STIX and other measures of seaworthiness

[SAWDOC]

Is STIX now regarded as one of the better measures of seaworthiness?
Can anyone provide a link to a list of STIX values for 32 -34 ft sailing boats?
Particularly interested in comparing a Beneteau 32s5 (i understand there are different keel types) with a wauquiez gladiateur. Would anyone advise using other parameters such as Displacement/length ratio etc?

 

[doug748]

See here:
http://www.rya.org.uk/infoadvice/regssafety/Pages/stabilitydata.aspx
Download - Listing of stability data.
There are a lot of gaps, some manufacturers being a bit coy.

 

[SAWDOC]

Doug,
Thanks - excellent spreadsheet with lots of numbers. Unfortunately the two boats of interest are not included. Still , good reference material.

 

[kiboteam]

Hi SawDoc,
I currently own a First 32s5, deep fin in South Spain.
I have a friend who is close friend of one worker at Berret-Racopeau Design, who drew the First 32s5. I asked them for all the info about this boat, included the Stix, and sent to me a lot of data. They even calculated the Stix for me, and said that the Deep fin version has 37.6, and the winged keek 33.5.

Hope this is helpful to you.

Regards from Spain.

 

[Tranona]

Is STIX now regarded as one of the better measures of seaworthiness?
Can anyone provide a link to a list of STIX values for 32 -34 ft sailing boats?
Particularly interested in comparing a Beneteau 32s5 (i understand there are different keel types) with a wauquiez gladiateur. Would anyone advise using other parameters such as Displacement/length ratio etc?

The RYA link provides you with a good discussion on the origin and value of the measurements. STIX as it name implies is a measure of stability, so if you believe ultimate "stability" is a measure of "seaworthiness" then it is a good comparator. However, it is heavily influenced by length so short boats generally have lower values than longer ones.

Empirical evidence (ie what boats have actually achieved) will tell you that many boats with low STIX have undertaken ocean passages in all weathers successfully, suggesting "seaworthiness" is not just a high number on a particular (composite) measure.

While all the common ratios provide useful information about the performance of a particular design and to an extent how it might behave in particular circumstances, the reality is that most of us will never get anywhere near the limits of its capabaility so, other than providing some sort of comparison between similar boats it is all rather academic.

If you have the experience and the desire to take a boat to the limits where STIX might be important I guess you would already have formed a pretty good idea of what you want in a boat.

 

[SAWDOC]

The RYA link provides you with a good discussion on the origin and value of the measurements.

The RYA page contains the following rext

To assist in understanding yacht stability, the above link to Monohull Ballasted Sailing Yacht Stability, will take you to an extract from an RYA Magazine entitled "Stability Explained". This extract explains stability assessment in general for monohull ballasted sailing boats, with particular relevance to the RCD and ISO 12217. A more detailed explanation of stability for all types of recreational craft, including powerboats, is given in the RYA booklet G23.

This seems to refer to the good discussion you mention but I see no link on the RYA page to their magazine extract, only the link to the spreadsheet for download.

In any event your post raises the query if stability equates to seaworthiness and you express your point well. If one looks looks for empirical data to compare different models, what other empirical measures would one use? In my case, skeg hung rudder, reasonable performance and medium displacement seemed to combine to give a good seaworthy boat. And a comparatively high STIX value. As suggested, some get to know you sail time on board was very illuminating for the choice I had to make.

 

[SAWDOC]

Hi SawDoc,
I currently own a First 32s5, deep fin in South Spain.
I have a friend who is close friend of one worker at Berret-Racopeau Design, who drew the First 32s5. I asked them for all the info about this boat, included the Stix, and sent to me a lot of data. They even calculated the Stix for me, and said that the Deep fin version has 37.6, and the winged keek 33.5.

Hope this is helpful to you.

Regards from Spain.

Kiboteam,

thank you - gracias. I had not seen your reply. I have opted for the Gladiateur principally due to the higher displacement, resulting in a higher stix value.

 

[Tranona]

If one looks looks for empirical data to compare different models, what other empirical measures would one use?

The data you are looking at is the very opposite of empirical - it is theoretical in the sense that it is creating a composite measure from selected design dimensions, some of which represent physical characteristics which empirical observation suggests give a boat certain characteristics.

