Side note, if there is such a thing as Heavy Ground Effect under a windsurf board, has anybody ever tried watersking with a windsurf board, if the HGE Hypothesis is correct it should be possible get True Flight. No?
Kinda like a Hoverboard? Except they don't work on water (unless you've got power)
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Naa Barn, waterskiing's never gonna hover it.....
'cos you haven't got the lift from the sail !
If you look on the pic.of West Kirby,you see that the board in front looks
against the wind. Maybe 10°. I worked a 22cm fin so,that she is in front
on the right side of the tuttle base.
The end of the fin is on the left side of the tuttle base. angle is 7°.The fin do
the work. How always,but the board looks to the finish.We need right and left fins.
It works good. I reduce a little the brake of the board. Wolfgang
here is a real simple test - next time its 30 knots of wind just put your board on the roof racks and point it into the wind - wait and see how long the board stays there
dont forget to bring a tape measure to you can measure the distance away from the car it lands.
Putting it on the roof won't have the HGE.
A board is just over 1m2 in area, there is a limit to the amount of lift it can generate in free air.
This is a Shaka in 30knots, half way around your body weight is committed to the sail, and the nose of the board is pointing directly into wind, starting at a small angle, then an extreme angle, and then back down again.. I can guarantee there is not crazy lift from the board in free air, it's very controllable (the sail, less so).
The wing loading for a hanglider is 6.3 kg per square metre
en.wikipedia.org/wiki/Wing_loading
and a typical hang glider of 14.4 sq metres has a best glide speed of 50 kph ~ 30 knots for an all up weight of ~ 110 kg.
deltaplane.brozs.net/doc/scandal_spec.pdf
The picture of a hanglider further back in this thread shows the similarity of design to a windsurfer sail - so a similar wing loading is quite likely - ie about 30 kg of lift all up. But I think we'd arrived at a figure something like that earlier on in the thread when we put the idea of being able to be largely supported by the sail to bed.
The amount of lift that can be provided by the board is still being sorted out, but on the way an application of the bluff body drag formula might be useful. Drag = Area * Cd * 1/2 rho V squared.
This is the force the board feels once the nose is fully lifted and it's square to the wind.
in 30 knots = 15 m/sec, air density of 1.2 kg per cubic metre, the Cd for bluff bodies is usually around 1. The drag formula gives 13 kg for a 1 sq metre board. That's about what it feels like trying to get it on the roof of the car. It's all drag of course so not very useful, but interesting to compare that with the aerodynamic lift of the hang glider in similar wind speed, ie. 6.3 kg/m squared.
So the question is. If a purpose designed wing in 30 knots generates 6.3 kg of lift per square metre how much more lift, if any, does a compromised wing ( the board) develop in 30 knots with ground effect?
The previously referenced article
www.dsto.defence.gov.au/publications/2058/DSTO-GD-0201.pdf
gives a clue but is not definitive
"WIG craft have been championed on the basis that they are more efficient than
equivalent aircraft and quicker than equivalent marine vessels. The efficiency argument
is somewhat speculative. While theoretically an improvement in efficiency is gained by
flying in ground effect, this efficiency is reduced by design compromises required of the
WIG craft. Such compromises include strengthened hull structures, reduced aspect
ratios and larger control forces. The degree to which total efficiency is improved can
only be determined by the direct comparison of optimised designs of equivalent WIG
and aircraft. Only through such a comparison would the value of the improved
efficiency and the cost of gaining this efficiency be determined."
^
I'd used a bluff body in reference to the previous post commenting on how they get hurled off a car roof, Cd rather than Cl. But the formulas have the same form, no argument there.
Not sure what you mean by
This conversation is complete double dutch to me.
Not one sentence can i make head nor tail of.
All i know is,if you sheet in,in the gusts,you go faster or you go over the handle bars.
yep mike- thats looking at it from the other angle... build it and see. The fastest boards from the last 5 or 6 years were designed to exploit aerodynamic nose lift.
We're all luvin' it Barn.
That was my weird way of encouraging you to continue.
So far 3667 reads of this topic - it's got us all (except Petermac) interested.
^^ hahah I don't need encouragement!
After the 30knot scenario broad reaching in 25knots, I can move onto sailing deep downwind at 45knots in 45knots of wind at SP on a 40cm wide board, that'll get interesting..
If my injury doesn't heal by next year I may have to take up this leisurely sport of Speedsailing, then I won't be all talk..
You need evidence?
Sandy Point 2010.
Mr Love takes off from the East bank on the Luv Mus Small, crosses the inlet and bears off hard down the run just at the end of the dunes. The wind is quite square , probably 30 knots and the water super flat. Next thing I know is Mr Love . 4 to 5 feet in the air, still in normal sailing position, nose of the board maybe 20 to 30 degrees from Horizontal, but no looping here , just sailing 4 feet off the water.
Mr Love bails, lands on his arse and if not for the wetsuit would have received intense colan cleansing. GPS reported the incident took place at 41 knots of board speed.
Witnessed by Seahorse who was standing on the bank right where it happened and who could not stop laughing for the rest of the day.
Where did all that lift come from????? 90kg me , 5 kg board, 10 plus KG rig, 4 feet plus in the air, almost mirror flat water, no ramps to launch off here.
No hypothesis from me about what type of lift it was, forward motion transfered into vertical lift ect ect, thats why I asked you rocket scientists 'where did that lift come from", but it happened. And yes it didn't last long, I s..t myself and bailed.
Where did you get the latest photo of the PiT crew, didn't think we had released that one yet!!! Knew I left my sandles somewhere.
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.
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?
remember there are other forces involved than just the board creating the lift.
When the sail is raked back and the gap between the board and sail is closed the sail itself is generating significant lift which is pullign up thru the mast base.
The amount of lift needed from the board itself is probably failry minimal. on a lot of baords the biggest problem speed sailers have is actually keeping the boards to stay down and flat.
You also need to take into acount the amount of lift the fin is generating.
looking from where the force is applied by both sail and fin the sail would be doing most of the lifting and the fin only really lifts the last 25% of the board.
Getting this balance right would give the board the right angle of attack to generate the ground effect that makes it feel like its hovering.
Too much angle and the board will flap up and down - too little wont see enuf air getting in under the board to do the job and it will slam thru chop.
At these speeds too there is a small amount of Vortex Shedding coming from your arms, legs and even body.