original Message -----
    From: "Robert Gainer" 
    To: 
    Sent: Friday, July 23, 2004 9:57 AM
    Subject: Re: Level of observation accuracy in medium seas
    
    
    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?
    >Security. http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963
    >
    >
    >Dave Weilacher
    >.US Coast Guard licensed captain
    >.    #889968
    >.ASA instructor evaluator and celestial
    >.    navigation instructor #990800
    >.IBM AS400 RPG contract programmer
    
    _________________________________________________________________
    Overwhelmed by debt? Find out how to 'Dig Yourself Out of Debt' from MSN
    Money. http://special.msn.com/money/0407debt.armx
    
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site