However, you only have to look at the wide variety of boats that are used to realise that there is no one design that is superior to all others. There is only a general consensus around the middle - and that moves over time. So, until the 1970's serious cruising boats were almost exclusively heavy displacement long keel. Then GRP changed the rules and other shapes became possible and within 10 years the old style boats virtually disappeared and lighter displacement fin and skeg became the dominant design. (All this before the RCD and STIX was even dreamed of!) These boats became the "standard" and accepted as the best compromise at the time. However 30 years later, shallow bodied, lighter displacement fin keel, spade rudder forms had taken over.

Your preference seems to be for a boat that typifies the last generation, but I am not sure that using numerical indicators leads you exclusively to the design characteristics you are quoting. You are fortunate in that 30 year's production of boats like that are available for you to choose from. However, if your preference is for a new boat in that style, your choice is very limited!

 

[jdc]

Is STIX now regarded as one of the better measures of seaworthiness?

I'd say that, while it may possiby approximate quite well to stability if the boat is left alone in huge oceanic seas, this is almost entirely irrelevant to 'seaworthiness' in the context in which most of us sail or spend the preponderance of our time on board.

Seaworthiness must include other factors, the relative importance of which will depend on where and how one sails. I think that few yachts founder due to being overwhelmed by oceanic seas compared to:
- abandonment following failure of components such as rigging or rudders
- secondary damage as a consequence of the above
- damage through striking debris (containers, whales, etc)
- catastrophic failure of hull or keel through engineering defects.

And these are in turn infrequent compared to simple human errors like:
- being run down by ships
- stranding or striking rocks
- getting into dangerous places close inshore.

So a measure of seaworthiness should include (in order of decreasing importance, ie starting with the most important imho):

1. ability of the boat to keep the crew from getting too exhausted or exposed to think clearly (the weakest part of any boat is probably the crew!)
2. reasonably fast passage making, even in nasty seas (so you're not out there too long / have few options to make shelter)
3. Sheer toughness, hence the oft mentioned interest in skeg hung rudders etc
4. The ability of the boat to still be directed in really bad conditions
5. the ability to stay reasonably upright and water tight in survival seas.

It seems to me that STIX relates only directly to number 5. And I think that 1, 2 and even 4 are not so bad for modern designs - these tend to do relatively poorly only in point 3.

There is a parallel with aircraft design here (I have been re-reading prof Gordon's famous book on structures): during WW2 the losses of aircraft were only about 1:10,000 per sortie due to airframe failures, but 1:20 due to enemy fire. However what was, to Gordon, a perfectly rational idea: to sacrifice some airworthyness of the frame in exchange for an ability to fly faster or carry better armaments, was very much rejected by flying crew despite it being likely to increase their chances of survival. I think something of the same calculus of risk applies also to yachting.

 

[Bosun Higgs]

I'd say that, while it may possiby approximate quite well to stability if the boat is left alone in huge oceanic seas, this is almost entirely irrelevant to 'seaworthiness' in the context in which most of us sail or spend the preponderance of our time on board.

Seaworthiness must include other factors, the relative importance of which will depend on where and how one sails. I think that few yachts founder due to being overwhelmed by oceanic seas compared to:
- abandonment following failure of components such as rigging or rudders
- secondary damage as a consequence of the above
- damage through striking debris (containers, whales, etc)
- catastrophic failure of hull or keel through engineering defects.

And these are in turn infrequent compared to simple human errors like:
- being run down by ships
- stranding or striking rocks
- getting into dangerous places close inshore.

So a measure of seaworthiness should include (in order of decreasing importance, ie starting with the most important imho):

1. ability of the boat to keep the crew from getting too exhausted or exposed to think clearly (the weakest part of any boat is probably the crew!)
2. reasonably fast passage making, even in nasty seas (so you're not out there too long / have few options to make shelter)
3. Sheer toughness, hence the oft mentioned interest in skeg hung rudders etc
4. The ability of the boat to still be directed in really bad conditions
5. the ability to stay reasonably upright and water tight in survival seas.

It seems to me that STIX relates only directly to number 5. And I think that 1, 2 and even 4 are not so bad for modern designs - these tend to do relatively poorly only in point 3.

