Inputs
P and E define the base triangle. Reef reductions are approximate vertical luff reductions, not distances along the leech or positions copied from another sail.
Mainsail planning
Mainsail reefing-area calculator
This simplified geometric estimate cannot determine safe reef locations, loads, balance or hardware engineering.
Worked example
Example: two slab reefs
With P 13.2 m and E 4.5 m, the triangular base area is 29.7 m². Reducing effective luff to 11.4 m gives about 25.7 m² before leech geometry; 9.8 m gives about 22.1 m².
Tool guide
How to use and interpret this tool
It shows how simplified mainsail area falls as the effective luff is shortened at each reef.
Decision logic
Base triangle = ½ × P × E. At each reef, P is reduced by the entered vertical amount and the simplified triangle is recalculated.
Read the result
Use the comparison to ask about reef depth and balance. The tool cannot calculate safe loads, real reefed geometry, leech position or the headsail combination.
Worked example
With P 13.2 m and E 4.5 m, base area is 29.7 m². Effective luffs of 11.4 m and 9.8 m give simplified areas of 25.7 m² and 22.1 m².
Common mistakes to avoid
- Copying reef depths from an unrelated boat
- Assuming percentage area reduction predicts helm balance
- Ignoring reef hardware, line angles and reinforcement
Professional verification
This tool supports briefing and comparison. It does not replace measurement on the boat, structural engineering, class or safety rules, or a sailmaker's final design.
Continue the decision