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 Boat Design Forums True wind/apparent wind?

#1
12-08-2004, 08:03 AM
 pygmalion Junior Member Join Date: Dec 2004 Rep: 10 Posts: 6 Location: Norway
True wind/apparent wind?

Can anyone give me the formula fore calculating:
1: true wind, when you know apparent wind and boatspeed? (speed and angle)
2: apparent wind, when you know true wind and boatspeed?

If possible on a exel file...? ;-)

(General) Thanks to everyone that contribute in this forum !
#2
12-08-2004, 01:21 PM
 pygmalion Junior Member Join Date: Dec 2004 Rep: 10 Posts: 6 Location: Norway
I dont have to have it on a ecel file!
I can do that my self, but I cant find the formula anywhere!
#3
12-08-2004, 03:17 PM
 Skippy Senior Member Join Date: Nov 2004 Rep: 12 Posts: 568 Location: cornfields
Quote:
 Originally Posted by pygmalion I dont have to have it on a ecel file!
Good, that makes it a lot easier.

Taking the angle between the boat's heading and the direction the wind is coming FROM,
first get the longitudinal (x) and lateral (y) components of the wind velocity:
(V means speed, a means apparent, t means true)
(A means angle in DEGREES)

Vay = Vtrue*sin(3.1416*Atrue/180)
Vax = Vtrue*cos(3.1416*Atrue/180) + Vboat

Vapparent = square root of (Vax squared + Vay squared)
Aapparent = (180/3.1416)*arctan (Vay/Vax)

To go the other way, switch true and apparent everywhere, and put a minus sign in the Vx equation:

Vty = Vapparent*sin(3.1416*Aapparent/180)
Vtx = Vapparent*cos(3.1416*Aapparent/180) - Vboat

Vtrue = square root of (Vtx squared + Vty squared)
Atrue = (180/3.1416)*arctan(Vty/Vtx)
#4
12-08-2004, 06:05 PM
 pygmalion Junior Member Join Date: Dec 2004 Rep: 10 Posts: 6 Location: Norway
Truble

(Im a bit tired now, it's 23.59 here)
I'll have a closer look tomorrow, if not anybody can have a look and help me in the mean time?
Attached Files
 Wind.xls (14.5 KB, 639 views)
#5
12-08-2004, 10:19 PM
 Paul B Previous Member Join Date: Aug 2003 Rep: 0 Posts: 0 Location: California
I put this sheet together quickly a couple of years ago. Seems about right until App Angle >90. I just input your calc in RED. Doesn't seem to work. Maybe someone can check both calcs.
Attached Files
 Apparent Wind Calc.xls (14.0 KB, 592 views)
#6
12-09-2004, 07:52 AM
 cgorton Junior Member Join Date: Aug 2003 Rep: 10 Posts: 50 Location: Massachusetts
http://www.sailingusa.info/true_wind_calculator.htm