Hi,

Someone posted this on the phantasm foil discussions:

But the issue with big fat wings like an i99 is that they hold you back because they are much less efficient, and therefore you can't build apparently wind apparently wind. Apparent wind = more power = more speed = ability to fly through lulls.


Can someone help me understand how buiding apparent wind can really help with ability to fly through lulls and get more power ? This terms is thrown here in there in conversations about high aspect foil and how it is benefitial to glide through lulls, like this would be a "magic" source of energy...

I understand that a high aspect wing is more slippery, and allows to achieve a higher top speed, which both greatly help to reach the next gust or fly longer. But this is has nothing to do with apparent wind. It has simply to do with "stored kinetic energy" and friction.

The way I see it, the true benefit of apparent wind is comfort and stability, since it tends to reduce the impact of fluctuations of the true wind felt on the sail.

Besdies a more slipped wing, from a sail perspective and power through lulls, it seems the only thing that can be done is use a Camber sail, which would ensure to keep the most efficient profile possible in the sail when the wind drops below 8-9 knots. (i.e I believe any non cambered sail starts to loose it's profile around that threshold, regardless of the sail size). Which would tend to explain why cambered sail are only relevant for larger sizes : small sails are used in higher winds for which there is always enough true wind to keep a good profile...


Am I missing something here about this apparent wind concept ?

My 2 cents:


i think to a degree youre pretty much right. What the OP is talking about is higher speeds generating more apparent wind speed, and a greater shift in apparent wind angle when travelling downwind. All useful if youre travelling upwind/downwind and you want to get there fast.

But a bigger/fatter wing will have a much lower stall speed, so youre trading peter for paul if youre talking soley about maintaining flight. But as you say, a cambered sail really helps. A non cambered sail really doesn't in this scenario.

Edit: i can't say i've seen a fat slingshot wing yet. Theyre big but relatively speaking of what they are, theyre all reasonably slim. Maybe the 99 is fatter?

Well, apparant wind won't help you in a lull, if anything it will slow you down because there's more drag from the "induced" wind at higher speeds. However, what swoosh said is the following:

"Apparant wind = more power = more speed = ability to glide through lulls"

Which I agree with in the sense that going faster increases the odds of being able to cross the lull.

However, the magical cricle of windfoiling "more speed = more apparant wind = more power = more speed etc.etc.etc." which is more effective with efficient / fast wings, has little to do with it in that regard when the lull has been entered.

This has a lot to do with vector physics. If you have a 10 knot cross wind and are traveling at 10knots, your new apparent wind is at 45 degrees.
With big fat large wings, you do have a lot more drag than high aspect wings, so: having a cambered sail will keep the power up as your apparent wind is less. (16 knots on a 10 knot cross wind, for example)
With high aspect wings (racing foils), you'll have a high degree of apparent wind, so you'll need a more flattened sail. I've found I can do 20 knots on a 10 knot cross wind.

Lulls and what to do: When you go from a cross wind of 10 knots into a 4-5 knot lull, your apparent wind might be almost straight on. You'll lose the lift (power) in the sail (it becomes pure drag). So you need to bare off, turn downwind, etc, to keep the the right apparent wind to the sail to keep it powered. Hence the reason you can point extremely well in gusts. Racing foils with flatter sails do much better going up wind, probably 10-15 degrees better than a big foil with a cambered sail.

There is a difference between HA and LA foil wings when it comes to "glide". The further you can glide the more chance you have to get through a lull and stay on foil.

Here's a comparison regarding the "glide" capability of two large foil wings -- one high aspect and the other low aspect. The difference was 5 metres (a little over 16 feet) of extra glide in favour of the HA wing. So there's more to getting through a lull than just your apparent wind.
To some extent (under certain conditions) the HA foil is a "magic" source of energy (additional glide).

Low drag foils respond to pumping better, if you can pump.
Bigger foils can foil at slower speeds.
So where is the balance for you?

Poimax05 I have sailed alongside guys with full cambered sails and race foils (~1000cm2) and I'm on a Naish 1570cm2 wing (similar to design of SS i76/i84) in light winds (10-12 knots). Our conditions are often 10-12 knots with gusts a few knots more. So you can use the gusts to get up and sail though the lighter winds. I also use a Naish lift 5.7 which is a low performance sail - grunty but no top end. In really light winds, what I found was I was able to pump up onto the foil in winds a little lighter than them (10 knots). Once we were all up and going, they had MUCH higher speeds than me as their foils are so much more streamlined and efficient for speed. I THINK I would come off the foil a little earlier than the 'race foilers' when the wind died off (sub 8 knots) and I think it's because they had a much higher speed than me, so were able to sail through the lulls easier. My stall speed would have been lower than them, but they could generate so much more speed than me, that they had to hit an extended lull for them to loose enough speed to drop off the foil. Again, just observations.

Yeah it's not really meant to be a scientific formula. Pretty much what the guys have said above is what I meant.

You are 100% correct, once you hit a lull, you are basically relying on the speed that you have and how slippery the foil is, plus also how well you can pump.

The apparent wind part is more about how having efficient foils, and "apparent wind" sailing, is what helps you get more speed so when you do enter the lull you can travel further. More speed also means more energy for maneuvers.

The wiki article on it is OK, tho maybe a bit dense.
en.m.wikipedia.org/wiki/High-performance_sailing

If you ever raced course (such as formula) you know all about apparent wind. Yes, it is vectors.

With foiling nowadays, we also have the vertical component. One slick way to glide through a lull is to stay really high (a long mast helps), then glide downhill through the lull. This also works when pumping to get going. Pump to get high, then glide downhill to build speed.

Always a balance between power, glide, pumping ability, drag, and reading the wind.

All i know is that there is a wind strength where the iqfoil squad are going at high speed and never dropping off the foil, yet I cant sustain flight on i99 with any sail up to 7.5. Huge differences in skill, weight, and fitness as well as gear - but if you ask them the answer is all about low drag, high speed, exploiting the apparent, and the right type of judicious pumping.