Hi,

I was out at the train today, having a lovely time pottering along, watching everyone blasting past me, occasionally getting onto a plane but mostly happy just to be shlogging. After a couple of hours of this I got up the confidence to go for the straps... and MY GOD don't you move when your feet are locked in!!!

Gotten Himmel!!

My question is: why Why do you seem to go so much faster when the feet are just in a different position? It's not like they're touching the water, my weight is still the same, the board would have to displace the same amount of water (I'd think). The only thing I can think of is that maybe it's related to wetted surface area, when you're in the straps then only the back of the board touches the water and therefore doesn't have as much skin friction... but then again I'd expect a flat board to go quicker than one that's cocked up at a bit of an angle.

'Tis weird this windsurfing physics

Well said Oldie,

What is even more amazing is the difference moving your straps back and out does. Even more speed.

then start sliding the mast base back towards the end of the track.

Tail walk! bring it on

then it's time to start shortening the harness lines, and start lifting with the front foot.

Then start bearing off, sometimes the walk back is pleasent reflection time.

Best winds
Tony L

certainly with the force of the sail taken by the harness, that leaves the arms to just trim for max speed.
Feet in straps allows much more force to be transmited thru sailor, (more than sailors weight, I know this from experience, wrenched front foot out of strap to gybe, just got mega catapaulted).
Also feet instraps allows for better board trim, you can lift the windward rail, until fin lift takes over and does it for you.
I find it easier to pump in the straps, the feet can get involved as well. That's the other thing of course if the wind is marginal, it might not blow you onto the plane, but if you pump there, there's often enough to stay there.

Harness + straps = speed, why?

Because

Out at the train today I got another piece of advice from some dude rocketing past me... "Sheet in!!" I had a chat to him at the beach, and he said that I was way undersheeted, I needed to pull the sail in, then rake it back for max power.

So I tried it, and whaddayaknow? It works!! I can get into the straps sooo much sooner now, then with my weight forwards, the sail raked back and sheeted in, I find myself going the same speed as all the other guys. I even beat a guy with a 7m sail, when I was on a 5.7!!!!
YEEEEEEEHHHHAAAAAAA

That machine gun noise is getting a lot more frequent now, and I can't believe how well you go upwind when planing... doesn't make sense to me but then again I'm just going to have to resign myself to not understanding how the "fiendishly efficient machine" works. Just go with the flow

Nebbian,

perhaps we are forgetting the apparent wind effect and the lift it supplies.

As the wind flows faster over the sail the Bernoulli forces increase - these are the same forces that give an aeroplane lift. The wind flowing over the outside of the sail travels a greater distance than the wind on the inside of the sail - this creates a pressure differential (low force in the sail luff) which manifests itself as lift (desire for pressure to equalize from low to high creates a force that is directed away from the sailor (inside luff pocket to outside luff pocket) and this is transmitted through the mast base (and our body when we are in the straps) Hence when we are in the straps our ability to harness the forced moves from one point to 2 or 3 and we have greater control and usually greater power delivery and ultimately greater speed.

(the faster you travel the more the lift - hence thto get the board moving to provide lift from the Bernoulli forces.

This force is counter balanced by the fin by directing our weight through the fin which needs to be be big enough to overcome the sideways force and convert this to forward motion. Too small a fin = lots of sideways "drift" If the fin is too large then you can harness too much power and the sideways counter balance of the fin can lead to a vertical lift- which leads to "tail walking".


This also means that if you can work the board ont the plane through pumping or running downwind you can maintain the plane as the lift from the sail generates its own apparent wind. This is fine until you decide to gybe, get halfway through the gybe and discover the wind had disappeared.!!!

This is why luff pocket shape is so important in a sail and why cambered sails which induce and hold the surved shape have more power than RAF sails - especially at the top end and when sail >110 degrees off the wind. ALso why balancing fin and sail size is so important.

Of note - I think of the sail twist at the top of the leach relating to the same principles. In normal conditions, the sail has no/minimal twist providing max power, as the power - wind gust hits the leach twists to reduce the power load from a gust - in this manner the twisting leach sails provde good low end power and high wind control.

