Misleading stability

[Polux]

Some moths back I had wrote a letter to Yachting Montlhy magazine and as I believe the subject is important I would like to share it and have your comments and opinions about the issues at stake.

That's the letter that was sent by email:

"Before I had a boat I was already a Yachting Monthly reader (I had my first boat 30 years ago) and occasional subscriber and I am grateful for the several articles on boat stability that were published along the years and the inclusion of boat stability data in your boat tests.

This information has led me to take an interest in the subject.

In my opinion Yachting Monthly have contributed to give its readers a better comprehension of boat stability: initial stability (for carrying sail), reserve and final stability (for safety). That information as well as the AVS and the inverted stability are important criteria that any boat buyer would have interest in knowing and understanding before choosing a boat.

However I would like that on your boat tests the information on reserve stability would not be resumed only to the printing of a stability curve. The boat testers can only experience (while sailing) the initial stability, the one that is used for sailing and they do so, commenting on the boat performance (carrying sail ability, stiffness and power), but they normally remain silent in what regards safety stability. They cannot experience it (for example the capacity of recovering from a heavy knock down or capsizing) but they have the data to comment on that. So, why remain silent? Why not make that assessment as a part of all boat tests, instead of only printing the stability curve that many readers cannot understand?

I would also like to call Yachting Monthly attention for the discrepancies on the stability curves that are provided by the manufacturers.

I explain:

When you tested the Hanse 430 I was amazed by the bad stability curve that was printed. I thought that there was a mistake because the boat had a better than average Ballast/Displacement ratio and a deep keel with a bulb and it could not have an AVS under 110º, a lot worse than all French boats that have a lower Ball/Displ and similar characteristics.

I was interested in the boat so I talked with the boat designers on a boat show. The Naval Architect who talked to me said that the Hanse numbers where correct and that the French boats could not have the AVS (and stability curves) that were published and served to certify the boats. I thought that he was kidding. Everybody was wrong and he was right?

This summer I visited a boat factory and talked with the resident Naval Architect. I wanted to know why the boat I was interested in had such a good Ball/Displ ratio and only an average AVS (117º). Well, he basically said to me the same as the other naval architect. Basically he said that the stability curves that were submitted for the boat certification were not supervised by anybody and that the different programs used for calculating stability could give completely different stability curves because they didn’t use the same parameters and that many curves where “artificial”.

I was skeptical. It looked too bad to be true.

But then he showed me a lot of ORC AVS (LPS), the ones taken from stability curves that are used for rating the boats for offshore racing … and I was finally convinced. Those stability curves are all calculated the same way (for all the boats), in a credible manner.

http://www.orc.org/rules/Stability and Hydrostatics Datasheet Explanation 2010.pdf.

After all, his boat had a good stability curve, if compared for instance with a Dufour 40e, that suddenly lost about 10º on the AVS to show just a poor 110,7º.

(http://www.ffvoile.org/ffv/public/habitable1/orc/certifs/pdf/10065a.pdf).

And that is not the most worrying. The Dufour 40e is a performance boat and has a not so bad Ball/Displ ratio (32%). What would be the AVS of an Oceanis 43, if calculated by the ORC rules, knowing that the boat has a Ball/Displ of only 27%? It would be probably a lot worse than the 107º that Hanse claims for the 430.

It seems that something is very wrong with the stability data that are used to certify the boats and that are given to the public. It seems that some boat builders (or their designers) use the correct data while others use programs that give inflated results. Anyway it is completely inacceptable that the data that serves to certify boats, specifically the stability curves, are not obtained the same way in all boats, as it is the one that is used on the stability curves that are used for rating the boats for Offshore Racing (ORC).

I felt cheated and I believe that after reading this, I will not be the only one.
I leave a suggestion for Yachting Monthly: Compare the LPS (AVS) that is on the ORC rating files with the one they use to certify the boats and ask the manufacturers why the difference. Let us know what they say and please tell us in what manufacturers the difference is more substantial. "

Regards

Manuel de Carvalho

 

[Seajet]

One thing which strikes me straight away, and I don't know how or if it is included in AVS curves, is coachroof buoyancy.

