The Southern Cross 39 Sailboat
Specs & Key Performance Indicators

Published Specification for the Southern Cross 39 Sailboat

Keel & Rudder Configuration: Fin keel with rudder on skeg
Hull Material: Fiberglass
Length Overall: 11.9m (39'0")*
Waterline Length: 9.5m (31'0")*
Beam: 3.9m (12'1")*
Draft: 1.6m (5'4")*
Rig Type: Cutter
Displacement: 9,525 kg (21,000 lbs)*
Ballast: 3,482 kg (7,676 lbs)*
Sail Area: 71.9m² (774ft²)*
Water Tank Capacity: 454 L (120 gal)
Fuel Tank Capacity: 189 L (50 gal)
Hull Speed: 7.7 knots
Designer: Thomas Gillmer
Builder: C. E. Ryder
Year First Built: 1981
Year Last Built: 1990
Number Built: Unknown

* Used to derive the design ratios referred to later in this article - here's how they're calculated...

Options & Alternatives

The Southern Cross 39 sailboat remained largely unchanged throughout its production run, but there were some refinements over time.

Early models featured a V-drive transmission with an exposed propeller, while later versions moved the propeller to an aperture in the rudder for better protection.

Additionally, the first 13 hulls were factory-built, while subsequent boats were available as kits for owner completion. Some of these owner-finished boats were later designated as Gillmer models to distinguish them from factory-completed versions.

Sail Areas & Rig Dimensions

Sail Areas & Rig Dimensions

Sail Areas

• Mainsail: 30.9m² (332ft²)
• Foresail: 41m² (441ft²)

Rig Dimensions

• I: 15.5m (50'10")
• J: 5.3m (17'4")
• P: 13.7m (44'11")
• E: 4.5m (14'9")

Published Design Ratios
The Key Performance Indicators (KPIs)

The design ratios of the Southern Cross 39 reveal its characteristics as a stable, bluewater cruiser with balanced sailing performance:

• Sail Area/Displacement Ratio (16.33): This indicates moderate performance, meaning the boat is neither sluggish nor excessively fast. It provides steady power without being overly demanding on the sails.

• Ballast/Displacement Ratio (36.55%): A relatively high ballast ratio suggests strong stability, allowing the boat to recover well from heeling and providing confidence in offshore conditions.

• Displacement/Length Ratio (315): This places the Southern Cross 39 in the heavy displacement category, which translates to comfortable motion, absorbing wave impacts smoothly rather than bouncing excessively.

• Comfort Ratio (35.2): With a high comfort ratio, the boat is well-suited for extended cruising, ensuring a stable ride in rough seas and reducing fatigue for crew members.

• Capsize Screening Ratio (1.8): A low capsize screening ratio suggests strong resistance to capsizing, reinforcing the boat's capability as a safe ocean-going vessel.

Overall, the Southern Cross 39 is designed for long-distance cruising, with a predictable, comfortable motion, solid offshore stability, and reasonable sailing performance without sacrificing safety.

The Hidden Depths: Why Sailboat Ratios Only Tell Part of the Story

Sailboat design ratios like Displacement-Length Ratio (D/L), Sail Area to Displacement (SA/D), and Ballast Ratio offer useful starting points. They can give you a quick glimpse into a boat's potential. But don't be fooled; these numbers fall far short of truly defining how a sailboat handles. There are some significant blind spots.

Beyond the Numbers: What Ratios Miss

Think of these ratios as shorthand – they're just averages. They boil down intricate hydrodynamic and aerodynamic interactions into a single figure, often missing the subtle elegance of a well-designed hull shape, the precision of keel and rudder profiles, or the finely tuned sail plan distribution.

• Dynamic Behavior Isn't Static: Sailboats are alive on the water, constantly shifting in three dimensions. Ratios, by their nature, are static calculations. They simply can't predict how a boat will truly behave when battling waves, gusts, or sudden shifts in wind.

• Weight's Secret Influence: Yes, displacement is factored in, but where that weight sits is crucial. Its longitudinal, vertical, and athwartships distribution profoundly impacts pitching, rolling, and overall stability. For instance, a low center of gravity (CG) from deep ballast is far more effective than the same weight perched higher up.

• "Real-World" vs. "Design" Loading: Published ratios often rely on "light ship" or "design" displacement. The reality? Most cruising boats rarely sail like that. Pile on the gear, provisions, and crew, and the actual displacement changes dramatically, completely altering how the boat performs.

The Crucial Context: When Ratios Fall Short

• Not All Boats Are Equal: Ratios are most insightful when comparing similar types of boats with aligned design philosophies. Try to compare a sleek, light-displacement racer with a beefy, full-keel cruiser using just ratios, and you'll likely get a misleading picture.

• Purpose Matters: What's "good" for a racing machine might be a nightmare for a comfortable cruiser. A high SA/D is fantastic for chasing light air around the cans but could mean a perpetually overpowered boat that demands constant reefing on an offshore passage.

• Designer's Dilemmas: Every sailboat design is a dance of compromises. Ratios won't tell you about those deliberate trade-offs, like optimizing for raw speed over seakindly comfort.

Unseen Details: The Parts Ratios Ignore

• The Keel and Rudder Story: Ballast ratio just tells you how much ballast there is, not where it is or the shape of the keel. A deep fin keel with a lead bulb provides far more righting moment and better upwind performance than the same weight of internal ballast or a shallower keel, even if the ballast ratio is identical. The aspect ratio, foil sections, and planform of keels and rudders are critical for lift and drag—details simple ratios can't capture.

• Beyond Basic Sail Area: SA/D often uses a standard calculation (e.g., 100% foretriangle plus mainsail). This might not reflect the actual sail area a boat uses, like overlapping genoas, staysails, or Code 0s. And let's not forget the crucial role of individual sails' shape and efficiency.

• Subtle Hull Forms: Ratios won't reveal the nuances of hull form, such as the prismatic coefficient (how full the ends are), the waterline shape (U-shaped versus V-shaped sections), or the half-angle of entry. All of these deeply influence a boat's resistance and seakeeping ability.

The Human Element: Feel and Experience

• Comfort in a Seaway: Ratios give no hint of how a boat feels when the waves kick up. Its motion, pitching, rolling, or whether it's a perpetually wet ride—these are subjective but vital for crew comfort and endurance on long voyages.

• Handling Her Temperament: How easily a boat steers, tacks, or maneuvers in tight spots isn't found in a ratio. That's down to rudder size, balance, keel placement, and overall hull trim.

• The Craftsmanship Factor: Even a boat with "good" ratios can be a pain to sail if it's poorly built, badly rigged, or fitted with inefficient hardware.

The Whole Picture
Ultimately, design ratios are a decent starting point for broad comparisons. They provide a quick quantitative snapshot. But understand this: they're a massive simplification of a truly complex, dynamic system.

To genuinely grasp a sailboat's characteristics, you need to dig deeper: examine the full design intent, detailed lines plans, stability curves (like the GZ curve), and, ideally, get some real-world sailing experience aboard.

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