Hugo Boss and skywalk.

[BurnitBlue]

Why would a kite surfer go higher because he is tethered to Hugo Boss. I thought that an untethered kite surfer had no limits and the sky is the limit. I have looked at the youtube video and I just dont understand how Eric Thomson soared so high with a tether to Hugo Boss's masthead. I am missing something basic here but what?

 

[flaming]

Why would a kite surfer go higher because he is tethered to Hugo Boss. I thought that an untethered kite surfer had no limits and the sky is the limit. I have looked at the youtube video and I just dont understand how Eric Thomson soared so high with a tether while he was skidding across the water while un-tethered. I am missing something basic here but what?

When a kitesurfer leaves the surface of the water there is nothing stopping him being blown downwind. So although they can do massive jumps, the airtime is limited as the kiter swings under the kite, where it is in its unpowered state. When tethered to the boat he could go up and not disappear off downwind, keeping the kite in the power zone.

 

[BurnitBlue]

When a kitesurfer leaves the surface of the water there is nothing stopping him being blown downwind. So although they can do massive jumps, the airtime is limited as the kiter swings under the kite, where it is in its unpowered state. When tethered to the boat he could go up and not disappear off downwind, keeping the kite in the power zone.

Thanks for the explanation. I was looking at the problem from a conventional yachts point of view where forward momentum is supplied by the sails irrespective of grip on the water. Your explanation brings up another point though. A kite surfer who is airborn is like a yacht sitting up on foils surely. Again to me the fact that a yacht on foils loses its grip on the water must create an advantage cancelled out by the reduced grip on the water surface. Excuse my ignorance, laugh if you must but how much of this disadvantage does a yacht on foils suffer. My puzzle has moved from kite surfers to yachts on foils.

 

[JumbleDuck]

Why would a kite surfer go higher because he is tethered to Hugo Boss.

As Flaming suggests, kites need to be tethered to fly. Otherwise they are gliders and gliders, by definition, come down.

Again to me the fact that a yacht on foils loses its grip on the water must create an advantage cancelled out by the reduced grip on the water surface.

The vertical part of the foils gives plenty of grip on the water. See: keels.

 

[BurnitBlue]

As Flaming suggests, kites need to be tethered to fly. Otherwise they are gliders and gliders, by definition, come down.



The vertical part of the foils gives plenty of grip on the water. See: keels.

Ah bells ringing. I forgot about the dihedral turn up in the foils. Pity, i imagined that discovering such a connection between kite surfer and foils would cause a massive shift in cosmic physics and cause all those fancy boats to immediatly start sailing backwards. I will have to be content with learning something new today.

 

[Ningaloo]

Whatever the physics, you have to admire Alex and these stunts. I have my fingers crossed for him in this year's Vendee.

 

[mjcoon]

As Flaming suggests, kites need to be tethered to fly. Otherwise they are gliders and gliders, by definition, come down.

Give or take finding a thermal up-draft. I imagine they are rare over water...

 

[JumbleDuck]

Give or take finding a thermal up-draft. I imagine they are rare over water...

They are indeed, and even when you find one, the glider is still coming down ... it's just that the air is going up faster.

 

[flaming]

Thanks for the explanation. I was looking at the problem from a conventional yachts point of view where forward momentum is supplied by the sails irrespective of grip on the water. Your explanation brings up another point though. A kite surfer who is airborn is like a yacht sitting up on foils surely. Again to me the fact that a yacht on foils loses its grip on the water must create an advantage cancelled out by the reduced grip on the water surface. Excuse my ignorance, laugh if you must but how much of this disadvantage does a yacht on foils suffer. My puzzle has moved from kite surfers to yachts on foils.

If you're talking about IMOCA then remember that they don't really foil upwind, and that a significant amount of the foil will be vertical, or nearly vertical, at upwind heel angles.

If you're talking about the AC75 boats, then the answer is a bit different in that they are foiling upwind, however as well as utilizing the vertical section of the foil arm, the foil will not be flat in the water, but at an angle so that the lift has both a vertical and horizontal component. In fact, this does actually result in the boats having negative leeway when going upwind.
INEOS team UK have just launched their new boat with what seems to be significantly larger (and therefore more draggy) foils than the competition. There is some speculation that one reason for this will be to allow them to run with more cant on the foil whilst remaining airborne, resulting in more lift to windward when going upwind. It should also allow them to lift of more quickly and lower down the wind range.

 

[BurnitBlue]

If you're talking about IMOCA then remember that they don't really foil upwind, and that a significant amount of the foil will be vertical, or nearly vertical, at upwind heel angles.

If you're talking about the AC75 boats, then the answer is a bit different in that they are foiling upwind, however as well as utilizing the vertical section of the foil arm, the foil will not be flat in the water, but at an angle so that the lift has both a vertical and horizontal component. In fact, this does actually result in the boats having negative leeway when going upwind.
INEOS team UK have just launched their new boat with what seems to be significantly larger (and therefore more draggy) foils than the competition. There is some speculation that one reason for this will be to allow them to run with more cant on the foil whilst remaining airborne, resulting in more lift to windward when going upwind. It should also allow them to lift of more quickly and lower down the wind range.

Thanks. It seems that, to understand foils, it is better to start from the beginning. Short descriptions begat more questions. I would think that the science and limits of foils must be based on known parameters from aeronautic airplane wings translated to water so the trade offs between the drag of extra wide foils and the hull drag saved by lifting the boat out of the water must be known. For instance if the ballast keel could be swung out of the water when the boat is riding the foils it would be a race winner. Are designers close to achieving that?

 

[flaming]

For instance if the ballast keel could be swung out of the water when the boat is riding the foils it would be a race winner. Are designers close to achieving that?

You'd think so, but no...