If you recall, the classic case of the Fastnet 1979 found flush deck, wide beam racing boats, probably with poor ballast ratios too, had stayed inverted for quite some time after B2 knockdowns.

Some cruising boats designed using the Fastnet Enquiry results, the Anderson 26 is one example which springs to mind, therefore adopted high buoyancy coachroofs, so that if the boat became inverted the buoyancy of the coachroof would assist righting; exactly the same thinking as with modern lifeboats.

It would be interesting to know if this is factored into AVS curves; I had always, perhaps lazily judging by what the OP says, thought it was ?

 

[Tranona]

Think you are attacking the wrong people here. It is unreasonable to expect a magazine to carry out the tests you suggest.

If there is a problem with the data provided by designers/builders, or the variability of methodologies, then you should draw this to the attention of the regulators and certifying bodies.

 

[Phoenix of Hamble]

But perhaps the magazines could well play a part in raising a noise?

There are other factors too, besides the coachroof bouyancy mentioned by Seajet, such as Bridgedeck height to prevent downflooding....

 

[Seajet]

A while ago the ex-M.D. of Andersons and self were seriously thinking of putting the Anderson back into production; one of the things to sort was RCD clearance, I got the very strong impression we'd have to pull a test boat over to at least 90 degrees, in fact I discussed this with the RYA and they have a place to do this.

I suppose a complete inversion as one needs for a full AVS curve would be difficult, and expensive in several ways ( I'm saying I think it was only a 90 degree test, memory fades and we had a sample boat we could do anything with anyway ) - one might think a computer program could estimate the inverted part, though as the OP says, it would be jolly handy and fair if everyone used the same software / formula.

 

[dt4134]

If YM (or PBO etc.) got their teeth it a debate like this it might make it an interesting read.

 

[Seajet]

It might lead to the loss of some advertising revenue too though...

 

[Polux]

Think you are attacking the wrong people here. It is unreasonable to expect a magazine to carry out the tests you suggest.

If there is a problem with the data provided by designers/builders, or the variability of methodologies, then you should draw this to the attention of the regulators and certifying bodies.

I am not attacking nobody. I like Yachting Monthly and I am a subscriber.

The reason I have posted it here was not for attacking anybody but just to attract the attention of the right people to call the attention of of the regulators and certifying bodies.

I don't believe that the certified bodies ignore what is happening. The boat designers certainly know about that. It is not only necessary to call the attention but to join the necessary muscle to make them do something about it, against the interest of the majority of boat manufacturers and believe me, those have a lot of weight.

Regards

Manuel

 

[Polux]

One thing which strikes me straight away, and I don't know how or if it is included in AVS curves, is coachroof buoyancy.

If you recall, the classic case of the Fastnet 1979 found flush deck, wide beam racing boats, probably with poor ballast ratios too, had stayed inverted for quite some time after B2 knockdowns.

....

It would be interesting to know if this is factored into AVS curves; I had always, perhaps lazily judging by what the OP says, thought it was ?

No, this is not anything to do with coachroof buoyancy that is normally included on the stability curves.

If you look at the links on the letter you will see that the AVS are completely different. No what happens is that most calculations for certifying a boat are based on the stability curve. The stability curve is provided by the manufacturer through the boat designer, that assumes the responsibility for its accuracy. What happens is that they use different softwares to make the stability curve, based on slight different criteria and the results are very different.

It would not be too bad if the German boats were not always penalized and if the results that come from the comparison of the designer stability curve and the stability curve needed to certify the boat for ORC racing did not show always a difference on the French boats (and others) of 10º or more on the AVS. The German boats normally don't show a significant difference between the two AVS, meaning that the stability curves are close enough.

