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    Re: Course to steer at a given speed.
    From: Lu Abel
    Date: 2012 Nov 28, 12:19 -0800
    Good point, Greg!

    In my previous, I also forgot to mention that the "follow the bouncing ball" screen on a GPS with its attendant cross-track error (XTE) and ETA displays obviate the need for current calculations.  Set the destination into your GPS, the display will give you your ETA based on current (no pun) SOG, adjust boat speed to make the calculated ETA the desired time of arrival.

    Lu


    From: Greg Rudzinski <gregrudzinski@yahoo.com>
    To: NavList@fer3.com
    Sent: Wednesday, November 28, 2012 11:37 AM
    Subject: [NavList] Re: Course to steer at a given speed.

    Lu,
    This is where the GPS becomes a useful piloting tool when combined with the knot meter. With course made good, speed made good, course steered, and speed through the water all known then set and drift can then be known immediately.
    Greg Rudzinski
    [NavList] Re: Course to steer at a given speed.
    From: Lu Abel
    Date: 28 Nov 2012 11:02
    Ah, but how does one "calculate leeway" to adjust course? Every single text I've seen, from Bowditch to something like Sweet's "Weekend Navigator" all assume you know that leeway will be and then simply tell you to subtract or add it to course steered to get CMG (or vice versa). Leeway depends on one's point of sail -- maximum when close-hauled, non-existent when going downwind. More important, it also depends on the boat's underwater configuration. Sailboats are supposed to go straight ahead and not sideways, so I suspect this has been an issue for naval architects ever since they started doing scientific design of boats.
    As a long-time navigation instructor, I've always found current calculations a bit like a solution searching for a problem. While potentially appropriate while well offshore, in coastal waters current will shift with the tide. So if I do the standard calculation for set and drift based on the difference between my DR and Fix, how is that relevant except as determining an average value over the exact period of time and tidal conditions for the calculation? If I'm trying to determine course-to-steer (Track) as in Sean's problem, how do I know what value to use for Drift (speed of current) when it's changing minute-by-minute over a current cycle?
    Let me make it clear that, as someone with a graduate degree in engineering, vector math (which current calculations are) do not intimidate me, they were part of my education from my freshman year onward. So my comments above are absolutely not because I can't do the problems, it's just that magic number called Drift that's the problem. And its cousin Leeway.
    Lu
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