Next message: Bill: "ReRe: Level of observation accuracy"
----- 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:
>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:
>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:
> >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:
> >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?<G>
> >
> >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.<G> 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.<G>
> >
> >But you could certainly figure the math. A sphere (close enough<G>)
>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.<G>
> >
> >
> >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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>
>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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