Sailing boats' Stability, STIX and Old Ratios

Interesting to note that the MCA at its Marine Guidance Note 280 (M) "Small Vessels in Commercial Use for Sport or Pleasure, Workboats and Pilot Boats – Alternative Construction Standards" stablishes two "Permitted Areas of Operation" for RCD's Design Category A. One of them is Code "0", allowing boats unrestricted area, and the second one is Code "1" that goes up to 150 miles from a safe haven. It depends on the range of stability of the vessel. This is applicable only if RCD Category assessement has ben done using Options 1 or 2 of Section 6.1 of ISO 12217-2, this is, performing Stability Tests.

They categorize RCD's Category A into two zones/codes, as above mentioned, category B into two zones/codes and Category C into three zones/codes. So there is at least a national authority who thinks RCD's categorizations are not enough and need a deeper and more careful one, including further stability criteria.

In Spain, France and Portugal also exists this further categorization, by means of the so called 'sailing zones'. I do not know the criteria for Portugal or France, but in Spain authorities only consider the safety equipment carried aboard to fix the 'sailing zone' asigned to every particular boat, not extra stability requirements.

Here the general stability criteria used (sailing monohull) by the MCA:

1.- the area under the righting lever curve (GZ curve) should be not less than 0.055 metre -radians up to 30 degrees angle of heel and not less than 0.09 metre - radians up to 40 degrees angle of heel or the angle of downflooding if this angle is less;
2.- the area under the GZ curve between the angles of heel of 30 and 40 degrees or between 30 degrees and the angle of downflooding if this less than 40 degrees, should be not less than 0.03 metre - radians;
3.- the righting lever (GZ) should be at least 0.20 metres at an angle of heel equal to or greater than 30 degrees;
4.- the maximum GZ should occur at an angle of heel of not less than 25 degrees; and
5.- after correction for free surface effects, the initial metacentric height (GM) should not be less than 0.35 metres.

(Interesting to note they coincide, almost textually, with IMO's fishing vessels stability criteria).

The GZ curves should have a positive range of not less than the angle determined by the formula 90+60x(24-LOA)/17, for Codes "0" and "1", and or 90°, whichever is the greater.

In the damaged condition, the residual stability should be such that the angle of equilibrium does not exceed 7 degrees from the upright, the resulting righting lever curve has a range to the downflooding angle of at least 15 degrees beyond the angle of equilibrium, the maximum righting lever within that range is not less than 100mm and the area under the curve is not less than 0.015 metre radians. This damage should not cause the vessel to float at a waterline less than 75mm from the weatherdeck at any point. Proposals to accept reduced freeboard or immersion of the margin line may be accepted subject to special consideration.

Curves of statical stability (GZ curves) should be produced for:-
1.- Loaded departure, 100% consumables;
2.- Loaded arrival, 10% consumables;
3.- Anticipated service conditions; and
4.- Conditions involving lifting appliances (when appropriate).
In addition, simplified stability information in the form of a Maximum KG Curve should be provided, including a worked example to illustrate its use.
Maximum free surface moments should be included within the Loaded Departure condition

 

You have something here, although probably data are mixed:
http://www.rina.org.uk/rfiles/HISWA/1994/Investigation of minimum stability requirements for.pdf
Scroll down for table number 2.

There is also an interesting list of boats' data (Table 3) of which more than 5 units were built, having had no stability related problems. There we can ascertain how AVS is bigger for the smaller boats and lower for the bigger.

 

Yea, that's what I meant. I thought "finer" was the correct term, but I'm relatively new.

Yea, that confirms the imrpessions I was getting comparing classically designed boats to more modern production boats. Often boats of the same length can differ by 2' or more in beam.

I have to admit, beamier boats make sense for weekender or a cabin cruiser where cabin space and speed are probably the more sought after virtues. Alternatively, for an ocean cruiser, the seagoing comfort of a "narrower" hull and the safety factor seem to be essential.

Additionally, wouldn't a narrower hull of similar design to a beamier cause less turbulence? Or would the increase in draft due to smaller displacing surface cause more drag?

 

Well, I guess you get me wrong. I don't think that good modern boats, what we have being calling beamier boats are inadequate to cross oceans, if they are designed for that. They can be also a safe boat, with a huge positive stability (much bigger that in a traditional narrow boat) and that without compromising an adequate negative stability.

