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Originally Posted by astroglobe
If anyone has a quicker and dirtier method of ship interception let me know, I was thinking about trying interceptions based knots per hour, distance travelled of the contact compared to my boat's course and distance and making a rougher guestimate of where they might intersect....rather than the more elaborate protractor angles......
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The only way I can think of is if you are better at punching buttons on a scientific calculator, than doing the geometry thing. As with the graphical method you need to know his speed and general course. And do it based on his current position if it is now later than when the contact appeared on the map. So you need to advance the position along it's course based on speed and time since. Then calculate or measure(protractor) the angle between his course and the direction to your Uboot (so target is at the corner), this is known as AOB.
Your lead/intercept angle = arc_sine ( (Sine_AOB * Targetspeed)/Yourspeed )
So you punch in, in this order:
AOB, sine_button, multiply, Target_speed, divide, Your_speed, arc_sine_button
To set course easily: Draw another protractor (last one, honest!), first leg from target position to you (being at the corner), and the other leg to the resulting angle ahead of the target's position. Drag it to also cross the target's course, so you know you won't dig into an island or something along the way. Or takes an absurd long time to reach it. If ok, drag a waypoint over this leg to set course and go.
If the calculator ends up giving an error result, then you will never be able to catch the target at your intended speed. It is going to fast. Go faster if you can, or forget about him. (ofcourse, check you didn't make an error while calculating if you are not sure)
Arc_sine is sometimes called inv_sine or sine^-1 depending on calculator brand, consult the manual; you need to go back from a fraction between -1 and 1 to an angle between -90 and 90 degrees.