The criteria for the stability curve used for ORC certification is well defined and mandatory to all boats.

Those 10º difference in AVS would turn in Class B a lot of boats that actually are certified as Class A boats.

Regards

Manuel

 

[Polux]

A while ago the ex-M.D. of Andersons and self were seriously thinking of putting the Anderson back into production; one of the things to sort was RCD clearance, I got the very strong impression we'd have to pull a test boat over to at least 90 degrees, in fact I discussed this with the RYA and they have a place to do this.

I suppose a complete inversion as one needs for a full AVS curve would be difficult, and expensive in several ways ( I'm saying I think it was only a 90 degree test, memory fades and we had a sample boat we could do anything with anyway ) - one might think a computer program could estimate the inverted part, though as the OP says, it would be jolly handy and fair if everyone used the same software / formula.

That is not needed. You only need paperwork (stability curve and all the calculations made by a program on the computer) and a cerified designer to provide the signatures.

 

[Ubergeekian]

http://www.orc.org/rules/Stability and Hydrostatics Datasheet Explanation 2010.pdf.

Which ends with

Note that the theoretical predictions of stability are affected by these several important assumptions and
restrictions:

1. The vertical position of the center of gravity is derived from the inclining test, and therefore may be subject to any possibile measurement errors.
2. No account has been given to the positive buoyancy from deck camber, house, and other structures above the rail which, depending on their integrity, may improve the stability at large heel angles.
3. The negative effect due to cockpit volume has been neglected.
4. Internal fluid free surfaces, either in tankage or in bilge water, will serve to reduce both the positive and negative righting arm.
5. It is impossible to account for many of the factors which affect the dynamics of a hypothetical capsize in a steep breaking wave.
6. Stability information not derived from a full measurement and inclining test – as may be the case for some ORC Club certificates - may be prone to inaccuracy.

There's your answer right there, I suspect. Particularly the last one.

 

[Polux]

Which ends with

Note that the theoretical predictions of stability are affected by these several important assumptions and
restrictions:

1. The vertical position of the center of gravity is derived from the inclining test, and therefore may be subject to any possibile measurement errors.
2. No account has been given to the positive buoyancy from deck camber, house, and other structures above the rail which, depending on their integrity, may improve the stability at large heel angles.
3. The negative effect due to cockpit volume has been neglected.
4. Internal fluid free surfaces, either in tankage or in bilge water, will serve to reduce both the positive and negative righting arm.
5. It is impossible to account for many of the factors which affect the dynamics of a hypothetical capsize in a steep breaking wave.
6. Stability information not derived from a full measurement and inclining test – as may be the case for some ORC Club certificates - may be prone to inaccuracy.

There's your answer right there, I suspect. Particularly the last one.

That is a general explanation. Please don't assume that I don't know what I am saying. The ones that called my attention to this were Naval Architects from some reputed companies with a name in design and boat building.

It is quite the opposite of what you conclude. Basically to certify a boat (CE certification) you don't need to do any measurement or inclining experiences to determine values. It is all made in a computer and they don't use the same program to determine the AVS (or LPS) and the stability curve. Different program using slightly different criteria give different results.

For certifying a boat for ORC racing they will not accept the stability curve or the AVS that was used to certify the boat (guess why not?).

You have to determine the AVS from a measured righting moment and from there you make a stability curve (I mean, a Naval Architect does that). It is all very well defined and it is equal for all boats:


"106 - Limit of positive stability (LPS) as calculated by the LPP from the measured righting moment...

