While I usually write articles on the rules and tactics of radio sailing - this one is quite different.

The wind that powers our boats is invisible.

What is Wind? What causes it? How does it affect how we trim our sails?

I discuss these and related topics in a new article "Wind - the invisible engine" which may be found on my web site.

https://sites.google.com/site/johnsrcsailingrulesandtactics/

and scroll to the bottom of the home page where I have some short items in the Appendix.

It's is also attached below.

I hope it helps you understand what wind is!

Enjoy!

John

Wind - the invisible engine.pdf

New to RC sailboat - this is a helpful website - Thanks

Edited June 7, 2025Jun 7 by Kev

John, 

As usual a succinct and clear explanation.  A point from me; the forecast windspeed is generally calculated at 10m Above Ground Level (AGL).

Secondly, the major part of weather forecasts is that they are based on computer models, and the cost of computing means that there are relatively few models available world wide.  The most significant part of weather modelling is that each model is based on a square grid (actually cubic, but we'll ignore that for this), and all the variables - pressure, pressure tendency (increasing/decreasing), temperature, precipitation, wind speed, wind direction to name the obvious - are all assumed to be identical across the whole grid square.  As some of the grids (e.g. the American GFS Model basic) are large   (22.5x22.5km) and others (UK Met Office) are small (1.2x1.2km) you need to choose a relevant model within the forecast provider.  My recommendation is the Windy app, together with the Premium subscription where you can compare the results of different models and the different resolutions.

Hi Larry,

thanks for the feedback.

Yes, I agree on your observations about forecast models. I just use them as a simple guide. Is there going to be wind? From which general direction (our local site is fine except for due north and due south), and the temperature, and if rain is forecast.

The screen shot below shows me to expect a good sailing day with steady winds during our race window and an acceptable direction, comfortable temp and no rain! A persistent shift is indicated late in the day, long after we have quit.

Using the forecast tool over time builds experience and confidence in its accuracy for our local water.

I still use my eyes and experience reading the water and gusts to select my rig.

John

I have regular conversations with (big boat) sailors who say (e.g.) that a forecast is excellent - when the forecaster may only buy two updates a day from GFS - and observation says that it's patently inaccurate, but that's sailors for you!

In the interest of balance I'd like to propose an alternate theory regarding sail twist.  I am not saying I'm right and you're wrong just proposing another possible explanation of why sail twist might work.

The wind does indeed increase in average speed with height and it also rotates in direction with height due to the rotation of the earth. but these effects are very small over the height of a model yacht sail.  More importantly the wind at ground level is far from stable in either speed or direction due to the turbulence caused by trees  etc inland and waves at sea. I find it hard to believe that the flow is stable enough (lamina if you will) for you to be able to tune the sail twist to the variation in velocity.  What is certain is that the more you sheet in the more force you produce (until the sail stalls), but the more you sheet in the more this force is directed sideways rather then forwards. Sail twist can be used to 'unload' the upper section of the sail thereby reducing the heeling moment and hence the hull drag   In other words the lower section of the sail is producing more lift and but also more heeling moment.  The upper section is producing less lift, but since it is directed further towards the intended direction of travel, the loss in 'drive' is more than compensated by the reduction the heeling moment.

I actually sail my boats with very little sail twist, although you can have a reasonable amount of twist upwind it just looks wrong to have a lot of twist downwind when the sail is stalled.  With a stalled sail I think you want to present the maximum projected area to the wind so this means having very little twist.

On the subject of how a wing produces lift, it would be a disservice to all those people who tried to teach me aerodynamics if I didn't point out that the "air has to travel further over the top of the wing therefore it must travel faster" argument is a common mis-conception or perhaps more correctly: only part of the story.   This argument struggles to explain how an aircraft with a symmetrical wing can fly at all or how an aircraft with a conventionally cambered aerofoil can fly upside down.  It also struggles with a yacht sail (technically a cambered flat plate) as there is no difference in the distance the air must travel between the windward and leeward sides. 

