Misleading stability

[Twister_Ken]

PS I admire your choice of the Arcona. Its one that would be on my shopping list of possibles but do you know whether the inner frame is included in the ballast weight as per X boats?

Not entirely sure. My impression is that my 1900 kg of ballast is comprised entirely of the lead hanging off the bottom, but will ask for clarification. I guess X's attitude is defensible if they consider the frame and keel to be a combination of internal and external ballast

From the builder...

"The Arcona Yachts ballast weights are always solely the weight of the fin keel.

The Steel frame does of course add even more low down ballast but we do not take its weight into account when quoting our ballast weight."

 

[Polux]

Have you considered, Manuel, that it could be that the ORC method is not accurate, whereas most of the manufacturers data is quite accurate (excepting for, maybe, a few errant cowboys) ???

Yes, off course. It was my first assumption till some Naval Architects explained me that I was wrong. Not any naval architects but one from the prestigious Judel/Vrolijk design cabinet and another, the house architecht of a medium boat manufacturer company that makes performance cruisers.

If I am not mistaken, the ORC method uses a simple heeling test, and conjures up from that a value for AVS. This would only be correct, and accurate, if the hull shape and weight distribution was an exact replica of that underlying the ORC model. This is most unlikely for most modern boats. Hence, a correctly calculated Stability Curve and AVS using actual hull shape data and weight distribution is probably far more accurate than the ORC method.

You forget that some naval designers that use computer generated stability curves using actual hull shape data and weight distribution end up with stability curves and AVS that are similar or very close to the ones that are provided by the ORC method.

Also it is inexplicable that the ORC model gives similar results as the ones provided by a generated computer in some boats (Dehler 39, X 43) and in another boats, that have similar design characteristics (Beam, Ballast/Displacement, length and shape of the hull and house) gives AVS results that are 10º to 20º off (first 40.7 and First 44.7).

I admit that the ORC model can favor one type of hull, but if the hulls keels and ballast are identical, than the model should provide the same results, compared to the ones that are provided by a computer generated stability curve (and AVS).

The ones that have said to me that different computer programs, that have a different parameterizations, give different results were the two architects that I was talking about and they know a lot more then me about the issue and about the softwares available for boat design. And that conclusion is clear if you take a look at the comparison that I have posted:

This is not about inadequacies of the standards. This is about inadequacies on the global criteria to generate stability curves that generate wrong assesments about a given boat.

This is about all stability curves not being made the same way (contrary of what happens to the ones needed to certify the same boat for ocean racing (ORC) - they don't accept the ones provided by the builders).

This is about this knowledge being public on the techinical levell and nobody doing anything to levell the play.

This is about misleading data that is given to the consumers, aggravated because not all manufacturers give misleading data and the ones that give similar information to the one that is obtained by the ORC methodology about the stability performance of their boats, have the consumers thinking their boats perform worse than the ones that offer pimped up performance. We are not talking about 2 or 3º, we are talking about a 10º diference on the AVS and that's huge, so huge that there would be a lot of boats that would not have been aproved as class A boats if the stability data was acessed using the ORC methodology that is the only one based on real inclining experiences with the boat on the water.

This is not about the inadequacies of the standards, this is about the right that the consumers have to have correct and significant data (obtained the same way for all) about the stability of the boat they are going to buy.


Let me specify:

red a divergence equal or superior to 8º, Brown, between 5 and 8, Green, 5 or less than 5.

First the ORC AVS, taken with an inclining experiment, last the AVS provider by the builder or the Designer and computer made)

X43 - 115,3 - 119, Arcona 43 -115,3 - 125; Elan 410 - 120,2 - 127; Elan 340 -116,8 - 132; Faurby 424 - 110,3 -118;; Dufour 40 - 109,9 - 122; First 36,7 - 113,3 - 126; First 40,7 - 114,6 - 126; First 44,7 - 107,6 - 130; Dehler 39 -117,6 -118; Hanse 430 - 115,4 -109;; Oceanis 423 - 110,5 -119; Oceanis 473 - 108,5 - 119; ; Swan 56 - 122,0 - 127; Sun Odyssey 49 - 110,1 - 120.

