A "J-Class" yacht is a single masted racing sailboat built to the specifications of Nathanael Herreshoff's Universal Rule, The J-Class are considered the peak racers of the era when the Universal Rule determined eligibility in the Americas Cup.
The J-Class is one of several classes deriving from the Universal Rule for racing boats. The rule was established in 1903 and rates double masted racers (classes A through H) and single masted racers (classes I through S). From 1914 to 1937 the rule was used to determine eligibility for the Americas Cup. In the late 1920s the trend was towards smaller boats and so agreement among American yacht clubs led to rule changes such that after 1937 the International Rule would be used for 12-metre class boats.
Universal Rule formula
The Universal Rule formula is:
- is boat length (a number itself derived from a formula that includes Load Waterline Length L.W.L in feet)
- is sail area
- is displacement
- is rating
- Herreshoff initially proposed an index of .2 but ratifying committees of the various yacht clubs changes this to, at various times, .8 or .85. This is, essentially, a 'fudge factor' to allow some boats designed and built prior to the adoption of the Universal Rule, to compete. 
The numerator contains a yacht's speed-giving elements, length and sail area, while the retarding quantity of displacement is in the denominator. Also the result will be dimensionally correct; R will be a linear unit of length (such as feet or meters). J-Class boats will have a rating of between 65 and 76 feet. This is not the overall length of the boat but a limiting factor for the variables in the equation. Designers are free to change any of the variables such as length or displacement but must reduce the other variables if the changes derive a different rating (or they must designate the craft as belonging to another class).
J Class examples
What follows is a table of well-known J-Class yachts. Note the difference in the variables like displacement, sail area, etc. Plugging these variables into the Universal Rule will result in a rating between 65 and 76 feet.
|1930||Shamrock V||Camper and Nicholsons||119 ft 1 in||81 ft 1 in||20 ft||14 ft 9 in||134 tons||7,540 ft²|
|1930||Weetamoe||Herreshoff Manufacturing Company||125 ft 9 in||83 ft||20 ft||14 ft 6 in||7,550 ft²|
|1930||Yankee||George Lawley & Son||126 ft||83 ft||22 ft 6 in||14 ft 6 in||148 tons||7,288 ft²|
|1930||Whirlwind||George Lawley & Son||139 ft||86 ft||21 ft 9 in||15 ft 6 in||158 tons||7,335 ft²|
|1930||Enterprise||Herreshoff Manufacturing Company||120 ft 9 in||80 ft||23 ft||14 ft 6 in||128 tons||7,583 ft²|
|1933||Velsheda||Camper and Nicholsons||127 ft 6 in||83 ft||21 ft 6 in||15 ft|
|1934||Endeavour||Camper and Nicholsons||129 ft 6 in||83 ft 6 in||22 ft||14 ft 9 in||143 tons||7,651 ft²|
|1934||Rainbow||Herreshoff Manufacturing Company||127 ft 6 in||82 ft||21 ft||15 ft||141 tons||7,535 ft²|
|1936||Endeavour II||Camper and Nicholsons||135 ft 6 in||87 ft||21 ft 6 in||15 ft||162 tons||7,543 ft²|
|1937||Ranger||Bath Iron Works||135 ft||87 ft||21 ft||15 ft||166 tons||7,546 ft²|
History and Evolution of the J Class
Prior to the adoption of the Universal Rule, the Seawanhaka Rule was used to govern the design of boats for inter club racing. Because the Seawanhaka Rule used only two variables: Load Waterline Length () and Sail Area, racing boats at the time were becoming more and more extreme. Larger and larger sails atop shorter and wider boats leading either to unwieldy, and ultimately unsafe, boats or craft that simply was not competitive.  In order to account, in some ways, for the beam and the relationship of the length over all () to the load waterline length the universal rule was proposed, taking into account displacement and length, which itself was a result of a formula taking into account such things as "quarter beam length". As different boats were designed and built, the notion of classes was derived to maintain groupings of competitive class.
