Hi guys

I'm trying to understand the dead downwind faster than the wind thing.

Can anyone on here say that they have definitely achieved any of these speeds?

at 110 degrees 2.92 times the speed of the wind
at 120 degrees 2 times the speed of the wind
at 130 degrees 1.55 times the speed of the wind

eg going 40 knots in 25.8 knots of wind at 130 would count
going 30 knots in 19.4 knots of wind at 130 would count
going 20 knots in 12.9 knots of wind at 130 would count

i'd back myself to do 20 knots at 130 off the wind on a formula board. it would be close but i think possible. maybe some of the guys with vmg data can confirm.

Select to expand quote

ginger pom said...

Hi guys

I'm trying to understand the dead downwind faster than the wind thing.

Can anyone on here say that they have definitely achieved any of these speeds?

at 110 degrees 2.92 times the speed of the wind
at 120 degrees 2 times the speed of the wind
at 130 degrees 1.55 times the speed of the wind

I don't know if surfers can do it, but land-yachts, ice-boats and multihull-sailboats can achieve a downwind VMG > windspeed:
en.wikipedia.org/wiki/Sailing_faster_than_the_wind

This is how it works:



Animated:


In theory this should work with boards too (kite or sail).

Here is more:
mirror.wiederhoeft.info/DownwindForDummies/DownwindForDummies_10.zip

ok, I'm not convinced that it has been done on water but that's just a drag thing. It could be done because it definitely has been done on ice ...

Select to expand quote

ginger pom said...

ok, I'm not convinced that it has been done on water but that's just a drag thing.

It is more difficult on water, but has been done.

From: en.wikipedia.org/wiki/Sailing_faster_than_the_wind

It sailed 20 nautical miles (37 km) downwind in 1 hour 3 minutes, so its velocity made good downwind was about 2.5 times windspeed, consistent with being able to sail about 14 degrees off the apparent wind when sailing downwind

You just need a huge sail, much much larger than the drag-creating elements (like the surfer)

Didn't BMW Oracle and Alinghi in the last Americas Cup sail both upwind and downwind faster than the wind. I remember the commentators saying that if you let a balloon go at the top mark, both boats would beat it to the bottom mark.

It is just a matter of the speed of the air flowing over the sail combined with low enough drag to do this isn't it?

Select to expand quote

Bluedog76 said...

Didn't BMW Oracle and Alinghi in the last Americas Cup sail both upwind and downwind faster than the wind. I remember the commentators saying that if you let a balloon go at the top mark, both boats would beat it to the bottom mark.

Yes, the wikipedia quote above refers to BMW Oracle

Select to expand quote

Bluedog76 said...
It is just a matter of the speed of the air flowing over the sail combined with low enough drag to do this isn't it?

Yes, just a matter of overall efficiency. A combination of sail lift/drag-ratio & sail size vs. hull drag (hydro & aero).

Select to expand quote

ginger pom said...

ok, I'm not convinced that it has been done on water but that's just a drag thing. It could be done because it definitely has been done on ice ...

It's been achievable on water for a long time and was recently taken to another level as Bluedog76 said by BOR90 in the AC 2010, unless you were referring specifically to windsurfers? BOR90 had a VMG of about 2 times the true wind speed upwind and about 2.5 times the true wind speed downwind. Case in point, as Bluedog76 eluded to, the example they gave during the AC was: If you let a balloon go at the top mark, BOR90 could start at the bottom mark at the same time, sail to the top mark, then sail back down to the bottom/start mark and still beat the balloon.

We won't see a sailing boat like it built for quite some time I think, BOR90 was the ultimate sailing craft that could be achieved with out current technical knowhow...baller!

GP, come to Sandy Point on a south-westerly... it can be blowing (only) 25kn, yet some guys will be getting 40kn on the dial. And we know its 25kn as we have taken a long walk with a wind meter...

Formula sailing can easily do 2x windspeed, ie: 12kn + flat water should result in 25kn peaks.

I would like to see more definitive work done as to just how strong the wind is at a given moment. eg the wind varies greatly at various heights say from 300 mm off ground level to 3 meters. Gusts are coming and going all the time.
IMHO it is very difficult to say how strong the wind was when a particular speed was achieved.
Herby

On numerous occasions sailing deep downwind on a Formula board in gusty conditions ( Northerlies at Green point) I have sailed out of the front of the gust, slowed down and then had the gust catch up to me and take off again.
You would have to be going faster than the wind to do this I would think!!!!

Select to expand quote

mr love said...

On numerous occasions sailing deep downwind on a Formula board in gusty conditions ( Northerlies at Green point) I have sailed out of the front of the gust, slowed down and then had the gust catch up to me and take off again.
You would have to be going faster than the wind to do this I would think!!!!

or just running out of the edge of the gust

Select to expand quote

Herbylyn said...

I would like to see more definitive work done as to just how strong the wind is at a given moment. eg the wind varies greatly at various heights say from 300 mm off ground level to 3 meters. Gusts are coming and going all the time.
IMHO it is very difficult to say how strong the wind was when a particular speed was achieved.
Herby

true. its tricky to know exactly how strong the wind is. I've got a really good reference and that was at SP. The MI guys had their 5 calibrated telemetry wind meters spaced along the run at about boom height. Tim was watching the gauges on his mobile laptop as I went past . He reported the wind was between 29 and 31kts on all the meters as I went past. It was one hell of a steady run and I did a 44.9kt 500m with ~46kt peak. Typically for wind averaging between 18 and 25kts I go 1.5 times wind speed. SP was a special place like that. On a good day the wind filled in right down to the water. I havent seen wind that steady there in the last 5 years due to rapid dune growth continuing to wreck the speed run.