The data (AVS) was taken from the ORC international certificates (net) and from the RYA information on stability, from stability curves (provided by the manufacturers) published in magazines and from stability curves that the manufacturers display on their sites.

Others may say that some manufacturers spruce up their stability curve and calculated AVS value. If you have hard evidence that that has occurred, please make it known to the competent authorities, as that probably constitutes some form of crime, as to misrepresent a vessels capabilities is a serious matter. ..

I have felt the need to come public and to say to the ones that should be interested in these matters what is happening. If they don't care or think this is not important, fine. I could not care less. Now that I know what is happening I know enough to make my own assessment of a boat stability even if I have to look at distorted stability curves. The boats I am interested in are cruiser racers and those have ORC international certificates that provide an accurate measure that will permit me to correct the manufacturers provided stability curves. Unfortunately cruisers don't have normally ORC international certificates, but that's not my problem.

By the way, someone said that there is needed some knowledge to read a stability curve and to take conclusions. For the ones that want to have some insight about it, take a look here:

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

To have meaningful conclusions comparing different boats you should look at RM curves, not GZ curves.

Regards

Manuel

 

[dt4134]

To have meaningful conclusions comparing different boats you should look at RM curves, not GZ curves.

Thanks for starting an interesting debate Manuel.

One question though, isn't the RM curve simply the product of the GZ curve and the boat's mass? So simply put it indicates that bigger, heavier boats will resist a breaking wave better, which is intuitive anyway. But for boats of roughly the same size and weight then comparing GZ curves is just as good as comparing RM curves.

 

[Ubergeekian]

You forget that some naval designers that use computer generated stability curves using actual hull shape data and weight distribution end up with stability curves and AVS that are similar or very close to the ones that are provided by the ORC method.

Remember that beyond the range of the heeling test the ORC curves are computer generated too.

Also it is inexplicable that the ORC model gives similar results as the ones provided by a generated computer in some boats (Dehler 39, X 43) and in another boats, that have similar design characteristics (Beam, Ballast/Displacement, length and shape of the hull and house) gives AVS results that are 10º to 20º off (first 40.7 and First 44.7).

It's perfectly understandable. All it means is that for some hull shapes different methods and assumptions give similar results and for some they don't. Or that some manufacturers use the ORC methods, or results. It still does not necessarily mean that one method is any better or more reliable than any other.

I admit that the ORC model can favor one type of hull, but if the hulls keels and ballast are identical, than the model should provide the same results, compared to the ones that are provided by a computer generated stability curve (and AVS).

Only if they make the same assumptions about loading, crew positions, hatch and windows open/closed and so on.

The boats I am interested in are cruiser racers and those have ORC international certificates that provide an accurate measure that will permit me to correct the manufacturers provided stability curves.

You are deluding yourself. There is absolutely no reason to believe that the ORC figures are any more reliable than the manufacturer's ones. They may be, or they may not. I don't know. You don't know.

 

[boomerangben]

I take Polux's point that installing standard cruising kit such as furlers and radars is going to reduce a boats resistance to roll over and righting moment after knockdown. But what I find interesting is what those numbers are. Given a simple GZ/RM curve you cannot quantify the effect of changing the location of the Centre of Gravity - you need to rerun the analysis behind the curve. Now to a certain extent you can compare similar boats of similar hull forms and weight distribution, but how similar do they have to be? Are "minor" differences significant or not? So my question to Polux would be what AVS does he consider safe and on what basis did you arrive on that number.

I have to confess that I don't read much in the way of reports in mags for buying new boats. I am not in the market for a new boat. But if I was concerned about AVS and so on and I was about to spend 10s of thousands of pounds/euros I would want the manufacturer to give me that information and basis on which that analysis was done.

It is also interesting to note that increased stability makes for a rough ride - the greater the righting moments, the greater the accelerations and the greater the vomit factor. One comment I seem to recall being made in reviews is the motion of the boat in a seaway. Of course on a long passage that motion has a huge impact on the performance of the crew and generally it is the crew that is first bit of kit on a boat to faulter in rough weather.