There is a parallel with aircraft design here (I have been re-reading prof Gordon's famous book on structures): during WW2 the losses of aircraft were only about 1:10,000 per sortie due to airframe failures, but 1:20 due to enemy fire. However what was, to Gordon, a perfectly rational idea: to sacrifice some airworthyness of the frame in exchange for an ability to fly faster or carry better armaments, was very much rejected by flying crew despite it being likely to increase their chances of survival. I think something of the same calculus of risk applies also to yachting.

Interesting and thoughtful post !

Leaving aside the issue of damage through striking an object, the other 3 of your first 4 factors relate to build quality and STIX is very much aimed at design quality not build.

But I do agree with your 5 seaworthiness factors if not the order - do you really think speed is more important than being water tight? Problem is that 1,2 are subjective, 3 is mostly build quality whilst only 4 and 5 are STIX type issues.

As I understand it, STIX was developed post the Fastnet disaster as a screen to prevent inappropriate boats being taken ocean racing, and the data and characteristics chosen for the calculation were those proved to be of benefit in a Fastnet type storm. But it defintely doesnt cover build quality - thats the territory of things like the ABS standards. Nor does it cover crew quality.

And since a high STIX raises production costs / makes boats less attractive for flotilla, its not surprising that some of the the AWB builders avoid giving the data

 

[noelex]

Stability is one of the more important properties of any boat, particularly a monohull sailing yacht. I don’t understand how it can viewed it as irrelevant to seaworthiness.

I'd say that, while it may possiby approximate quite well to stability if the boat is left alone in huge oceanic seas, this is almost entirely irrelevant to 'seaworthiness' in the context in which most of us sail or spend the preponderance of our time on board.

There are many other factors to consider, but a measurement of stability (I would prefer a stability curve rather than a single Stix number) is helpful or even essential in determining a yachts suitability for its intended task.
If more perspective owners demanded this information I think we would end up with better yacht designs.

 

[Marsupial]

Stix numbers are only an indication of how a boat will/may behave in a seaway but as my recent experience shows a bigger stix number is better than a small one. To be Cat A Ocean you need a stix of 32 or more, and a boat thats bigger than 34ft (I believe). To all the doubters out there I challenge you to sail a stix 32 then a stix 40 - 50 AWB in a blow and then find the 32 superior in seakeeping ability! and yes I know a Jen37 sailed 100miles without a keel and its stix is 33 ish! (with a keel).

just a thought.

Also as far as I know the RCD does require manufacturers to include stability curves in a pack to the first owner, my Jenneaus were both supplied with them.

 

[johnalison]

I'm not sure about the 34ft rule. My HR34, which is under 34ft is rated Cat A but I've never been able to find a STIX value. Being an older design, with less freeboard than most current boats, I suspect the STIX would be relatively low, though with about 40% lead ballast I'm not unduly concerned.

The trouble with STIX, which I have some respect for, is that you can't reduce a boat's reaction in a dynamic situation to a single number, any more than you can reduce intelligence to a single IQ. I have a friend who specifically chose a shallow draft boat for an Atlantic crossing (which never happened) on the grounds that it would be less liable to knockdown. That I expect is another can of worms, but I am at least partly grateful that there is someone in Brussels trying to look after my safety.

 

[Tranona]

Stix numbers are only an indication of how a boat will/may behave in a seaway but as my recent experience shows a bigger stix number is better than a small one. To be Cat A Ocean you need a stix of 32 or more, and a boat thats bigger than 34ft (I believe). To all the doubters out there I challenge you to sail a stix 32 then a stix 40 - 50 AWB in a blow and then find the 32 superior in seakeeping ability! and yes I know a Jen37 sailed 100miles without a keel and its stix is 33 ish! (with a keel).

just a thought.

Also as far as I know the RCD does require manufacturers to include stability curves in a pack to the first owner, my Jenneaus were both supplied with them.

Yes, you are right, the RCD requires publication of stability curves and some builders also publish their STIX numbers.

You can get a 31 or 32ft boat into Category A and given that LWL is a big factor in STIX, overhanging ends are disappearing fast! A modern "32 ft" could have an LWL of 30 ft - compared with around 24 for a Contessa 32.

 

[FullCircle]

I have been to look on the RYA spreadsheet, and Isee many derivatives numbers.
I find that my Jeanneau Sun Odyssey 35 with Lift Keel, has a greater STIX (32) than the deep fin 36i which replaced it(28). However, I was also looking at STOPS categories, and mine is Cat 2 (60 miles from safe haven) whilst still being certified at RCD Cat A Ocean.
Both classifications however are Beaufort force 8, with wave height capability of 4m plus.

