SailNet Community - View Single Post - Bluewater defined?
View Single Post
  #9  
Old 02-02-2005
PCP's Avatar
PCP PCP is offline
Senior Member
 
Join Date: Dec 2004
Location: Portugal, West Coast
Posts: 16,188
Thanks: 21
Thanked 98 Times in 81 Posts
Rep Power: 10
PCP will become famous soon enough
Bluewater defined?




Even if I agree with a lot of things that Jeff has said (about oceangoing boats) I consider that his opinions on AVS, Stix numbers and in Stability are in the least misleading. In many cases they are simply untrue and they don’t contribute at all for the general understanding and the importance of the issue in the global evaluation of a boat, with special incidence on safety.

Let’s see what I mean:

Jeff said:

“In following the research process that resulted in STIX, it should be understood that the purpose in developing the CE Directive for Recreational Watercraft, of which STIX is a component, was never to extablish an absolute standard for vessels going offshore. “

I say:

That, in my opinion, is not true. The purpose of that directive (that has the value of a law and is mandatory to all state members) was precisely to establish absolute standards that all boats have to conform to be classified in four different categories . Those categories are defined by parameters regarding the minimum safety characteristics a boat has to have, regarding uses in different sea conditions. From the Directive:

“Definitions:

A. OCEAN: Designed for extended voyages where conditions may exceed wind force 8 (Beaufort scale) and significant wave heights of 4 m and above, and vessels largely self-sufficient.
B. OFFSHORE: Designed for offshore voyages where conditions up to, and including, wind force 8 and significant wave heights up to, and including, 4 m may be experienced.
C. INSHORE: Designed for voyages in coastal waters, large bays, estuaries, lakes and rivers where conditions up to, and including, wind force 6 and significant wave heights up to, and including, 2 m may be experienced.
D. SHELTERED WATERS: Designed for voyages on small lakes, rivers, and canals where conditions up to, and including, wind force 4 and significant wave heights up to, and including, 0,5 m may be experienced.”

” Boats in each Category must be designed and constructed to withstand these parameters in respect of stability, buoyancy, and other relevant essential requirements listed in Annex I, and have good handling characteristics.”


So category Class A means unrestricted ocean going boat.


Jeff said

“There is often a tendancy to focus on such items as the AVS … or STIX (CE Stability Index) as key elements of the overall safety of a boat offshore. These numbers represent a very small snapshot of the real safety of a boat and as such can be grossly misleading………….”


I say:

For practical effects, what provide the real separation of boats in class are the stability requirements. Many of the boats in other classes can satisfy all requirements of ClassA except Stability requirements as they are defined by ISO 12217-2:

RCD stands for Recreational Craft Directive, an EU Directive setting down minimum safety standards for the construction of recreation craft .

“RCD Category A boat limits are a minimum mass of 3.0 tonnes and an AVS greater than (130 – (2 x mass)) but always equal to or greater than 100º.

RCD Category B boat limits are a minimum mass of 1.5 tonnes and an AVS greater than (130 – (5 x mass)) but always equal to or greater than 95º.

.The required RCD STIX limits which are applied in addition to the above limits on mass and AVS are:-

Category A equal to or greater than 32

Category B equal to or greater than 23 “


“STIX, which scores a boats stability on a scale of 1 to 100, uses a boats length as it’s prime factor adjusting this by seven other factors including assessment of a boat’s

ability to withstand a capsize by considering the area under it’s GZ curve.

recovery from inversion by looking at it’s AVS and mass.

recovery from knockdown by overcoming water in the sails.

displacement-length factor giving credit for a heavy displacement for a given length.

beam-displacement factor recognizing problems associated with topside flare and excessive. Beam.

wind moment representing the risk of flooding due to a gust and
the risk of downflooding in a broach or knockdown.”


STIX number was made from and by :

“ It was from work on these two screening formulae (The RORC Triple S Numeral developed in the 1980s and the RYAs own STOPS number used for smaller MCA Code vessels since the 1990s) that the ISO working group drafting the new stability standard developed its own stability index screen known as STIX. The RYA played a major part in this work.”

