Ian K said..
I was in the Wimmera once when you fellas were having a windy day at Sandy Point. It was windy there also. Flat as, and a few irrigation ditches around. Do a google search of "Wimmera irrigation channels". This google earth shot seems to have a dry irrigation channel in it?

Yeh they have the pipeline now so they don't fill dams anymore in the Wimmera...although Green lake is low and heaps of weed...and yabbies :-)

Speaking of other fluids the trick is actually warm water. Have a look at the viscosity of water, it drops a lot as temp goes up. I remember talking with some guys about trying to do some speed runs in the warmies, the warm cooling water discharge channels of a big power station. But the only thing going for it was low friction. Wind, obstructions, angles,acess everything else was bad. But if you had a ditch, you could put a pool blanket on it. I reckon thats fair game. Maybe that helps at luderitz. What is the water temp there?

Isn't it swings and roundabouts, lift and drag are both affected in a similar fashion?

Definitely flat water. Its not swings and roundabouts with water temp though. Higher temp water is lower viscosity but virtually the same desity. Cavitation is temp dependant but can be designed for. Reynolds number changes and so does lift drag ratio. Planing drag will be lower at higher temp. Cold humid air is much denser and better than hot dry air. Also tends to be less turbulent.
density of water at 4 deg c 1000kg/m3 at 50c 988kg/m3 ie 1% less
at 20c abs viscosity 1e-3Ns/m2
Abs viscosity at 4degc 1.57e-3Ns/m2 at 50c 5.47e-4 about 66% less than 4c water.
ethanol densityat 20c 789kg/m3 abs viscosity 1.2e-3 about 20% higher than water.
methanol at 20c 792kg/m3 abs viscosity 5.8e-4 about 40% less than water. But dont drink it you will go blind.
pretty much useless info hey

sailquik said..

decrepit said..
Isn't it swings and roundabouts, lift and drag are both affected in a similar fashion?

I thing that is correct. Denser water = more lift and more drag.

However, if the water is denser you can use a smaller fin....... Not sure if the relationship is linear though. --Ian??

Anyhow, it's probably a moot point because there is no doubt in my mind that the best sort of water is FLAT water!

Don't think it'd be linear with density. Only guessing, maybe someone with a fin analysis program can chip in, but I'd think the kinematic viscosity, the ratio of absolute viscosity to density, is in there somewhere. The kinematic viscosity of water is 1.787 X 10^-6 and air is 1.568 X 10^ -5. So although the the Kinematic viscosity varies by almost a factor of10 and the density varies by almost a factor of 1000 the shape of the foils for air and water are similar and the lift to drag ratios that we can achieve in these two fluids are in the same ball park. So definitely not linear.

I don't think there's enough in it to worry about chasing minor variations in viscosity. The density of air and kinematic viscosity of air varies quite a bit with temperature, we don't notice, maybe we just subconsciously hold on to a bigger sail in hot weather, but as far as I can tell we but don't go noticeably slower.

FLAT water, and shallow water. There was a big discussion sometime back on whether there should be a regulation on water depth for speed sailing to control the benefits of ground effect. Regulation seemed to stall. A water depth greater than the width of the hull was suggested as the point at which the benefit is fading away.

But while the loophole still exists, a precision-build channel of 5 -10cm depth, boards 50cm wide, with multiple 4.5 - 9.5 cm fins has got to have potential.

Ian K said..

FLAT water, and shallow water. There was a big discussion sometime back on whether there should be a regulation on water depth for speed sailing to control the benefits of ground effect. Regulation seemed to stall. A water depth greater than the width of the hull was suggested as the point at which the benefit is fading away.

But while the loophole still exists, a precision-build channel of 5 -10cm depth, boards 50cm wide, with multiple 4.5 - 9.5 cm fins has got to have potential.

The WSSRC specify a minimum depth of 50cm for their records. Thus the Luderitz Canal was supposed to be that minimum depth and was certified as such by their 'observer'. I am pretty sure there were bits of it that were only just fin depth though, and sometimes slightly less! I am not sure about the eyesight of the 'observer'.

The WGPSSRC has no rule (yet) regarding water depth as we consider it self regulating. With the width of our windsurfing boards and the depth our fins need to be for effectiveness (16-20cm), it seems pointless. Not sure if that will always apply to kite boards though...

Multiple fins have a big efficiency disadvantage over a single fin that normally completely rules them out for speed. However, the advantages of 'ground effect' might overcome that in extreme situations.

I wonder if there is any study of the effects on lift of weed filled water?

On air density. Going from one extreme to the other: Sandy Point in winter to Burrum heads, I could really feel the difference in sail size. A good example was the final Sunday after the event when it was a steady 20-22 knots. At Sandy Pt in that wind I could just sail my CA40 with a 5.8m sail. At Burrum I was about the same on my 6.2m sail (which I would never use on this board at home). When a squall came through at around 25 knots I could just crack a 39kt 2 second. In the same strength at Mallacoota a few months ago I did the same with my 5.8m. I certainly think there is around a half a meter difference in it, but as Ian says, it is very difficult to quantify with all the other variables.

mr love said..

Ian, not sure that we don't notice the difference in air temperature. I will swear a 25 knot Southerly bluster on Port Phillip in a 10 deg Winters day feels noticeably stronger than a 25 knot gust in a hot Northerly 30 deg plus day, but maybe it is just perception??

I was considering it from the point of view of maximum speed potential Martin. The air at 30 deg is 7% less dense. So could go just as fast by rigging 7% larger ? Rig a 7.0 rather than a 6.5? If the speed potential is pretty much equal provided you tune for the new conditions, it's one little indicator that there may not be much to gain by fiddling with the water either. Just a guess, don't think there's a simple theory that'll tell us.

There is also air and sand entrained in the flow. They have a very big effect on fin performance - to the point of causing premature cavitation.

Mmmm thread seems to have been hijacked.
on holidays now red and trying to organize your way but getting Penny past warrnambool shops will be tricky.

found Mal F's mud shoes made from cut up kite board under the van . will give them a go this weekend.