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    Re: Set, drift, and leeway
    From: Eddie C. Dost
    Date: 2006 Jan 29, 00:31 +0100

    Hi Bill,
    you usually have a "polar diagram" for the vessel, giving you a
    speed through water for each wind angle. With this you can optimize
    for maximum speed towards destination. This is what tactics software
    The hard part is getting this polar diagram correct for the boat
    and crew in question to optain useful results.
    On Sat, Jan 28, 2006 at 05:19:07PM -0500, Bill wrote:
    > This initially started out as a quest to see if it mattered which order
    > leeway, and set and drift were applied when calculating the new course to
    > steer to make good a rhumb line course to destination.  So far, it does not
    > seem to matter.
    > The lovely part of the vector diagrams and equations in my texts is that
    > they assume constant speed, or the ability to increase speed in some
    > situations.
    > It occurs to me that a sailboat is usually running at the best speed she can
    > make, conditions permitting.  A change of course, assuming wind direction
    > remains constant, will alter speed unless she is able to run at hull speed
    > on all points of sail.
    > This is where red flags started popping up.  Assume a wind from the north,
    > 0d, 10 knots.  Rhumb-line course to destination 90d.  She is on a screaming
    > beam reach at 7 knots.  Leeway 6d.  Set 180d, drift 3.
    > To make good the intended track of 90d, she will have to come up
    > approximately 29d, steering a course of approx. 61d.  She would also have to
    > increase speed through the water.  But by coming up towards a close reach,
    > she loses speed through the water.  As she loses speed, the angle from the
    > rhumb line to compensate for leeway, and set and drift, increases. As the
    > angle increases she goes to yet a higher point of sail, so speed decreases
    > again....and on and on.
    > Which brings up the question, is there some slick way to solve this problem?
    > Bill
    Christian Dost          brainaid GbR              Bluecher & Dost
    software                Monheimsallee 45          phone +49 241 5151 138
                            D-52062 Aachen            fax   +49 241 5151 139
    ecd@brainaid.de         Germany                   cell  +49 172 9312808

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