I wonder what ever happened to Hansen?

That is a LOT of battens!

mr love said..
A bit late to the party Choco

It's like trying to come up with new chords someone somewhere has already done it in the music industry

sailquik said..
I wonder what ever happened to Hansen?

There is an interview with Hansen at americanwindsurfer.com

fangman said..
^ I think this is the one:

www.americanwindsurfer.com/articles/wingmaster/

Thanks Fangman: If I had an industrial sewing machine I would play around with this concept

choco said..

mr love said..
A bit late to the party Choco

It's like trying to come up with new chords someone somewhere has already done it in the music industry

I find the same thing Choco....every time I copy something I find it's been done before, very frustrating.

USA46 said..
I see lots of people try find out why deep downwind speed raiders use so much twist.

-Much more apparent wind shift happening between foot and head on downwind course than on upwind.

Graphical explanation with two vectors sum:
-very easy to notice that on downwind courses twist angle beta will be allways larger than on upwind courses




(This explanation can be controversial, I can tell why...)

I agree that your basic concept is correct if the wind strength increases with height above the water. If this is true, then in a gust, the change in apparent wind direction is also going to be greater at the head than at the foot.

However, your vector diagram implies that the wind at the foot is half the strength of the wind at the head. Do you have any data to back that up, or in fact to show that there is any significant difference in wind strength from foot to head?

There's plenty of profile measurements on google images. They all vary depending on conditions but with enough fetch over uniform roughness (or smoothness) they all settle to pretty much logarithmic. Half the strength between 1 metre and 4 metres looks possible from the google returns. These are all averages over minutes though. At any instant it could be reversed. Sailquick reports that the Luderitz channel has the low wind quite affected by the earthworks.

Speed raiders, ( who coined that phrase? most apt) are not going all that much faster than the wind. Fiddling with USA46's diagram ( is the black arrow head on the wrong end?) you can see you'd theoretically need less twist on a relatively faster, taller craft where the foot of the sail might already be 1 metre above the water. That's above the most rapidly changing section of profile. That's probably why Sailquick reports not much twist on Macquarie innovation.

Not that we've shown profile is the main reason for twist, but no harm in allowing for it. If profile was the only reason for twist the rate of twist would vary with height with most of it happening at the bottom. Our sails, with twist biased at the top, are upside down, but nothing is new.

Ian K said..
There's plenty of profile measurements on google images. They all vary depending on conditions but with enough fetch over uniform roughness (or smoothness) they all settle to pretty much logarithmic. Half the strength between 1 metre and 4 metres looks possible from the google returns. These are all averages over minutes though. At any instant it could be reversed. Sailquick reports that the Luderitz channel has the low wind quite affected by the earthworks.

Speed raiders, ( who coined that phrase? most apt) are not going all that much faster than the wind. Fiddling with USA46's diagram ( is the black arrow head on the wrong end?) you can see you'd theoretically need less twist on a relatively faster, taller craft where the foot of the sail might already be 1 metre above the water. That's above the most rapidly changing section of profile. That's probably why Sailquick reports not much twist on Macquarie innovation.

Not that we've shown profile is the main reason for twist, but no harm in allowing for it. If profile was the only reason for twist the rate of twist would vary with height with most of it happening at the bottom. Our sails, with twist biased at the top, are upside down, but nothing is new.

"If profile was the only reason for twist the rate of twist would vary with height with most of it happening at the bottom. Our sails, with twist biased at the top, are upside down, but nothing is new."

Very very good observation.This is way I said this is controversial explanation.
If we look at wind gradient picture 1 ,we can see that most of wind speed change happening from 0-1m.That mean we must have maximum twist in this zone.Obviously foot-power max camber zone(0-1m) do not have twist at all.So from this wind gradient picture 1 sail power zone(0-1m) must be in goood stall condition(airflow separate from low pressue side/leeward side of sail)..
In reality that is not happening for sure.So we are on the beggining of controversial topic again!


wind gradient picture1



twist angles 0-1m and 1-5m



Experimet with rope and tell tales will tell the truth.
There is two possible solutions;
a)no wind gradient,telltales will stay parallel
or
b)wind gradient in reality at that heights is not logarithmic function as theory picture 1 show,must be some diffrent function(maybe more linear) ,than everything become OK

The truth is out there!

olskool said..
I think youre onto something there Pacey Im guessing here, but a certain AOA is best for lift. So due to the decreasing draft of sail from bottom to top. The AOA must increase towards the top due to the shallower draft?? Please explain some more.

Not exactly. The AoA must stay the same for best lift. But that angle may change as you move up the mast higher. Hence the sail is twisted. That is IF this increase in wind speed is actually significant where we sail.

sailquik said..>>>>That is IF this increase in wind speed is actually significant where we sail.

It's certainly much less on mirror water, when there's laminar flow over the water.
Mal Flew his kite at lake george and said he couldn't feel any difference in wind strength between water level and 25m up.

Use this to generate profiles.
wind-data.ch/tools/profile.php?lng=en

The lowest value of roughness length you can put in is .0002 for water. Which is smoother than mown grass despite the chop build up. Not a precise science of course but this is the graph of the appropriate log profile for the smoothest you can get.

Water is surprisingly low on the roughness scale despite the chop. I asked a fellow from environmental mechanics who happened to be a windsurfer about this. He thought some sort of harmony between the wind and the chop that is generated by the wind made it act as if smoother than it looks. .Not as rough to the wind as what you might think when comparing it to similarly-sized random obstacles on land. He put down the drop of onshore wind in the surf zone to the waves pitching up and breaking into a non-harmonious state.

Ian K said..
Use this to generate profiles.
wind-data.ch/tools/profile.php?lng=en

The lowest value of roughness length you can put in is .0002 for water. Which is smoother than mown grass despite the chop build up. Not a precise science of course but this is the graph of the appropriate log profile for the smoothest you can get.

Water is surprisingly low on the roughness scale despite the chop. I asked a fellow from environmental mechanics who happened to be a windsurfer about this. He thought some sort of harmony between the wind and the chop that is generated by the wind made it act as if smoother than it looks. .Not as rough to the wind as what you might think when comparing it to similarly-sized random obstacles on land. He put down the drop of onshore wind in the surf zone to the waves pitching up and breaking into a non-harmonious state.

That's a terrible curve! It means only kiters have a chance to go, since there's no wind near the water! It would be interesting to know how wind strength and gradient relate. In the early summer, we often have stable gradients in lighter winds, meaning (like the graph shows) very little wind on the water, while the meters that are mounted 10 m above the surface show good numbers. Once the wind picks up beyond 20 knots, the wind seems to reach the surface a lot better.

USA46 said..

No need for expensive wind meters ,everbody who has boat and one rope can make measurment.Cost nothing.
Read my post or look at this link:

http://www.ockam.com/docs/ShearOmometer.pdf

But Pacey already confirm that telltales will not show shift form apparent wind..

No, I said the experiment was done in the 1930s and got negative results, doesn't mean it will always give negative results

USA46 said..
Do you have link about this topic?

No, I only have pre-link technology for this. I'll have to go through my Uffa Fox books page by page

Windshear! You've gone off on another tack. I suppose Speed Raiders can tune twist for just one of the two, but the topic heading does include slalom.