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Subject: Re: Rusty!!!!
From: Eric Haberfellner (ehaberfe@XXX.XXX)
Date: Mon Dec 03 2001 - 21:47:25 EST
I am also confused about what method to use for solving question 1. Here is
a copy of a note that I sent to Dan asking for help. Perhaps some of the
other listers can give me a hand.
-----Original Message-----
From: Eric Haberfellner [mailto:ehaberfe@XXX.XXX]
Sent: Sunday, December 02, 2001 10:15 PM
To: dhhogan@XXX.XXX
Subject: RE: Silicon Sea: Leg 82
OK, I am already having a problem on part 1 of this problem. The initial
course that I calculate for the great cricle course is 282.7466d, not the
given answer. How I got there is this.
The formula for calculating the great circle initial course angle given in
Bowditch is:
Where L1 is lat. of departure
L2 is lat. of dest.
DLo is difference in long.
C = arctan ( sin DLo / ( (cos L1 * tan L2)-(sin L1 * cos DLo) )
So we calculate:
DLo = -22.7466d (decimal degrees)
L1 = 11.4967d
L2 = 19.5000d
We plug these valuse into the equation and we get
True Course = -66.7735 = 293.2265d.
Now we uncorrect for deviation and variation. Timid Vrigins Make Dull
Companions and Uncorrecting east is least, so:
Magnetic course = True course - 10.0d E. Var. - 1.0d E. Dev. = 282.2265d.
Now for the current, The Annapolis Book of Seamanship gives the formula for
correcting magnetic course for a current as:
Where RL is magnetic Rhumb-line course = 282.2265
drift is speed of current in knots = .7 knots
set is direction of current in magnetic degrees = 285d - 10.0d E. var. =
275d M
boatspeed is in knots = 9.7 knots
Mag course = RL - arcsin ((drift * sin(set - RL))/boatspeed)
Plugging in the values we get magnetic course 282.7466d.
Where the heck did I go wrong??
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