MARITIME – AILING PERFORMANCE CALCULATOR
Forestay Sag
A precise tool.
What is the Forestay Sag & How does it work?
The forestay is the primary forward‑supporting cable on a sloop, keeping the mast upright and transmitting aerodynamic forces from the headsail into the hull.
When wind pushes on the headsail, the forestay experiences a distributed load that causes it to sag; this sag reduces the effective mast rake and can alter the boat’s pointing ability.
y = frac{w L^{2}}{8 T}
y = sag (m)
By estimating the sag using the simple beam‑deflection formula, sailors can decide whether to increase tension, reef the sail, or adjust the rig to maintain optimal performance.
Parameters
Result
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Frequently Asked Questions
What does forestay sag affect?
Forestay sag reduces mast rake and can alter the boat’s pointing ability.
How do I calculate forestay sag?
Use the formula y = (w L^2) / (8 T), where w is wind load, L is length, and T is tension.
What happens if forestay sag is too much?
Excessive sag can decrease the mast’s stability and affect the boat’s performance.
How does increasing tension help with forestay sag?
Increasing tension reduces sag by distributing the load more effectively along the forestay.
Can I use this formula for any sailboat type?
This formula is primarily for sloops, but similar principles apply to other sailboats with forward supports.
What units should I use in the formula?
Use meters for sag (y), newtons per meter for wind load (w), meters for length (L), and newtons for tension (T).
How often should I check forestay sag?
Check forestay sag regularly, especially after changes in sail configuration or weather conditions.
Results are for informational purposes only and do not constitute professional advice.