The Southern Cross 31 Sailboat

The Southern Cross 31, a long-keeled cutter, was designed by Thomas Gilmer and built in the USA by Clark Ryder.

A Southern Cross 31 sailboatThe Southern Cross 31 long-keeled cutter

Published Specification for the Southern Cross 31

Underwater Profile: Long keel with transom-hung rudder

Hull Material: GRP

Length Overall: 31'0" / 9.5m

Waterline Length: 25'0" / 7.6m

Beam: 9'6" / 2.9m

Draft: 4'6" / 1.4m

Rig Type: Cutter

Displacement: 13,600lb / 6,169kg

Designer: Thomas Gilmer

Builder: Clark Ryder (USA)

Year First Built: 1975

Number Built: 150

Owners Association: Southern Cross Owners Association


Published Design Ratios for the Southern Cross 31

1. Sail Area/Displacement Ratio: 12.6

  • Less than 16 would be considered under-powered;
  • 16 to 20 would indicate reasonably good performance;
  • Over 20 suggests relatively high performance.

2. Ballast/Displacement Ratio: 32.4

  • Under 40: less stiff, less powerful
  • Over 40: stiffer, more powerful

3. Displacement/Length Ratio: 389

  • Under 100: Ultralight
  • 100 to 200: Light
  • 200 to 275: Moderate
  • 275 to 350: Heavy
  • Over 350: Ultraheavy

4. Comfort Ratio: 37.6

  • Under 20 indicates a lightweight racing boat
  • 20 to 30 indicates a coastal cruiser
  • 30 to 40 indicates a moderate offshore cruising boat
  • 40 to 50 indicates a heavy offshore boat
  • Over 50 indicates an extremely heavy offshore boat

5. Capsize Screening Formula: 1.6

  • Under 2.0 (the lower the better): Better suited for ocean passages
  • Over 2.0: Less suited for ocean passages

read more about these all-revealing numbers...


Summary Analysis of the Design Ratios for the Southern Cross 31

1. A Sail Area/Displacement Ratio of just 12.6 suggests that the Southern Cross 31 will need a stiff breeze to get her going. In light conditions, unless you've got plenty of time on your hands, motor-sailing may be the way to go.

2. A Ballast/Displacement Ratio of 32.4 means that unless the bulk of the ballast is concentrated in a bulb at the foot of her keel, the Southern Cross 31 will have a tendency to heel excessively in a gust, and she'll need to be reefed early to keep her sailing upright in a moderate breeze. 

3. A Displacement/Length Ratio of 389, tells us the Southern Cross 31 is firmly in the ultra-heavy displacement category. Load her up as much as you like and her performance will be hardly affected, not that it was ever startling. Few if any sailboats are built to this displacement category these days - but they remain popular with some long-distance sailors.

4. Ted Brewer's Comfort Ratio of 37.6 suggests that crew comfort of a Southern Cross 31 in a seaway is similar to what you would associate with the motion of a moderate bluewater cruising boat - a predictable and acceptable motion for most seasoned sailors.

5. The Capsize Screening Formula (CSF) of 1.6 tells us that a Southern Cross 31 would be a safer choice of sailboat for an ocean passage than one with a CSF of more than 2.0. 

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