Langth and Sail Area Rule

The Length and Sail Area Rule

In 1887 the YRA decided that Tonnage Rules of any sort were no longer acceptable and introduced the Length and Sail Area Rule. This was the brain child of its Secretary, Dixon Kemp, who wanted to introduce a Rule that paid more attention to the factors that actually contributed to the speed of a boat: the length and (for the first time) the sail area.

The formula was :

LxSA

6000

where L was the waterline length in feet and SA was the sail area of the largest rig in square feet.

The introduction of the Rule for the first time required the limited sail area to be used in the most efficient way and the hull resistance to be reduced as far as possible by more efficient design. This coincided with the general acceptance of Froude's work on resistance and resulted in hulls becoming much shallower, with the profile cut away as far as possible. Because beam was not taxed, stability was sought by increasing the beam and carrying the lead ballast on the end of an attenuated fin. The shallow hull form and large beam facilitated the design of boats whose actual sailing length increased substantially as they heeled and designs were developed that exploited this by using long overhangs both fore and aft.

These trends were carried furthest in the smallest classes, the Half, One and 2.5 raters that made up the extremely competitive 'Solent small classes'. These were all open keelboats and centre board craft, used exclusively for racing

In the larger classes many owners found the Rule unsatisfactory because the shallow hulls it produced meant that only in the very largest yachts was it possible to provide the standard of accommodation that the late Victorian yachtsman demanded. The designers and builders of the larger class of yacht soon found that orders were drying up because boats built as racers had no resale value or later use as cruisers. They lobbied the YRA in 1892 and again in 1894 to suggest that elements should be introduced into the Rule that would enforce a fuller hull able to give 'proper' accommodation and to preserve the link between racing and cruising yachts. The YRA were initially unreceptive to this idea, partly because they knew that the Prince of Wales was planning to have a large yacht built to the Rule. This was the famous Britannia. Despite the Prince's interest, by 1896, they had had enough and went over to the first of a series of Rules that did indeed seek to enforce 'wholesome' hull forms capable of providing accommodation (even in classes so small that they were not fully decked).

When modellers began to look at the LSA Rule, they did so very tentatively. No club wanted to be the first to break away from the '1730' Rule, on which nearly all interclub competition was based. There was also uncertainly over whether a 15 rater or a 10 rater was the most appropriate replacement for the 10 Tonner, and even doubts whether the requirement to measure the sail area could be accurately met in all clubs. The result was that, despite a widespread desire to sail to the same Rule as their full size counterparts and the efforts of Model Yachtsman and Canoeist, which ran design competitions for both 15 and 10 rater classes, there was little enthusiasm for the new Rule, until about the time that it was being abandoned by full size sailors. When it was adopted by a majority of clubs, the 10 rater emerged as the most popular class, though there were also clubs, such as the London, that sailed 15-raters and examples were also seen of 5, 20, 30 and even 40 rater models. It is noteworthy that, while the 10-rater became the most popular model class for the next fifty years or more and the locus of much important development in model yachting, all this design development took place after it had become obsolete as a full size Rule.

The Rule essentially invites the designer to trade the two main speed factors of a boat, length against sail area to produce the most effective combination. Initially however the main problem that modellers faced was how to produce a hull that was cut away to reduce frictional resistance, while at the same time capable of being sailed without minute by minute helm corrections. A model that is fast but wayward does not win many races. Early designs to the Rule were little different in style from the preceding Tonnage Rule boats except in being more beamy. As the need to reduce wetted area pressed harder, attempts to overcome steering tendencies relied on a solution of brute force and ignorance, the twin or even the triple fin.


Zebra	Triple Fin

Though the twin fin was extremely popular, it was not a real answer and was clearly a brake on boat speed. This was demonstrated when a full size 1-rater was built with this configuration, but quickly altered when she was found to be uncompetitive.

Other attempts at a solution included the development of the fin and skeg configurations, designed to give a cutaway hull sufficient deadwood to preserve its tracking ability. This was originally developed by William Paxton, a professional model builder and keen model yachtsman, and was briefly known as 'the Paxton keel'. The illustration Illustration 8 Deerhound. is taken from Bassett-Lowke's 1912 catalogue, but the style dates from a good deal earlier. The boat, designed by Paxton, probably dates from the very early years of the century. Though it has a very pronounced skeg, it still uses a weighted tiller to give a form of automatic steering, despite the introduction of Braine gear in 1904. In an auction of items from Paxton's workshop in 1999, though there were many different types of model yacht fittings, there was nothing that looked like a Braine gear.

The final solution to the problem was Bill Daniels' XPDNC of 1906. This 10-rater, which Bill claimed had had five keel configurations and fifteen rigs before he realised that he had to produce a hull that was volumetrically balanced, marks the effective start of modern model yachting. Employing calculated hull balance and effective automatic steering from the Braine gear, it swept all before it in very keen competition in London and was followed by the even more successful Onward of 1911.


XPDNC lines	XPDNC sails

Initially the sail area had been measured by assessing the total area of cloth, but in the early 1920s the MYA adopted the IYRU method of measurement in which the main sail was measured as a simple triangle, or quadrilateral in the case of gaff sail, and the roach controlled by limits on the size and positioning of battens. The foresails were measured by taking 85% of the fore triangle measurement. This system continued until the Rule revision of 1969, discussed below.

So successful was this concept that 10-rater design stagnated for twenty years, with typical boats having waterlines of 36 to 39 inches and top suits of up to 1600 square inches, before Jim Steinberger produced Phoenix in 1930. This boat introduced a very high Bermuda rig, which was both more efficient and, because of the way the sail area was measured, gave additional unmeasured area in the roach of the sails. More efficient use of the sail area allowed the boat to be longer (43 inches lwl, 1395 sq. inches SA) and this trend continued for free sailing boats through the remainder of the life of the Rule as changes in sail materials and better rig design permitted longer water lines and the re-introduction of the bulb keel allowed hulls to be refined

Phoenix

. A rule change in 1969 to cope with the use of wing masts reverted to measuring the actual area of cloth and added the side area of the mast but, by also changing the constant to 7500, allowed existing boats to continue to be competitive and did not disturb the trend of designs to get longer, up to 65 inches on the waterline, and sail area to get smaller. This was not reversed until the introduction of radio into the class, where the lack of a spinnaker for down wind work and the difficulty of accurately trimming high aspect rigs by radio prompted a return to shorter boats with more sail.