I think Polux is right that some form of standard is used for determining these details, perhaps there is for producing the RCD paperwork. But I don't think it is appropriate to demand a single piece of software - whose software should it be, and what about the other software houses who would go out of business because their software is obsolete?

 

[doug748]

It sounds a bit rum to accept that the figures are fiddled but nobody reads them, or if they did they would not be understood.
It would be nice to see the left hand side of the graph of righting moments by practical ORC experiment, superimposed on those divined by the makers. The scientists may still have their doubts, the rest of us might reach pretty firm conclusions.
Anyway thanks Polux for bringing this interesting matter up. And God bless us all for not bringing anecdotes, about this boat or that, into an academic discussion.

 

[Polux]

Thanks for starting an interesting debate Manuel.

One question though, isn't the RM curve simply the product of the GZ curve and the boat's mass? So simply put it indicates that bigger, heavier boats will resist a breaking wave better, which is intuitive anyway. But for boats of roughly the same size and weight then comparing GZ curves is just as good as comparing RM curves.

Yes you are right, if the boats have the same displacement. But even on similar boats, for example the available big production cruisers, you have normally different displacements and therefore if they all had the same GZ curve you would have different RM curves. The new Bavaria 40 weights 8680kg, the new Jeanneau 407 weights 7450kg, the Oceanis 40 8260kg, The Dufour 405 8000kg and the Hanse 400 8400 kg. There are significant differences among some of these boats. Between the lightest and the heavier you have a difference of more than a 1200kg and that will make a big difference on a RM curve.

Besides you know about the differences between RM and GZ curves but most people don't and they just assume that a boat with a better GZ curve is more stable than another with an worse one and that can be not the case and it will not be probably be the case if the boat is heavier.

If someone wants to compare the overall stability of different kinds, types and sizes of boats, a GZ stability curve is not the right tool. You will need RM curves. The RM curve should be the one provided for public information.

Here you have this a lot better explained by the naval architect that designs Malo yachts. He seems pissed because the public tend to assume the GZ curve as a measure of a boat stability:

http://www.maloyachts.se/YACHTS/Stability/tabid/134/Default.aspx

Regards

Paulo

 

[Seajet]

Calibration and accurate measurements & readings are rather close to my heart, having been involved with this with aircraft...

It's quite staggering to see the difference between 'pro' ( say military and by no means perfect ) and civilian kit, call me pedantic but I'd quite like to know if I'm close to stalling speed, or a few hundred feet above or below the runway I'm approaching !

This is why I've asked a lot about depth-sounders and their beam angle, the general consesus is about 45 degrees, a fair enough scan for a boat I think; though some highly qualified Test Pilots reckoned 6 degrees, which would be a serious snag...

Any true test should have the boat realistically loaded, ie stores, water, fuel and crew.

NB, a crane induced inversion in calm water doesn't replicate the dynamic forces on a boat in Mid-Atlantic, but such a test is probably as good a datum as anywhere to start.

There is no substitute for real - life inversion tests ( with the above mentioned loading ), with independent observers and film crews present.

When we were thinking about putting the Anderson 22 back into production, we thought we might have to submit our test example to roll-over tests, and were happy to comply; it's called having faith in the design !

 

[Polux]

I take Polux's point that installing standard cruising kit such as furlers and radars is going to reduce a boats resistance to roll over and righting moment after knockdown. But what I find interesting is what those numbers are. Given a simple GZ/RM curve you cannot quantify the effect of changing the location of the Centre of Gravity - you need to rerun the analysis behind the curve.

Yes of course and the negative effect of a radar dome in the mast and furling main will be bigger in boats with a smaller RM curve (smaller boats or lighter ones). That analyses have been made, for a given boat and I have read the results. There was an article that mentioned that on YachtingMonthly, many years ago. The number of degrees lost on the AVS (for a 40ft boat), if I remember correctly, was 10º, but that will depend on each boat stability.

But if I was concerned about AVS and so on and I was about to spend 10s of thousands of pounds/euros I would want the manufacturer to give me that information and basis on which that analysis was done.