Thanks Slowboat....I've been hoping for some such info for years.
I have not heard of any other serious efforts to accurately measure the wind
and compare it to the speed of a passing sailor.

Select to expand quote

Herbylyn said...

I would like to see more definitive work done as to just how strong the wind is at a given moment. eg the wind varies greatly at various heights say from 300 mm off ground level to 3 meters.

For kiteboards you would have to measure the wind even higher up, for a fair comparison. As for the gusts, you need a longer track and a clear margin to the average, to be sure that you have beaten the wind downwind.

Select to expand quote

slowboat said...

Herbylyn said...

I would like to see more definitive work done as to just how strong the wind is at a given moment. eg the wind varies greatly at various heights say from 300 mm off ground level to 3 meters. Gusts are coming and going all the time.
IMHO it is very difficult to say how strong the wind was when a particular speed was achieved.
Herby

true. its tricky to know exactly how strong the wind is. I've got a really good reference and that was at SP. The MI guys had their 5 calibrated telemetry wind meters spaced along the run at about boom height. Tim was watching the gauges on his mobile laptop as I went past . He reported the wind was between 29 and 31kts on all the meters as I went past. It was one hell of a steady run and I did a 44.9kt 500m with ~46kt peak. Typically for wind averaging between 18 and 25kts I go 1.5 times wind speed. SP was a special place like that. On a good day the wind filled in right down to the water. I havent seen wind that steady there in the last 5 years due to rapid dune growth continuing to wreck the speed run.

What angle to the wind was that?

I'd say it was between 125 and 140 degrees over 600m of run (bank is curved)

i think it's a moving target though because of drag. effiencies are higher at lower wind speeds because drag is less due to reduced speed.

when i had my formula gear and gps, ratios were always high from 10-15 knots and then decreased drastically as wind speed increased.

Select to expand quote

slowboat said...

I'd say it was between 125 and 140 degrees over 600m of run (bank is curved)

You can work out almost exact AVERAGE sailing angle with respect to wind over say 500m (or any distance) using your GPS Results file as I described in the post below:

www.seabreeze.com.au/forums/Windsurfing/Gps/Quick-Question-Whats-the-fastest-sailing-angle/

Select to expand quote

DaGodfather said...
You can work out almost exact AVERAGE sailing angle with respect to wind over say 500m (or any distance) using your GPS Results file as I described in the post below:

www.seabreeze.com.au/forums/Windsurfing/Gps/Quick-Question-Whats-the-fastest-sailing-angle/

For most places sailing on both tacks, back and forth evenly, this feature works great.

But it relies on an estimation of the wind angle using the assumption that the average angles sailed during the session are similar on both tacks. Walking back up the bank will screw this up. Doing long runs and tacking back would also screw up the predicted wind angle. Also the wind angle changes as much as 10deg during some runs in rain squalls. So its not going to be very accurate in some places (like SP). Good for formula training and checking out which parts of the course have the lifts and knocks.

Select to expand quote

slowboat said...

Walking back up the bank will screw this up....()...So its not going to be very accurate in some places (like SP).

Yeah, you're right, I forgot you guys walk back at SP...

Simply not possible.
If the first post of the thread is adhered too.ie.Faster than the wind DEAD DOWNWIND,its easy to go faster than air speed both downwind and up,but everyone is talking about a degree off dead downwind.Going from a reach,at higher than windspeed to dead downwind may show a momentary over windspeed reading but this will disappear real quick.
By all means claim to go faster than the wind,it happens every day but going DEAD DOWNWIND faster than the wind for any extended period of time,doubt it.

Select to expand quote

k4s said...

Simply not possible.
>>>>>>>>>>>>>>>>>
By all means claim to go faster than the wind,it happens every day but going DEAD DOWNWIND faster than the wind for any extended period of time,doubt it.

This conversation has been had before, and a vehicle has been built that will go between 2 and 3 time faster than the wind dead downwind.
Granted you can't do it on a standard windsurfer.

Select to expand quote

k4s said...

Simply not possible.
If the first post of the thread is adhered too.ie.Faster than the wind DEAD DOWNWIND,

The OP is asking about a downwind VMG > wind speed, at broad reach, in windsurfing.

He just stated, that he asking it, because he is trying to understand how DEAD DOWNWIND faster then wind works:
en.wikipedia.org/wiki/Sailing_faster_than_the_wind

Obviously it doesn't work with conventional sailcraft. But it works if your sails are continuously moving relative to the hull. Here for comparison:

Broad reach with downwind VMG > wind speed:



Dead downwind faster than wind, with a propeller coupled to wheels:

If we apply some high school trigonometry to the Sandy Point example given by slowboat ie.

avg speed = 44.9 knots
avg angle = 132.5 degrees

The downwind sailing speed works out to be 30.3 knots. The actual wind measurements in the above example were between 29-31 knots, so it's... too close to call it a proof.

However, I do agree with Gestalt that the drag is much lower at lower speeds, so it will be much easier to beat the wind at sailing speeds of around 20 knots then at 45knots. Has anyone got any reliable wind/speed/angle measurements for lower winds?

Edit:

Even better, if you have some data you want to try, copy this formula into Excel spreadsheet:

=COS(RADIANS(180-132.5))*44.9

except substitute your own sailing speed and angle instead of Chris's. This will calculate the downwind component of your sailing speed, which you can then compare with your actual wind measurements and see if you went faster.
(for better accuracy avoid using instantenous speeds, just stick to some average like 500m).