Wednesday, October 09, 2013

Sci-Tech watch, 2: The America's Cup AC 72 and AC 45 -- the revolution of winged, hydrofoil catamarans capable of 40+ knots and sailing up to 20 degrees off the wind

AC 72 Team New Zealand's Aotearoa
in San Francisco Bay, 2013, showing
hydrofoils in action. The mainsail,
at back, is a wing. [HT: Wiki]
For years, I have hardly watched television, so I missed the catamaran revolution in the America's Cup in 2010 and now 2013. But then one afternoon I visited some friends in an office where ESPN was on on a small office TV. 

Sailing races.

I soon realised this was the America's Cup finals (a replay) 2013.

I was amazed to see wing sail catamarans, a revolution since the days of the classic monohulls with the familiar Bermuda Rigs.

Then, I noticed: they are hydrofoils too -- and of course the dominance of black told me these were carbon fibre matrix composite multi-hulls. Light, strong but running close to the limits of materials and structures, very fast. No wonder the crew were wearing hard protective helmets.

I decided to follow up, and found this Wiki article helpful (this is not a very political topic, Wiki tends to do much better on such). Clipping:

Following the 2010 America's Cup, where the Golden Gate Yacht Club's USA 17 trimaran defeated the catamaran Alinghi 5, it was decided by the winners that the next America's Cup competition would be sailed in catamarans in the hope of making the sport more attractive to television audiences.[2] At the same time a smaller sister class, the AC45, was developed to allow teams to practice and adjust to the new formula as well as create a greater exposure of sailing to the general public with the America's Cup World Series . . . . 

[The AC 72] was expected to sail faster than the wind: upwind at 1.2 times the speed of the true wind, and downwind at 1.6 times the speed of the true wind.[4][5] But in fact it proved faster, averaging about 1.8 times the speed of the wind with peaks slightly over 2.3.[6] Emirates Team New Zealand sailed at 44.15 knots (81 km/h, 50 mph) in 15.8 knots of wind (2.79 times the wind speed) on July 18, 2013. [7]

Typical racing speeds are over 30 knots (55 km/h, 34 mph) with the boats capable of sailing well over 40 knots (74 km/h, 46 mph) in the right conditions. The fastest race speed recorded was on Emirates Team New Zealand which was 47.57 knots (88 km/h, 55 mph) in 21.8 knots of wind (2.2 times the wind speed) on September 24, 2013.[8]
Such numbers are very impressive, but do not come without a price. The Swedish entry capsized and was "[d]eemed a "complete loss" in 9 May 2013 capsize resulting in the death of crew member Andrew Simpson."

Doff caps for a minute of silence, in memory to the price technological breakthroughs so often extract before they become routine -- and even after that.

Then, let us watch a brief vid:

And, when you have time, enjoy the Final, race 19 -- observing how the various aspects of the technology and the underlying science of sailing based on the lift on what is in effect a wing (and sometimes of course simply a surface for wind to press against) as well as the way the hydrofoil wings provide life by flying though the water and thus lifting hulls out sharply reducing friction and enhancing speed potential dramatically all come into play -- here:

As we watch, let us reflect on the underlying science and technology on display, and what this says to us as a region about where we need to go to be up to world class speed for C21.

The points of sail. A. in irons (into the wind);
B. close hauled; C. beam reach;
D. broad reach; E. running;
Shaded: "no go zone" [Wiki]
To give an idea of the revolution involved, sailing at 20 degrees to the wind is sailing -- at 32 points to 360 degrees, or 11.25 degrees per "point" -- within two points of the wind direction. Sailing at 45 degrees to the wind is hard, indeed, old fashioned sailing boats had a hard time tacking upwind. In the days of  Paul's famed voyage to Rome of Ac 27, 6 points off the wind was probably an effective limit, 4 points being 45 degrees.

 "Tacking" of course describes the zig-zagging pattern of sailing on one side then the other side of the wind that allows boats to progress upwind. But this is inherently wasteful as the side to side components are wasted. 

While I am at it, yes, sail boats have been using wings since the days of the old Lateen rigged triangular sail, and since the Polynesian "crab's claw" sailed catamarans that allowed settlement of the Pacific.  

Indeed, the classic Genoa and Bermuda sailing rigs do just that. Bermuda Rig:

Bermuda rig (HT: Wiki)

But then, the old fashioned kite is a wing.

What the C21 rigid wings do, is they enhance efficiency (how in 2010, the wing sailed trimaran beat the defending Catamaran Alhingi 5.)

 As Wiki discusses:

Sailboats cannot sail directly into the environmental wind, nor on a course that is too close to the direction from which the wind is blowing. The range of directions into which a boat cannot sail is called the no-go zone. Its width depends on the design of the boat, its rig, and its sails, as well as on the wind strength and the sea state. Depending on the boat and the conditions, the no-go zone may be from 30 to 50 degrees either side of the wind, a 60 to 100 degree area centered on the wind direction.
When attempting to sail into the no-go zone, the boat's sails do not produce enough drive to maintain way or forward momentum through the water. Therefore the boat will eventually coast to a stop, with the rudder becoming less and less effective at controlling the direction of travel.
A sailboat's ability to sail close to the wind is referred to as its "pointing" ability. A yacht designed primarily for racing can typically point within 30 or at worst 40 degrees from the wind direction, whereas one designed for cruising may only be able to point within 40 to 50 degrees of the wind direction. Working boats may not be able to point this well and square rigged vessels certainly cannot. The size of the no-go zone varies, but all sailboats have one.
"Within 30" degrees tells us all we need to realise.

We are now talking 20 degrees, almost straight into the (apparent) wind. (There is some interesting vector addition stuff at work when a yacht sails at various angles to the wind, of interest to students and fans of physics, cf. here. In effect, the vessel's speed and direction and that of the true wind combine to give an apparent wind speed and direction. Blend in lift and drag, and voila, boats that can sail up to twice as fast as the wind.)

Here is a discussion, with a CAD visualisation:

Telegraph has a less nerdy discussion, and this vid is worth a look:

And so, we see here a lot of science and technology, and not a little art and strategy too. Well worth pondering, and expanding on in due course. END