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Old 27-10-2011, 08:40   #1
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Calculation of Crossing Layline

Can someone point me at a reference for an equation to work out the latitude and longitude at which the current boat track will cross a marks layline?

Given that the location of the mark, bearing of the layline, vessel's location and vessel's course over ground are known.

Is it safe to assume that for short course racing, the navigation may be simplified to being planar rather than spherical?

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Old 27-10-2011, 08:44   #2
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Re: Calculation of crossing layline

what you're asking for would change for every boat, and would also vary with racing conditions. How I lay a mark in a flood tide is sure as heck going to be a different vector than how I lay it in an ebb. And the layline I was able to fetch with five crew on the rail probably won't work with half as much rail meat. Buy a new suit of sails, and you'll change your ability to fetch a mark.

Sorry, you'll never replace a good tactician with a formula. There are just too many variables, and it takes a lifetime of racing to learn them.

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Old 27-10-2011, 08:58   #3
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Re: Calculation of crossing layline

Bash is correct. However, that doesn't stop one from wanting to do the mental exercise of the theoretical point.

I would think that you would be able to use the section labeld "Intersection of two paths given start points and bearings" on this Calculate distance and bearing between two Latitude/Longitude points using Haversine formula in JavaScript page.

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Old 27-10-2011, 09:33   #4
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Re: Calculation of crossing layline

This one may be the ortho.

Alternatively, convert lat to Nm and calculate plain geometry?

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Old 27-10-2011, 09:40   #5
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Re: Calculation of Crossing Layline

Spherical trig is only relevant for distances covering hundreds of miles. For shorter distances Mercator charts are fine. You could also use a plotting sheet instead of putting unnecessary lines on your chart.

I agree with Bash in that for short course sailboat racing this is something that is best done visually using a mental calculation and experience as to when you have reached the lay line.

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