Well as I understand it, assuming a steady wind, there is an optimum sail size for the conditions, going bigger or smaller than that will slow you down.

You can only extract the amount of power from a sail that you can counterbalance with your weight.
Once the sail is big enough to give you this maximum power, going bigger only adds more drag, and decreases control.
If you go smaller than this, you aren't getting full power.
Probably a bit over simplified, but that's how I understand it.

Travelling dead down wind is a special case though, as the faster you go the less power you have, so you can't go as fast as the wind unless there's 0 drag.
Not sure about kites, but on a windsurfer this isn't an easy direction to travel. It's much faster and easier to go on a broad reach. The fastest is supposed to be 130 - 140 degrees. And on speed gear you can do about 1.5 X windspeed, ie in 20kt winds 30kts is possible, but I don't think that's linear, nobody's done 60kt in 40kt winds

Hi Buster,

You may need more downhaul. This lowers the centre of effort (where the sail pulls from) which means that you will get catapulted less. The sail also reacts to gusts better.

Get one of the faster guys to check out how your sail is rigged.

busterwa said...
so what ur sayn is that there is no advantage having a bigger sail to produce top speed if your body mass cant offset it

You'll just be overpowered, out-of-control and catapulted. Treading water a lot.

is it fair to say that if i cant offset my sail in the wind drop down a sail size

Yep. Definately. This is why there are different sail sizes. Generally you'd want at least two sails in your quiver.

just when i went out a few weeks back there was people using the same sail as me but everytime i sheeted in it would catapult me, i had trouble getting back in i couldnt waterstart or uphaul and when i did ended up gettn slung/catapulted even when i harnessed in

A textbook definition of being overpowered!

in sayn that i was under the impression that if i drop my sail size to meet my body mass i wouldnt produce the same speed os the others

Anything is faster than just treading water.

i guess i was under the impression that being catapulted meant that i was tryn to meet fullspeed to quickly.

Quite possibly, but more likely you were simpy using too big a sail for your skill, weight and conditions.

There is also the factor of how you rigged it. Suggested downhaul and outhaul are printed on your sail as Luff and Boom respectively. Increase both to depower the sail to match the conditions.

Also (correct me where wrong) for faster, downwind runs more downhaul, less outhaul, smaller fin. Opposite for upwind.

Search youtube for something like "windsurf rigging" and you'll see plenty of examples on how to do it.

if im travelling dead down wind as fast as my sail/kite can go and its blowing 20 knots does that mean ill be travelling down wind at 20 knots at maximum peak speed?

Dead downwind, yes. <dons firesuit>

As you approach 20 knots for this example you apparent wind will drop lower and lower. As you hit 20 knots you will be doing same speed as the wind, apparent wind will be 0.

Things are different at, say, 135 degrees.

Things may also be different if you are at 135 degrees, going faster than the wind, and then bear off to 180. In practise you can certainly go faster than wind at 180, but not for long, for me anyways.

It's all about the apparent wind.


Where:
V = velocity (boat speed over ground)
W = true wind velocity
a = true pointing angle in degrees (0 = upwind, 180 = downwind)
A = apparent wind velocity

More:
http://www.physclips.unsw.edu.au/jw/sailing.html

Land yachts travel many times faster than the wind by using the apparent generated.
Often sailing at 60-80 km/h in 20km/h winds.

Inertia is the idea that an object keeps moving unless acted upon by an outside force. Much of the initial work written about inertia was done by Isaac Newton in the 17th century and became known as his first law of physics. It is one of the basic principles of physics and has a number of real-life applications, as it helps to explain much of why objects behave the way they do.

The idea of inertia is quite simple. Inertia is the principle that is that an object will tend to keep moving at a certain speed and direction unless something else acts to change it. That external force could be almost anything. Friction is a common external force that can change the speed and direction of a moving object. Many other objects can also affect motion, however, from gravity of the earth's pull to another person. The key idea is that an object will keep moving (or stay still) unless something else influences it otherwise.

thankyou evilpanda 10/10............first seabreese physics lesson

there is a method in the madness......................it involves some higly equated maths transposition.

Ian K said...
Why are eighteen footers always sailing upwind?

I understood that as relatively speaking.

Common example:

Wind is coming from the North at 20 knots.

I am on a reach going East, at 20 knots.

The apparent wind, for me, is now NE at 20 knots. Relatively I am sailing upwind as the wind from my perspective is coming at me at 45 degrees. ie on a windsurfer you are (when going) always sailing upwind, relatively.

In a fast boat, there's no point going straight downwind: you can never go faster than the wind. So you travel at an angle..."

It's a fine point whether you differentiate between going directly downwind and tacking downwind.

If by tacking downwind you beat the parcel of air that started at the same time to the bottom of the course, you have sailed downwind faster than the wind. - you could release a smoke bomb to test this -

Maybe it's not possible in current model boats, but it's not a theoretical impossibilty, we just have to make better boats with lower drag.

http://www.physclips.unsw.edu.au/jw/sailing.html
On that link scroll further down to the section "How can boats sail faster than the wind?". You have to sail at an angle though as if you are going dead downwind there is no wind for the sail to deflect, thus 135 degrees is about the fastest angle.

</My 2 pesos.>

^ what d'ya expect for 2 pesos?

well done buster, it works to a certain extent the opposite way as well, when under powered you can point a little downwind to help you get up.