Square metre boat: A matter of length

Anything flat-hulled with long overhangs, made of wood, ideally with a glossy finish? This is how most sailors answer this question. In principle, that’s true, since the open design class launched in 1908 for racing in the Stockholm archipelago or Skärgård initially left a lot of freedom. Only the sail area was limited. If you wanted to win with a Square metre boat, you had a long, thin and light one. The longer, thinner and lighter the better.

This had led to extreme and short-lived boats. To adjust this, the so-called Square metre boat rule, as the class regulation is still called, was improved in 1916 and again in 20. One of the last thirty 30 sqm boats of the previous 1920 rule was 44 feet (13.40 m) long and less than six feet (1.75 m) wide. This development and also the hardware battle had to be reined in.

Repeated refinements of the Skärgårdskryssare Rule

The Squaremetre boat was popular at the time. Here the Germans managed to get involved in international regattas again after World War I. The Olympic games of 1920 were sailed in the 30 and 40 sqm class. Henry Rasmussen designed and built them in Bremen, his rival Gustaf Estlander in Berlin. A substantial and eagerly renewed 40 sqm fleet was sailing on the Berlin lakes. Ambition and competition in the sqm classes was hot.

Finally revised square metre bot rule 1925

Prof. Karl Ljungberg, one of the inventors of the type, revised the Rule so comprehensively that in 1925 the Squaremetre boat became kind of reinvented. Ljungberg devised a clever corset of related dimensions around the hull. In favor of the durability of the boats, it was supplemented with detailed scantling regulations. Since then, you could no longer win a race with just the next longer boat. It had to be a smart compromise fitting to the conditions.

According to the 1925 rule, length was to be bought with correspondingly more weight, more freeboard, more keel length and added beam. Since the sail area was limited to nominal 15, 22, 30, 40 or 55 etc. square meters, it was worth having a bigger, beamier and heavier boat in a breeze. In light air it is difficult to move additional weight and water resistance with that same sail area.

A closer look at the 1925 rule shows how clever Ljungberg’s measures were. It also shows that a Squaremetre boat is more than a flat-hulled sleek boat, as we sailors today assume seeing the long, slender planks. The drawing with the multitude of measuring points, levels and terms gives an idea of how sophisticated the rule and the result, the class, is. To begin with, a closer look at two essential measurements, the length and beam, might help.

Which length is considered

The length does not consider the total length of the boat or the waterline, as it is common. Instead the measurement is taken at a certain height above the waterline. In the course of the first two decades, the squaremetre boat sailors had discovered how to achieve little wetted area in light air and a high hull speed in stronger winds. They had taken this to the extreme with light and slender boats with long ends hovering beyond the short waterline. The overhangs don’t add water resistance when wind and water is calm. When it starts blowing, they automatically stretch the short waterline to the effective waterline of the moving boat – adding hull speed.

Where that length is measured

Hence the length considered by the rule is taken above the waterline. The waterline is named Horizontal plane 0 in the drawing. The length considered by the rule is Horizontal plane 1. It approximates the effective waterline of the hull when underway in a building breeze. The Horizontal plane 1 is taken depending on the actual waterline length, namely two percent of the Horizontal plane 0. According to the table below, it is 15 centimetres above the waterline at a 22 sqm boat and 18 cm at the 30 sqm class.

This is an example how demanding the design and construction of a Squaremetre boat was (and is). It had to float exactly on the intended waterline. The calculations for the floating position of the boat had to be correct and the boat builder had to adhere strictly to the weights and wall thicknesses of all components. The length of horizontal plane 1 is included in the measurement in the following table as the ideal length (L1). Since the ballastratio is optional for the Squaremetre boat up to 55 sqm (beyond it is limited to 50 percent), the designer and boatbuilder could build the boat as light as possible – while adhering to the detailed specifications. At the same time they could give it as much ballast as they were able – with the boat finally floating at the intended waterline. A high ballastratio helps any slim boat with little hull stability upwind.

The beam issue

Beam was a sensitive issue in the class. The slimmer the boat, the better in light wind. With the previous rule changes in 1916 and 20, attempts had already been made to enforce minimum widths. One idea was to achieve wider boats with an imagined rectangle inside the mainframe, the so-called suitcase size.

The 1925 rule now measures the main frame, called Main Section in the drawing, on three levels. First below deck as measurement b0, then at a certain height above the waterline as b1 and slightly below the waterline as b2. The largest beam above the water (b1) is taken into account with factor 4. Now these three beam number are used by the 1925 rule as mean breath at main section. The third row of the table Mean breath at main section outlines the values for the different sqm boat classes.

Apart from marginal additions, Ljungsberg’s big hit, the fourth version of the Sqm Boat rule, is existing since 1925 and governs the open design class of all nine Squaremetre boats from 15 to 150 sqm until today. It is accessible as a 47-page pdf in English at the Svenska Skärgårdskryssare Förbundet (SSKF) website with a mouse click. The result of the 1925 rule is a beautiful, remarkingly durable sailing toy. Squaremetre boats are kept, preserved and sailed with endless enjoyment from generation to generation.

Photo above by Ulla Prötel: 15 and 22 sqm boats in Lemkenhafen on the island of Fehmarn.

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