.....I think Polux is right that some form of standard is used for determining these details, perhaps there is for producing the RCD paperwork.

Not concerned only about the AVS, the AVS is only part of the information that a Stability curve will provide. I would say concerned about a boat stability. And are we not all, when we buy a boat?

The stability curve provides information about the sail carrying potential (initial stability) about the energy needed to capsize the boat (area under the positive part of the RM curve), downflooding angle and the energy needed to re-right the boat when it is inverted. It provides also very important information, like the force the boat is making to right itself up at 90º of heel...and of course, it also provides the AVS.

... So my question to Polux would be what AVS does he consider safe and on what basis did you arrive on that number.

Safe for doing what? A day sailer, a coastal cruiser and an offshore boat will have different requirements.

That is a tricky question and not one that I want to discuss in a forum like this. Different naval architects would give you probably different answers and besides there are many factors to consider:

1-The stability curve only gives information about static stability. We would have to look at dynamic stability also.

2- The AVS is only a part of the requirements (assuming we are talking of an offshore boat) for a safe stability. Others would be the down flooding angle, the force the boat is making to re-right itself at 90º of heel, the total energy needed to capsize the boat, the total energy needed to re-right the boat when it is inverted and the proportion between the positive area under the RM curve and the negative area under the RM curve.

If we are talking just about the AVS I would say a minimum of about 120º (real AVS). The average time to re-right a boat with a 120º AVS (assuming a typical stability curve), in sea conditions that were able to capsize that boat, is between 1 and 3 minute. And the reason I would like to have a boat with a minimum AVS like that is because I don't think I can hold my breath longer than that

It is also interesting to note that increased stability makes for a rough ride - the greater the righting moments, the greater the accelerations and the greater the vomit factor. One comment I seem to recall being made in reviews is the motion of the boat in a seaway. Of course on a long passage that motion has a huge impact on the performance of the crew and generally it is the crew that is first bit of kit on a boat to faulter in rough weather.

Well, yes and no. what gives a rougher ride is the increased initial stability, the one that is used to power the boat, to offer resistance to the wind on the sails. That one goes only till about 30º and on modern boats most of it is given by form stability (beamy boats with large transoms). You can increase substantially safety stability without increasing much initial stability.

Regarding comfort in a sea way there would be a lot to be said. There are people that feel more uncomfortable on the slow but bigger movement of a slow and heavy boat (up and down) than on the sharper but less wide movements of a modern fast boat, and vice versa (providing the boat has narrow entries and a sleek hull). Another factor to consider is that speed gives stability.

Regards

Manuel

 

[boomerangben]

Hi Polux,

This is interesting and got me thinking about yacht stability and so on. You have clearly done a lot of research and want a scientific means of choosing a boat. The engineer in me can understand that and have wondered about building a stability booklet for a boat (if and when I actually get one!). Ironically though, my choice of boat is rather more on looks and reputation of designer, builder and the class itself - the literature has no stability curve! I am however looking a boats that have been around a long time and well proven in practice - a luxury not available to someone wanting a latest creation.

The cynic within me would caution the use of stability curves published in marketing material. Whilst of course a published stability curve must (one hopes at least) reflect a snapshot in the design life of a boat, it is just that - a snapshot of a given hull geometry and load case. I have not looked in detail at stability curves provided by manufacturers, but would be surprised if they go into much detail on the assumptions made when doing the analysis. Moreover, this material one imagines is from the marketing department and may not be the what a Naval Architect would provide or at least expect to see as a prospective purchaser. I personally would not accept the marketing blurb for making scientific comparisons.

What I find interesting to your approach is that on the one hand you want a quantitative comparison but on the other hand (and I suspect you are well aware of this) the data you have is for an unknown load case. In other words, might not bear any resemblance to how you are going to configure the boat.

What would be really interesting to know would be how the stability of the boat is affected by different loadings. For that you need cross curves of stability which allow the stability curve to be generated for any know position of the CoG and displacement. It would be interesting to see if there any available for production boats since only then could you determine how sensitive a boat's stability is to its loading.

