truewind.pdf

TrueWind

UserÓs Guid by

David Burch

About TrueWind...................................................................2

How to use TrueWind...........................................................3

Definitions

Wind direction............................................................4

Apparent wind............................................................4

Apparent wind angle..................................................4

Apparent wind speed .................................................. 5

True wind angle .......................................................... 5

True wind speed ......................................................... 5

Speed .......................................................................... 6

Heading ...................................................................... 6

Discussion

Telltales ...................................................................... 7

Electronic wind instruments ....................................... 7

True wind versus apparent wind ................................ 8

Tips, tricks, and special cases .................................... 8

How to find true wind

Reading the water ....................................................... 9

Formulas and diagrams .............................................. 10

Graphical method ....................................................... 12

Starpath TrueWind INDEX

About Starpath TrueWind

Starpath TrueWind is a Windows program designed to compute very conveniently

true wind from apparent wind for use in maneuvering and weather analysis. It is

also graphically enhanced for studying the vector solution in general to learm

more about how the individual components of the solution are interdependent. A

general discussion of wind analysis is included for background.

The program has been produced and copyrighted by Starpath School of Naviga-

tion which retains all rights to the product. We do, however, distribute it free of

charge as a service to mariners and as a means of promoting our other products

and services which can be seen at www.starpath.com.

The program may be distributed freely provided it is not modified and it is pre-

sented in full with its original install file which includes the discussion document

(truewind.pdf) and so long as it is not part of any commercial product or service,

directly or indirectly.

A valid legal edition of this program is called "Starpath TrueWind" and the file

name for distribution of the self extracting zip file is sp-tw101.exe.

Starpath School of Navigation

3050 NW 63rd St.

Seattle, WA 98107-2521, USA

206-783-1414, fax 206-783-9209

info@starpath.com

www.starpath.com

INDEX UserÓs Guide

How to Use StarPath TrueWind

You can compute true wind two ways , either enter the apparent wind data

numerically and then press ÐCompute True WindÑ or drag the yellow dots in the

diagram with the mouse to create the appropriate vector diagram. The computa-

tions will execute when you release a dot. The apparent wind angle and apparent

wind speed are shown in green. True wind values are shown in blue.

All wind speeds are intended to be in knots , but any unit can be used so long as

the boat speed and the wind speeds are in the same units, ie miles per hour, km

per hour, or meters per second.

To drag yellow dots , left click a dot and hold down the left mouse button as you

move the dot to the desired location, then release the mouse button. You can move

the wind from port to starboard side or vice versa.

Adjust the scale as needed with the set of radio buttons, which determine the

scale of the full picture. The scale values will apply to the speed units you choose,

even though still marked in kts.

To move the entire diagram on the page, right click anywhere on the diagram

and then drag it into position and release the right mouse button.

Quick help buttons are available on the screen itself.

Definitions of the various terms are given below, as well as discussion of a few

general issues of true wind versus apparent. Note restrictions mentioned in defini-

tions of speed and heading.

Starpath TrueWind INDEX

Definitions

Wind direction

This name generally refers to the true wind blowing across the land or across the

water. In this case, it is always the direction from which the wind is blowing. A

north wind blows from the north, toward the south. Unless otherwise specified,

these directions are assumed to be true directions. Usually it is best to specify

wind direction numerically as, say, 280 for a wind from 10 north of west. In

particular, for careful weather analysis, we do not want to refer to, or to log a wind

direction as just ÐwestÑ or Ðwesterly.Ñ when we know it to be, say, 275. When less

than 100, it is best to add a leading zero, i.e., Ðthe wind is from 050,Ñ said as Ðoh-

five-ohÑ rather than from 50 (five-oh or fifty). A southerly wind is from 180, if

asked the wind direction the answer would be one-eight-oh. ÐWind directionÑ is

distinguished from Ðwind angle.Ñ

Note that although the final goal is to have wind direction in terms of true bear-

ings, we often keep track of directions with compass headings, and in these cases

at some point we have to apply the local magnetic variation to get the desired

result. Once we find the true wind angle, we have to either apply it to a corrected

compass heading (ie the true heading of the vessel), to get true wind direction, or

we apply it to the compass heading itself to get the compass bearing of the true

wind direction. In this latter case, it would be best to then immediately correct this

to get the true bearing of the true wind.

Apparent wind

The wind we actually feel or the vessel feels when moving. It is a combination of

the true wind and the effective wind created by our motion. If our speed is zero,

the apparent wind is the same as the true wind. The apparent wind is described by

the apparent wind speed and the apparent wind angle.

Apparent wind angle (AWA)

The direction of the apparent wind relative to the head of the vessel, usually listed

as port or starboard. It varies from 0 (wind on the bow), through 90 (wind on the

INDEX UserÓs Guide

beam), on around to 180 (wind on the stern). The apparent wind angle is always

smaller (forward of) the true wind. The faster you go, the more the apparent wind

moves forward on the vessel. The apparent wind is the actual wind you feel on

your face. If you are facing 30 to the right of the bow, and the wind is in your

face, then the apparent wind angle is 30 . Note that unlike wind direction, leading

zeros are not used for wind angles. It is AWA = 30 , not 030. Since apparent wind

angles are relative angles, it does not make sense to refer to them as ÐtrueÑ or

Ðmagnetic.Ñ

Apparent wind speed (AWS)

The speed of the wind in knots that we measure from a moving vessel. It is a

combination of the true wind speed and the effective wind we create with our

motion. Motoring at 8 kts straight into a 12-kt true wind, we would experience an

apparent wind speed of 20 kts. Sailing dead downwind at 7 kts in a true wind of

12 kt, the apparent wind would be reduced to 5 kt. When sailing diagonal to the

true wind direction, up or down wind, the resulting apparent wind speed cannot be

determined from simple arithmetic, but instead vectors must be solved as done by

this computer program. In general, though, sailing into the wind, the apparent

wind speed is always higher than the true wind speed and sailing downwind it is

always lower.

True wind angle (TWA)

The direction of the true wind relative to the head of the vessel, usually listed as

port or starboard. It varies from 0 (wind on the bow), through 90 (wind on the

beam), on around to 180 (wind on the stern). The true wind angle is always larger

(aft of) the apparent wind. If the apparent wind is on the bow, the true wind is

more toward the beam. If the appraent wind is on the beam, the true wind is more

toward the quarter, and if the apparent wind is on the quarter, the true wind is

more toward the stern.

True wind speed (TWS)

This is the actual speed of the wind over the water. Generally it is expressed in

knots. 1 knot = 1.15 miles per hour = 1.85 km/hr = 0.514 meters per second.

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