Following Sir Thomas Lipton's near success in the 1920 America's Cup, he challenged again for the last time at age 79, in 1929. The challenge drew all the novelties developed in the previous decade on small boats to be ported onto large boats, and pitted British and American yacht design in a technological race. Between 1930 and 1937, the improvements brought to the design of sailboats were numerous and significant:
- The high-aspect bermuda rig replaces the gaff rig on large sailboats
- Solid-rod lenticular rigging for shrouds and stays
- Luff and foot grooved spars with rail and slides replacing wooden hoops
- Multiplication of spreader sets: one set previously (1914), two sets (1930), three sets (1934), four sets (1937)
- Multiplication of the number of winches: 23 winches, Enterprise (1930)
- Electrical navigational instruments borrowed from aeronautics with repeaters for windvane and anemometer, Whirlwind (1930)
- "Park Avenue" boom (Enterprise, 1930) and "North Circular" boom (Rainbow, 1934) developed to trim mainsail foot
- Riveted aluminium mast (4,000 lb (1,800 kg), Duralumin), Enterprise (1930)
- Genoa Jib (Rainbow, 1934) and quadrangular jib (Endeavour, 1934)
- Development of nylon parachute (symmetric) spinnakers, including the World's largest at 18,000 sq ft (1,700 m2) on Endeavour II (1936)
- Duralumin wing-mast, Ranger (1937)
All these improvements may not have been possible without the context of the America's Cup and the stability offered by the Universal Rule. The competition was a bit unfair because the British challengers had to be constructed in the country of the Challenging Yacht Club (a criterion still in use today), and had to sail on their own hull to the venue of the America's Cup (a criterion no longer in use today): The design for such an undertaking required the challenging boat to be more seaworthy than the American boats, whose design was purely for speed in closed waters regattas. The yachts that remain in existence are all British, and probably log more nautical miles today than they ever did. This would not have been possible if Charles Ernest Nicholson did not obtain unlimited budgets to achieve the quality of build for these yachts.
Yacht designer Clinton Hoadley Crane noted in his memoirs that "America's Cup racing has never led to good sportsmanship. The attitude of the New York Yacht Club [...] has been more that of a man in the forward position at war who has been ordered to hold his position at all costs – at all costs." In 1930, Thomas Lipton spent $1,000,000 for his Shamrock V challenge when America was facing a stock market crash, but the NYYC still built four cup defenders. The rivalry lead both countries to put on a display of true technological innovations using the maximum load waterline length authorised by the rule for Endeavour II and Ranger in 1937.
Most J-Class yachts were scrapped prior or during World War II because steel and lead had become precious to the war effort. In the post-war era, J-Class racing was deemed far too expensive, so no challenge for the America's Cup was placed until 1958 with the smaller third International Rule 12mR class.
The original yachts carried 165 ft (50 m) masts, but they dismasted frequently in conditions other than the lightest of winds. As a consequence, British yachtsman Sir Richard Fairey (Chairman of Fairey Aviation, and owner of Shamrock V at the time) suggested an America's Cup challenge in the smaller K-Class (less expensive with a more manageable rig), but the New York Yacht Club refused the drop in size. In 1937, disaster struck on the delivery trip of the Vanderbilt's defense candidate, Ranger, from Maine to Newport, when rigging parts fell from the mast whilst under tow. Nothing could be done to save the top 30 ft (9.1 m) from breaking off. Fortunately, a new aluminum mast built for the 1934 defender Rainbow (a candidate for the 1937 defense) was loaned to the project and used throughout the Defender selection series until Ranger's mast could be repaired.
The J-Class rule was amended in 1937 to force rigs to weigh a minimum of 6,400 lb. The larger scantling would prevent the frequent dismastings that had been previously observed in the British Big Class seasons.
In the early part of the 21st century, as part of the celebration of the 150th "jubilee" of America's Cup, several existing J Class racers, and many replicas, were brought to the Isle of Wight for a round the island race. 
