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    Re: Compass Course
    From: Dan Hogan
    Date: 1997 May 04, 19:23 EDT

    At 17:07 5/4/97 -0700, Millard Kirk wrote:
    >Maybe I need to know the difference between:
    >Compass Course, Magnetic Course, and Course to Steer, or at least
    >between Compass Course (CC) and Course to Steer.
    Compass Course= the True Course adjusted for Var and Dev.
    Magnetic Course= the TC adjusted for Magnetic variation only.
    Course to Steer= The Compass Course to follow adjusted for Dev, Var, and
                     current. Usually used with the Sailings in calculator and
                     computer programs.
    >These three problems all appear to be the same to me. My interpretation
    >of these problems is to find the Course to Steer to arrive at the TC or
    >True Course.  I have worked all and plotted them and get the same answer
    >via way of plotting and using a calculator.
    TVMDC (uncorrection) covers changing from True Course to Compass Course
    that you steer.
    VDMVT (correcting) covers changing from Compass Course(Course Steered) to
    the True Course.
    The correction for current is a seperate calculation. Either with a Trig
    formula or a vector graph.
    >Here are the problems restate with answers given and the ones that I
    >have calculated on the basis of Course to Steer to maintain the True
    >Leg 25
    >We depart the Suez Yacht Club, as we exit the Gulf of Suez we take a
    >departure fix from two points of land at 06:35:00 giving us a position
    >of (29d 51.0'N  32d 33.0'E). We set a TC of 164d down the Gulf of Suez.
    >Wind is Force 4 from 330d. Current 5.0 Kts, Set 155d. Var0.0d Dev4dE.
    >Speed 9>Kts.
    Take TC 164d apply the Var and Dev to get the Compass Course. Then run a
    vector for the current set and drift(5Kts@155d) for the Course Made Good
    and/or Course to Steer.
    For a Celesticomp it should return the CCmg or Course to Steer depending
    what it calls it. Compass Course is the TVMDC answer.
    >2)  What is our Compass Course(CC) and Speed Made Good(SMG)?
    >Ans. (SMG) 14.0 kts.
    >Ans. (CC)  160.0d
    >Comment:  When I worked this problem I assumed the given parameters
    >would be used the (TC) 164d , Current Speed 5.0 kts, Set 155d, Dev 4dE
    >and log speed of 9 kts to give me:  Using the above parameters I get:
    >(SMG)  13.9
    >(CC)  164.9d  or Course to Steer to maintain a TC of 164d
    >If I am being set 5 kts  at 155d there is noway steering 160d will hold
    >a course of 164d. I am being set in a way to drive me away from my
    True, the question asks only for SMG and CC rather than CC made Good(CCmg).
    CCmg which is probably what your calculator calls Course to Steer. We can
    change to CCmg/Course to Steer, but it will have different names different
    names on the various navigation programs/calulators.
    This is a copy of Navig94, the program I use to verify the answers:
    2)CC & SMG
    ??????????????????????? [1] VELOCITY TRIANGLE ?????????NAVEX SYSTEMS (c)??
    ?                                                                        ?
    ?            Variation  0.0? W         Deviation  4.0? E                 ?
    ? -----V1------            ------V2------            ------V3------      ?
    ?Speed      9.0            Drift      5.0            SpMdGd    14.0      ?
    ?TrueCrse 164.0?           Set      155.0?           TCMdGd   160.8?     ?
    ?                                                                        ?
    ?CompCrse 160.0?                                     CCMdGd   156.8?     ?
    ?                                                                        ?
    [Snip Legs 26 & 27 data]
    OK, what we have is a failure to communicate...The problems ask for answers
    that would be visible on a plotting sheet. When you use a dedicated
    navigation calculator or a computer navigation program designed for "on
    board" use you don't see a lot of the work accomplished to answer the
    question. Just the final answer that you would need on the boat.
    The problems can be adjusted but we need to agree on nomenclature. This
    gets us into using DR (Dead Reckoning), EP(Estimated Position), Course to
    Steer(CTS(?))/Compass Course Made Good(CCmg), etc, etc.
    Dan Hogan 			Yacht GACHA
        West Covina, CA		Catalina 27
        dhhogan@XXX.XXX	San Pedro, CA
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