Took this pic a week or so ago in 12 to 15kts.





You can see where the rig is touching the water, on the downwind side ripples are forming.
If you lift the rig, the ripples move downwind and all is glass again. Put the rig back down and ripples appear.

I have 2 theories, anybody else care to comment/have a better idea?

theory 1, the rig is acting like an anti pollution sausage at an oil spill, and interupting the slow movement of oil downwind, allowing the ripples to form.

theory 2, the rig is causing turbulence in the air flow, disturbing the laminar flow over the water, which creates the ripples.

Nice photo Mike, bastard
Wise old me thinks theory 2

So Firie, are you coming over, we'll probably be here for a couple more weeks.

Hi Mike, looks like it now, we have a caravan being built in Melbourne which has had a couple of delays, today I got an estimated delivery date (31st) which gives us time to do the lake. So I'm guessing we can be there the last week of March, hope you're still there or someone else to hold my hand. I'll just bring my slalom board and smallest fin and hope that'll do, if not I'll just enjoy the vib, watch and yack

PS We'll tent LG and caravan home.

Looks unbelievable flat , like ice ring with a bit of snow at the edges

My guess for the glassy water is that the wind doesn't touch it, it must start flowing a little bit higher than water level. That could be tested with a windmeter. What was also intriguing is that some parts of the lake had ripples and some not. Not sure if the surrounding geography is the explanation...

Being fairly ignorant I found that on the very light wind days you could see the wind/ripples on the water way over the other side from Cocky's past the weather station but it did not feel any stronger than the glass near the launch. In fact I got better speeds on the run nearer the launch.
Not used to this phenomenon and takes awhile to get used to. Sure not complaining though!!
Is it the fact that the water is shallow, has weed growing up from the bottom etc??? NFI.

I've noticed something similar in the Lake Illawarra shallows in 12 to 15 knots. Patches of weed near the surface and maybe not so near in other places induce some regions to ripple and others to remain glassy. Not sure what it is. A bit of Internet tunneling has unearthed the capillary wave effect. Not sure if it has anything to do with it. But it's where surface tension has an effect on wave formation and there are thresholds involved but I can't find an article that has real numbers, like 12 knots in it. This is the sort of gear you need to investigate further.


Figure 5. The US-LTA Model 138S airship with the suspended meteorological platform of the Naval Research Laboratory below it. APL/UW's Ku-band microwave system is mounted in the gondola of the airship.

Measured Ku-band cross sections plotted versus wind speed showed a threshold wind speed somewhat below that predicted by Donelan and Pierson, JGR, 92(C5), 4971-5030, 1987 (Figure 6). The reason that the measured threshold is below the predicted one is due to the variability of the wind over the measurement time. This variability reduces the impact of the threshold wind speed on spaceborne scatterometer measurements as shown in the next section.

Mike, you just like teasing those of us who can't get there, don't you?

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Windxtasy said..
Mike, you just like teasing those of us who can't get there, don't you?

He bloody does A

Mike i think if i was there i could give a answer but all i can say is you lucky legend enjoy it and i hope you get it good and get a pb !!!!!!!!!!!!!!!

Strange phenomena. What I can remember (sign - it's a fading memory) is that the glassy areas are not dead flat (like a mill pond) but the surface shape has very shallow angles - sorta like shallow dimples, and the other (grey) areas have sharper surface angles. Maybe this means that once air flow vector at the water boundary reaches a threshold the water surface forms micro breaking waves leading to the "grey" patches. Given that the "average" air flow is the same on both water regions there must be a turbulence that is being setup by the water surface itself (a self sustaining action), and as seen by Mike's sail in the photo. To remain a "grey" (non - glassy) area the surface fetch must have to be long enough to maintain the turbulence. I think I remember that if the sail gets removed the grey area downwind becomes glassy again - because the fetch of the disturbance is not long enough to maintain the disturbance.

As Ian says above, I have seen this phenomena at Lake Illawarra once or twice near the shore at a depth deep enough to sail on, so maybe the existence of a lot of growing weed somehow affects the surface properties of the water such that it resists formation of micro breaking waves - oil as Mike says? Certainly the foaminess of the LG water suggests some sort of chemical (natural) phenomena.

Anyway - just thinking aloud

My Theory - probably Sh!te.

Due to the movement of air across the a sail, a very small localised high pressure system forms over the the total sail area.

As the a higher pressure travels anticlockwise, the slightly accelerated air spills of the top of the leach in the direction of the original air movement.

Photoshop???

I've never been to LG but have read the following exert from a Windsurfing SA Fact Sheet on LG.