In the IMOCA class they definitely do not want to get the keel out of the water. The reason is extremely counter intuitive, but logical once you realise the bigger gains. The keel pin is not level with the waterline, but instead the front of it is higher than the aft end. This means that when the keel is canted the keel fin has a positive angle of attack and creates lift. This gives 2 benefits, firstly it increases the amount of lift in total, allowing the boat to lift further from the water, but more importantly, since it is on the opposite side the submerged foil it makes the boat much more stable in "flight". In the IMOCAs this is especially important as they are driven almost all of the time by autopilots. This helps reduce the amount of touchdowns and therefore significantly increases the average speed.

 

[flaming]

I think this pic best illustrates what I mean.

 

[RJJ]

Why would a kite surfer go higher because he is tethered to Hugo Boss. I thought that an untethered kite surfer had no limits and the sky is the limit. I have looked at the youtube video and I just dont understand how Eric Thomson soared so high with a tether to Hugo Boss's masthead. I am missing something basic here but what?

This chap was my great great grandfather. Alex Thompson and those clever foil designers are all very well but Hargraves' box kite experiment (seen in context of his time, and "given" to his fellow inventors) is rather more interesting. To his dying day he was asked if the Wright brothers' failure to acknowledge him rankled....and always said he was delighted to have been of assistance ?

Lawrence Hargrave - Wikipedia

 

[BurnitBlue]

You'd think so, but no...

In the IMOCA class they definitely do not want to get the keel out of the water. The reason is extremely counter intuitive, but logical once you realise the bigger gains. The keel pin is not level with the waterline, but instead the front of it is higher than the aft end. This means that when the keel is canted the keel fin has a positive angle of attack and creates lift. This gives 2 benefits, firstly it increases the amount of lift in total, allowing the boat to lift further from the water, but more importantly, since it is on the opposite side the submerged foil it makes the boat much more stable in "flight". In the IMOCAs this is especially important as they are driven almost all of the time by autopilots. This helps reduce the amount of touchdowns and therefore significantly increases the average speed.

Absolutely fascinating. I hope the thread continues as the whole foil thing has been way outside my experience so naturally I have previously ignored it. This is a good way of learning what the future holds. One last puzzle. These foils increase measured beam so if no one cares, why not be as wide as say a catamaran with foils extending so far the ballast keel can be left off completely.

 

[mjcoon]

They are indeed, and even when you find one, the glider is still coming down ... it's just that the air is going up faster.

That's a very relativistic definition of "coming down"!

 

[BurnitBlue]

This chap was my great great grandfather. Alex Thompson and those clever foil designers are all very well but Hargraves' box kite experiment (seen in context of his time, and "given" to his fellow inventors) is rather more interesting. To his dying day he was asked if the Wright brothers' failure to acknowledge him rankled....and always said he was delighted to have been of assistance ?

Lawrence Hargrave - Wikipedia

I recall a childrens story (Gullivers Travels?) where balloons were used to lift a boat clear of the water. Quite appropriate to this thread. I reckon I could have thought of that. But I wallow in obscurity.

 

[TLouth7]

Absolutely fascinating. I hope the thread continues as the whole foil thing has been way outside my experience so naturally I have previously ignored it. This is a good way of learning what the future holds. One last puzzle. These foils increase measured beam so if no one cares why not be as wide as say a catamaran with foils extending so far the ballast keel can be left off completely.

Certainly this is a thing, see the previous generation of America's cup boats, or any number of foiling catamarans. The reason that the current America's cup boats and IMOCA 60s are monohulls with ballast is simply that they are racing classes, and the class rules specify that they have to be monohulls.

Lifting a boat clear of the water by any means (balloons, kite etc) has the fundamental problem that you then cannot use the power of the wind to go in any direction than directly downwind. You need foils (sails and some sort of keel, daggerboard etc) in both media to be able to sail in a direction of your choosing.

 

[BurnitBlue]

Absolutely fascinating. I hope the thread continues as the whole foil thing has been way outside my experience so naturally I have previously ignored it. This is a good way of learning what the future holds. One last puzzle. These foils increase measured beam so if no one cares, why not be as wide as say a catamaran with foils extending so far the ballast keel can be left off completely.

Oops I can answer my own question. The answer is leverage if jt is a monohull.

 

[flaming]

Absolutely fascinating. I hope the thread continues as the whole foil thing has been way outside my experience so naturally I have previously ignored it. This is a good way of learning what the future holds. One last puzzle. These foils increase measured beam so if no one cares why not be as wide as say a catamaran with foils extending so far the ballast keel can be left off completely.

Several reasons.

Firstly the Keel is prescribed by the rules, it's the same on every boat. (Well, every boat built since that rules came in) as is the mast. So once you have that you're looking at the foils having the best trade off between the lift they give you and the drag they cause.
With another constraint, which is that OD mast. Currently it is the limiting factor on the whole boat, as if you increased the size of the foils (and therefore their righting moment) too much you will overload the mast and that's the end of your race. Even as it is the top boats have a lot of load monitors in the mast and rigging to sound warnings when the loads are getting too high and telling the skippers to back off a bit...

Then consider the actual conditions you will be sailing in. Teams do enormous amounts of study into where the race will be won and lost, and therefore where they want to be the absolute fastest and where they are happy to be simply ok.

 

[flaming]

Another factor is what happens when the boat is in especially strong winds, or very light winds. Some of the foils cannot be retracted very much at all. So will always be deployed. This is actually suspected to make them slower in both light and strong winds. Lighter because they will cause a lot of drag, and heavier because the skipper will have to back right off to avoid overloading the foils or boat and breaking things. Alex on Hugo Boss is therefore expected to have an advantage here over his rivals, as his constant radius foils can be essentially fully retracted.