From Scandinavian Waters to the Med - Stability

From Scandinavian Waters to the Med

Stability

One of the most often raised questions when we tell about our swing keel is: but how about stability, doesn’t she capsize easily? The short answer is ‘no’  but in order to convince the skeptic I have collected some facts (se also: RYA article, "Stability matters" and Graham Radford Yacht Design: Stability Discussion

 

A boat floating on its waterline is in balance, as its centre of gravity (CG) and centre of buoyancy (CB) are both on the centerline of the boat and outweighs one another (the boat is in stable equilibrium).

When the boat is heeled (i.e. by pressure of wind on its sails) the CB will move towards the heeling side of the boat 1) due to keel-weight moving to windward and 2) because buoyancy increase due to the changing underwater-shape of the boat (ie. the increased size of the submerged area). Eventually the boat will find a new balance where the increased buoyancy outweighs the pressure of the wind.

Move the cursor over the picture to se the effect of heeling the boat

CG=Centre of Gravity
CB=Centre of Boyancy
AH=Angle of Heel
GZ=Righting moment (se text)

 

The above diagram demonstrates the relationship between angle of heel and GZ, i.e. the orthogonal distance between the CB and CG: The righting moment (cf. above), changing as a boat is heeled. The x-axis indicates AH and the y-axis the GZ value.

As AH increases, the righting moment GZ changes: Initially it increases, until the so called angle of maximum stability (AMS) is reached; for most boats this happens at an AH 45° and 65°. Further heel will gradually decrease the GZ value until it becomes 0 (as before heeling the boat, time however, in unstable equilibrium); this heeling angle is referred to as the angle of vanishing stability (AVS). Further heel will make the boat capsize and the boat enters into its area of negative stability and it will not be able to right itself. This of course is critical to safety
 
Above I have gathered GZ curves from a number of different sailing boats including the Southerly 110. According to the EU Recreational Craft Directive (RCD) all new boats must be supplied with a stability curve and a classification (se later on this page).

Two points should be made: Note how some modern cruising Yachts as for instance the Dufour 385 reaches its AMS relatively early i.e. it has a high initial stability but the AVS occurs at a mere 120° thus leaving a full 80° (2x180°-120°) of negative stability in case of a capsize.  Also note the completely different characteristics of the much smaller, shallow draft Scanyacht 290 which doesn’t reach its AMS until approximately 70° (the little hump un the curve indicates when this boats rather large superstructure is submerged) also note that the Scanyacht does not have any area of negative stability its stability comparable with a tumbler so to say. Finally it is worth while noting that the completely different characteristics of the GZ curves alone are due hull shape and position of CG. Displacement does not play a role.

 

As mentioned above the GZ curve itself only expresses hull shape and does not take displacement into consideration. All other things being equal though, a larger displacement boat is more stable than a low displacement, simply because the buoyancy of a larger hull is bigger than that of a small one. Thus, to compare stability, the GZ value must be multiplied with the displacement of the boat and that is exactly what is done in the stability diagram above. In the diagram the y-axis indicates righting moment in kilo Newton (kN).

Note how the heavy Malö 41 clearly separates itself from the other boats in the diagram. Also note the difference between the 41 foot and the 33 foot IMS respectively, two boats with basically similar hull shapes but of different size.
Also note how the AMS, AWS and area of negative stability is unchanged (with respect to the GZ curve) thus leaving the 38 foot Dufour potentially hazardous if capsized and the much smaller Scanyacht with stability characteristics almost as a modern lifeboat.

Finally, and this is really why this page on stability as made, note the Southerly 110 which is a medium displacement boat with stability characteristics better than the larger Dufour 385 and IMS 42 (which are certainly quicker boats) but also how little difference there is in its stability characteristics with the keel up or down. 

STIX:

The so called STIX number (varying from 1 to 100) is generally recognized as the best all-round indicator of a boats stability. The primary factor of STIX is boat length (i.e. size) adjusted with other factors such as:

  • The ability to withstand capsize (the area with negative stability, cf. above)
  • The ability to recover after a 180° capsize (AVS and displacement)
  • The ability to recover after knock down (overcoming effect of water filled  sails)
  • The displacement-length factor (high displacement is favored)
  • Beam displacement-factor (excessive beam and topside flare taken into consideration)
  • Risk of flooding due to wind gusts
  • Risk of down flooding in case of knock down
 
 

EC-ISO classification:

  • Class A: Off-shore i.e. extended sailing on open sea with waves >3m and wind > 8 – 10 beaufort.
    (STIX > 32)
  • Class B: Coastal (Not more than 500 nm from the coast waves< 4m and wind < 8 beaufort
    (STIX > 23)
  • Class C: Inshore (STIX > 14)
  • Class D: Protected waters (STIX > 5)
  • Offshore boats should have a LPS/AVS > 120º

    The diagram above shows the analysed boats in relation to EU-RCD classification.

 

Latest update October 2015

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