RORC stands for Royal Ocean Racing Club, RYA for Royal Yacht Association and MCA for Maritime & coastguard Agency.


I say:

These technicians, (the ones responsible for the technical safety stability requirements that the boats need to fulfill to be to classified in each one of the four EU categories) the naval architects that work for RORC and RYA, know a lot more about stability and safety than me or Jeff or any amateur.

They are professionals that have been working in this field since that race of bad memory, and have been responsible for the previous systems used to make offshore racing and small commercial vessels (including commercial sail boats) safer.

If they consider AVS , STIX numbers and Mass as the key elements of the overall safety of a boat , it is in my opinion very misleading to say , as Jeff has said: “These numbers represent a very small snapshot of the real safety of a boat and as such can be grossly misleading………….”

They say about the previous systems used for measuring stability (for ocean racing and used in Commercial vessels), the:

“RORC Triple S Numeral developed in the 1980s and the RYAs own STOPS number used for smaller MCA Code vessels since the 1990s start to approach the problem but both in a very rudimentary manner (nevertheless both are very successful).
It was from work on these two screening formulae that the ISO working group drafting the new stability standard developed its own stability index screen known as STIX. The RYA played a major part in this work..”

With a curriculum and a background like that, you can believe them when they say about the STIX system:


“STIX is arguably the most sophisticated stability screening tool yet available”


Jeff said:


“AVS suffers from another problem as well. As has been pointed out many times on this forum, there is no uniform standard for calculating AVS.”

I say:

This is not true, at least to boats sold in EU.

About the AVS, and we are talking about boats sold in the EU (including American boats), they all have to have their Dynamic Stability and Static Stability (the numbers that are later expressed in a GZ/RM curve that shows the AVS , and from where it is then calculated the STIX number) measured according to ISO 12217-2. This norm guarantees that all boats are measured exactly the same way, including specifications regarding weight taken from a boat in sailing condition, as defined by the rule.

Those measurements are not made by the builder but by independent, technical and very specialized private agencies holding a permit given by the EU that attests their independence and competence to do the job.

Obviously all the numbers of AVS and STIX that I have quoted in the previous post are all EU validated data, numbers that can be compared and that give reliable information on safety and stability of each boat.



Jeff said:

“key calculations and measurements were omitted from the standards because member nations considered them to be onerous. Instead simplified surrogate formulas were substituted for actual more sophisticated calculations resulting in very loose and sometimes missleading approximations”

I say:

This is, in my opinion, nonsense.

The Naval Architects and technicians that produced the stability requirements and the way to measure them were the most experienced in the field and based their work in the most meaningful work (and extended experience) on the subject, namely the RYA STOP numbers and the RORC SSS numeral system, both systems widely used with success for many years.


Jeff said:

Regarding to AVS and STIX numbers

“ None of these numbers take into account the weight distribution and buoyancy of the vessel in the inverted condition which can greatly alter the relative stability of individual vessels as the approach their limits of positive stability.”


I say:

This is not true.

As shown above the two first correction factors of the STIX are:

ability to withstand a capsize by considering the area under it’s GZ curve,

recovery from inversion by looking at it’s AVS and mass,

When you take into account the negative part under the GZ curve you are considering the inverted stability of the boat. When you consider mass and AVS in a recovery, you are considering global weight distribution. You are not considering only the weight of the empty boat( because all the data that permits those calculations were not taken from an empty boat), but the weight of a boat in sailing condition, as defined by ISO 12217.

Jeff said:

“An extremely high … STIX can be easily achieved simply by designing an excessively narrow boat with lots of freeboard , but with that excessively narrow beam and high tophamper, comes a greatly increased likelihood of a capsize or roll over”

I say:

This is not true.

As shown above the STIX calculation takes into account, besides length, seven different correction factors.

These three completely prevent that an easily capsizable boat could have a high STIX number :

ability to withstand a capsize by considering the area under it’s GZ curve,

displacement-length factor giving credit for a heavy displacement for a given length.

beam-displacement factor recognizing problems associated with topside flare and excessive beam.


Fact is that an easily capsizable boat, no matter its AVS, will always have a low STIX number:




Paulo
Reply With Quote Share with Facebook