ma said...

Soft.
When I see my wrist do that, I just go faster.
My neck did that once. Should have seen how fast I went after that.

Ian K said...

You wouldn't think water was that hard, but then 10 metre tower divers who hit the water at sqrt(2 X 9.8 X 10 ) m/sec = 14m/sec = 50.4 kph often use wrist braces just in case they don't break the water exactly as planned. ma was doing 75kph!

Hard one to calculate if possible at all. These divers are hitting the water square on, he was traveling parallel, Would you rather ride a bike into a wall at 50km/h or high side/drop it at 75. But then the divers use a water jet or air stream to break the surface tension (makes a huge difference) and the sailer has wind chop so......The divers are in fresh water, salt water has a higher density. The divers are free falling where if your still hooked into the harness you get driven in hard, I've had a few instant headaches. Nit picking I know but was bored
I'd put my money on Macrosciens theory that he hit something else first.

Ian K said...
[

Drag = 1/2 * 0.015*1000*21*21 = 3300 newtons = 337kg

assuming that surface area of the foot is only the same as hand ( I suspect is bigger ) or head equal to two hands
tower jumpers should brake all legs and heads every time they enter the water.
Not all pools always had bubbles and some jumps even higher then 10 meters
-what even more interesting kiters permanently jumping up and down landing on their no so small boards should be squashed

Macroscien said...

Ian K said...
[

Drag = 1/2 * 0.015*1000*21*21 = 3300 newtons = 337kg

assuming that surface area of the foot is only the same as hand ( I suspect is bigger ) or head equal to two hands
tower jumpers should brake all legs and heads every time they enter the water.
Not all pools always had bubbles and some jumps even higher then 10 meters
-what even more interesting kiters permanently jumping up and down landing on their no so small boards should be squashed

Without confusing things with calculations, I would guess that's why the divers would enter the water with their fingertips, instead of with a flat palm...

If you haven't noticed, kiters are still some what powered up when they land a jump. They are not just ditching the kite and free-falling.

Ian K said...

Yes Macroscien, it is paradoxical. But you've checked the equation haven't you? You're always up for an equation check.

yee, our intuition is sometimes missleading. I always though that better jump from dead air plane to the water then on the ground. Wrong. On the ground you may land on the tree or shed. On the water you are dead for sure.
I wonder what is max survivable high jump to the water for average person.
Anyway that is quite interesting topic maybe for next thread?
What do you think Ian?

FormulaNova said...
divers would enter the water with their fingertips, instead of with a flat palm...

and there I can see a problem..
open hand with fingertips pointing straigh ahead ... one mistake and it changes to open hand flat dragging this awful 350 kg
..I would prefer to punch the water with fist..
10m jump ... I never tired I am afraid of heights

Macroscien said...

FormulaNova said...
divers would enter the water with their fingertips, instead of with a flat palm...

and there I can see a problem..
open hand with fingertips pointing straigh ahead ... one mistake and it changes to open hand flat dragging this awful 350 kg
..I would prefer to punch the water with fist..
10m jump ... I never tired I am afraid of heights

It seems I was wrong anyway, and that they do actually try and enter with flat palms, albeit reinforced with the other hand.

Sailhack said...


One hand raised with clenched fist - waving's not really the go, but hard to resist when trying to get someone's attention.

One arm up with clenched fist is widely used by waterskiers to increase their visibility when they fall. It means "I'm perfectly OK but I am in the water so please don't run over me with your ski boat."

Waving the arm will be more noticable to people on shore and sustained slow waving looks different to most people than a hello wave. Afterall, kids are all taught at school that it is the signal to use when they need help in the surf.

+1 what Mat said.

If there is little or no movement, or erratic movement, get there QUICK!

If someone is badly injured they probably won't be able to give ANY signal!

Any time you witness a crash, watch and monitor. If possible get there ASAP. If all is good, nothing lost. If not, seconds may count!

For some one sailing past and checking up on you, thumbs up (especially with eye contact) gets the message across that you are OK, but if you think you see this signal, always double check your first impression to make sure you got it right.

Ian K said...

Yes, up to the point where they talked about breaking surface tension. A wet paper bag has just as much surface tension and they're famously easy to break through.

I'm not sure what you mean by that.... soap / bubbles / etc do reduce the surface tension.

Water acts like a non-newtonian fluid at high impact velocity, part of which is attributable to surface tension - while a wet paper bag generally doesn't as it contains fibrous material which (among other things) changes the density and surface tension.

