Sailboat Ratios: A Guide to Understanding Performance and Stability

When evaluating sailboats, especially for cruising or racing, understanding the various ratios can provide valuable insights into a boat's performance characteristics. These ratios help you compare different designs and make informed decisions based on your sailing goals.

A Crucial Preliminary Note: These ratios are excellent tools for initial screening, but they are not the final word. Hull shape, keel design, rudder type, build quality, and rig configuration also have a massive impact. Nothing replaces a personal inspection and, most importantly, a sea trial.

The Ratios

1. Sail Area to Displacement Ratio (SA/D)

This is the "horsepower-to-weight" ratio for a sailboat. It indicates how much power the sail plan has relative to the boat's mass.

• Formula: SA/D = SA / ( (D / 64) ^ (2/3) )
  • SA = Sail Area in square feet (usually the area of the mainsail + 100% of the foretriangle)
  • D = Displacement in pounds
  • 64 = The approximate weight of one cubic foot of seawater in pounds
• Interpretation:
  • Under 14-15: Underpowered. Typical of heavy, motorsailer-type offshore cruisers. Will require motoring in light winds but can be very forgiving.
  • 16 - 19: Moderate. The sweet spot for most coastal and offshore cruising boats. Enough power to sail well in a variety of conditions without being overwhelming.
  • 20 - 24: Performance-oriented. Common for cruiser/racers. The boat will feel lively, accelerate quickly, and perform well in light air, but may need to be reefed sooner.
  • Above 25: High performance/Racing. Expect thrilling acceleration. These boats can be twitchy and physically demanding, requiring an experienced crew.
• For the Owner: Are you looking for a gentle floating cottage (low SA/D) or a spirited sports car (high SA/D)? Your answer will guide your target range.

Sail Area to Displacement Ratio Information and Calculator

2. Displacement to Length Ratio (D/L)

This ratio relates the boat's displacement to its waterline length, indicating how heavy a boat is for its size. It's a primary indicator of its potential speed and how it will move through the water.

• Formula: D/L = (D / 2240) / ( (0.01 * LWL) ^ 3 )
  • D = Displacement in pounds
  • LWL = Length of Waterline in feet
  • 2240 = Pounds per long ton
• Interpretation:
  • Under 100: Ultra-light displacement (ULDB). Will plane easily but can have a jerky motion. Pure racing boats.
  • 100 - 200: Light displacement. Modern performance cruisers and racers. Can surf or plane in the right conditions.
  • 200 - 300: Moderate displacement. The majority of production cruising boats fall here. They can't plane but offer good load-carrying ability and a smoother ride.
  • 300 - 400+: Heavy displacement. Classic full-keel, bluewater cruisers (e.g., Westsail, Island Packet). They carry momentum well, track straight, and have a comfortable motion in heavy seas, but are slow in light air.
• For the Owner: A low D/L suggests a boat that's quick and nimble for coastal sailing or racing. A high D/L suggests a boat with a steady, seakindly motion well-suited for long-distance ocean passages.

3. Ballast to Displacement Ratio (B/D)

This ratio shows what percentage of the boat's total weight is dedicated to ballast. It's a key indicator of stability and stiffness (its ability to resist heeling).

• Formula: B/D = (Ballast Weight / Displacement) * 100%
• Interpretation:
  • Generally, higher is better for stability. A B/D of 35-40% is considered good for a cruising monohull. Ratios above 40% suggest a very stiff boat.
  • This ratio is highly dependent on keel design. A modern fin keel with a lead bulb at the bottom is much more efficient at creating righting moment than a long, shallow encapsulated keel. Therefore, a modern boat might have a lower B/D (e.g., 30%) but be just as "stiff" as an older design with a 40% ratio.
• For the Owner: If you dislike heeling and want a boat that stands up to its canvas, look for a higher B/D ratio, but also consider the keel type.

4. Capsize Screening Formula (CSF)

Developed by the Cruising Club of America, this formula is a rough measure of a boat's seaworthiness in extreme conditions. It favors heavy, narrow boats.

• Formula: CSF = Beam / ( (D / 64.3) ^ (1/3) )
  • Beam = Maximum beam (width) in feet
  • D = Displacement in pounds
• Interpretation:
  • Lower is better.
  • 2.0 or less: Generally considered suitable for offshore ocean passages.
  • Above 2.0: May be less suitable for offshore work. Many modern, beamy coastal cruisers will have values higher than 2.0. This doesn't mean they are unsafe, but it indicates they derive their stability more from their wide hull form than from weight and may be more vulnerable when hit by a breaking wave on the beam.
• For the Owner: If your primary goal is bluewater cruising, a CSF below 2.0 provides an extra measure of confidence. For coastal cruising, this number is less critical.

5. Comfort Ratio (CR)

Devised by naval architect Ted Brewer, this attempts to quantify the comfort of a boat's motion at sea based on its displacement and size.

• Formula: CR = D / (0.65 * (0.7 * LWL + 0.3 * LOA) * Beam ^ (4/3))
  • D = Displacement in pounds
  • LWL = Length of Waterline in feet
  • LOA = Length Overall in feet
  • Beam = Maximum beam in feet
• Interpretation:
  • Below 20: Very light racing boats. Motion will be quick and jerky.
  • 20 - 30: Typical of coastal cruisers. A tolerable motion.
  • 30 - 40: Mid-range offshore cruisers. A slower, more comfortable motion in a seaway.
  • 40 - 50+: Heavy, bluewater passagemakers. Considered very comfortable with a slow, gentle motion.
• For the Owner: A high comfort ratio suggests a boat that will be less tiring on long passages. A low number suggests a boat that might feel more like a dinghy in choppy water.

6. Hull Speed (S/L)

This is not a comparative ratio but a simple calculation of a displacement hull's theoretical maximum speed, which is limited by its waterline length.

• Formula: S_L (in knots) = 1.34 * sqrt(LWL)
  • LWL = Length of Waterline in feet
• Interpretation:
  • This is the speed at which the boat's bow wave and stern wave are one LWL apart. Exceeding this speed requires a huge amount of power, or for the hull to begin planing (lifting out of the water), which is only possible for boats with a low D/L ratio.
  • A longer LWL directly translates to a higher potential top speed for a displacement boat.
• For the Owner: This helps set realistic speed expectations. Don't expect a 25-foot LWL boat to cruise at 8 knots.

Putting It All Together: A Comparison

We'll compare two hypothetical boats: a Bluewater Cruiser (e.g., Island Packet 32) and a Cruiser/Racer (e.g., J/105). The ratios for these boats are illustrative and should be verified with actual specifications.