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yoyo said...

In 2006 Sam Parker did a table top similar to Martin in front of us as Chris and I were walking back up the bank. In his case he got about 2-3 feet up, board stayed horizonal and he landed it. Initially I thought he must have hit a bit of chop that wasn't apparant. On reflection, the bank was quite steep and I think the wind bounced off the surface. An inverted microburst so to speak.

Talking to him afterwards, admiring his skill I got the impression he was just as surprised as us, froze for a second by which time he had landed, so he continued on as if nothing had happened.

Remember that moment well.
It happened down the bottom of the run as I was trying to slow down from I think a 42ish to 44ish run.

Biggest mistake I made was sheeting out to try to wash off speed. Only let the rig off a little and just lifted and hovered. It all happened fast but was lucky enough to think to sheet in again and get some well needed mast foot pressure back. Came down a little wiser for the lesson.

Sam.

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Te Hau said...

Ian K said...


So ground effect or not I have trouble seeing any wing of 1 square metre developing much more than 6.3 kg of lift efficiently in 30 knots. In attempting to generate 25 kg the air on the underside would be brought to a stand still or the air on the upper surface coaxed to do more than 50 knots, or a combination of both, unlikely, I can't visualise a flow pattern consistent with this.

6.3kg of lift would obviously never get the gear out of the water, so why does it happen and how do we hover it all for 1-300 metres at a time?

The most simple explanation is always preferred.

Amos, is climbing a hill, he's literally hauling arse. Half way up he visits a Liposuction clinic, (it's one of those mountain clinic/retreats) and he gets 20kg sucked out of his arse. Next, he walks into a detox centre and gets an enema, loosing a further 6.3 kg of half digested Macca D's..

Fatty Amos continues up the hill, but is not so fat anymore, and he literally feels like he is floating up the hill. Amos gets to the top and tells everyone he fells 100kg lighter.

But it was an illusion. Taking a little weight off the top is important..

I kind of like to share some of my experiences. About flying the fin, its a great feeling, but never brought me a awesome record or soo... In the past I had this magic combination fanatic falcon 91 + sonntag fins sl-s 34cm + RS:Slalom MK3 6.2, and with this combination I could really fly the fin (between 25-35kn).

It was possible because the fin was as soft as it can be in the tip, BUT still maintained good lift when flexed (otherwise it would be spinout when flying the fin). When compared to a normal bendcurve fin I could use SL-S 31-32cm... So extra lenght to cope with the flex when loaded.

The trick was to slowly balance my weight to the back and close the gap with the sail, somehow I could hang my weight for a very big % on the boom without pushing the board down. By the fin-sail pressure the fin bended and the board slowly climbed out of the water. It felt like the fin only lifted the boardweight, and the boardshape+airflow did the rest.

In almost lab-conditions I managed once hundred meter or so flying the board, and it was most strange to look at the tail (didn't know this is possible). Most of the time it was vice-versa 10 meters flying and 10 meters water contact.

But basically when flying the fin there was a little speed problem too, when entering a gust in flying mode it was almost guaranteed the fin increased angle of attack AND drag (with the most sensitive backhand). In other words to accelerate to higher speeds I had to make the board keep contact on the water. On topspeed pulling the board out of the water did give me the edge because you barely slow down even in much less wind (because on higher speed the fin has often enough lift/efficiency to do crazy things).

BTW the combination falcon 91- sl-s34 - RSS 6.2 did not my best speeds, BUT it was a very good distance-NM-hour weapon (easy on the legs). Lateron the same SL-S 34 did an awesome session with starboard futura 93 in highwind, but when I hitted a fishing net close to 40kn it was the end of my good carbon friend. But I have a new sonntag sl-s 34cm (updated model) waiting in the finbag for the moment I can buy a 90 liter board again [}:)]

I'd tried to get a rough estimate of the equilibrium forces on a board in the past, but with 4 forces,

1. Back foot
2. Front foot
3. Mast foot pressure
4 Hydrodynamic lift

there's just two many combinations that might give an equilibrium.


(gravity is a fifth force on the board but the c of g is so close to the mast foot we can combine those two into one)
And I've left out aerodynamic lift from the board because I'm playing devil's advocate on that one.

But a recent photo in seabreeze showed that with reasonable skill you can sail with the front foot free. Therefore I assume it must only be used for minor trim and we can neglect the force it applies. With 3 forces the whole lot becomes much easier.



