South Atlantic Windvane: Comparative Technical Analysis

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Key Negative Observations
The Physics of Off-Center Mounting
Auxiliary Rudder Submersion Requirements

Key Negative Observations: Hydrovane, South Atlantic (S 500/600), and Windpilot (Pacific Plus)

Negative analysis of auxiliary rudder systems for offshore sailing

Steering Independence: Hydrovane, South Atlantic (S 500/600), and Windpilot (Pacific Plus) are the only systems that feature an independent rudder, which does not rely on the vessel's main rudder to steer.
Hydraulic Steering: For vessels equipped with hydraulic steering, auxiliary rudder systems (Hydrovane, Windpilot Pacific Plus, South Atlantic S 500/600) are the technically correct choice. This is because pure servo-pendulum systems (such as Monitor or Aries) struggle with the "creep" (internal fluid bypass and heading drift) inherent to hydraulic setups.
Power vs. Sensitivity: Servo-pendulum systems (Aries, Windpilot Pacific, SA S 301-470) generate massive steering forces that increase proportionally with boat speed, making them ideal for heavy displacement vessels with manual tiller or mechanical wheel steering.
Installation Simplicity: South Atlantic stands out with a design that allows for owner installation in a record time of 2 to 4 hours, utilizing highly versatile mounting arrangements.

The Physics of Off-Centre Mounting: Why Hydrovane's Recommendation is Misleading

Photograph of a boat's heel affecting the immersion of an off-centre rudder

The Hydrovane is frequently recommended by its manufacturer for off-centre installations (away from the centerline) on the grounds that it is an independent auxiliary rudder system and that, according to University of Southampton tests, it is "completely indifferent" to its location as long as it receives "clean water". In fact, the manufacturer claims that over 75% of their current installations are off-centre.

However, from a technical and physical standpoint supported by other industry experts in the reference material, this recommendation can be considered misleading and false, based on the following points:

Hydrovane itself acknowledges in its (technical manual and FAQ) that the blade can rise too far out of the water. What they do is minimize the practical importance of that fact ("but beating is the easy part to steer", implying that less rudder power is needed at that point of sail) — they do not deny the physical phenomenon, they reframe it as operationally irrelevant. This is a direct admission, in their own technical literature, of precisely the phenomenon we describe: beating + heel + windward side (it is precisely the scenario that the formula h=d·sinθ predicts as the worst-case scenario).
The Laws of Physics are Universal: Experts like Peter Förthmann (Windpilot) point out that basic physical principles cannot be ignored for marketing convenience. If a vessel heels to the opposite side of where the equipment is mounted (the windward side), the rudder blade will rise partially or completely out of the water.
Loss of Effectiveness: A rudder surface that is not submerged "is as if it weren't there." Paul Elmers of South Atlantic warns that if a rudder is installed on the port side, for example, it will be very efficient when heeling to port, but useless when heeling to starboard as it will rise out of the water.
Strict Off-Centre Limits: While Hydrovane promotes unrestricted off-centre mounting, South Atlantic establishes a maximum limit of approximately 30 cm to avoid critical performance degradation. On their part, Monitor manufacturers are even more categorical, describing off-centre mounting as potentially "disastrous" because the blade would submerge too deeply on one tack and rise completely out of the water on the other.
Laminar Flow Issues: In the case of multihulls, warnings are issued against installing the equipment on one side of a hull (off-centre), as this exposes it to turbulent laminar flows that alter the system's operation. The only location with symmetrical flow is the centerline.

Conclusion

Both the geometry and the images themselves demonstrate measurable physical effects.

An off-center auxiliary rudder installation is not merely an aesthetic choice. It introduces a geometric asymmetry that can reduce steering effectiveness, with the effects becoming more significant as the lateral offset and the vessel's angle of heel increase.

For a system whose entire purpose is to keep a boat on course — day and night, in changing conditions — these effects are not something that should be dismissed or ignored.

There is a substantial and established body of research — in the Netherlands (Delft, one of the leading centers in the field), Australia, and the United States — specifically addressing the relationship between heel angle, rudder immersion, and the steering force a rudder can generate.

This is not an unstudied subject; it has been an active area of sailing yacht hydrodynamics research for decades.

None of these papers was specifically intended to study off-center windvane installations. However, taken together, they independently demonstrate, through different approaches and experimental data, the fundamental physical principle discussed at the beginning:

The immersion of the rudder blade is a key variable governing its ability to generate hydrodynamic force, and vessel heel directly alters that immersion.

Auxiliary Rudder Submersion: The 1:3 Rule and Physics

Close up of an auxiliary rudder blade submersion in water

If an auxiliary rudder blade with a surface area of 0.30 square meters has half of its area out of the water, the chances of it steering effectively are extremely low to non-existent.

