Sailing boats' Stability, STIX and Old Ratios

Extracted from a 2003 report from Richard Slater to the Australian Yachting Federation:

"The use of STIX in Australia is limited at this time. Mainly because the highest level STIX (design category A) has the following parameters:
Wave height: up to approx 7 m significant (highest 1/3rd of waves)
Typical Beaufort wind force up to 10
Calculation wind speed (m/s) 28
When considering many Ocean Races include sailing in Southern Australia / Bass Straight these parameters seem low. One option that we have canvassed to RORC is the possibility that a design category can be set with the above variables increased."

 

I'm not sure what the problem is. As I understand, the only relevence of Design Category A to racing, is that the ISAF in OSR 2006 3.04.4 says that ISO 12217-2 Cats MAY be used to show compliance with the Race Organisers STIX requirement. But this would only be if the Race Organisers felt those STIX numbers were sufficient.

However, this OPTION comes after the REQUIREMENT (OSR 3.04.3) for Race Organisers or National Authorities to set the Minimum STIX Requirement at whatever level they feel appropriate for that race. Surely the Australians are at liberty to set a minimum STIX at whatever level they feel appropriate ? I'm sure I've seen races here (OSTAR ?) where the minimum STIX is higher than the minimum required for ISO Cat A.

 

Paulo brought to my attention a mistake in RM 1200 data: Her AVS is indeed 119º and not 116º. My mistake comes from the scanning and digitizing of her Stability curve from PBO magazine: a distortion was produced, so the wrong angle. I'm sorry for that.

(See post 175)

I did all the process again, and now my digitized curve is, I think, pretty close to the one at PBO (which may be, on its side yet distorted, absolutely). As Dfl (downflooding) angle is the only data missing to calculate her STIX, let's find out her various STIX at several Dfl angles:

100º ---- 33,521
120º ----- 36,423
130º ----- 36,423
Etc.

120º is very close to her AVS angle (119º)

From 120º up the FDF factor keeps constant at its maximum value of 1,25 and so STIX values over that downflooding angle remain the same, 36,423

AS we can see her STIX at 100º is now of 33,521 instead of 32,92 as calculated before (Not a huge difference, but a difference)

Paulo says he has been told RM 1200 STIX is 38,33 for the MSC (at this moment named MOC) condition, and 38,7 for MxSC (I understand this is the loaded condition. Nowadays MTL).

As STIX shall be taken as the lowest of all encountered values, I asume 38,33 as being its official STIX. The difference is now from 38,33 to 36,43 (almost two points).

To reach that value, I should:
-diminish her sail area from 86,67 m2 to 53,4 m2, which seems unrealistic.
- increase her MOC to 7720 mkg instead of the 6876 kg used.
- (taking 7800 kg as MOC, as published at PBO magazine, her STIX raises to 38,514, other values kept constant).
- use 7800 kg as MOC, and bring down Bwl to 3,73 m instead of my estimated 3,8

So this boat with a 120º downflooding angle, 3.73 m Bwl and a MOC of 7800 kg, may very well reach the informed 38,33 STIX value.

I have to recognize my numbers were not accurate and misleading, so I apologize to everybody, specially to Marc Lombard and RM boats.

I change my final statement in post 175 on this boat, to the following:

"An interesting boat, no doubt, with some interesting features. But, with that length/beam ratio, light displacement, high negative area at the GZ curve (32% of the positive), maximum negative GZ slightly bigger than the 50% of her maximum positive GZ, and maximum GZ at around 50º, this boat has no desirable stability characteristics, in my opinion, for an all around oceanic cruiser"

Cheers.

 

The RM 1200's STIX figure, as well as the POGO 40's one (as per numbers posted at the Seaworthiness thread, see post 422 there with all reserves of the case) are heavily dependant on their downflooding angle, which depends on the boat's beam as well as on flooding point height. So we realize here that, although STIX directly penalizes beam at FBD factor, it directly favours it at FDS and FDF ones trough the downflooding angle, one of the single elements that more heavily influences STIX by itself.

