B Getting sailing boat velocity from wind speed and direction

1. Sep 2, 2016

kubassek

I am trying to figure out how to calculate the boat velocity using wind speed and wind direction. I have tried to find sources online but this is the only useful site I managed to find:
Unfortunately it has been a while since I did maths and I'm struggling with understanding how the VB (boat velocity) in the examples on the website is calculated and was wondering if anyone could explain it at all? The results don't have to be 100% accurate so as long as a rough estimation of speed is achieved that's fine.

The part I am most struggling with is how the VB is calculated using a - apparent wind as I though you needed the boat velocity (VB) to figure out a.

Thank you in advance for any help.

2. Sep 2, 2016

Staff: Mentor

You need to do a vector sum. Not just the speeds, but the speeds and directions. The pictures below illustrate. Does that answer your question?

3. Sep 2, 2016

A.T.

What about the course? Do you have a target that you want to reach as fast as possible?

Or do you want to maximize speed no matter the direction?

https://en.wikipedia.org/wiki/Saili...heorem_and_Maximum_speed_course_sailing_angle
http://www.onemetre.net/Design/CourseTheorem/CourseTheorem.htm

You will need more parameters, at least the lift/drag ratios to the air and to the surface (efficiency).

4. Sep 2, 2016

sophiecentaur

I can't decide what information you have about the situation. You are basically dealing with a vector equation with three vectors. You need to know two of the vector quantities in order to work out the third.
You can measure apparent wind speed and direction on the boat but what about true windspeed and direction? Would you assume that the Windfinder information is accurate enough? It sounds to me that it's an ill conditioned problem. Buy a second hand Garmin gps and that will tell you what you want.

5. Sep 2, 2016

CWatters

It is impossible to calculate boat speed from just the wind speed and wind direction. The crew also have a say in such matters!

If you have info on the true and apparent wind then it's a simple vector sum as the drawing anorlunda posted shows.

6. Sep 3, 2016

BillTre

I agree with CWatters.

The design of the boat (think about those American Cup boats) will also matter, as well as other things.

I think of the vectors more as reflecting the relative amounts of force derived from the wind, which through the action of the boat, crew, water conditions, etc. generates a speed.

7. Sep 3, 2016

sophiecentaur

I had the luxury of a boat that went so slowly that real and apparent wind were never very different. "A bit for'ard" was accurate enough for me.

8. Sep 3, 2016