I too have wondered what roller reefing and radars have on stability. To be honest, regardless of the numbers I wouldn't fit in mast reefing (but again I am thinking of smaller boats than yourself), partly because of stability, but mainly because of reliability - it is one thing having all that kit up the mast, quite another having 100kg of me up there too trying to fix it!

Best of luck with your research and I hope you get the right boat for you.

 

[Tranona]

Interesting to consider what influences a buying decision and the factors that people consider. In an earlier thread on this subject, somebody put in a plea for a return to more "wholesome" boats, by which they meant (I think) longer keels, heavier displacement etc where a judgement was based on the shape and how it matched one's expectation of what a boat should look like.

In this month's YM is a test of the latest Dehler - a 45ft racy cruiser. Its general shape is totally unlike a "wholesome" yacht, but is aimed at people in a previous generation who might have bought something like a Sigma 38 or earlier a Nic 36 - both state of the art in their era. Ignoring any rating issues, to my mind you can only accept the Dehler concept on the basis of the numbers as intuitively it does not look "right" - although the "test" was very positive.

Perhaps in a few years' time we will all be used to the new shapes and be able to determine their value without the worry of trying to understand the theory behind the designs.

 

[Polux]

Interesting to consider what influences a buying decision and the factors that people consider. In an earlier thread on this subject, somebody put in a plea for a return to more "wholesome" boats, by which they meant (I think) longer keels, heavier displacement etc where a judgement was based on the shape and how it matched one's expectation of what a boat should look like.

That is not my approach. I think that in the last 10 years boat designers have made an outstanding job improving sailboats in all regards, including safety. Of course, not all boats are designed for offshore work and I would say that compared to 20 years ago there are a lot more boats that are used strictly for day-sailing, small coastal trips and to be at the marina, as a second house. The market just reflects what consumers want from a boat.

In this month's YM is a test of the latest Dehler - a 45ft racy cruiser. Its general shape is totally unlike a "wholesome" yacht, but is aimed at people in a previous generation who might have bought something like a Sigma 38 or earlier a Nic 36 - both state of the art in their era. Ignoring any rating issues, to my mind you can only accept the Dehler concept on the basis of the numbers as intuitively it does not look "right" - although the "test" was very positive.

Perhaps in a few years' time we will all be used to the new shapes and be able to determine their value without the worry of trying to understand the theory behind the designs.

I agree with you about the targeted public, but if you have a look at the new Dehler 41 you will see that it has a more modern hull, with a broader stern and the beam brought aft. So even Dehler, that always was a conservative brand, is changing

But regarding the 45 and pure performance let me remind you that particular hull form still excel in some conditions. That hull form is not dissimilar from the one from the Beneteau 40 and as you know on the last Sydney to Hobart race and on the last one, the Benetau 40 (and the 45) performed admirably. Last year the winner (First 40) leaved a Class 40 Pogo at 8 hours and this year the same boat would have been also ahead of the Pogo if he had not retired (engine problems) but the second F40 on both years finished less than 10 minutes behind the Pogo, that is a pure racing boat. That race, as you now, is mostly an upwind race, most of the time, in bad weather.

This type of shape is optimized for upwind sailing and give, as a bonus, a more comfortable ride. In my opinion it makes sense on a 45ft cruiser that has already enough interior space and that is aiming more fast cruising than racing. Of course that boat, narrow as it is, could have the beam brought aft and have it larger without losing any of its upwind potential and winning a better downwind control.

Sorry, this is not the subject of this thread, but I could not resist.

If you like to talk about boats and design have a look here:

http://www.sailnet.com/forums/boat-review-purchase-forum/62341-interesting-sailboats-66.html

Regards

Paulo Manuel

 

[Polux]

Hi Polux,

This is interesting and got me thinking about yacht stability and so on. You have clearly done a lot of research and want a scientific means of choosing a boat. The engineer in me can understand that and have wondered about building a stability booklet for a boat (if and when I actually get one!). Ironically though, my choice of boat is rather more on looks and reputation of designer, builder and the class itself - the literature has no stability curve! I am however looking a boats that have been around a long time and well proven in practice - a luxury not available to someone wanting a latest creation.

