Capsize Screening Calculator ⛵
Calculate the Capsize Screening Formula (CSF) for your boat. Assess stability risk for offshore sailing based on beam and displacement.
How to Use the Capsize Screening Calculator
Enter your boat's maximum beam and displacement to calculate the Capsize Screening Formula (CSF). This formula was developed after the 1979 Fastnet Race disaster.
CSF below 2.0 indicates good stability for offshore use. Values between 2.0 and 2.2 are marginal. Above 2.2 suggests the boat may be susceptible to capsize in heavy weather.
CSF is a screening tool, not a definitive stability measure. It works best for monohull sailboats. Wide, light boats (like beach catamarans) will score poorly but may have other stability characteristics. Always consult a naval architect for a complete stability analysis.
Boat & Marine Calculator - Introduction
This tool calculates the Capsize Screening Formula (CSF) using two inputs—your boat's beam in feet and displacement in pounds—and tells you whether your hull meets the offshore safety threshold. Developed after the 1979 Fastnet Race disaster, a CSF below 2.0 is considered acceptable for offshore passages, 2.0–2.2 is marginal, and above 2.2 indicates higher susceptibility to capsize in breaking seas.
How It Works
The formula is CSF = beam (ft) / (displacement (lbs) / 64.0)^(1/3), where 64.0 lb/ft³ is the density of seawater (freshwater uses 62.4). A 38-ft cruiser with a 12-ft beam and 18,000 lb displacement gives CSF = 12 / (18000 / 64.0)^(1/3) = 12 / 281.25^(1/3) = 12 / 6.55 ≈ 1.83, which is offshore-safe. Reducing beam or adding ballast both lower the score.
Usage Scenarios
- Bluewater passage planning: A sailor considering a 38-ft sloop for an offshore Atlantic crossing measures beam at 12 ft and displacement at 20,000 lbs. CSF = 12 / (20000/64)^(1/3) ≈ 1.77—well below 2.0, confirming the hull is suitable for open-ocean conditions without modification.
- Racing boat check before an offshore event: A 35-ft racing boat with a 12.5-ft beam and 10,500 lbs displacement returns CSF = 12.5 / (10500/64)^(1/3) ≈ 2.14, landing in the marginal zone. The skipper uses this to decide whether to add water ballast before the offshore qualifier.
- Comparing two used boats: A buyer is choosing between a 42-ft cruiser (13-ft beam, 25,000 lbs) scoring CSF ≈ 1.85 and a 40-ft boat (14-ft beam, 18,000 lbs) scoring CSF ≈ 2.21. The wider, lighter hull fails the offshore threshold—useful context before negotiating or planning the intended sailing area.
Frequently Asked Questions
What is the Capsize Screening Formula?
The CSF was developed after the 1979 Fastnet Race disaster to quickly screen boats for capsize susceptibility. CSF = Beam (ft) / (Displacement (lbs) / 64.2)^(1/3). A value below 2.0 is considered safe for offshore use. Above 2.0 suggests higher capsize risk.
What CSF value is considered safe for offshore sailing?
CSF below 2.0 is generally considered safe for offshore passages. Most purpose-built bluewater cruisers have CSF values between 1.7 and 1.9. Racing boats may have higher values. The US Sailing Association uses CSF < 2.0 as one criterion for offshore race eligibility.
Does a high CSF mean my boat will capsize?
No. CSF is a screening tool, not a prediction. A high CSF indicates the boat is more susceptible to capsize in severe conditions. Many factors affect stability: hull shape, keel design, ballast ratio, center of gravity, and seamanship. CSF works best for comparing monohull sailboats.
How can I improve my boat's stability?
Reducing beam or increasing displacement lowers CSF, but these are design-level changes. Practically: keep weight low (heavy items in bilge), reduce topside weight (smaller mast, less deck gear), ensure proper ballast, and avoid overloading above the waterline. Good seamanship in heavy weather is the most important factor.