Yacht Stability – What is STIX?

What does STIX mean? The Stability index (STIX) of a yacht defines its dynamic stability.  Monohull sailing yachts between 6 and 24 meters get a STIX value between 1 and 100 and all manufacturers have had to offer this information under law in Europe for several years now. The higher the STIX value, the more stable the boat when under way (hence dynamic stability). Note that there is a relation between the STIX value of a boat and it’s CE Category. A CE Category A requires a minimum STIX value of 32 (Sydney-Hobart race 35), while CE category B requires a minimum STIX value of 23. Calculating STIX values is rather complex. The base value is calculated using the length, where the longer the boat the higher the base value will be. This base value is then multiplied with 7 factors, each having an impact on the stability of the boat.  A factor will increase the boats overall dynamic stability.

As an example the Sun Odyssey 33i has a STIX of 36.75, well in excess of the ‘Category A’ requirement and considerably more than any of its competitors, and just about every Jeanneau model is similarly very stiff. The new Sun Odyssey 349 has a STIX value of 42 and the Sun Fast 3600 49! The STIX value is a much more accurate measurement of the actual stability of a yacht underway in normal conditions than the traditional reference which is the point of vanishing stability.

Why are Jeanneau yachts so stiff? Well, it’s a combination of hull design and form, low centre of effort of the keel, and the lower weight distribution due to how the yachts are structured, particularly with the lighter injection moulded decks. Compare with other manufacturers and you’ll see the difference.