Sailing downwind - faster than the wind?

Ubergeekian said:
I refer the Rt. Hon. Gentleman to my post #51.
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Sorry - I must have missed your post! The idea of proof of concept of two cats and beam was given to me years ago by a fellow in California. Having sailed very fast cats it's so obviously true that you can get downwind faster than the wind.
 
Ubergeekian said:
I find it vaguely depressing to see how many people are claiming the laws of physics and thermodynamics to support their contention that exceeding the windspeed downwind is impossible when it has already been done.

There are lots of good arguments to be had about implementations, efficiency, theoretical limits but the basic idea is beyond any doubt, because it has already been done.
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Show us
 
Ubergeekian said:
Drifting doesn't count.
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Why not? a boat making way dead downwind with wind and tide as described might be drifting but its still sailing within the context of the OP - YOU don't get to make the rules.

Ubergeekian said:
I'm afraid not, or at least not quite in that way. Sure, if you have a 10kt tide and a 10kt wind in precisely the same direction you can get a sailing boat released from rest to do something. It will only be transient, though: the steady state result is the boat drifiting downwind and downtide with the sails empty and the rudder useless.
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If you have a 10kt tide and a 10kt wind in precisely the same direction you can sail across it perfectly happily, why do you think it would only be transient.

Ubergeekian said:
If you want the sails to work steady state then you must have a wind-water speed differential.
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Nope! if you want the sails to work steady state (produce a force) you need sails -air differential. To be able to STEER or not simply drift downwind you need water - boat differential

Ubergeekian said:
OK, it's a flat calm day and the tide is 10kt heading due south. You drift south with it and voila! you have a 10kt apparent wind from the south. Which you can use to sail. Why? Because there is a difference in velocity between wind and water. It wouldn't work in a 10kt northerly wind and it wouldn't work at slack water.
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No, it's because you have a difference in velocity between air and sail - no air/sail action = no movement.
The water / hull effects are neccessary for sailing but are a separate set of forces generated by movement through the water. (Or water past the hull - if you want another frame of reference)
Each of the two sets of forces are controlled independently of each other (within bounds) by the sailor. Don't try to bundle them up into one statement and then jump through hoops trying to justify it - what's the point?

Look, the most important part of my previous statements and the bit you completely ignored was the "frame of reference" bit.
Pick an element to be your frame of reference, the boat would be best, but it doesn't actually matter, you could use the seabed or even the water surface or a specific lump of air (though the last two make visualisations difficult)
But don't pick pointless slippery arguments based on a shifting viewpoint to try to prove a moot point that is in itself off-topic.
 
Ubergeekian said:
The underwater bits wouldn't be. That's the point.
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Please give a more understandable response!
What I wrote was that if you were going directly downwind at windspeed, you would effectively be in calm conditions.
I don't see how your reply relates to that or how you might get propoulsion from the motion of a boat through water if the boat was getting propulsion from the apparent wind which was causing it to move through the water at all!!!!!!
You may be muddled on this one but check back to Newton who established that "For every reaction there is an equal and opposite reaction". Back to basics and you will be fine.
 
mikemonty said:
No,

Show

US

prove you have seen a boat do better than windspeed straight downwind.
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OK here we go.

This link is to video of the 1st America's cup race which was sailed over a 20-mile upwind/downwind course. The wind speed during the race was 6 knots.

BMW Oracle round the upwind mark at 99 mins 20 secs and cross the finish at 162 mins 34 secs. So, 20 miles in 63 mins 14 secs which I make 18.97 knots.

You have been shown.
 
First law of thermodynamics

To all those who are citing the law as proof that the proposition is impossible -

Look behind you, there is an elephant in the corner of the room.

There is a source of energy being drawn on here - the movement of air relative to surface. No matter what speed the vehicle is doing or in what direction it is travelling, there is a differential between wind and surface speed. The question then is - will the mechanism described successfully turn that source of energy into movement greater than the wind speed?

Now have a look again at this clip. A lot of people have assumed, without looking carefully, that the prop is turned by the wind at the start but that is not the case. Look carefully at the prop at about 18 secs into the clip. It is clear that the vehicle has to be pushed to get it going
 
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snowleopard said:
OK here we go.

This link is to video of the 1st America's cup race which was sailed over a 20-mile upwind/downwind course. The wind speed during the race was 6 knots.