This isn't the place to explain aerodynamics (and I'm not the best person to do it anyway) but perhaps if you look at the diagram you drew and replace the cambered section with a symmetrical section, think of the fluid as water rather than air and call it a rudder not a wing:  then maybe it's easier to see that the change in direction of the fluid flow must cause a force on the rudder?

Good stuff.  But the business of the wind speeding up to follow the upper wing camber so that it meets up with the wind below is no longer thought a useful explanation.  Instead,

"[...] we imagine we are sitting with a bundle of fluid particles as we flow over the upper surface of a cambered wing.  Starting from the nose, we are forced up by the curve of the wing.  As the curve of the wing eases off, we stop being pushed up, and we travel straight and level for a moment or two.  And now as the wing surface begins to drop away from us, where do we go?  Well, we just want to keep going straight because of momentum and inertia.  On the other hand, if we all just stop following the surface, we are going to create a perfect vacuum.  So some of our bundle starts to follow the surface down, some of our bundle continues more or less straight on, and the remainder of the bundle spreads itself between those two.   Technically, this results in decreased fluid pressure on the upper surface.  A pressure decrease is the same thing as a partial vacuum, meaning that there is a suction on the upper surface of the wing, lifting it up.  The amount of sucking – lifting – force depends on the amount of camber;  more camber, more suction."

https://www.onemetre.net/Design/Lift/Lift.htm

Try this:

It's a bit frantic and you may not follow all of it (unless you are familiar with Bernoulli's equation and know what the Navier-Stokes equations are about) but the thing to take away is that the explanation of how a wing produces lift is complex and can only properly be explained using some fairly complex maths and therefore all the simple "molecules speeding up" or ^bouncing off the surface" explanations are nonsense.

Thank you all for your feedback. I have updated my article to include it.

Here is v2.

John

Wind - the invisible engine v2.pdf

Edited June 9, 2025Jun 9 by John Ball
update to credit Lester's web site

Hi John

> unless you are familiar with Bernoulli's equation and know what the Navier-Stokes equations are about

Yup, check, and, ah, check.

> all the simple "molecules speeding up" or ^bouncing off the surface" explanations are nonsense

These are perfectly good explanations, though some are considered better.  The lift produced by a wing is described, but not explained, by some maths which is just a computable model of the situation in the realm of engineering that gives rather useful results.  An explanation is a different matter from the realm of the philosophy of science, and a useful explanation is when a human says, "Ah, got it, I think I understand."

And I think it is considered offensive in polite discussion to tell anyone they are talking nonsense.

 Not wishing to agree or disagree with the pure theory of flow over the wing/sail but I do find Lesters description easier to understand and possibly easier to help people see a source of the pressure drop

On 08/06/2025 at 16:05, John Ball said:

Hi Larry,

thanks for the feedback.

Yes, I agree on your observations about forecast models. I just use them as a simple guide. Is there going to be wind? From which general direction (our local site is fine except for due north and due south), and the temperature, and if rain is forecast.

General consensus with our members is that the wind is always several knots less than the forecasts (bbc, even predictwind) predict. Backed up by our on-site gear (http://cswindstation.duckdns.org:5772/public-dashboards/a81154154edc4a028f24ae236e1a7919)

Even the direction often ends up being 10s of degrees away from 'expected'. At which point, as John says, the forecasts are taken just at hints of the coming trends rather than absolutes!

On 10/06/2025 at 14:33, Colin Helliwell said:

General consensus with our members is that the wind is always several knots less than the forecasts (bbc, even predictwind) predict. Backed up by our on-site gear (http://cswindstation.duckdns.org:5772/public-dashboards/a81154154edc4a028f24ae236e1a7919)

Even the direction often ends up being 10s of degrees away from 'expected'. At which point, as John says, the forecasts are taken just at hints of the coming trends rather than absolutes!

Remember that surface wind is forecast at 10 metres above ground level.  Friction with the land reduces windspeed as you get close to the ground...