Based on the physical and technical principles described in the industry sources, here is the detailed breakdown:

The 1:3 Proportion Rule: For an auxiliary rudder system to function correctly, its effective surface area must be approximately one-third (33%) of the vessel's main rudder area. If the blade measures 0.30 m² but only 0.15 m² is submerged, the ratio relative to the main rudder becomes insufficient to generate the hydrodynamic lift required to hold a course.
Unsubmerged Surface is Non-Existent Surface: Industry sources are uncompromising on this point: "any rudder surface out of the water is as good as not being there." A rudder that is not fully immersed cannot process laminar water flow correctly and loses its ability to generate the necessary lift to swing the vessel's stern.
Loss of Control Due to Heeling: This issue becomes critical when the vessel heels to the side opposite the gear installation (the windward side). Experts note that under these conditions, the rudder becomes "useless" as it clears the water, leaving the vessel without mechanical self-steering.
Operational Consequences: If the auxiliary rudder becomes under-proportioned due to lack of immersion, it will fail to counteract the forces causing the boat to yaw. To regain any semblance of steering control, the skipper would be forced to drastically reduce sail area, severely impacting cruising speed and performance.

In conclusion, an auxiliary rudder designed with a surface area of 0.30 m² must be fully submerged to be properly effective. With only half of the blade in the water, the system ceases to be a reliable self-steering gear and becomes, at best, an unstable directional aid.

Technical Resources & Guides

Explore our complete technical library in the FAQ and Technical Reports sections. Our technical documentation covers:

Technical Resources & Bibliography

Technical documentation covers the following references:

Polytechnic University of Madrid (ETSIN), Department of Naval Architecture.
Zamora-Rodríguez, Izquierdo-Yerón and Botia Vera conducted experimental tests in a 100 m towing tank, complemented by CFD (Computational Fluid Dynamics) simulations, to investigate how vessel heel affects rudder hydrodynamic performance. The study demonstrates that the lateral force generated by a rudder varies significantly with the vessel's heel angle: it increases when the rudder is on the windward side and decreases when it is on the leeward side. The results also show that the rudder's degree of immersion and its orientation relative to the incoming flow are critical factors governing the steering force it can generate.
TU Delft (Netherlands) — the most established research group in the field Keuning, Vermeulen, Katgert and others, from the Ship Hydromechanics Laboratory at Delft, have a specific line of research on this subject, with two relevant papers:
“The Yaw Balance of Sailing Yachts Upright and Heeled” — uses data from the DSYHS (Delft Systematic Yacht Hull Series) and the DSKS (Delft Systematic Keel Series), systematic series of tests involving different hulls, keels and rudders, both upright and heeled. • “Optimization of Upwind Sailing Applying a Canting Rudder Device” — tests conducted at the Delft laboratory using a 1992 America's Cup model, measuring rudder force separately under upright and heeled conditions. Delft is probably the world's most widely cited academic center for sailing yacht hydrodynamics. Its systematic series (DSYHS) have been a standard reference in sailing yacht design since the 1970s.
Australian Maritime College
“The Effect of Heel Angle and Free-Surface Proximity on the Performance and Strut Wake of a Moth Sailing Dinghy Rudder T-Foil” — experimental tests conducted in the towing tank at the Australian Maritime College. This is the most directly relevant study of all: it explicitly measures how heel angle and proximity to the free surface — that is, how close the rudder blade is to the air — affect rudder lift and drag, while varying immersion depth, angle and speed.
Naval Surface Warfare Center (Carderock), USA
A paper presented at the 18th Chesapeake Sailing Yacht Symposium (2007) describes tests in which sailing yacht rudders were instrumented with strain gauges and tested in a towing tank at different speeds, heel angles, and wave conditions. The study explicitly documents the case in which, at approximately 15° of heel, the upper part of the rudder reaches the free surface — in other words, it experimentally examines precisely the phenomenon in question: what happens to the rudder blade as heel brings it closer to emerging from the water.
University of Duisburg-Essen (UDE), Germany, in collaboration with the Federal Waterways Engineering and Research Institute (BAW) and the Development Center for Ship Technology and Transport Systems (DST).
Numerical and Experimental Investigation of Rudder-Induced Hydrodynamic Forces

Approach:

Experimental investigation in a towing tank combined with CFD simulations to measure lateral forces, hinge moments, and pressure distributions at high angles of attack.

Key contribution:

Fundamental to understanding the behavior of a primary or auxiliary rudder when a vessel undergoes severe yawing and extreme maneuvering conditions, such as broaching — a critical situation encountered particularly in lightweight performance sailing yachts and ocean-going sailing vessels.
Technical University of Denmark (Danmarks Tekniske Universitet – DTU), Department of Mechanical Engineering.
Core research team:
- Stig Staghøj Knudsen — Principal researcher and author of the PhD thesis.
- Jens Honoré Walther — Professor and principal project supervisor.
Technical University of Denmark (DTU)
Dynamic Fluid-Structure Interaction (FSI) of Racing Sailboats

Approach:

Developed at DTU, this work applies fluid-structure interaction models incorporating flexible hulls, keels, rudders, and foils operating in waves.

Key contribution:

Demonstrates how rudder blade flex under extreme loading conditions alters the effective angle of attack and overall drag. In high-performance sailing yachts, the torsional stiffness of the rudder blade helps prevent cavitation and loss of steering control (stall).