By my scientific education all this stuff can be derived from 1st year Uni Physics 101 and if you require the high level very fine tuning a higher degree in maths may be required.

Dr J.

Neb, who gives a rats a..e why, just get in the straps, hook up and yell yeeehaaaaaa!

quote:

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Originally posted by nebbian

The problem as I see it is that lots of people have a lot of experience with windsurfing fast, they know where to put their bodies to go fast, but have little idea of the physics behind what's happening... Take the guy in the shop where I bought my windsurfer, who tried to tell me that if my arms were bent, they would heat up, and that was because I was absorbing the power in the sail into my arms, therefore I would go slower as the 'power' was being 'absorbed'. "It's simple physics", he said. Right end result, but he had no idea what the actual mechanism was. (btw the heating up is because of the way muscles work, it's nothing to do with whether a solid object is bent or not, no power from the sail is 'absorbed' in your arms)

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Actually he is correct. Nebbian you just started windsurfing and still have your feet infront of the mast. Stop questioning the experienced people who know what they are talking about. If you want to go fast, get a harness, beach or water start head down wind get in the harness then footstraps and stop wondering why the ocean swell is angling at a different angle than the other day and enjoy it!

quote:

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Stop questioning the experienced people who know what they are talking about.

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Tsk tsk tsk. I see we've another conformist in our midst.


Wise, that Vando is.

There are those that blindly enjoy, and those who want to know why, and neither understands the other all that well.
Me, I want to know why!!! then you have a chance of improving the experience!!
Good on ya nebs, keep us thinking! Some times a newby can give a fresh insight

Ben,

I have given this some thought and even reviewed my lecture notes from physics 101 - Apologies to Melb Uni - you taught me right - I just remembered incorrectly.

However, the principle was right and so was Bernoulli. The curvature (or flat sail) at and angle to the wind will provide differential pressure across the sail to provide a net force on the sail.

We use our body weight to counteract this force and the 3rd force (in a very simple approximation) is the lateral force from the fin.


The sum of the forces equals a nett force foreward which provides propulsion.

Footstaps at he back of the board add additional load points to transmit the power. Think of the sail a a lever. In this scenario, you can acheive greater leverage the further down the boom you apply force.

another way to think of this:


Think of that old playground favourite a "see-saw"

If you put 100kg on one end of the see-saw and try and stand in the middle of the see-aw around the pivot point it is very hard to get that 100kg off the ground. If you now put 80 kg at the pivot point and gradually move it to the other end of the see-saw the amount of effort required to lift and move the 1000kg weight at the other end of the see-saw is reduced the further out you go.

Putting you feet in the staps allows you to lever you weight further away from the board. - Hence you can get more power from the rig


- J

aahhhh Nebbian, I now know who you are my friend!

mate you are doing just fine out there, believe me! and I must say I have noticed an increase in speed from you in the past week especially, mmmm, been studying the advice given here I see, well done.

continue the speed!!

md

quote:

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If I'm standing forwards then I have to sheet out because otherwise over the front I go, if I stand further back then I can sheet in more and have more force pushing me forwards, 'cos I can balance it, being further out on the 'seesaw'.

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There's also the shifting center of lateral resistance effect, when you're forward going slow. the rails are a big part of lateral resistance, so it's center is forward as well, so the mast has to be more vertical otherwise you go too far upwind. As you move back, the center of lateral resistance also moves back as the forward rails come out of the water. This means the mast has to lean further back or you go too far downwind. So as the the gap closes, increasing power, wetted area decreases, lowering forward resistance. As you go faster fin also develops more lift, shifting center of lateral resistance further back still. So the faster you go, the faster you can go! For a while anyway, till all the forces reach equilibrium.
And while I'm on fin lift, you're right about it having to be off the vertical to have any horizontal lift, but this does happen, there's a certain amount of flex for a start, ( the fin's lift is counterbalancing the sail's lateral force,) and once really powered up it also forces the windward rail up against the sailors weight. There was a link on rec.windsurfing to a photo of a formular board almost on it's rail and the fin almost horizontal, they were talking about fin loads I think.