Conference unifix::sailing

    re. .0
    	First sentence should read, "I have a question for the..."

    Unless the wind is blowing directly down the channel, your plot
    should look like a zig-zag. Depending on wind direction, the zigs
    will be shorter than the zags, or vice versa, since one tack will
    be more favored by the wind. If the you're fortunate enough to get
    a windshift that blows across the channel, perpendicularish to the
    channel, your plot will become a straight line as you move onto a
    reach.
    
    Ed

	Navigation has a purpose: to get you there safely and
	predictably. If you are going down a channel, whether to
	plot the zig-zag depends on whether it will be helpful!
	If you are in fog and you won't be able to get any shore
	fixes while working your way down the channel, plotting
	the zigs and zags may be helpful to figure out where you
	are. Hopefully you will take soundings to confirm your
	position as you go down the channel. 

	If the weather is good, and you can get fixes from the
	shore features, there is not really any need to plot zigs and
	zags down the channel. The most important thing is to get
	a good fix as you exit the channel, so your next leg can
	be accurately plotted. Usually the easiest way is pass close
	to a channel buoy on the way out.

	Getting a good DR position from a sequence of tacks
	down a channel is extremely difficult, because your
	plotting error, boat leeway, and channel current may create
	a substantial difference between your plotted vs. actual
	progress down the channel.


	--RS

	
	caution: book-learned idiot speaking....

	According to my navigation instructor "A Dead Reconing plot is the 
	line between known waypoints, you plot your fixes to show where you 
	are and use the DR/Fix relationship to tell you where to go next. 
	The end of each DR segment is where you want to end up, so you can
	get to the next point."

	on a common sense side, why would you want to plot a DR according
	to the wind unless you plan to redo your DR with every wind change?


	(and people ask me why I sail on lakes! "Just aim for the 
	brown house, then the red flag pole, then the gazzebo, then...")
		;-) greg

    A DR is an estimate of where you think you are based on the time,
    speed, and direction travelled since the last point you plotted. The
    last point may also be a DR point, or it may be a fixed point (visual,
    or electronic). 
    
    If you have fixes to coincide with the DR, you can measure set/drift
    and apply that to course steered or speed to remain closer to the
    desired track.
    
    If you dont have fixes but know or can estimate leeway, and set/drift 
    of current from tables, that may be added to the DR to create
    a better estimate, but you should have some record keeping system 
    to indicate those assumptions.   

    Walt

    In answer to your question, by performing a series of zig-zags,
    for example beating into the wind, you would be traverse sailing.
    This form of navigation is performed by finding the single equivalence
    course and distance of these zig-zags.
    
    I consists of building a table of the zig-zag courses and distances
    and solving each leg to determine the equivalent single course.
    
    For a detailed example and explaination, read "American Practical
    Navigator", by Nathaniel Bowditch, a text commonly known to navigators
    as "Bowditch", article 814.
    
    Hope this helps,

    Bob

    I remember, back in programming days, an algorithm that I used to
    compute the distance travelled using the Lat and Long changes over
    time.
    
    I dont have a copy of the algorithm now and my geometry is too rusty to
    figure it out..... does anyone out there know it ?

    
    You may be referring to Traverse Sailing, also known as "The
    Sailings". This is valid for short distances (<< 600 miles) and uses
    plane (flat) triangles. Slightly more accurate is Mercator Sailing.
    You need other techniques for distances greater than 600 miles.
    
    bernie