I'd read somewhere on the internet (great reference) that the hydrodynamic force on a planing hull is concentrated very much towards the leading wet edge. This makes sense, parcels of water are just seeing a flat ramp. Once the initial change of direction has taken place, at the leading edge, the ride to the trailing edge is easy. Just as if the suspension on a car survives the hit of the leading edge of a flat ramp jump the rest is easy.


So I've put the centre of hydrodynamic lift between the rear foot and the mast track.



With the centre of hydrodynamic lift that bit further forward than we might imagine, we can see that the vertical components of all the forces on the board can be balanced in translation and moments if the weight is distributed equally between mast track and the back foot.

So that's about 40kg of downward pressure by the back foot and 40kg of downward mastfoot pressure, 80kg of hull lift right between them. (This is only the vertical component of force at these 3 points, but they still must balance. I've subtracted 10kg out of the equation for sail lift).

But 40 kg is a pretty easy job for one leg, ( OK, the lateral fin reaction makes that sqrt (40^2 + 30^2) = 50kg) , but just walking about requires 80 kg every second step, so I'd say it's easy to get the impression you're floating along. Mind you after trying to go flat out for a Nautical mile on one tack the back leg gets a bit tired.

Any nose lift would relieve the back foot of more pressure, but I don't think it is needed to explain the lightweight feeling.

Today for 15 minutes,i tried a 30cm vector slalom fin in my sonic 90litre,60cm wide board, in powered up conditions of around 15-25 knots.

It provided insufficient lift, felt i was sailing side-ways,and going much slower.

Could not maintain lift of windward rail for more than few seconds,so board was sitting very low in the water.

Swapped over to my 34cm vector rockit fin and was able to lift windward rail and surf the chop easily.

Tried a 36cm vector rockit fin in similiar strength winds with same 5.6m sail and was going slow. My ankles were getting overworked trying to control the excess lift.

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Ian K said...
.......
So that's about 40kg of downward pressure by the back foot and 40kg of downward mastfoot pressure, 80kg of hull lift right between them. (This is only the vertical component of force at these 3 points, but they still must balance. I've subtracted 10kg out of the equation for sail lift).

......., but I don't think it is needed to explain the lightweight feeling.

You're starting to make sense of this now methinks Ian.

But what if your back foot downward pressure is only say 20 or 30 kg like when you put as much weight as you can into the mast foot - that lightweight feeling when you feel as if you are lifting and pushing sideways (against the fin) with the back foot?? Like when you bear away, hanging out as wide and as far back as you can.

That would/could mean 50 or 60 kg mast pressure downwards. Is all of this counteracted/balanced by hydrodynamic lift?

The front foot often has no downward pressure but is there as a balance/stability thing. It also is the anti-catapault security measure - for when you feel that swing & lift ....then panic.

Did I say lift?

So what is this lift that our weight is counterbalancing?

Oh!
.
.
Maybe the lift is coming from the sail [}:)][}:)]

But hasn't someone just spent the last 4 pages of this thread PROVING beyond ALL reasonable scientific doubt that this is impossible?

Must be time for a Xmas drink

Cheers everyone, luving the discussion...to be continued

Lordy. Don't anyone tell the wavesailors about this thread - it's pink-in-the-face embarrassment, although informative and entertainment for most of us.

This may sound selfish but hell I'm glad someone else crunches this stuff so I can spend most of my precious time on the water enjoying the thrill that is windsurfing. Thanks a million. Respect!

What's I think is perhaps most important thing to understand is that the body's relationship to equipment is by far the most unique in all sail craft - the key to the attraction of windsurfing...and kitesurfing to a degree.

Happy sailing and happy holidays!

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Wineman said...

Ian K said...
.......
So that's about 40kg of downward pressure by the back foot and 40kg of downward mastfoot pressure, 80kg of hull lift right between them.

.......,

But what if your back foot downward pressure is only say 20 or 30 kg like when you put as much weight as you can into the mast foot - that lightweight feeling when you feel as if you are lifting and pushing sideways (against the fin) with the back foot?? Like when you bear away, hanging out as wide and as far back as you can.

That would/could mean 50 or 60 kg mast pressure downwards. Is all of this counteracted/balanced by hydrodynamic lift?

The front foot often has no downward pressure but is there as a balance/stability thing. It also is the anti-catapault security measure - for when you feel that swing & lift ....then panic.

Sounds like you're still OK with a 3 forces on the board assumption Wineman. If you reduce backfoot pressure to 20 or 30kg and transfer it to mast base pressure of 50 or 60kg then that see-saw pictured above about the board's centre of hydrodynamic lift is not balanced.

