New developments in rudder layouts

ledNew developments in rudder layouts may be found in the application of twin rudders. They are typically applied on boats with beamy aft sections. These boats would lift the centreline rudder out of the water to a considerable extent when heeled.

The twin rudder arrangement yields more control when heeled because in those conditions always one rudder always remains fully submerged and almost vertical. Also when running downwind the twin rudders supply the necessary directional control. A disadvantage is found in the fact that these twin rudders are placed off centre and therefore are no longer protected against collision with debris etcetera by the keel placed in front of them, the advantage being that the chance of damaging them both at the same time is small.

In general it should be mentioned that any rule, if applicable, does not allow the use of materials for ballast with a higher specific weight density then lead. So in a way the choice is limited. When the keel was an integral part of the hull the material of choice was the same as for the hull, i.e. wood and only the ballast part of the keel was fabricated from either lead or cast iron. In the transition from wood to newer hull construction materials for yachts, such as steel, aluminium alloy and (glass) fibre reinforced polyester, also new construction material for the appendages were used. In steel and aluminium alloy hulls the keels were often made also of the same material as the hull and the ballast was poured in from the outside after finishing of the hull. This implied that usually lead was used. In more modern hulls from these metals still the keels are made from the same materials, welded to and being integral part of the hull. Even when the keel is not faired into the hull over a considerable of its length as with the wooden hulls The choice of materials for the keels, both when the keel is a separate appendage and when it is structurally independent, is influenced by a number of considerations: i.e.:

  • the structural requirements of the separate fin,
  • its connection to the hull,
  • the specific weight density of the materials used
  • the cost involved.

Two materials dominate the field:

  • Lead. Lead has the higher specific weight density of the materials allowed. It comes close to 11 tons per cubic meter depending on its purity, but is quite expensive. In particular because some 4% of its weight has to be added as antimony to the pure lead to give it more strength. Typical application of this choice is in the more performance orientated designs. also the health and environmental precautions, which have to be taken during melting and pouring, contribute to the cost.
  • Cast iron. Cast iron has a specific weight density of around 7.8 tons per cubic meter but is structurally sound on its own. It is much cheaper, both in acquisition and in manufacturing. Also casting is much more common practice and less of a hazard. Typical application of the cast iron keel is therefore in the cheaper series production boat market though it often leads to corrosion problems.

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