List of J-Class yachts
Ten yachts were built to the J-Class rule between 1930 and 1937, six in America and four in Great Britain. All three which survived were designed by Charles Ernest Nicholson: Shamrock V, Endeavour and Velsheda, of which the latter never served for an America's Cup challenge.
Other boats raced in J-Class regattas: The yachts Katoura (Starling Burgess, 1927), Resolute (Nathanael Herreshoff, 1914) and Vanitie (William Gardner, 1914) served as trial horses and most International Rule 23mR yachts were converted to the J-Class, of which three remain in existence: Astra, Cambria and Candida.
The creation of the J-Class Association in 2000 and the launch of a new replica of Ranger in 2004 accelerated the revival of the class. Several replicas and original designs were subsequently built and the association now organises races for the J-Class in Newport, Falmouth and Cowes.
|J-Class conversions did not compete or qualify Challengers Defenders replicas|
|Launch||Name||Sail||Designer||First ship-owner and Yacht Club||Description|
|1893||Britannia||1||K1||George Lennox Watson||Prince Albert Edward, RYS||YRA first class rater converted to the J-Class (1931). scuttled (1936)|
|1907||White Heather II||B1||7||K7||William Fife III||Myles Kennedy, Royal Albert YC||23mR converted to the J-Class (1930). scrapped to cast the lead for Velsheda (1932)|
|1914||Resolute||J1||Nathanael Greene Herreshoff||Henry Walters syndicate, NYYC||Universal rule 75-footer defender (AC1920). converted to the J-Class (1931). scrapped (1939)|
|1914||Vanitie||I1||William Gardner||Alexander Smith Cochran, NYYC||Universal rule 75-footer defender trials (AC1920). converted to the J-Class (1931). scrapped (1939)|
|1928||Astra||K2||JK2||Charles Ernest Nicholson||Sir Adam Mortimer Singer, RYS||23mR converted to the J-Class (1931). refitted (1987)|
|1928||Cambria||K4||William Fife III||Sir William Berry, RYS||23mR refitted (1995, 2001).re-rated as a J-Class (2003)|
|1929||Candida||K8||Charles Ernest Nicholson||Hermann Anton Andreae, RSYC||23mR converted to the J-Class (1931). refitted (1989)|
|1930||Shamrock V||JK3||Charles Ernest Nicholson refit Dykstra Naval Architects||Sir Thomas Lipton, RUYC||Unsuccessful challenger (AC1930). refitted by Pendennis shipyard (2001).|
|1930||Weetamoe||1||Clinton Hoadley Crane||George Nichols syndicate, NYYC||defender trials (AC1930, AC1934). scrapped (1938)|
|1930||Yankee||2||JUS2||Frank Cabot Paine||John Silsbee Lawrence syndicate, NYYC||defender trials (AC1930, AC1934, AC1937). scrapped (1941)|
|1930||Whirlwind||3||Lewis Francis Herreshoff||Landon Ketchum Thorne syndicate, NYYC||defender trials (AC1930). scrapped (1935)|
|1930||Enterprise||4||Starling Burgess||Harold Vanderbilt syndicate, NYYC||successful defender 4:0 (AC1930). scrapped (1935)|
|1933||Velsheda||JK7||Charles Ernest Nicholson refit Dykstra Naval Architects||William Lawrence Stephenson, RYS||restored by Southampton Yacht Services (1997)|
|1934||Endeavour||JK4||Charles Ernest Nicholson refit Dykstra Naval Architects||Sir Thomas Sopwith, RYS||unsuccessful challenger 2:4 (AC1934). restored by Royal Huisman (1989) and refitted by Yachting Developments (2011)|
|1934||Rainbow||J5||J4||Starling Burgess||Harold Vanderbilt syndicate, NYYC||successful defender 4:2 (AC1934). defender trials (AC1937). scrapped (1940)|
