South Atlantic Windvane: Comparative Technical Analysis

• 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 (Monitor, 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 Hydrovane is frequently recommended by its manufacturer for off-center (offset) installations under the argument that it is an independent auxiliary rudder system and that, according to testing by the University of Southampton, it is "completely indifferent" to its positioning as long as it receives "clean water." In fact, the manufacturer claims that more than 75% of its current installations are offset.

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

• 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 away from the side where the equipment is mounted (the windward side), the rudder blade will be partially or completely lifted out of the water.
• Loss of Effectiveness: A rudder surface that is not submerged "is as good as not being there." Paul Elmers from South Atlantic warns that if a rudder is installed on the port side, for example, it will be highly efficient when heeling to port, but useless when heeling to starboard as it lifts out of the water.
• Strict Off-Center Limits: While Hydrovane promotes unrestricted offsetting, South Atlantic establishes a maximum limit of approximately 30 cm to prevent critical performance degradation. For their part, the manufacturers of Monitor are even more definitive, labeling off-center mounting as potentially "disastrous" because the blade would plunge too deep on one tack and lift completely out of the water on the other.
• Laminar Flow Issues: In the case of multihulls, warnings are issued against installing the gear on one side of a hull (off-center), as this exposes it to turbulent laminar flows that disrupt system operation. The only location with symmetrical flow is the centerline.

In conclusion, although the Hydrovane’s design mechanically permits off-center mounting to clear transom space (for swim ladders or gangways), the physical reality of heeling dictates that performance will be seriously compromised when sailing on the tack opposite to the mounting side.

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.