Interestingly, I remember increasing the SSS value of my Evo25 so that it could race offshore. 22 being the minimal benchmark then if I recall.

As all of this is not related to one feature, I have downloaded the whole regulations, and already discover that I could increase my capability score just by making a watertight bulkhead, amongst many other features. Interesting evenings reading ahead, and I may add to my annual jobs list if I see any eye catching suggestions I find worthwhile.

 

[fishermantwo]

Is STIX now regarded as one of the better measures of seaworthiness?
Can anyone provide a link to a list of STIX values for 32 -34 ft sailing boats?
Particularly interested in comparing a Beneteau 32s5 (i understand there are different keel types) with a wauquiez gladiateur. Would anyone advise using other parameters such as Displacement/length ratio etc?

I would suggest a look at the thread on Boat design forums and ask the question there. The thread has been running for some time and is several pages long. There is even a chap there that will calculate your Stix numbers for you. Most common vessels have been discussed and you will most likely to find what you want.

http://www.boatdesign.net/forums/stability/

 

[Lucky Duck]

Yes, you are right, the RCD requires publication of stability curves and some builders also publish their STIX numbers.

You can get a 31 or 32ft boat into Category A and given that LWL is a big factor in STIX, overhanging ends are disappearing fast! A modern "32 ft" could have an LWL of 30 ft - compared with around 24 for a Contessa 32.

My old First 31.7 was Category A provided you went with the deep draft version (1.9m), so it is possible even with a relatively lightly ballasted boat. The shallower variant was Cat B but I did wonder whether any of that type were ever sold.

I don't recall seing a stability curve with the information provided at handover for boats comissioned in 2001 and 2005, have I been short changed?

 

[Sans Bateau]

It may be no coincidence that the Cat A Stix rating is 32, when you consider that the Contessa 32, the benchmark, has a Stix of 31-33 (Source RYA). When the Contessa 32 was design there was no consideration of Stix or stability curves, it was designed as a good boat. It could be said that luck was on its side, and a good design transpired. Alas other boats that sailed on that same ill fated Fastnet race, were, perhaps, not so well designed.

Since then designers have been able to give a lot more consideration to what works and what will not. Today many respectable 32 fters will have a stix far in excess of 32. Is this just to get the A rating, or because of careful design, drawing on the mistakes of the past?

As big a legend in its own life that it is, maybe the Contessa just was lucky.

 

[SAWDOC]

The data you are looking at is the very opposite of empirical - it is theoretical in the sense that it is creating a composite measure from selected design dimensions, some of which represent physical characteristics which empirical observation suggests give a boat certain characteristics.

However, you only have to look at the wide variety of boats that are used to realise that there is no one design that is superior to all others. There is only a general consensus around the middle - and that moves over time. So, until the 1970's serious cruising boats were almost exclusively heavy displacement long keel. Then GRP changed the rules and other shapes became possible and within 10 years the old style boats virtually disappeared and lighter displacement fin and skeg became the dominant design. (All this before the RCD and STIX was even dreamed of!) These boats became the "standard" and accepted as the best compromise at the time. However 30 years later, shallow bodied, lighter displacement fin keel, spade rudder forms had taken over.

Your preference seems to be for a boat that typifies the last generation, but I am not sure that using numerical indicators leads you exclusively to the design characteristics you are quoting. You are fortunate in that 30 year's production of boats like that are available for you to choose from. However, if your preference is for a new boat in that style, your choice is very limited!

Excellent observation. Thanks for posting

 

[SAWDOC]

So a measure of seaworthiness should include (in order of decreasing importance, ie starting with the most important imho):

1. ability of the boat to keep the crew from getting too exhausted or exposed to think clearly (the weakest part of any boat is probably the crew!)

Since starting this thread I have enjoyed reading Hal Roth's "Two against Cape Horn" - very good read which I would recommend. He describes a very interesting episode where his 35 foot boat runs aground on an uninhabited island close to the cape. The main point of interest was how the four members of the crew responded differently to this crisis, with the strongest most physically able crew member being reduced to a bundle of nerves unable to contribute in any way to the refloating challenge. The two women crew on board rose to the challenge.