Downwind faster than the wind. Poll

[halfway]

RAI: To swap from gybing to tacking requires that the sail crosses the boat. It does not require the boat to alter course.

If the real wind behind Oracle was to reverse suddenly at a certain distance the following boat would have to change from gybing to tacking immediately. This would mean that the sail would cross the boat but the boat would still stay on the same 45 degree line. It has nothing to do with putting the bow through the apparent wind.


As for the apparent wind during a gybe. That travels across the bow. But it travels a much further distance than when tacking an apparent wind across the bow. Gybing produces a quicker and wider swing. The two manoeuvres are so different it is hard to confuse them. Anyone that does stays on the pontoon for the day.

 

[halfway]

xxyyzz: "It's more like asking if you should tack lots or as little as possible. "

The rotational speed is fixed to the forward speed so the ratio is nothing to do with the length on each tack. It is the angle of the tack.


Think about "cylinder world". The two ends are islands. Windward and Leeward. To sail from one to the other along the cylinder sizes (The sea) there is only ONE tack.

You stay on it the whole journey. If your fellow traveller is doing the same but on the other side of the cylinder you both mark out a 45 degree helix.

That is what the prop is doing.

 

[RAI]

Anyone that does stays on the pontoon for the day.

I'll talk to you again when you have watched what BMW Oracle does when it tacks down wind and when you have figured out how it manages to cover 20 nm down wind in 63 minutes with only 6 to 8 knots of wind available.

 

[fireball]

RAI: To swap from gybing to tacking requires that the sail crosses the boat. It does not require the boat to alter course.

If the real wind behind Oracle was to reverse suddenly at a certain distance the following boat would have to change from gybing to tacking immediately. This would mean that the sail would cross the boat but the boat would still stay on the same 45 degree line. It has nothing to do with putting the bow through the apparent wind.


As for the apparent wind during a gybe. That travels across the bow. But it travels a much further distance than when tacking an apparent wind across the bow. Gybing produces a quicker and wider swing. The two manoeuvres are so different it is hard to confuse them. Anyone that does stays on the pontoon for the day.

I'm staying well out of the Down wind faster than the wind bit - but your last paragraph is perhaps a little confused.

By convention:
A tack is when the wind (apparent) passes around the front of the mast - ie across the bow - you can tell this quite easily because the sails flap....

A Gybe is when the wind (apparent) passes around the back of the mast - ie across the stern - you can tell this quite easily because the sails do not flap

I can categorically state that I have never Gybed with the apparent wind passing across the bow - even in the fastest performance boat I've owned - RS800 - although it was best to get all the power out of the main by speeding right up down wind with the spinny up then the main ghosted across during the gybe.

As BMW Oracle does travel down wind and 'gybe' whilst still going faster than the true wind then it is really a tack ....

 

[halfway]

RAI: Find me a video of Oracle gybing down wind faster than the wind with the sails on the outside of the boat from the centre direct course line.

If you do not understand that request then find a video of Oracle going to windward tacking. Draw a line on the screen between the 2 race markers. Are the sails closer to the line than the mast or further from the drawn line?

Now do the same thing for downwind leg. Are the sails closer to the centre line than the mast?

If there is no difference then tell me the reference to the two videos. I'll go out and have a whole life now while you search for the impossible.

 

[halfway]

fireball & RAI:

The definition of a gybe is:

2 : to change a vessel's course when sailing with the wind so that as the stern passes through the eye of the wind the boom swings to the opposite side


At no time does it say APPARENT.


What about the situation of going 1.1 times the speed of the wind. As you gybe the boat will slow and the wind will neither go around the bow or the stern. Since it will have a zero apparent wind.

Now your stuck. You just invented a third way!

The solution is to use the correct definition of gybe and stop falling for all this bad sailing commentary. They are gybing and always will.

 

[Keen_Ed]

 

[Keen_Ed]

 

[fireball]

fireball & RAI:

The definition of a gybe is:

2 : to change a vessel's course when sailing with the wind so that as the stern passes through the eye of the wind the boom swings to the opposite side


At no time does it say APPARENT.


What about the situation of going 1.1 times the speed of the wind. As you gybe the boat will slow and the wind will neither go around the bow or the stern. Since it will have a zero apparent wind.

Now your stuck. You just invented a third way!

The solution is to use the correct definition of gybe and stop falling for all this bad sailing commentary. They are gybing and always will.

You said .... "As for the apparent wind during a gybe. That travels across the bow. But it travels a much further distance than when tacking an apparent wind across the bow."

Up until recently it mattered not whether you talked about true or apparent wind as the act of gybing slowed the boat down and boat speed was below wind speed - so the wind did pass the stern not the bow.
If the boat is in a stream which is flowing in the same direction but faster than the wind then the boat only sees apparent wind from the other direction - does this mean that when he tacks he's gybing?

Your paragraph about apparent wind travelling across the bow during a gybe is complete bolox - during a gybe apparent wind passes across the stern, otherwise it's a tack!! - you seem to have got yourself in such a knot over the concept of travelling faster than the wind that you've completely forgotten the basics!!

I could find out where you can buy a copy of 'A Dummies Guide to Sailing' if you like - it seems you might need it!

 

[Keen_Ed]

 

[Keen_Ed]

 

[Keen_Ed]

Your paragraph about apparent wind travelling across the bow during a gybe is complete bolox - during a gybe apparent wind passes across the stern, otherwise it's a tack!! - you seem to have got yourself in such a knot over the concept of travelling faster than the wind that you've completely forgotten the basics!!