Hi Boomerangben,

Yes I have done a lot of research but that was not only because I wanted to have the capacity to chose the right boat for me (I mean, the next one) but also because the architect in me wanted to understand all about hull shapes and sailing, including safety, speed and cruising potential.

But this research was not only scientific, as you call it, but mostly about understanding why the better naval architechts designed the new boats the way they do, with special incidence in racing and solo or duo racing (because it is the way I sail, and I would say, most sail, I mean, with a small crew).

Yes you can buy a 20 years old boat, a "well proven boat" but you would be naive if you think, that regardless of your type of sailing, you can not have a modern, safer and faster boat. You have just to know how to choose the right type of boat for your sailing...and have the money to buy it

As I have said on the previous post, yacht design have progressed a lot on the last 20 years, with new materials permitting solutions that increase speed and safety.

The cynic within me would caution the use of stability curves published in marketing material. Whilst of course a published stability curve must (one hopes at least) reflect a snapshot in the design life of a boat, it is just that - a snapshot of a given hull geometry and load case. I have not looked in detail at stability curves provided by manufacturers, but would be surprised if they go into much detail on the assumptions made when doing the analysis. Moreover, this material one imagines is from the marketing department and may not be the what a Naval Architect would provide or at least expect to see as a prospective purchaser. I personally would not accept the marketing blurb for making scientific comparisons.

Yes, I agree.

This thread is precisely about marketing blurb. The point is that us, as EC citizens pay a lot to bureaucrats to have the right to have credible information, as consumers. The EC boat certification is all about that. I resent (a lot) having to pay and don't have the right to have credible information anyway. The message is: Someone should oblige the Eurocrats to do their job properly!

What I find interesting to your approach is that on the one hand you want a quantitative comparison but on the other hand (and I suspect you are well aware of this) the data you have is for an unknown load case. In other words, might not bear any resemblance to how you are going to configure the boat.

What would be really interesting to know would be how the stability of the boat is affected by different loadings. For that you need cross curves of stability which allow the stability curve to be generated for any know position of the CoG and displacement. It would be interesting to see if there any available for production boats since only then could you determine how sensitive a boat's stability is to its loading.

Yes I am well aware of that. But if I have the right data I can do the rest of the homework. Besides, in what regards stability, you can have curves on minimum sailing conditions and curves with full load (the max load of the boat determined by the designer). Some time ago they were both necessary for the EC certification. I think something is changed, last year or so, but I did not yet understood what (medium weight stability curve?).

Of course, this indicates to you what is the load the boat can carry, how it will affect stability, but doesn't tell you what will happen to the boat sailing characteristics and speed. On some very fast cruisers, that have semi-planning hulls and that can go easily over hull speed, you would have to travel light, or very light, to maintain that potential.

And of course, all that talk about stability curves is only half-picture. You would have to have a good look at dynamic stability also and in what regards to that, modern performance boats offer good advantages over older designs: Thirty years ago it would be unimaginable that a race like the Transat from France to Brasil, with 85 22ft solo sailing boats could happen regularly without capsizes or any other major life threatening incident.

Best of luck with your research and I hope you get the right boat for you.

Thank you, I will...the bigger problem is not choosing but having the money for it

I wish to thank all the good "treatment" I have received in this forum, being a rookie and all. This is not "my forum" but being this subject an European problem it seemed to me more adequate to bringing it here and not on an international forum.

If you or any of you want to continue to chat about boat design with me, you are very welcomed to this thread:

http://www.sailnet.com/forums/boat-review-purchase-forum/62341-interesting-sailboats-66.html

For the ones that think that I am a stability freak, you will find that I rarely talk about stability, at least on the "scientific terms", utilizing a boomerangben expression.

Regards

Paulo Manuel

 

[boomerangben]

I would love to be able to browse the glossy mags and brochures for a new boat. However as you say, it is the money and my budget wouldn't get anything more than a new dinghy!