KEY POINTS:
The skirts came off the America's Cup boats this week, revealing a range of different designs. On display were long bulbs, short bulbs, fat bulbs, skinny bulbs, horizontal wings, not so horizontal wings, and many different hull shapes.
But what does a keel bulb do, what is the
difference between a long bulb and a short bulb and how do the different hull shapes come into play?
What does the keel do?
The keel provides the lift to power the boat forward and stop it sliding sideways. The bulb, which weighs around 19 tonnes, provides the stability to counterbalance the enormous force of the sails and prevents the boat from falling over.
What is the difference between a long bulb and a short bulb?
A long, slender bulb reduces the centre of gravity of the keel by placing more material lower down, which increases stability and hence increases the power that can be developed. The tradeoff for the longer bulb is that it has more wetted surface which means it can be stickier in light wind. Short, fat bulbs have less wetted surface area than long bulbs and therefore present less drag in light winds. Short, fat bulbs are also better in rough water when the boat is pitching as they present less drag.
Why did some teams reveal one long bulb and one shorter one?
Auckland yacht designer Greg Elliot says that hydrodynamically the differences between the two shapes are not as great as people like to make out. Having two different bulb shapes may mean the teams are trying to cover one another - if you are racing someone you may want to match their configuration - or it may be because of the wind conditions. During April and half of May the wind is expected to range between seven and 20 knots. From late May until July it is expected to settle between seven and 13 knots.
What are the wings on the bulb?
Positioned on the bulb, the wings help to improve the performance of the keel fin upwind by making the fin think that it is longer than it really is. Teams can alter their upwind and downwind performance by using different length wings. The angle of the wings and whether they have any twist along their length can also have a big effect on performance. The longer the wings, the better the potential upwind performance. The downside is that wings do not provide an advantage downwind. Instead, the longer the wing, the more drag. Most wings revealed were positioned near the middle of the bulbs, however Oracle's USA87 had its on the back. The winglets on Team NZ's NZL84 were very low.
What about the rudder?
The rudder is a vertical hinged piece that goes into the water at the stern and is used to steer the boat. Teams are able to experiment with its shape and length, bearing in mind drag.
And different hull shapes?
A lot of teams appear to have optimised their second boats towards lighter breezes with narrower hulls and more slab-sided topsides. Both features allow the boat to heel more quickly to the breeze, which in turn helps them achieve a longer waterline length more quickly and generate more speed. Of the top four, the Oracle boats were more rounded in sectional shape and the Luna Rossa the most boxed, which means they had flat bottoms and almost vertical sides.
The round shapes have less wetted surface and are better for light winds. The boxed shape has more stability by being slightly wider at the waterline. With narrow boats the buoyancy needed to support the boat has to be placed elsewhere. The result is that the ends of the boats are now generally fuller.
* The third day of the 13th pre-regatta was called off because of poor weather. Intermittent wind pockets blew across the course but a lack of consistent weather left the fleet waiting on open water nearly four hours before the fourth and fifth races were postponed. After three races, Alinghi lead the fleet racing regatta with 33 points. BMW Oracle Racing and Team New Zealand are tied second, with 27. With similar weather predicted for day four, teams are likely to race on the reserve day with points from Act 13 counted.