Moths do this when they're on foils - the apparent travels around the bow on the downwind turn, as well as the upwind turn. And yet they call a tack a tack, and a gybe a gybe.

http://www.youtube.com/results?search_query=foiling+gybe

 

[boomerangben]

boomerangben: "helicopter rotors go around a lot slower than an aircraft propeller. "

Yes, but you try getting a helicopter to fly directly up at any significant speed.

"The only commonality is aerofoil theory"

Absolutely. I have never said do not use aerofoil theory. However it is not necessary to use it. Just one fixed point real world lift/drag/angle of attack graph is enough to prove the concept. No theory is required when you use real life data. (Prop modelling is a different matter)


"will stall at around 18 degrees angle of attack"

But if I was driving the cart I would not need more than a few degrees of attack angle at any time. The prop pitch would be increased from zero, as the speed of the vehicle increased and so would track (just ahead of it) the apparent angle forward.

The whole point of using a helicopter as an example is that the rotor works where there is little or no airflow through it other than what the blades induce. This is as close an analogy to the cart propeller with no relative airflow (over the cart) as I can think of. Aeroplanes are generally moving forward and have substantial air mass flow through their propellers, which isn't the case with this cart.

You can only use real life as proof if the scenarios are directly analagous. It is far more reliable to go back to first principles and look at what it is the machine is actually doing. Trying to compare this cart to sailing down wind (or vice versa) is like trying to explain how a suspension bridge works by watching how far your boat moves when at anchor. Same theory, different application.

My reply re 18 degrees angle of attack was trying to get you to explain the 45 degree helix.

I have no idea what you mean about if you were driving the cart..... the cart's propeller pitch is fixed. The angle of attack depends on rate of rotation, induced air flow and air flow through the propeller disc.

 

[Keen_Ed]

 

[RAI]

Would you know if this was a tack or a gybe if you were not told or could not see the true wind's direction? No big boom swing, no problem getting the foresails across. It doesn't look like the boat slows down to wind speed.

http://www.youtube.com/watch?v=N5cBTP7vI10

 

[Keen_Ed]

Well, the fact that they turn away from the wind is a bit of a clue - to port when they're on starboard, and to starboard when they're on port....

 

[snowleopard]

In the days when our present terminology was invented, a gybe was when true and apparent wind crossed the back of the boat and the boom went across with a bang. When true and apparent wind went across the bow and the main flapped then re-filled on the other side was a tack. For most of us that is still true today.

What we have now is a situation where true wind crosses the stern and apparent crosses the bows, the mainsail flaps and re-fills. The ancients never met such a circumstance so their terminology can't be directly related.

We could call it a tack or a gybe though neither would be a clear description of what is happening. We could even invent a new name for it. To say one term is definitely right and the other definitely wrong is not justified. Nor is calling someone who chooses the other option a 'dummy'.

 

[Bosun Higgs]

So you're saying that if you are on the cart and the road is zooming underneath you at 10mph you can't get any energy from it? So if you put a wheel down on a moving treadmill you couldn't get any energy from that wheel? When a cog in a gearbox engages with another cog it cannot transfer energy?.........!

You are trolling arent you?

But to respond. If the road were to pass energy onto the cart, then the road would be in a different state after the cart had gone. Is it? No it isnt. The road is stationary. Its at the same potential energy as it always was. Its at the same temperature.

The treadmill is different. What is happening there is that a motor is driving the treadmill and as soon as you put a wheel down on the treadmill and take energy from it either of two things happen depending on the trteadmill design. Either the treadmill slows down slightly ( less kinetic energy) or the power draw from the motor goes up a bit. Either way energy from the treadmill motor is being diverted to turn the wheel.

Two meshing gears only transfer energy if one gear is inputting it and the other is taking it out. Just like the treadmill.

Unless you are getting to nuclear processes where matter is turned into energy and vice versa, energy is always conserved. Might be turned from one form into another, but its still exists in the same quantity.

 

[Pierrome]

You are trolling arent you?

But to respond. If the road were to pass energy onto the cart, then the road would be in a different state after the cart had gone. Is it? No it isnt. The road is stationary. Its at the same potential energy as it always was. Its at the same temperature.

What you're saying is nonsense. Of course it is possible to transfer the energy of its movement into the cart.

I will give you an example and I ask you to explain it.
Some jet airliners have an emergency system which is called RAT (ram air turbine) it is a wind turbine that can be deployed into the airstream in the event of electrical failure, and it provides enough power for essential services in the aircraft. It works because the aircraft is travelling through the air and there is a relative difference in velocity between the air and the aircraft.
You therefore are saying that it cannot work because the air is static although the aircraft is moving through.

I don't know why they would choose to fit such things on planes when it is impossible for them to get energy from (static) air going past!

Please explain why the (static) air going past the moving aircraft is different from a (static) road rolling beneath the moving cart.

 

[Bosun Higgs]

The energy in this system is not constant and energy is not being conserved. The energy comes from the wind over ground speed and the mass of the air passing the cart. There is an excess of it and most of it is wasted. Mark Drela's analysis provides an illustration of the relationships involved.

Of course energy is being conserved - there is neither fission nor fusion taking place. Basic rule of physics.

Yes the energy to drive the cart would come from the wind passing the cart but as soon as the cart gets up to wind speed, there is no wind passing it. So no transfer of energy can take place. In fact the cart will reach steady state at below wind speed when there is sufficient energy being transferred to match that being dissipated through rolling resistance etc.

Interestingly you can see this happening in the videos by watching the flag thing on the cart. Whilst for a few seconds as the wind speed drops and momentum carries the cart forwards you will see the cart going faster than the wind but it soon slows down to just below wind speed again.