The J40 Sailboat
Specs & Key Performance Indicators

The J40, a performance cruising boat, was designed by Rod Johnstone and built in the USA by Tillotson-Pearson.

'Blue Jay', a J40 performance cruising yacht with bluewater capability

Published Specification for the J40

Underwater Configuration: Fin Keel & Spade Rudder

Hull Material: GRP (Fiberglass)

Length Overall: 40'0" (12.2m)

Waterline Length: 34'0" (10.4m)

Beam: 12'2" (3.7m)

Draft: 6'6" (2.0m) - deep draft version, 5'6" (1.7m) - shallow draft fin keel version, 5'5" (1.6m) - wing keel version.

Rig Type: Masthead Sloop

Displacement: 18,000lb (8,165kg)

Designer: Rod Johnstone

Builder: Tillotson-Pearson

Year First Built: 1984

Year Last Built: 1994

Number Built: 86

Published Design Ratios for the J40

1. Sail Area/Displacement Ratio: 17.9

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: 36.1

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

3. Displacement/Length Ratio: 204

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

4. Comfort Ratio: 27.9

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: 1.9

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

Summary Analysis of the Design Ratios for the J40

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1. A Sail Area/Displacement Ratio of 17.9 suggests that the J40 will, in the right conditions, approach her maximum hull speed readily and satisfy the sailing performance expectations of most cruising sailors.

2. A Ballast/Displacement Ratio of 36.1 means that the J40 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 204, tells us the J40 is clearly a light displacement sailboat. If she's loaded with too much heavy cruising gear her performance will suffer dramatically.

4. Ted Brewer's Comfort Ratio of 27.9 suggests that crew comfort of a J40 in a seaway is similar to what you would associate with the motion of a coastal cruiser with moderate stability, which is not encouraging news for anyone prone to seasickness.

5. The Capsize Screening Formula (CSF) of 1.9 tells us that a J40 would be a safer choice of sailboat for an ocean passage than one with a CSF of more than 2.0.