Keel fin
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Under a keel fin is defined as the connector between the hull of a yacht and the severe, often made of lead produced, keel bulb .
The keel fin of an America's Cup yacht is around three meters long and weighs around a ton. It is made of high-strength steel in a few special factories in Germany (BMW Eisenach plant for the BMW Oracle Racing team ), France and New Zealand. The keel fin and the keel bomb have to balance forces of up to 500,000 Newtons that act on the approx. 800 square meter sail area of an AC yacht. The AC teams regularly do a lot of secrecy about the manufacture of the keel fin and the connection between keel fin and boat hull.
In normal sailing conditions, the keel fins are subject to permanent mechanical vibrations. The reason is that the boat designers try to concentrate as much weight as possible in the deepest possible keel bomb (approx. 20 tons for America's Cup yachts) in order to ensure sufficient stability of the boat through the righting moment . The regatta yacht can sail more upright and faster. Furthermore, one tries to achieve that the shapes of the keel fin and the keel bomb offer minimal resistance to the flow of the water, i.e. allow the yacht to glide through the water with optimal efficiency. The shape of the keel fin is designed in such a way that it tapers from the hull to the keel bomb.
Sailing on a regatta yacht - even on a straight course through the water - creates vibrations that put stress on the keel fin in particular. Heavy seas and hard turns (in the pre-start phase in the match race ) also place high loads on the keel fin due to the inertia of the heavy keel bomb.
Another complication is related to the fact that on modern racing yachts the keel fin can be moved sideways in order to swing the keel bomb out to the side by means of the motor (in an emergency also by hand crank operation). In this way, the righting forces can be increased in higher winds and larger sail areas can be sailed. The aim is to achieve higher wind speeds. As was recently shown in the course of the Vendée Globe 2008/2009, which was contested by Open 60 yachts, the upper connections between the keel fin and hull in the area of the swing-out motors are in danger of breaking in rough weather, which led to several failures at the regatta .