I would really want to know the RM 1200's downflooding angle used to calculate the 38,33 STIX figure. 120º is higher than her 119º AVS, which may be, absolutely, but I would like to check it. Well, in fact I would like to have access to the whole calculation, as per some doubts about the accuracy of some STIX related data given at the Seaworthiness thread. See post 436 there.

About the influence of downflooding point height on STIX, see again post 87 of this thread.

Cheers.

 

More info on CE's RCD categorization of boats:
We've seen Multichine 28 (http://www.yachtdesign.com.br/02_ingles/plans/mc28/tech28-2.html) is said to be a Category A boat. Now I learn Nauticat 38 is a Category B boat! (http://www.nauticat.com/Default.aspx?id=436151&BoatId=7&ShowView=Specifications). May we that easily asume Multichine 28 is a more seaworthy boat than Nauticat 38? Is this surprising categorization related with Nauticat 38's STIX calculation or is there any other reason? Anyone knows?

Cheers

 

Well, well, well....!
Yesterday I learnt POGO 40 has a delta factor within her STIX (Paulo dixit), because of its built in unsinkability (watertight subdivisions. I should have realized that by myself! I'm losing faculties...! ). So to be able to compare its stability characteristics with those of other boats it's fair to take out this delta factor from the STIX number, or, to say it in other words, compare the rest of the STIX factors, not taking delta into account.

So taking out the 5 enters from what it seems to be POGO 40 Cruiser's MOC STIX of 41.3 (the one relevant to its categorization) we get a figure of 36.3 formed only by the geometry and weight related factors of STIX, this is LBS, FDL, FBD, FKR, FIR, FDS and FWM.

Surprisingly we find out now this figure is lower than the 38.33 of the RM 1200, which is a boat not unsinkable as far as I know, so its STIX being formed only by the beforementioned factors, and comparable to Beneteau 373's one. So, what is wrong here? STIX method...? POGO 40...? Me....?

About delta factor (which is not mathematically a factor, but an addend with a fixed value of 5 enters, whatever the boat if unsinkable under ISO 12217 and with positive lever arm at 90º), I think (And I have said this before in this thread) it would be interesting to take it out of the STIX figure to make different boats' STIXs more fairly comparable, and mention it as a separated bonus. Something like adding an U letter or the like to a 'not delta' STIX, or either to the standard STIX, just to make it clear its figure is influenced by delta.

 

Guillermo, I agree with your suggestion, stability and seaworthiness should primarily be assessed as pre-event and secondarily as post-event. 36.3U would show a clearer picture than 41.3 and is the way this should be shown

Mikey

 

A portion of this post appears in the seaworthiness thread, but it also has relevance within this thread.

I don't think that eliminating "pre-event” and "post-event" circumstances is a proper methodology for evaluating seaworthiness. As Guillermo has pointed out, it has merit when comparing two boats, to evaluate other parameters, but the fact that the Pogo 40 has watertight compartments is a significant safety feature that can not go overlooked. It is indeed in keeping with safety features already implemented in the open 60 class boats, and is an example of how safety features that compliment a more extreme design can, and should go hand in hand. Just less than a month ago Mr. Greenwood out of New York has made the following comment:

Originally posted by DGreenwood:
Imagine a cruiser having to meet the buoyancy rules for example (water tight bulkheads). That right there would make them way too expensive.

Source, post # 29:
http://www.boatdesign.net/forums/showthread.php?t=15206&page=2

and now here we have an example of just that.

May I also call your attention to the fact that a boat rolling over is partly pre-event, and partially post-event; yet we don’t discount the post-event inversion as trite. No, indeed we attribute a greater necessity for the boat to self right. So if there is some other catastrophic event, say downloading, or a breach in the hull, I would think that not sinking should rank pretty high up on the seaworthiness scale; especially since it is one of the big complaints of the multihull proponents.