BMW Oracle round the upwind mark at 99 mins 20 secs and cross the finish at 162 mins 34 secs. So, 20 miles in 63 mins 14 secs which I make 18.97 knots.

You have been shown.
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Wow. I didn't find that justification in the videos. However, if the upwind mark was exactly up wind of the finish line, and your figures are correct, then the boat gets over three times wind speed VMG down wind!

So if they had set off a smoke generator on the stern as they left the up wind mark, the smoke would always remain up wind of the boat.
 
snowleopard said:
OK here we go.

This link is to video of the 1st America's cup race which was sailed over a 20-mile upwind/downwind course. The wind speed during the race was 6 knots.

BMW Oracle round the upwind mark at 99 mins 20 secs and cross the finish at 162 mins 34 secs. So, 20 miles in 63 mins 14 secs which I make 18.97 knots.

You have been shown.
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No I haven't.

I'm not sifting through an hour and twenty of this but I doubt that at any point was the boat going on a sustained dead run.
I can accept that, if the hull is efficent enough and the wind strong enough it can sail on apparent wind at planing speed (or hydrofoilling or whatever the thing does) close to a dead run - dependant on its best pointing abiity. (wouldn't want to see what would happen if they fell off planing speed)
But this is not the same as the original statement.
 
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Keen_Ed said:
http://www.youtube.com/watch?v=1pSYALWQ-nI

http://www.youtube.com/watch?v=aJpdWHFqHm0

Been done on land. Questions as to whether it can be done on the water, what with the inefficiencies of water propellers compared to wheels.

Threads here:
http://forums.sailinganarchy.com/index.php?showtopic=31824
http://forums.sailinganarchy.com/index.php?showtopic=82175
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I just looked at the second link and have to say it was convincing. The principle appears to be different from my own assumptions - the wheels are driving the prop, causing an airmass to react against the wind - not the prop driving the wheels which was my mistaken assumption.
(In fact I got more useful background out of the guys webpage than the video.)
However for a boat, it still isn't going to work for displacement sailing, and it certainly isn't the principle that drives the Oracle boat!
 
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mikemonty said:
I just looked at the second link and have to say it was convincing. The principle appears to be different from my own assumptions - the wheels are driving the prop, causing an airmass to react against the wind
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Well put, that's exactly what's happening.

True enough, it would be more difficult to do for a boat, due to the drag of the water, but maybe possible using hydrofoils.

Although it may not appear so, it is the same principle as BMW Oracle exceeding the windspeed VMG downwind.
 
mikemonty said:
No I haven't.
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OK so you can't be bothered to look at the evidence but you really should re-read this thread before making comments like that. No one was claiming that a boat with sails could exceed windspeed while sailing on a dead run. They achieve the VMG by tacking downwind.

Now go back to post 51 where Uber shows how a mechanism can be devised that converts that into motion along the direct line.
 
RAI said:
So if they had set off a smoke generator on the stern as they left the up wind mark, the smoke would always remain up wind of the boat.
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Indeed. If they had released a balloon as they rounded the top mark it would have still been drifting down the course 2 hours after the boats had packed up and gone home!

Of course that's on the water. A land yacht tacking downwind can do as much as 80 knots.
 
snowleopard said:
OK so you can't be bothered to look at the evidence but you really should re-read this thread before making comments like that. No one was claiming that a boat with sails could exceed windspeed while sailing on a dead run. They achieve the VMG by tacking downwind.

Now go back to post 51 where Uber shows how a mechanism can be devised that converts that into motion along the direct line.
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I think you'll find that the entire thread is based on the dead run scenario.

Pierrome said:
Strange as it may seem - (and some have difficulty accepting this). It is possible to devise a wind-driven craft that sail directly downwind faster than the wind. It's been done with models, and in the next few weeks a team in America are planning on demonstrating a man-carring version. They have a website on http://www.fasterthanthewind.org/
Admittedly, this is on land, but it is theoretically possible to do it on water too.
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Another clip

I think this one deserves a look.

I see two possible responses: (a) I don't see how it could work so the evidence must be false or (b) I wonder why it works.

We have proved, for those prepared to read the evidence, that downwind faster than the wind motion is possible, so as far as I am concerned the question is - how does this particular device work? It has to be drawing its energy from the difference in velocity between air and surface. My first thought would be to find out whether the prop or the wheels are providing the forward motion.
 
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