107 - Righting Moment
107.1 When an inclining test is performed with weights that are transferred once from starboard to port
side and the angle recorded four times in succession, the measured righting moment shall be
calculated as follows:
(1 4)
(1 4) (1 4) 0.0175
-
- - = × × ×
PD
RM W WD PL
4
1 2 3 4 RM RM RM RM RM measured
+ + +
=
107.2 When an inclining test is performed with four weights that are transferred one by one from starboard to port side, the measured righting moment shall be calculated as follows:
SLOPE
RM WD PL measured
= × × 0.0175
where
PL = PLM/(1+GSA/RSA)
SLOPE = (4.0*SUMXY-SUMY*SUMX) / (4.0*SUMXSQ-SUMX^2)
SUMX - the sum of the inclining weights W1+W2+W3+W4
SUMY - the sum of the pendulum deflections PD1+PD2+PD3+PD4, referenced to datum point.
SUMXSQ - the sum of the squares of the inclining weights W1^2 + W2^2 + W3^2 + W4^2
SUMXY - the sum of the products of the inclining weights multiplied with their corresponding pendulum deflections PD1*W1 + PD2*W2 + PD3*W3 + PD4*W4
The slope of a least squares fit straight line through the inclining weight vs. pendulum deflection is determined iteratively, plotting in turn each of the five possible combinations of four selected data points, as referenced to the fifth point. Of the five alternative plots, that yielding the fit with the highest correlation coefficient determines RM."

Please see points 106 and 107 - ORC rating rules:

http://www.orc.org/rules/ORC Rating Rules 2010 - v1.01.pdf

Regards

Manuel

 

[dt4134]

Which ends with

Note that the theoretical predictions of stability are affected by these several important assumptions and
restrictions:

1. The vertical position of the center of gravity is derived from the inclining test, and therefore may be subject to any possibile measurement errors.
2. No account has been given to the positive buoyancy from deck camber, house, and other structures above the rail which, depending on their integrity, may improve the stability at large heel angles.
3. The negative effect due to cockpit volume has been neglected.
4. Internal fluid free surfaces, either in tankage or in bilge water, will serve to reduce both the positive and negative righting arm.
5. It is impossible to account for many of the factors which affect the dynamics of a hypothetical capsize in a steep breaking wave.
6. Stability information not derived from a full measurement and inclining test – as may be the case for some ORC Club certificates - may be prone to inaccuracy.

There's your answer right there, I suspect. Particularly the last one.

You have a point there, however the stability data is essential for the RCD classification of all modern EEA boats so, given that, it is surprising that there's so much variation (indeed any variation) in the methods manufacturers use to calculate stability curves. Sounds like a weakness in the RCD there.

And if I may answer Seajet's post at the same time: maybe if YM went about it the wrong way they may drive advertisers away (but given the near monopoly of IPC I'm not quite sure where they'd go) but from Polox's post there's a definite issue for them to get their teeth into. I'm a believer in the RCD, but if it has weaknesses (which I'll admit it has) why should a yachting magazine be scared away from investigating and reporting on those issues. And if it leads to an interesting and informative read so much the better.

 

[Seajet]

dt,

I'd really like to think you're right !

Not quite sure who could sort getting RCD's etc onto a level playing field, as it would apparently require enough muscle to influence foreign builders too; seems a job for the EU but we know what people's reactions to, and suspicions of, that will be, and with the best will in the world it would take ages to implement.

I suppose there may be something in people realising these RCD's and published AVS's are at best a rough guide, and just as in the past must exercise their own research and judgement when looking at boats, which I suspect most experienced people do anyway, half the time without realising it.

A bit rough on inexperienced people though, just the sort of people who would benefit if these given statistics and capability ratings were trustworthy, the OP's point...

So maybe, also the OP's point, an expose by the magazines, followed by rather more informative reviews saying something like " this is the published AVS, BUT..."

 

[Twister_Ken]

Also worth bearing in mind that the boat builders' numbers are for a bare boat. Start doing typical cruising things to a boat - roller reefing with sails up in the air, stuff mounted at the masthead, radar on the mast, davits on the transom, dinghy on the coach roof, etc, and the numbers (possibly dodgy in the first place) are only going to get worse.

 

[Tranona]

But perhaps the magazines could well play a part in raising a noise?

