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STIX or Stability Index for yachts Print E-mail

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 + 2.25LBS          

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

where LH = Hull Length and LWL = Length at the Water Line.

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

FDS: Dynamic Stability Factor

FIR: Inversion Recovery Factor

FKR: Knockdown Recovery Factor

FDB: Beam-Displacement ratio Factor

FDL: Displacement-Length ratio Factor

FWM: Wind Moment Factor

FDF: Downflooding Factor

 The final formula to calculate the STIX is:

STIX = ( 7 + 2.25LBS) x (FDS x FIR x FKR x FCB x FDL x FWM x FDF)0,5 + Q

where Q is 5 for unsinkable boats and 0 otherwise. 
 
Passage planning Print E-mail

 

Passage planning is a very broad subject and how much you will actually need largely depends on the kind of trip you're doing. A Sunday afternoon coastal trip around some nearby buoys like you've done so many times before does not require the same planning as an ocean going journey to a new destination. Below are a few categories, in no particular order, that should somehow get your attention before you commence any trip:

 

  • Boat / equipment check-up
  • Weather forecasts
  • Tidal considerations
  • Food and beverages
  • Navigation plan

 
 
SART Print E-mail
SART stands for Search And Rescue Transponder.  A SART is meant to help rescuers locate you in an emergency situation.
 
When activated a vessel (or rescue helicopter) that is equipped with a 9Ghz bandwidth radar will be able to home on your SART device.  The SART will show on the radar as a series of 12 dots.  When you get closer to the SART the dots will become bigger and bigger circular lines until they are fully closed concentric circles.  By this time you are no further than 1 mile away from the SART.
 
When spotting a SART on your radar you should check with the nearest coast station if they know about an emergency situation and offer assistance when required. 
 
When, in an emergency situation you have to leave your vessel for a life raft you should always take your SART with you.  Mount the SART as high as possible in your life raft for it to be "visible" as far as possible.
 
Boom Vang Print E-mail

A boom vang or kicking strap (kicker) allows vertical adjustment of the boom.  It is an important tool in shaping the main sail.  

The vang typically runs from the base of the mast to a point about a third of the way out the boom.

Tensioning the vang tightens the leech of the sail, flattens the sail and bends the mast.  By controlling leech tension, the boom vang is one of the three methods of controlling sail twist.  The other two are the main sheet and the traveler.  

Cruising sailors use the vang to keep the boom from rising downwind.

While under sail, the opposite force to the vang is supplied by the sail itself. When the sail is reefed, a topping lift supplies the upward force on the boom. Some line vang systems incorporate a piston to provide the topping lift force and to damp oscillations. 
 
Rigging Dimensions Print E-mail
I, J, P, E: what do they stand for and what knowledge do they give you ?
 

"I" is measured along the front of mast from the genoa halyard to the main deck. The main deck is where the deck would be if there were no deckhouse.

"J" is the base of the foretriangle measured along the deck from the headstay pin to the front of the mast.  
 
"P" is the luff length of the mainsail, measured along the aft face of the mast from the top of the boom to the highest point that the mainsail can be hoisted or black band.

"E" is the foot length of the mainsail, measured along the boom from the aft face of the mast to the outermost point on the boom to which the main can be pulled or to the black band.

 

When you know those values you can use the following formulas to calculate approximate areas for: 

 

  • Mainsail = (P x E) / 2 (add 10% for racing mains)
  • 135% Genoa = (( J x I ) / 2) x 1.44
  • 100% Jib = ( I x J ) / 2
  • Racing Spinnaker = 1.8 x J x I 
  • Cruising Spinnaker = 1.65 x J x I  

 
 
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