US SAILING Course, Design, Stability and Heavy Weather

 
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Sailboat Design                      Heavy Seas

A number of measurements and formulas are used to describe the sailing characteristics and stability of sailboats.  Some of the most commonly used descriptors are discussed  below:

Sailboat Design & Characteristics      Go To Top

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LOA  Overall length.

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LWL  The length of the hull at the water line (load waterline length).

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Bm   Width at the widest part of the boat (Beam).

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Disp  Displacement or weight of the boat.

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Ballast   Weight of the bilge or keel.

Maximum Hull Speed Formula (Ref 1):  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.)  
Go To Speed Calculator

SA/D Ratio is the sail area/displacement ratio (Ref 1).  This ratio indicates how fast the boat is in light wind.   The higher the number the faster the boat. 
Go To SA/D Ratio Calculator

     * 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.

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).   Go To D/L Ratio Calculator

Ballast Displacement Ratio (Ref 1):   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. Go To Motion Comfort Calculator  

Reference 1:   Reference:  Rousmaniere, J,  The Annapolis Book of Seamanship  Simon & Schuster, New York, New York,  Chapter 1:  The boat p26-35, 1999.

Reference 2:   Ted Brewer Yacht Design  http://www.tedbrewer.com/yachtdesign.html  

Heavy Seas and Boat Stability        Go To Top  


A 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.
    

   
     
Go To Capsize Screening Formula Calculator

Go To Angle of Vanishing Stability Calculator

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Where are you going to sail the boat ?  Heavy weather in the open ocean, with a long fetch and high waves, is much more dangerous than that encountered in a sound or lake.  High winds alone can easily capsize a small boat, but a larger sailboats may only heel to a near 90 degree angle.  However when combined with high waves even large boats can easily be capsized.    
 
Thus, if one is sailing in an inland waterway, it is less likely that the boat will be capsized.  A boat with a larger beam, which heels less and has more living space may be desirable in this setting.

 

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A longer boat is desirable in heavy weather and high waves.  No matter how big a sinusoidal wave is, it should not capsize a boat.  A breaking wave is a different story, these waves have a slope of 70 degrees and if higher than a boat is long, the boat cannot go over them.  An extreme breaking wave hitting the boat's stern or bow may cause it to tumble over end on end (pitchpole or pitchpoling).   If a breaking wave hits the boat on the beam, it can more easily broach or capsize the boat.  Thus, one of the most important factors in surviving high waves is the length of the boat. 
  

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A high stability is most important when hit beam-on with a breaking wave and may only give a relatively small advantage.  

Boat "stability" or resistance to being capsized is most important when a boat is hit beam-on with a breaking wave.  Boats with a wider beam are less stable that those with a narrow beam.  However, boats with a wider beam are stiffer (heel less) and have more living room below.


According to Andrew Claughton  in Heavy Weather Sailing 30th ed. p 21  "This (the test data presented in the chapter) suggests that alterations in form (of a sailboat) that improves capsize resistance may be rendered ineffective by a relatively small increase in breaking wave height."

If a boat is positioned into a breaking wave, most boats (wide and narrow beamed) can survive a 55% LOA (overall boat length) breaking wave.  However, a 35% LOA breaking wave hitting a wide-beamed boat beam-on can easily capsize the boat.  All yachts tested rolled to 130 degrees.  No yacht, no matter how stable, could consistently resist capsizing when hit, beam-on, with a 55% LOA breaking wave.   (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,  Camden, Maine) 

Putting this in perspective in a 40 foot (LOA) sailboat:  In a highly stable boat wave survivability would increase by 8 feet, if hit beam-on by a breaking wave.  A 40 foot sailboat no matter how stable will not consistently survive a 22 foot breaking wave.  Thus, in a strong gale with 22 foot seas and breaking waves, a 40 foot sailboat is at risk of capsizing no matter how stable.

         

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Most important factor is an experienced crew:  Of all the factors, it  is far more beneficial to have an experienced crew that can either avoid or position the boat into large breaking waves.  

The 1998 Sydney - Hobart race was one of the worst sailing disasters in recent maritime history.  And from it many lessons were learned regarding the functioning of boats and crews in heavy weather.  115 boats left Sydney and were hit by an unexpected typhoon.  Seven boats were abandon and five were lost.  The 1998 Sydney to Hobart Race Review Committee report, summarized by Peter Bush, committee chair, reported the following as one of the significant findings:  "There is no evidence that any particular style or design of boat fared better or worse in the conditions.  The age of  yacht, age of design, construction method, construction material, high or low stability, heavy or light displacement, or rig type were not determining factors.  Whether or not a yacht was hit by an extreme wave was a matter of chance."    (Ref:  Rob Mundle in Fatal Storm,  Publisher's Afterward p 249.  International Marine/McGraw-Hill  Camden, Maine.)

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.

1.  Capsize Screening Formula.  This formula estimates the boat's resistance to capsizing. (Ref. 1)  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" (Ref. 3)  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.  Go To Capsize Screening Calculator

1)  Rousmaniere, John   The Annapolis Book of Seamanship   Boat Selection.   Chapter 1   p35   Simon & Schuster,  New York, New York.

2.  Ultimate (Latent) Stability.  This is the resistance to capsize and heel.  One of the best predictors of ultimate stability is the "angle of vanishing stability" or the angle to which the boat can heel and still right itself. A dingy will have a stability range of about 80 degrees, an inland water boat should have a stability range of 100 degrees, and an offshore boat of at least 120 degrees.  Boats which have a stability angle of less than 140 degrees may be left floating upside down once capsized.  Boats with a higher angle will usually right themselves (Ref. 3).  The formula on the website used to estimate the Angle of Vanishing Stability does not fully take into account the position of the vertical center of gravity.  Thus, boats with a long lead keel or a lead bulb at the end of the keel may have a higher angle of vanishing stability than that predicted by the formula.   Go To Calculator of Angle of Vanishing Stability.

3)  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,  Camden, Maine

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Web page last updated:  04/02/2005