The Westerly Tempest 31 Sailboat
Specs & Key Performance Indicators

The Westerly Tempest 31, a masthead sloop, was designed by Ed Dubois and built in the UK by Westerly Marine Ltd.

Westerly Tempest 31

Published Specification for the Westerly Tempest 31

Underwater Profile: Fin Keel & Spade Rudder

Hull Material: GRP (Fibreglass)

Length Overall: 30'7" (9.3m)

Waterline Length: 24'6" (7.5m)

Beam: 10'10" (3.3m)

Draft: 5'0" (1.5m)

Rig Type: Masthead sloop

Displacement: 9,019lb (4,019kg)

Ballast: 2,690lb (1,220kg)

Designer: Ed Dubois

Builder: Westerly Marine Ltd (UK)

Year First Built: 1987

Year Last Built: 1993

Number Built: 107


Published Design Ratios for the Westerly Tempest 31

1. Sail Area/Displacement Ratio: 14.8

  • 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: 29.8

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

3. Displacement/Length Ratio: 274

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

4. Comfort Ratio: 22.2

  • 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: 2.1

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

Read more about these Key Performance Indicators...


Summary Analysis of the Design Ratios for the Westerly Tempest 31

1. A Sail Area/Displacement Ratio of just 14.8 suggests that the Westerly Tempest 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 29.8 means that a sailboat like the Westerly Tempest 31 (which doesn't have a stiffness-enhancing bulb keel), is likely to benefit from being reefed early to keep her sailing upright in a moderate breeze.

3. A Displacement/Length Ratio of 274, tells us the Westerly Tempest 31 is a moderate displacement cruiser, which means she'll carry all your cruising gear without it having a dramatic effect on her performance. Most of today's sailboats intended for offshore cruising fall into this displacement category.

4. Ted Brewer's Comfort Ratio of 22.2 suggests that crew comfort of a Westerly Tempest 31 in a seaway is similar to what you would associate with the motion of a coastal cruiser with moderate stability, which is not the best of news for anyone prone to seasickness. 

5. The Capsize Screening Formula of 2.1 tells us that a Westerly Tempest 31 would not be as good a choice of sailboat for ocean passage-making, owing to the increased risk of capsize in strong winds and heavy seas when compared to a sailboat with a CSF of less than 2.0.

Other sailboats in the Westerly range include:

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