Rigging Dimensions
The four foretriangle measurements sailmakers need to quote a new suit.
I ?
26.75 ft
J ?
8.00 ft
P ?
31.00 ft
E ?
13.00 ft
Sail Area Breakdown
Calculated from rigging dimensions. Use these as your starting point when ordering a new suit.
| Sail | Area (ft²) | Area (m²) |
|---|---|---|
| MainsailP × E ÷ 2 | 201.5 | 18.7 |
| 100% ForetriangleI × J ÷ 2 | 107.0 | 9.9 |
| 150% GenoaTypical light-air headsail | 160.5 | 14.9 |
| Storm Jib~50% of foretriangle, high-cut | 53.5 | 5.0 |
| Symmetric SpinnakerEstimated from I, J | 385.2 | 35.8 |
| Total Working Sail AreaMain + 100% foretriangle | 308.5 | 28.7 |
Foretriangle Diagram
Drawn to scale from published I, J, P, E measurements — Fractional Sloop · Ben Seaborn design
Performance Ratios
How the Thunderbird carries its sail relative to its displacement.
SA/D Ratio
20.9
Powerful cruiser/racer — quick in light air, reef early in a blow.
Mast Height (above DWL)
— ft
Air draft not published — measure yours before any bridge transit.
Ballast Ratio
—
Ballast ratio not published.
Typical Sail Inventory
What Thunderbird owners usually carry and what's worth buying used vs. new.
Mainsail 201.5 ft²
Dacron cross-cut with 2 reef points is standard. Full-batten is a common upgrade.
Replace new
150% Genoa 160.5 ft²
The workhorse headsail. Most boats have one on a furler by now.
Replace new
110% Working Jib ~118 ft²
Good secondary sail for breezy days — used market is strong.
Buy used
Storm Jib 53.5 ft²
Bright orange recommended. Rarely used, hard to justify new.
Buy used
Asymmetric Spinnaker ~385 ft²
Popular downwind upgrade — easier than symmetric for shorthanded sailing.
Optional
Replacement Cost Estimator
Get a rough price range for a new mainsail and genoa for the Thunderbird — Dacron, laminate, and cruising performance tiers.