 

I totally agree, Tom. But what I'm proposing is to indicate things in separate, not to mix them in a single figure. Otherwise comparative may lead to confusion.
Cheers.

 

Because of its interest and for the sake of this thread, let me quote Rolf Eliasson words on the developing of STIX, in Professional Boatbuilder Magazine:

"In late 1991 John Moon, then head of RORC's measurement office, presented the ISO TC 188 Stability working group the idea of rating boats by a stability numeral. He was encouraged by the convener of the working group, Andrew Blyth of Great Britain. Needless to say the RYA representative, Ken Kershaw, was very supportive. Nevertheless, the proposal was received with silence. Too new, too unknown, and too radical an approach for a stability assessment, was the general response. So the group continued to look for common ground on minimum downflooding and vanishing-stability angles. Every country had its own ideas, and every country had good statistics to prove that it was right. Stalemate.

It was impossible for the group of 20 to 30 people to agree on a common approach, so a subgroup was formed consisting of the originator John Moon, Professor Peter van Oossanen from the Netherlands, Gregoire Dolto from France, and me.

We were charged with developing the stability-numeral method and validating it so that consensus could be reached within the main working group. Ours was fruitful cooperation. Equations were tuned and calibrated, and the most important were checked against a giant database of existing boats. Finally we were ready to share the results with the larger ISO 12217-2 group. The response was: Good work, but we cannot use this as the only method to assess the stability of a sailing vessel. Neither was was there any consensus regarding the fixed limits of downflooding or vanishing-stability angle.

This meeting took place in 1996 in Vienna, and it was there that the STIX method was eventually agreed upon - provided additional requirements of minimum downflooding heights and angles, together with minimum vanishing-stability angles, were also taken into account, and more validation was done. Several years later, STIX has become a part of an international standard. It seems like an unnecessarily long time, but I can tell you that to come to a consensus with so many countries is no minor feat. Ask the United Nations."

 

Two of the main business rules I work with every day are "keep things simple" and "don't spread important information around". The simpler, the clearer, the closer together important information is, the easier it is to pass on the whole message.

STIX 36.3U looks perfect to me

Mikey

 

I know I will fuel Guillermo ideas, but there is strong differences between sailboats and powerboats in the ISO rules.

Most hard factor in powerboats is the required righting moment at 30° of 25 000 N.m for category A and 7 000 N.m for category B.

If you take a mini 6.5, I think most with STIX in category B due to unsinkability, remove the mast and put an engine It would not be in category B in powerboats, because it would not have enough rigthing moment at 30 °. An empty 1 T boat would need 70 cm GZ at 30°.

And worse for category A, the multichine 28, category A sailboat, would be only slightly above half of the required rigthing moment of the minimum category A powerboat.

And unsinkability do not help you in rigthing moment or wind/rolling criteria. It just help you to have a bigger cockpit, and only in category B and lower.

 

It could also be 41.3U or 41.3*, or the like, to not diminish the attained STIX number for the boat, but clearly indicating the figure includes the effect of unsinkability. This would be particularly useful when comparing STIXs in a list.
Cheers.

 

Requirements based in righting arms and areas under the dynamic stability curves for certain angles, as well as criterias for the effect of wind and waves, are widespreadly used for commercial boats regulations, including sailboats. That's another way to look at the thing, useful for professionals, but that kind of information uses to be not available for owners of recreational boats and I'm not sure if they will be really useful for them (except for a few knowledgeable people). Have a look at the interesting and comprehensive rules under development in Australia:
http://www.nmsc.gov.au/documents/NSCV_Subsection_6A_2nd.pdf

Cheers.

 

STIX is often used to determine how safe a boat is, if pre-event is a more important state to communicate then it should be 36.3U, if post-event is more important then 41.3U.

I would say that pre-event is more important. The U is more than enough to communicate to the guy up-side-down that - OK, it won't sink anyway

Mikey