"Each year high tides and freshwater run off into drains that fill the Lake and encourage weed growth, which is both good and bad. The good point is that as Summer comes the Lake gradually recedes and leaves huge areas of the lake with weed close to the surface. At this time the sailing is unreal, the water like a mirror, not a ripple and the wind blowing at an average 20-25 knots, the conditions don’t getmuch better."

What I remember from science at school about wave formation is that half the wave is above the normal surface of the water and half is below the surface of the water. There is something below the surface of the water at LG (i.e. weed) that inhibits the wave formation below the water surface and hence inhibits the formation of the wave itself.

This however does not explain the effect of Decrepit's sail in the shallows, where there appears to be no weed. The ripples only occur where the leach of the sail is under water. It may simply be, as Davmen has suggested, a localised disturbance to the airflow.

combination of high salt levels and foreign matter in the water eg from WA, QLD, Tassie, NSW this foreign matter secrets an oil called PB's this PB oil causes the matter's head to swell which in turn creates a glassy mirror effect on the water where upon the matter can look at it's big head

Must agree Choco. It all mAkes sense now

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decrepit said..

Took this pic a week or so ago in 12 to 15kts.





You can see where the rig is touching the water, on the downwind side ripples are forming.
If you lift the rig, the ripples move downwind and all is glass again. Put the rig back down and ripples appear.

I have 2 theories, anybody else care to comment/have a better idea?

theory 1, the rig is acting like an anti pollution sausage at an oil spill, and interupting the slow movement of oil downwind, allowing the ripples to form.

theory 2, the rig is causing turbulence in the air flow, disturbing the laminar flow over the water, which creates the ripples.

I was going to say "theory 2" but the bigger question is if this is the case then why are the ripples only at the head of the sail and not the foot and middle?

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yoyo said..

decrepit said..

Took this pic a week or so ago in 12 to 15kts.





You can see where the rig is touching the water, on the downwind side ripples are forming.
If you lift the rig, the ripples move downwind and all is glass again. Put the rig back down and ripples appear.

I have 2 theories, anybody else care to comment/have a better idea?

theory 1, the rig is acting like an anti pollution sausage at an oil spill, and interupting the slow movement of oil downwind, allowing the ripples to form.

theory 2, the rig is causing turbulence in the air flow, disturbing the laminar flow over the water, which creates the ripples.

I was going to say "theory 2" but the bigger question is if this is the case then why are the ripples only at the head of the sail and not the foot and middle?

Foot and middle trailing edge are off the water, guessing

So what's the prize if we get it right??? A trip to LG!!

couple of extra things yo need to take into account.

That water is not stationry - the water is actually moving about 2-4 knots with the wind.
The lake bed itself is super flat with a hard limestone floor under the sand/mud. there are very few obstuctions on the bottom to disturb the above flow.
The wind is very consistant - nature abhors a vacumn but in this case the water flow and air flow creates a balance of pressure between them

In mikes ecample above he broke the surface tension holding this balance together and created a disturbance in the force!

Choco, that's probably one of the best comments in ages
All true as well.

I've witnessed similar effects whilst fishing in the ocean on a glassy day. My boat was creating (attracting?) a patch of wind that kept pushing me along at a slow pace. I started the motor and moved away from it about 50m. After about 10 mins it found me again!?

It sounds a bit trippy, but maybe the boat was creating some heat or effect that moved the wind across it?

Hahaha well said choco

Had another experience of glass today, well more like oil than glass. As somebody else said previously, the wind was stronger in the oily stuff.
In fact as soon as we hit the ruffled water that appeared to have more wind, we dropped off the plane, gybe and head back into the oily stuff, and as soon as you hit it you could get onto the plane again.
Very weird!

Mal had a theory last year, after he had a go on his kite on a light day. Going by the surface wind, thought he'd have enough. But the wind wasn't as strong as he thought, there was no increase in wind speed with height. So he reasoned that where there is lamina flow, the wind isn't impeded as much at the surface, but where there is turbulence, the surface wind is less.

But by ruffled water today, I'm only talking about a few centimeters, but there was a very pronounced drop in wind strength when you entered that zone.
12 to 15knts wind, (average 14), Jamie had a 31 and I was just under 30, great fun.

I'm no expert, but I'm guessing it's one of two effects:

Lee Waves or Karmen Vortex Street. I'm guessing Lee wave. Wiki em and see what you think

Beachport is in my back yard. A 6 hour drive for me. I'm going to have to make it down there soon :)

PS/ A bit of useless trivia for you. I only learnt of Karmen Vortex street effect last month. See this article about mystery Russian deaths:

science.slashdot.org/story/14/02/01/2058232/russias-dyatlov-pass-incident-may-have-been-explained-by-modern-science