There are other factors too, besides the coachroof bouyancy mentioned by Seajet, such as Bridgedeck height to prevent downflooding....

I think you will find that over the years the subject has been well covered by YM and YW in particular and all the issue raised by the OP have come up. Also the RYA has regularly published updates on developments.

The reality is that for most people this is a non-issue. CE marking (and for boats) RCD is a way of life and both manufacturers and buyers have come to terms with it, including its limitations. Enforcement is almost non-existent - many builders do not publish the data they are supposed to (how many times do you se N/A aginst STIX in particular) so one could argus for more regulation. However, there has not been a rash of disasters involving unsafe boats, particularly in larger mass production sizes because builders generally produce good products for the markets they target which are bought by discerning buyers.

Of course there are some marginal cases - certain boats from eastern europe and large cats from the southern hemisphere come to mind, but the RCD has had the effect of improving standards overall.

 

[stav]

What a sham modern yachting is. Seems to me everyone is in bed together scratching each others backs. All wrapped up by a group of EUcrats who are happy to write anything for their jobs. Further compounded by the numbers that have bought in to the modern system and it has got to be perpetuated to ensure their investment is acceptably secure.

Wouldn't it be lovely to read some indepth review of a product that was comprehensive in its testing and was related to the experince of the testers. Seems to me most articles are written with only the positive and you have to guess the spaces.

 

[Seajet]

Stav,

I have to agree with your comments about reviews, I can't remember seeing a bad one, but there are vague hints if you know where to look...

For real reviews, have a look at the 'pub and restaurant guide' forum here !

 

[Polux]

I think you will find that over the years the subject has been well covered by YM and YW in particular and all the issue raised by the OP have come up. Also the RYA has regularly published updates on developments.

... Enforcement is almost non-existent - many builders do not publish the data they are supposed to (how many times do you se N/A aginst STIX in particular) so one could argus for more regulation. ..

Of course there are some marginal cases - certain boats from eastern europe and large cats from the southern hemisphere come to mind, but the RCD has had the effect of improving standards overall.

I don't want more regulation, I want fairness. RYA has been doing a good work compiling boat characteristics with relevance to stability data, but it just publish what the builders sent them.

Regarding the boats out of EC I remember reading a test, I think in Yachting Monthly, about a Catalina 42. The Catalinas have a reputation of being seaworthy boats and have a good ballast/displacement ratio, but the published curve was just too bad, with an AVS like 107, if I remember right (it was about three years ago).
The testers said good things about the boat...and not a word about the incredible low AVS.

Some time later I have received a stability curve from a Dehler 39, a boat in which I has interested. I could not believe it. As all know the Dehler 39 is a very seaworthy boat, relatively narrow and with a high Ballast/displacement ratio. The AVS was only 118º and the stability curve was average. How could that boat have a worse AVS than a 40ft Oceanis, a beamier boat with a low B/D ratio? I have asked Dehler if they were sure about the curve...they were, it was the same they used to certify the boat on ORC.

I was also interested in the Hanse 430, a boat that has also a relatively high
B/D (if compared with the French cruisers) and again the stability curve was inexplicably bad, if compared with those boats and I have made that comparison. Take a look here:

http://www.myhanse.com/forum_posts.asp?TID=1326&PN=3

It is just too much to demand fairness on the data that is published by the builders? It is too much to ask that all stability curves (and AVS) to be made the same way, as the only form to provide relevant information?

If the Ocean racers, that take security seriously, have managed to have all stability boats assessed the same way, why can't we? Why can't we demand to the the EC regulators and certifying bodies to do their work properly?

Regards

Paulo

 

[Ubergeekian]

I have to agree with your comments about reviews, I can't remember seeing a bad one, but there are vague hints if you know where to look...

Sailing Today does some good bad comments (if you see what I mean), and I was dead impressed that the recent YM passage planning software review said (I summarise) "The hell with all of 'em. Use a chart."