The Stability index, STIX, of a yacht defines the dynamic stability of the yacht.  Only monohull sailing yachts between 6 and 24 meters get a STIX value between 1 and 100.  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 (see separate article).  A CE Category A requires a minimum STIX value of 32, 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.  Finally, boats with an internal buoyancy (Like Etap) get an extra 5 added to the STIX.

The base STIX value is calculated as:

7 + 2.25L_BS          

L_BS or Length Base Size was discussed a lot but the definition Rolf Eliasson gave is now generally accepted: L_BS = (L_H + 2L_WL) / 3

where L_H = Hull Length and L_WL = Length at the Water Line.

The 7 factors used to further specify the stability index are:

F_DS: Dynamic Stability Factor

F_IR: Inversion Recovery Factor

F_KR: Knockdown Recovery Factor

F_DB: Beam-Displacement ratio Factor

F_DL: Displacement-Length ratio Factor

F_WM: Wind Moment Factor

F_DF: Downflooding Factor

 The final formula to calculate the STIX is:

STIX = ( 7 + 2.25L_BS) x (F_DS x F_IR x F_KR x F_CB x F_DL x F_WM x F_DF)^0,5 + Q