faster than the wind katamaran

Question

http://www.bbc.co.uk/news/business-36824256

This is the catamaran in question, and in a separate article(on the same thing) that it can travel 3x as fast as the wind...

Under everything I understand about physics, this does not work, you can't get more energy out than you put in, then can anyone explain this?

There is a hint in the fact that this happens on reaches (paths with a significant component across the wind direction) rather than when running directly downwind. It tells you that in these cases the sails are doing something a lot more sophisticated than just blocking the wind. — dmckee --- ex-moderator kitten, Oct 02 '16 at 20:42
Possible duplicates: http://physics.stackexchange.com/q/931/2451 and links therein. — Qmechanic, Oct 02 '16 at 21:32
"you can't get more energy out than you put in" hmm, do not confuse energy conservation with speed. speed is not a conserved quantity, though it can be derived from conserved quantities as the answers show. — anna v, Oct 03 '16 at 04:46
3x is entirely possible. In fact the possible speed of a sail-vehicle has no limit, except for drag. It's essentially the same principle as squeezing a thin slippery wedge. Please google "downwind faster than the wind", and see a wind-powered car that goes about 3x wind speed downwind (it also goes upwind faster than the wind). There have been numerous questions and answers about this. — Mike Dunlavey, Oct 03 '16 at 14:06

score 5 · Accepted Answer · answered Oct 02 '16 at 21:33

5

When a sail boat does not move in the direction of the wind but lets say at a vertical angle to it, the wind force on the sail has a constant force component in the direction of the movement of the boat. So it depends only on the resistive forces of the boat in the water and air that limit the speed that can ultimately attained. Therefore it should be possible for a sail boat to achieve speeds that are higher than the speed of the wind that is driving the sail. This can, for example be seen in ice yachting, where speeds can easily exceed the wind speed.

answered Oct 02 '16 at 21:33

freecharly

16,046

Nice answer. My first thought was that the maximum speed would occur in the case that the boat would move in the same direction of the wind! – Oct 02 '16 at 21:43
And of course this an 100% valid answer to the question about the wrong estimation about energies involving as it is posted in the question, and of course without claiming any 'super sophisticated ' physics of our modern era. – Oct 02 '16 at 21:54
You just forgot to mention the critical role of the centerboard (in water) or skates (on ice) which constrain the vehicle's path to its "direction of movement". Other than that, you're right on the button!

Mike Dunlavey

Oct 03 '16 at 14:15

@ Mike Dunlavey - You are right, you need, of course, a well-designed sailboat with an appropriate keel to keep the leeward drift as small as possible. Good sail boat are usually built for that. – freecharly Oct 03 '16 at 15:31

dmckee --- ex-moderator kitten · Answer 2 · 2017-03-23T01:39:25.047

The physics of sailing it interesting. It's driven by few forces deriving their effect from a very limited subset of mechanics, but the full subject is deep and the technology that has arisen to take advantage of it is very sophisticated indeed. In a pure sail boat all the forces that matter to where and how fast the vessel goes are fluid forces: drag, lift, and buoyancy.

The simplest sail is just a piece of something put in the way of the wind. With that you only get drag and can only sail downwind. Angle it to deflect the wind¹ and you can sail to one side or another but still definitely downwind. Because of drag from the water you can't go even as fast as the wind.²

But if you let the sail bulge a little and tilt it relative the wind, then the air passing over it can run along both the up- and down-wind surfaces and suddenly the sail is acting like an airfoil:³ you have lift as well as drag. With the right set of the right sail and some help from rudder and sideboard, centerboard or keel and you can sail into the wind. The earliest technologies for doing so were not particularly good, and it took a lot of tacking to make much progress upwind, but it could be done.

You can also go faster with a lifting sail. In the early versions this was still slower than the wind owning to drag, but the technology improved.

Of course, with all kinds of sails as your vessel changes velocity the relative velocity (direction as well as magnitude!) of the wind changes too. You have to re-trim the sail(s). That can add to the force and change it's direction. Better lifting sails means both more speed and tacking closer to the wind.

The latter was the biggest driver of the technology for a long time in the history of civilization. Getting half-a-point closer to the wind could cut your travel time by tens of percent.⁴

Adding a second sail (a jib) is one of the ways to improve the lift effect of a sail: the big deal isn't the extra surface area, it's the way the staggered sails can re-direct the wind more than a single sail and therefore get more lift. That (or any other improvement in the lift effect) means the ability to come still closer to the wind or still more speed in reaches.

Once you have efficient sails, if you add some kind of low-drag hull (outrigger canoe, catamaran, ice-boat, or as in the case discussed above hydrofoiling (!) catamarans) and you can go faster than the wind.

¹ Okay. So this is a subset of lift. I'm going to ignore that and use "lift" to mean the force from redirecting boundary-layer flows. Not strictly correct, but useful to make this discussion flow nicely.

² Here I assume still water. The situation is a little more complicated in flowing water (be that from rivers, tides, or inter-sea choke points like the mouth of the Mediterranean).

³ A soft airfoil whose shape is adjusted by the flow that it re-directs and is adaptively trimmed by a sailor. A lot of complexity enters here. But even trial and error allows improvements.

⁴ In the classical Roman era cargo ships could make three trips from Rome to the mouth of the Nile and back in two years, and even that meant risking the autumnal storms.⁵ Lots of people died when they didn't make it to port in time. A few percent shaved off the up-wind leg of the journey could mean your life!

⁵ Of course that meant wintering over in different cities on alternate years.

Awesome! And I'd never really thought about the historical factors at all, but once you state them, it becomes clear how important it was to improve the technology. — Selene Routley, Oct 03 '16 at 08:05

faster than the wind katamaran

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