What is a Skerrycruiser or Skärgårdskryssare? 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. To win with a Square Metre Boat, you had to have a long, thin and lightweight one.
This had led to extreme and short-lived boats. To adjust this, the so-called Square Metre Rule, as the class regulation is called, was improved in 1916 and again in 20. One of the last thirty 30 m2 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 controlled.
Repeated refinements of the Skärgårdskryssare Rule
The Square Metre Class 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 m2 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 Skerry Cruiser classes was hot.
Finally revised Square Metre Rule in 1925
Prof. Karl Ljungberg, one of the inventors of the class, revised the Rule so comprehensively that in 1925 the Square Metre Boat became reinvented. Ljungberg devised a clever corset of related dimensions around the hull. In favour 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.
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. m2, it was worth having a bigger, beamier and heavier boat in a breeze. In light air, it is difficult to move additional weight and added water resistance with that same sail area.
Rules for Skerry Cruisers/Squaremetre Yachts by Svenska Skärgårdskryssare Förbundet (SSKF)
A closer look at the 1925 rule shows how clever Ljungberg’s measures were. It also proves that a Square Metre Boat is way more than a flat-hulled sleek boat, as we sailors today assume, admiring 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 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 Square Metre 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 comparably short waterline. The overhangs don’t add water resistance when the 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 in the 30 sqm class.
This is an example of how demanding the design and construction of a Square metre boat was and still 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 ballast ratio is optional for the Square Metre Boat up to 55 sqm (beyond it is limited to 50 percent), the naval architect and boat builder 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 to do — with the boat finally floating at the intended waterline. A high ballast ratio helps any slim boat with little hull stability upwind.
Rules for Skerry Cruisers/Squaremetre Yachts by Svenska Skärgårdskryssare Förbundet (SSKF)
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 considered with factor 4. Now these three beam numbers 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 Square metre 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 single mouse click. The result of the 1925 rule is a beautiful, impressively durable sailing toy. Square Metre Boats are kept, preserved and sailed with endless enjoyment from generation to generation.
Photo above by Michael Kurtz: 22 sqm boats in Flensburger Förde. Updated July 9, 25. You found this article worth reading? → Subscribe the free Newsletter and you won’t miss future publications.
