Understanding Yacht Design: Part Three

A three-part series: Part 3

Naval architect Filip Sochaj delves deeper into the Stability versus Comfort Motion debate.

Stability versus Comfort Motion

The Stability Index (STIX) and Angle of Vanishing Stability (AVS) are the two determining factors of masted hull safety used by yacht manufacturers and are the final elements of our yacht design series.

Other numbers and ratios mentioned in this article have been explained in Parts I (October issue) and II (November issues) of this series.

Stix

The Stability Index is the only stability standard regulated by a governing body, the International Standards Organisation (ISO), and it is obligatory for manufacturers of yachts built in the European Union (EU) or under Conformitie Europeene (CE) to publish this standard. It is therefore being adopted all over the world as one of the principal data references when assessing a yacht’s characteristics. 

STIX is based on a set of factors for a given average hull length (LBS). These include the righting energy the boat’s hull is built with; her ability to come back upright after capsize; and her ability to throw water out of the inverted sails. Other hull factors are; the effect of having a heavier displacement over a given length leading to an increased resistance to capsize and the susceptibility of yachts with flared sides to capsize. On deck, factors such as the size and position of cockpit openings are considered due to their effect on downflooding.

For a yacht to achieve a certain design category under CE it must have a minimal STIX number as the table below shows:

Cheating STIX?

Because STIX has many complex factors it can be ‘managed’ to produce a much higher stability rating than is perhaps reasonable. This can be very misleading. For example; putting a sticker on a deck hatch or port light saying ‘Do not open at sea’ will hugely affect the stability rating. Most people would assume reasonably, that the higher the stability rating the stiffer the boat will be, but, as you can see, this cannot be assumed to be true. As always, the devil is in the detail, and to gain anything at all from a yacht’s STIX’s rating you must understand all the elements that comprise it.

STIX Versus Seakindliness

Whilst stability is important if that is pushed too far, the vessel designed may have a very jerky uncomfortable motion, especially in bigger seas. Typically a catamaran, which is very stable in flat or light seas will be uncomfortable in heavy weather, particularly if it is trying to go to wind. To achieve the highly desirable comfortable motion needed for a blue water cruiser, the yacht needs to be able to roll a little with the wind and waves. An uncomfortable boat makes for a tired crew, one of the major causes of navigational error. 

Are STIX category ratings too low?

Many experienced sailors believe that STIX values are ‘loaded’ to allow some yacht builders to achieve a higher CE category for their yachts than they really deserve. As a result of having the bar set so low, unsuspecting and inexperienced prospective yacht owners are being deceived into thinking their yacht is suitable to cross oceans because their yacht has achieved CE category A Ocean status. Ridiculously CE has tried to avoid this criticism by trying to determine that ‘ocean status’ as related to category A rating, means winds of force 8 and wave heights of just 4 mtrs. Any ocean voyager will tell you they expect to experience winds and wave heights well above this on most ocean passages. From the table above we can see that to be considered a Category A yacht, you need a STIX figure of as low as 32. 

Even Rolf Eliasson, one of the creators of The Stability Index, advocates Category A should be a minimum of 40. You can read his article on the subject in Professional Boat Builder magazine, February/March 2003.

If, as we propose, the STIX for Cat A was raised to 40a whole host of production boats would lose their Category A ratings. As American’s say ‘go figure’.

Many voices in the industry consider STIX is over-rated as a safety measurement. However, if we must accept that STIX is here to stay, then the ratings should be increased to better reflect the yacht’s suitability for ocean passage making.

Pragmatically, yacht designers in general believe that STIX should be approximately equal to the yacht’s average hull length (LBS) for Category A Ocean.  A Kraken 50’s LBS is 45ft it’s STIX is 52.

AVS (Angle of Vanishing Stability)

The Angle of Vanishing Stability is the point beyond which the yacht will not return to the upright position when the heeling forces are removed and instead will continue to capsize and roll through 360 deg. This is caused when the centre of gravity of the yacht has overtaken the centre of buoyancy. 

The GZ graph shown above represents the amount of righting moment the vessel will have at any given heel point. The point where the graph crosses the X axis at 128 degrees, is where it loses all of its upright stability – The Angle of Vanishing Stability. What follows is inverted stability. 

The ISO requires the AVS for Category A yachts to be at least 110. A Kraken 50’s AVS is 128.

Achieving higher AVS

Just because a boat is hard to heel at first does not mean it will have a high AVS.

1. You need a good ballast ratio which helps keep the centre of gravity low.
2. Good form stability helps shift the centre of buoyancy outboard therefore increasing the righting moment. This is, unfortunately, the same form stability that will also keep you very stable when upside down, so you need some superstructure to counter that, as explained below. 
3. Your downflooding points need to be as high as possible, as they are directly related to how high and how far the peak of the GZ curve will be. The further the peak is the less likely you are to pass it. 
4. High volume in the superstructure. RNLI lifeboats are built to be self-righting: They achieve this due to very high and watertight superstructure. They have no inverted stability i.e. they won’t remain upside down. A lot of modern yachts rely on superstructure bouyancy for for increased AVS too, and rightly so, as it is very effective. However, when considering this you have to remember that a recreational yacht doesn’t follow the same fail-safe specifications for its hatches, windows and vents as a Lifeboat. Furthermore, if a portlight or hatch is left unlocked that greatly compromises the higher AVS rating that may have been achieved due to superstructure buoyancy, so in our opinion superstructure buoyancy should be considered as only a partial benefit when calculating a given vessel’s AVS.

AVS is an important statistic which contributes to assessing a yacht’s safety at sea. It must, however, be considered alongside all the other factors. Our criticism of Stix is that CE only requires the designer or builder to publish the STIX’s rating. They should be required to publish the seven factors that comprise it as well.

1. FDS – Dynamic stability factor. This factor represents the inherent righting energy that needs to be overcome before loss of stability occurs.
2. FIR – Inversion recovery factor; represents the yachts ability to recover after an inversion. 
3. FKR – Knockdown recovery factor. Represents the ability of the vessel to spill water out of the sails and hence recover from a knockdown.
4. FDL – Displacement-length factor. This factor accounts the favorable effect of heavier displacement for a given length increasing the resistance to capsize. 
5. FBD – Beam-displacement factor. Deal with the fact that yachts with significant topside flare are more susceptible to capsize from beam seas, and increased mean in relation to displacement. 
6. FWM – Wind moment factor. Refers to increased risk of downflooding caused my a wind gust on boats where the cockpit opening or major downflooding point is below 90 degree of heel. 
7. FDF – Downflooding factor quantifies the risk of downflooding in a knockdown. 

In Conclusion

No single set of figures will ever give you the complete picture, and more often than not, will provide a false or skewed conclusion if considered in isolation. Each ratio and statistic has its limitations and optimising just one element of them will lead to a design that is good in some areas but poor in others, for a blue water cruiser we need a balanced design comprising all the stability factors.

There is no hard and fast rule for any of the figures and ratios, there are ranges for each one, but not a definitive value. It is up to the designer to pick the right one for the yacht he is drawing. Yacht design is the art of accommodating and weighing up all factors to achieve the best outcome BUT the builder must first decide what their clients want and direct the yacht designer based on the priorities the clients will consider most important.

No yacht can be perfect for all purposes, but at Kraken Yachts, we believe it is possible to achieve perfection by solely focusing on building no-compromise blue water cruising yachts.