Sunday, July 15, 2007

More on VMG

I found Ulli's post so very helpful! Never was all that clear on how they made their navigation decisions. So I found this simple explanation of VMG (Velocity Made Good) that I thought I should share with you because I found that it helped me too:
VMG is a mathematical calculation for how much progress or speed you are making while you sail at an angle to the wind. In sailing it is impossible to sail directly into the wind. Therefore, the fastest way towards a particular point is not always a direct route. VMG measures how much speed you are producing toward your ultimate goal as you move at an angle to the wind.
Clear now? Well maybe this helps us understand why there are so many PhD sailors out there!


ulli said...

What is so damn difficult about VMG? It is simply the speed over ground multiplied with the cosine of the angle between direct direction and direction actually taken.
Huh? What's cosine? Never use cosine for shopping at Costco? Ok, let's do a simpler example.

Imagine you are at Kailua Beach and want to go to Waikiki Beach. No idea, why you would ever want to do that, but for the sake of argument, let's assume you want.

Direct connection via the Pali Highway is 10 miles, and you manage that in 10 minutes (German driving style). Your SOG (Speed over ground) is the same as VMG (Velocity made good), namely 1 mile/minute. Now imagine Pali is blocked by an accident, and Likelike scrambled by an earthquake. You would have to go via Hanauma Bay. That is 10 miles to Hanauma Bay, and from there 10 miles to Waikiki (I simplify). Still assuming a really mad driver making 1 mile/minute even on the coastal road, it takes you 10 minutes to get to Hanauma, and another 10min to get to Waikiki. Your SOG is now still 1 mile/min, but your VMG is only 0.5 mile/min, because it take you twice as long.

Valerie said...

You betcha. Got it!