|1936||Endeavour II||JK6||Charles Ernest Nicholson||Sir Thomas Sopwith, RYS||unsuccessful challenger 0:4 (AC1937). scrapped (1968)|
|1937||"77C"-Ranger||J5||Starling Burgess & Olin Stephens||Harold Vanderbilt, NYYC||successful defender 4:0 (AC1937). scrapped (1941)|
|2004||"77C"-Ranger||J5||Starling Burgess & Olin Stephens refit Dykstra Naval Architects||John A. Williams, NYYC||replica of "77C"-Ranger (1937)|
|2009||Hanuman||JK6||Charles Ernest Nicholson / Dykstra Naval Architects||James H. Clark||replica of Endeavour II (1936)|
|2010||"77F"-Lionheart||JH1||Starling Burgess & Olin Stephens||Harold Goddijn||original design (model "77F", 1937)|
|2012||Rainbow||JH2||Starling Burgess / Dykstra Naval Architects||Chris Gongriep, ZZV||replica of Rainbow (1934)|
|building||"J8"||J8||Frank Cabot Paine||original design (proposal"A", 1935)|
|building||"77B"-Cheveyo||J1||Starling Burgess & Olin Stephens||original design (model "77B", 1937)|
- universalrule.com History of the Universal Rule of Measurement
- Norman L. Skene (1904). Elements of Yacht Design. The Rudder publishing company. p. 146. ISBN 978-1-57409-134-2.
- Jacques Taglang. "1930–1934: Electric instruments". America's Cup AC-Clopedia.
- Jacques Taglang. "1930–1934: An exchange of ideas, from the 'Park Avenue' to the 'North Circular'". America's Cup AC-Clopedia.
- Jacques Taglang. "1934: The quadrilateral jib". America's Cup AC-Clopedia.
- Clinton Hoadley Crane (1952). Clinton Crane's Yachting Memories: 60 Years as Yachtsman, Designer and Racing Authority. D. Van Nostrand Sporting, New York.
- "J-Class Association".
- "Players of the Game; Charles E. Nicholson. Designer of Shamrock V. A Builder of Yachts. Designed Famous Schooner. Is in Sole Charge. Challenger Sleek and Handsome. Very Successful at Starts.". New York Times. August 18, 1930. Retrieved 2010-12-14. "Victory or defeat for Sir Thomas Lipton in his fifth at tempt to capture the America's Cup depends upon one man more than any other, and that man has been less in the limelight than all the rest in the preparations for the international yacht races off Newport next month."
- Harold S. Vanderbilt (1931). Enterprise: The Story of the Defense of the America's Cup in 1930. Charles Scribner's Sons Publishers. ISBN 978-1-178-54209-7.
- Harold S. Vanderbilt (1939). On the Wind's Highway. Charles Scribner's Sons Publishers.
- Gary Jobson (2000). An America's Cup Treasury: The Lost Levick Photographs, 1893–1937. Mariner's Museum Press. ISBN 978-0-917376-51-1.
- François Chevalier; Jacques Taglang (2002). J CLASS. Yachting Heritage Press. ISBN 2-9519121-0-2.
- Ian Dear (2004). Enterprise to Endeavour: The J-Class Yachts. Adlard Coles Nautical. ISBN 978-0-7136-6905-3.
- Dykstra Naval Architects - J-Class design, replica design and refit (Endeavour, Velsheda, Shamrock V, Ranger, Hanuman, Rainbow)
- Sparkman & Stephens - J-Class design
- Hoek Design - J-Class replica design
- New York Times - 1989 article about Elizabeth Meyer's refit of Endeavour
- David Glenn (2010-04-20). "Dr. Clark & Mr. de Waal's open letters". Yachting World (IPC Media). - letter 1 - letter 2 - letter 3
- David Glenn (May 2010). "The Enduring J Class". Superyacht World (IPC Media).
- youtube.com - videos of J Class yachts
- Classic Boat’s History of the J Class
- Photography gallery of Ranger courtesy of Jean Jarreau
- Photography gallery of J Class Hanuman (Endeavour II) courtesy of Jean Jarreau
America's Cup Competitions