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Subject: Re: Silicon Sea Leg 72
From: Noyce, Bill (William.Noyce@XXX.XXX)
Date: Tue May 29 2001 - 16:13:42 EDT
> > > 4) What is the Compass Course(CC)/Course-to-Steer for Aruba Gap
> > > from the DR position?
> > > -- ------------------------------------------------------------
> >
> > Using Law of Sines, I find I need to adjust my course North
> > about 1.5 degrees.
> > CC = 283.4 + 1.5 + 9 = 293.9d
>
> Mmmm..Barely OK. Try a current vector diagram.
I drew a diagram to get the orientation of the pieces right, but I
have a hard time measuring the resulting course adjustment, because
it's so small. Thus the attempt at a 'digital' method.
TC=283.4, Set=265.0, angle between = 18.4 degrees
sin 18.4 / speed = sin adj / drift
sin 18.4 / 8.5 = sin adj / 0.7
I did this with log tables before; doing it on the computer now
says adj = 1.490 degrees. The diagram indicates it needs to be
added to the course. Is there something inherently wrong with
this approach, or have I made a slip somewhere?
> >
> > Mid-latitude using 13d 42'
> > dLat = 15' N dLon = 183d W dep = 177.8 W
> > TC = 274.6d dist = 178.3 nm
>
> OK. Be aware that above 500 miles at 90d/270d Mid-Latitude can give an
error.
> Above 1200 mils it gets unreliable.
I saw a discussion initiated by Sam Chan about Mercator sailing
near 90 or 270, where the problem arises from rounding the course
before using dLat/cos(C) to compute the total distance. Is this
a similar problem? In Mid-Latitude sailing, where we already
have dLat and departure, the total distance can be computed either
as dLat/cos(C) or as departure/sin(C), so we don't have to divide
by a tiny rounded number.
Or are you talking about the error in distance that arises from
assuming a spherical earth? What method would be
better? I assumed we weren't planning to sail a great-circle
course here.
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