A few very helpful things to know about Sailboat Design & Characteristics
LOA Overall length.
LWL The length of the hull at the water line (load waterline length).
Bm Width at the widest part of the boat (Beam).
Disp Displacement or weight of the boat.
Ballast Weight of the bilge or keel.
Maximum Hull Speed Formula:
This formula estimates the theoretical maximum speed of a hull displacement boat. (1.34 times the square root of the length of the hull at the water line of a boat loaded with cargo
SA/D Ratio is the sail area/displacement ratio:
This ratio indicates how fast the boat is in light wind. The higher the number the faster the boat.
* Cruising Boats have ratios between 10 and 15.
* Cruiser-Racers have ratios between 16-20.
* Racers have ratios above 20.
* High-Performance Racers have ratios above 24.
SA / D = Sail Area / (Displacement in Cubic Feet )2/3
D/L Ratio is the displacement to length ratio (Ref 1). This indicates if the boat is a heavy cruiser (results greater than 325) or a light displacement racing boat (results less than 200).
D/L Ratio is the displacement to length ratio. This indicates if the boat is a heavy cruiser (results greater than 325) or a light displacement racing boat (results less than 200).
D / L = Displacement / ( 0.01 * LWL )3
Displacement is in long tons
Ballast Displacement Ratio:
This ratio is calculated by dividing the sailboat's ballast by the boat's displacement and converting the result to a percentage. This ratio indicates the resistance to heeing or a sailboat's stiffness. An average ratio is approximately 35%. A higher ratio indicates greater stiffness.
Motion Comfort (Ref 2): This ratio predicts the predicts the overall comfort of a boat when it is underway. The formula predicts the speed of the upward and downward motion of the boat as it encounters waves and swells. The faster the motion the more uncomfortable the passengers. The higher the number, the more resistant a boat is to movement. This ratio was created by boat designer, Ted Brewer. It is useful in comparing different boats. A higher value predicts a more comfortable ride. Asailboat with a LOA of 42 would be expected to have a Motion Comfort Ratio in the low 30's
Motion Comfort Ratio was developed by Boat Designer Ted Brewer. The formula predicts the speed of the upward and downward motion of the boat as it encounters waves and swells. The faster the motion the more uncomfortable the passengers. Thus, the formula predicts the overall comfort of a boat when it is underway. Higher values denote a more comfortable ride. As the Displacement increases the motion comfort ratio will increase. As the length and beam increases the motion comfort ratio will decrease.
MCR = Disp / (2/3*((7/10 * LWL)+(1/3 *LOA))*Beam4/3 )
Heavy Seas and Boat StabilityA sought after characteristic of sailboat design is the ability to withstand high winds and seas. Boat Stability is the resistance to capsizing and the Angle of Vanishing Stability is the degree the boat can heel and still right itself. That being said, the following factors should be considered when one is evaluating boat stability and design.
This formula estimates the boat's resistance to capsizing. This calculator gives only guidance. In general, heavy boats with narrow hulls are more stable. Results less than 2 indicate stability, greater than 2 the boat is relatively vulnerable to capsizing.
The following is the formula used to calculate
The Angle of Vanishing Stability:
Screening Stability Value ( SSV ) = ( Beam 2 ) / ( BR * HD * DV 1/3 )
BR: Ballast Ratio ( Keel Weight / Total Weight )
HD: Hull Draft
DV: The Displacement Volume in cubic meters. DV is entered as pounds of displacement on the webpage and converted to cubic meters by the formula:
Displacement Volume in Cubic Meters = ( Weight in Pounds / 64 )*0.0283168
The Beam and Hull Draft in this formula are in meters.
Angle of Vanishing Stability approximately equals 110 + ( 400 / (SSV-10) )
The following is the formula used to calculate The Capsize Screening Formula:
Capsize Screening Formula = Beam / (Displacement / 64)1/3
Displacement is in Pounds
Beam is measured in Feet
Sailboat Stability:
Two formulas are designed to give an estimation of the stability of a sailboat. The reliability and value of these screening formulas are disputed. Although reliable in the lab, they may not predict stability in severe heavy weather conditions.
Capsize Screening Formula.
This formula estimates the boat's resistance to capsizing. This formula does not take into account the vertical position of the center of gravity (VCG). The VCG can be lowered by a longer keel or by having more ballast (weight of the keel) at the end of the keel. However, according to Adlard Coles' "Heavy Weather Sailing" thirtieth anniversary edition, "The effects of large movements of the VCG on the propensity to capsize was surprising small". Nevertheless, a low VCG will greatly help the boat in righting itself once it has capsized. For example, the J-105 is a lightweight high-performance racer with a D/L of 135 and a SA/D of 24. Stability is not predicted by this formula but the J-105 has a 3400 lb 6.5' lead keel to stabilize the boat.
Adapted From: K. Adlard Coles' and Peter Bruce's (editors) Adlard Coles' Heavy Weather Sailing (30th edition) Stability of Yachts in large breaking waves. Chapter 2 pp11-23 International marine,