Could be 3 possibilities, because the moments must balance.

a. It's impossible to transfer that much weight to the mast base.

b. The centre of hydrodynamic lift is further forward than estimated in the
diagram. ie. It's closer to the mast base than the back foot.

c. There is in fact a strong aerodynamic lifting force, 10 or more Kg, up near the nose.

Guess I could believe 10kg nose lift. Just thinking how board trim affects jumps. Windward rail up prolongs the jump noticeably whereas windward rail down will send you into a power dive.

Got my board up on the fin for a short time today and I don't believe any of the vertical lift was due to the fin. It's a 45deg, 19.5cm, 9% chord to thickness timber and carbon. It's extremely stiff I'm sure my 70kg (+ all the stuff I've got on) isn't enough to bend it more than a mm or so.

Does this count?

Interesting photo as the camera POV is probably that of the wind. In this case the sail is at 45 degrees so I guess there is as much lift as drive...

In my opion if you are flying the fin then its what i refer to as secondary planing in otherwords fully powered up, without tail walking if you are flying the fin then everything feels perfect and very smooth, if it feels fast and your not feeling an easy sensation you are out of control and normally going slower than normal.

And basically there is no mathematical formula,boom height harness line length, fin angle, rocker or footstrap position and so on its time on the water and dedication to sailing really powered up.

I am not trying to preach because i am not qualified in this way but i am long enough in the tooth to know how it works.

Merry xmas.

Got an early mark from Xmas dinner yesterday and caught a good fresh noreaster. I believe at times I was doing what is known as "flying the fin". I made a point of estimating how much back foot pressure I was applying. If just standing around is 40 kg per foot, it was significantly more than that.

Judging by where the spray exits, I'd say during most sailing the wetted area begins just slightly behind the front footstrap. The interesting thing is that during times when the wetted area moves back for a few seconds, you have to apply more back foot pressure. This is consistent with the simplified 3 forces on the board diagram. If the centre of hydrodynamic lift moves back towards the back foot, weight must be transferred from the mast foot to the back foot for the see-saw to remain balanced.

If the board lift was a combination of hydro-aero dynamics then, when conditions got right for "flying the fin", wouldn't that centre of board lift be moving forward? In that case the back foot pressure would have to lighten for the board to remain balanced.

Have a think about how your back foot pressure varies next time you are flying the fin and report back. But be careful, over thinking it on the water will cause you to crash.

Photoshoot at the 2010 Formula Oceanics Hawks Nest. Right up at the sand spit very flat water, from memory around 15 - 20 knots and 9.8m RS Racing, SB HWR.

I was deliberately pushing hard on the back foot, and trying to roll the board up to fly the fin for the camera. I got a little more then expected.....fortunately landed without incident.

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yoyo said...



Interesting photo as the camera POV is probably that of the wind. In this case the sail is at 45 degrees so I guess there is as much lift as drive...

Looks like his bum is below the wetted area. Therefore I believe this is not a sustained position..

The better pics would be the ones close to 45 knots on dead flat water, I'm bored of the 30knot scenario, I can do that on my waveboard with my eyes closed..



If having the fin at an angle for some vertical lift is good then why not put the fin in at an angle? Seems obvious..

think my question is best on this thread, just dont want to create another science fiction thesis report?! Wow!
Santa blew down my xmas tree this year! And I had a great 1 hr blast on my own little secret 2k flat water slalom run..in 10-30knt gusts! Seriously windy at times!
Possibly sailed the fasted I ever have, and most definatley on my port run on several occassions was basically sailing fin only..and on one run so much so that eventually the board (and I!) just continued lifting out of the water resulting in an almighty high speed forward loop! knocked the xmas stuffing out of me.

Weapon of choice, my one design ..one board one sail, one fin, quiver!

JP Super Sport 65, 6.3m Loft Blade, correct mast curve, JP Stand fin,

Sail about centre of the track (not that there is musch track to set in) rigged well plenty of downhaul, tad out haul. actualy the sail was never the issue I had it under control and never felt like it was too much... the water was pretty flat and small chop if you could even say that, I was muscling the gear though at this point in a massive gust...long lines, way outboard stance, back straps in board a little (trying to get better at sailing off the wind for LOC) my weight is 85kg and I'm 6ft, was heading more off the wind than accross it.

I'm thinking needed a smaller more dedicated slalom fin? or it just really was freakin windy?!