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Weather Software => Other Weather Software => Topic started by: SB on June 13, 2017, 06:44:17 PM

Title: How to calculate relative wind for a ship
Post by: SB on June 13, 2017, 06:44:17 PM

I wonder if anyone can help.
 
I am trying to find an easy way to work out the relative wind direction and speed when one knows the ship course and speed and the forecast/actual wind sped and direction, preferably to do this in an excel format.  Can anyone help?

Many thanks

SB

Title: Re: How to calculate relative wind for a ship
Post by: hankster on June 13, 2017, 07:15:39 PM

Maybe this will help

http://www.bwsailing.com/cc/calculating-the-true-wind-and-why-it-matters/

Title: Re: How to calculate relative wind for a ship
Post by: SB on June 14, 2017, 06:28:46 AM

Thanks.  This is where I have got to but the formula(s) below do not work when the true wind speed is greater than the ship speed. Can anyone show me what I am doing wrong and how to correct this?  Many thanks.

       E                               F                   G                     H                            I                     J
Ship Speed (kts)   Track    Wind Direction    Wind Speed (kts)   Relative Wind Speed   Relative Wind Sp
    20                            130                  260                   18                           174                            26
    22                            168                  260                   24                           #NUM!                   #NUM!
   19                            288                  260                   15                           266                            4
   10                            341                  260                   10                           260                            0

I=F6+DEGREES(ASIN(H6/E6*SIN(RADIANS(F6-G6+180))))

J=(E6*COS(RADIANS(F6-I6)))+(H6*COS(RADIANS(F6-G6+180)))