Appendices explained. How keel and rudder act on a boat


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Appendages are the invisible but very important part on sailboats. In the photo, the keel of a ClubSwan 50 is clearly visible.

What happens below the waterline of a traditional sailboat? What is hydrodynamics and why do hull and appendages affect the speed of a boat? Starting with the basics, Professor of Naval Architecture at the University of Genoa, Paolo Gemelli*, former author for us of the articles “The Carrying Capacity of Sails,” “Mainsail and jib. How to regulate them with the help of science“, “Mainsail and jib interaction, boundary layer and forestay,” which have been widely read.

In the last episode we introduced you to the hydrodynamics of a sailboat, explaining the concepts of hull and drag. Today is the turn of the appendices: What is the best keel? What about the rudder blade? Better double or single? Not to mention foils, which are a game changer….

Hull appendages in sailboats

In the sailing world, hull appendages play a crucial role in both the performance, stability and maneuverability of boats. Their design requires careful consideration of hydrodynamics and the specific needs of the boat, whether cruising or racing.

The keel

The keel represents a fundamental element in a sailing boat: it allows for limiting the drift induced by the lateral component of the wind and limits the heeling of the hull due to air pressure on the sails. Over time, keels of different types were developed to meet usage requirements that gradually evolved over time.

Long and Full Keels: They are seen on traditional hulls and are characterized by considerable extension along the hull. They offer excellent course stability and provide good seaworthiness at the expense, however, of speed and maneuverability.

Fin keels: Shorter longitudinally and with greater vertical extension, fin keels improve boat performance especially in upwind sailing. They can be fixed or retractable, the latter useful for reducing draft in shallow water.

Bulb keels: They have a concentrated weight at the lower end (the bulb) aimed at maximizing transverse stability through increased righting moment, but minimizing increased draft.

This configuration is particularly advantageous for racing boats, where a low center of gravity improves performance without compromising safety.

Canting Keels (Canting Keel): They represented an innovation in the field of racing boats, allowing weight to be shifted sideways to increase the lever arm when the boat is tilted, significantly improving speed and performance in upwind sailing.

Appendices: rudders, single vs. double shovels

The choice between single- or double-bladed rudder strongly affects the boat’s maneuverability and control.

Single-Bladed Rudder: Traditional and commonly used on boats of all eras, it offers good course stability but can become less effective in heavy heeling conditions or with wide-beam boats.

Double-bladed rudder: Provides more control and responsiveness, especially in strong winds and rough seas. The double-bladed configuration is ideal for boats with wide beams, improving steering ability when the boat is tilted because at least one of the two rudders remains submerged, maintaining effective directional control.

The design of the appendices

The design of hull appendages represents a complex engineering process that requires the comprehensive analysis of the dynamics between the structural components of the boat and the aquatic environment in which it will be used. Through the use of advanced computational fluid dynamics (CFD) solutions and algorithm-based optimization methodologies, it is possible to simulate and examine the impact of different appendage configurations on overall boat performance.

These advanced technological tools allow targeted refinement of the morphology and dimensions of keels, rudders, and other appendages to achieve the optimal synergy between speed, stability, and maneuverability, meeting the specific needs of the crew and anticipating anticipated sea conditions.

Airfoil aerodynamic theory provides a solid foundation for qualitatively and phenomenologically interpreting and justifying fluid-dynamic interactions related to hull appendages. Such a theoretical basis facilitates an in-depth investigation of the interaction of these structures with the surrounding fluid, crucially affecting the boat’s performance and course stability.

Airfoils adopted in the marine and nautical context are characterized by a marked elongation in one of two dimensions, typically presenting a rounded leading edge and a sharp trailing edge.

Appendices - NACA 63-010
Fig. 1 – NACA profile 0012 at the bottom, NACA profile 63-010 at the top. Wing profiles such as 63-010 are particularly used for the design of hull appendages and are characterized by a more elongated shape than profiiles such as NACA 0012 put in for comparison.


When analyzing the forces exerted by a keel, given its remarkable similarity to an airplane’s wings, it becomes apparent that the fundamental properties of strength and lift are intimately related to the type of airfoil selected.

Fig. 2 – Trend of the lift-to-strength ratio of the NACA 63-010 profile as the angle of attack changes. It is observed that when the profile maintains an angle with respect to the fluid direction of less than about 4.5°, lift increases significantly with respect to drag and then drops sharply for higher angles of attack.

However, it is essential to remember that there are substantial divergences between the theoretical model of the two-dimensional section (or, more precisely, the wing of infinite aperture as depicted in Fig.2) and the practical context of a keel operating on a hull. The designer must therefore consider these distinctions at the design stage and take into account the main variations, which occur in terms of the strength and bearing capacity generated by the appendix attributable to various factors:

– The geometric configuration of the appendage
– The aspect ratio (in this case ill ratio of the length – how far it extends below the keel – to the average width in the longitudinal direction)
– The morphology of the end of the appendage
– The interaction with the free surface

The introduction of foils

However, the introduction of foils was perhaps the most significant innovation of recent decades, radically changing not only the design but the very concept of sailing.

These very special appendages require in-depth discussion to which we will devote ourselves in one of the next articles.

Who is our “prof”

*PaoloAndrea Gemelli is a lecturer in Naval Architecture in the Nautical Product Design degree program at the University of Genoa. From 1999 to the present, he has been involved in maritime security with a focus on weather routing and naval intelligence. He is a member of the expert panel of the European Maritime Safety Agency (EMSA) and the Italian Association of Intelligence and Geopolitical Analysts.



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