The reference to the Busta+hammer was to show some experimental evidence to support "fall from high - hurts"...

Bubble machines are great for diver training. They had one at the Sydney Aquatic Centre when I was there last as a sportsperson's parent.

The fine water jet directed on the water surface, which is allowed for competition, is a visual aid, to make it easier for divers to spot the water surface. As far as I know it doesn't make a bad entry less painful.

Indeed - the water jet doesn't make any appreciable change to the surface tension. The bubbles however do change it and it reduces the density of the water for few feet deep (making for softer impact).

Macroscien said...

mathew said...
The bubbles however do change it and it reduces the density of the water for few feet deep (making for softer impact).

Lets do such imaginary experiment. We inject bubbles under the ship, so all sides will be covered with emerging bubbles.
If the ship:
a) will sink deeper -because of lower water density
b) stay the same depth
c) emerge even higher
?

Wouldn't it sink because the density of the water is reduced dramatically (depending on how much aeration though). Could be an urban myth but I recall somewhere that boats have sunk when undersea floor vents have let massive air bubbles erupt and boat is directly over said bubble.

Had to look up non-newtonian fluids, water wasn't listed. But it looks like a non-newtonian fluid just means the shear force is not linearly proportional to the rate of distortion. ie. the 2nd term on the right hand side of the Navier Stokes Equation doesn't work as it should.



(This equation looks complicated but is just F=ma for fluids. The ugly looking left hand side of the equation just allows the mathematics to follow a particular fluid particle. The first term on the right is just the pressure force on the fluid particle of interest, the last is external forces on the particle of fluid we're following, usually gravity.

That 2nd derivative of the velocity in the 2nd term on the right is interesting. The 2nd term again is the viscous force on our particle due to velocity gradients in the surrounding fluid. At first you'd think the first derivative would be OK. But the 2nd is needed for the shear force because if the velocity gradient is linear shear drag from neighboring particles below = shear push from the fluid particles above above and our fluid particle gets no net viscous shear force so doesn't accelerate.

The trouble with the Navier Stokes equation is that even though it makes sense for
looking at one fluid particle swimming amongst all its neighbours, if you don't know what the neighbours are doing you've got a real problem. The Navier Stokes Equation is very difficult to solve.

Sorry got sidetracked by the non-newtonian fluids and tried to demystify the Navier Stokes equation. Took me ages to work out what that famous equation was on about years ago. Do windsurfers need to think about things like this? Maybe maybe not. I've got the red/yellow arrow day off work)

But back to our no.1 issue. The bubbling reduces the overall density but the biggest bonus is that the bubbles give it some compressibility, which means the above form of the equation is no help at all.

i dont think its that unreasonable to think that he broke his wrist by just hitting the water

just bend your wrist upwards and lean against the wall you can feel loads of pressure there, probbly only took a split second impact to snap it, plus it look shallow there as well

apparently the best method to destroy the strongest ship or submarine is to release suddenly adequately big gas bubble under the bottom of that ship....

Just found out that you guys are going very deep to see if this is possible by only touching the water... Even heavy scientific formules passing by. Very interesting !!

Unfortunately Angelo (who was capturing for Ghosts of Speeds) did'nt had my crash on camera.

Didn't touch anything but water. You can see on the video that i released my gear and that there is quite some distance between myself and my gear (didn't cut into that video fragment) in just 2 seconds. I also had to swim a few meters before reaching ground. The only thing I can think off is the forward motion (GPS showed 74,7 kmph analysed via GpsResults just a split second before the crash). Probably my right wrist touched the water first, bending my elbow and followed by the mass of my body (including 10 extra kgs of lead) which snapped my wrist.

I did had a similar crash a few months earlier damaging my hand as well (not same place on my hand and only a few weeks out).

That day we were with 3 having spinout at the same place... Very weird in the middle of your run, not shooting yourself off into the chop. Maybe seaweed? Currents? Couldn't get out of spinout, so i've choosen to let myself drop op my back (first releasing the harnesslines off course). It's a human response to make yourself 'big' and spread your arms (wrist down) while falling on your back into the water. Just try it out in a swimming pool! Best reaction would have been how ever : both arms crossed, making yourself as little as possible. Not easy to learn i guess...

Next year I'll be back on the water and yes with gps, but not as extreme as it was this year. Sold my smallest speedboard (43W, 54L). Guess i'll go for alpha, hour, distance and fun !!