I am not saying that I don't like "old" narrow boats. I like all of them (modern and old), if they are nice and well designed. I like the comfort and the easy movement of a heavy displacement narrow boat, but I like also the speed and the fun of helming a very fast boat.

It's a little bit like the difference between crossing America in a Mercedes or in a Ferrari.
Sure, the Mercedes is a lot more comfortable, but if you like driving, the Ferrari is a lot of fun. Of course, for many, a Ferrari would be unbearingly uncomfortable... and of course, if you drive the Ferrari at 140m/h, and the Mercedes at 70m/h, you will take more risks in the Ferrari.

That doesn’t mean that the Ferrari is more dangerous. If you drive both cars at the same speed, probably the Ferrari would have a better dynamic safety (safer while in movement) and the Mercedes a better passive safety (if you crash).

I think it is a bit like that between an old good traditional ocean cruiser and a fast good modern ocean cruiser, being the crash a capsizing, except that you don't get tickets for speed at sea.

I am not really talking about Beneteaus, I am talking about boats like the Lansart 47, the new French coqueluche. Take a look:


http://www.lansart.fr/index.html

http://www.guillaumeverdier.com/index.php?page=34

 

Looks like a big sister to the new Class 40 boats (cruiser/racer version). I very much like the concept- I think this is the next 'evolution' of the modern open class racing designs into the cruising arena. But still am (atleast intuiatively) concerned about the inverted stability. I guess if there is enough wave energy to capsize it-so to to bring it back upright again?

JohnO

 

To those interested in knowing how STIX shall be calculated, I attach an explicative text taken from "Principles of Yacht Design" (by Lars Larsson and Rolf Eliasson). Those formulae are the ones constant in the spreadsheeet I've posted before, which has been improved over the one originally posted by SailDesign.

Here an interesting article on sailboats' stability written by Matthew Sheahan some time ago:
http://www.yachting-world.com/yw/stability/stability97.pdf

And here a RORC's paper on STIX, RORC STIX, SSSN and some considerations on ISO/FDIS 12217-2
http://rorcrating.com/stix/stixpaper.pdf

From this last, commenting ISO/FDIS 12217-2 (Nowadays already ISO 12217-2):

Required angle of vanishing stability
Design category...Mass of Boat (kg).......Required angle of vanishing stability (φV(R))
A ..................... m > 3 000 kg ............φV(R) = (130 - 0,002 m) but always ≥ 100°
B ...................... m > 1 500 kg ...........φV(R) = (130 - 0,005 m) but always ≥ 95°
C .....................................................φV(R) = 90°
D .....................................................φV(R) = 75°

Examples of use.
1. Any boat with AVS equal to or greater than 75º satisfies the requirements for Design Category D.
2. Any boat with AVS equal to or greater than 90º satisfies the requirements for Design Category C.
3. The absolute minimum AVS for Design Category B is 95º.
4. The absolute minimum AVS for Design Category A is 100º.
5. A boat weighing less than 1500kg can never satisfy the requirements for Design Categories A or B.
6. A boat weighing less than 3000kg can never satisfy the requirements for Design Category A.
7. A boat weighing 5000kg has a required minimum AVS for:
Design Category B of: 130 - 0.005 * 5000 = 105º
and for Design Category A of: 130 - 0.002 * 5000 = 120º​

(An extra corolary: All boats with Disp over 15 tonnes are allowed a minimum AVS of 100º)

Now you may think 100º and 95º are absolute minimums for categories A and B. Yes? Well: Not so. AVS can even be less than those figures. Let's see:
As per ISO 12217-2, point 6.3.2 "Alternative requirement for design categories A and B", boats may also be assigned to category A or B provided that certain limitations are fulfilled. Simplyfying here only the more relevant ones:
a) AVS ≥ 90º for category A, or AVS ≥ 75º for category B
b) when the swamped or inverted boat is totally inmersed, buoyancy is sufficient to support the mass of the loaded boat by a margin
c) watertight compartments used to provide buoyancy shall be constructed to watertightness degree 1 in accordance with ISO 11812 and hatches and doors satisfy watertightness degree 2 of ISO 12216
....
e) the recommended maximum wind strength for a given sail area shall be determined on the basis that the upright wind heeling moment in a gust of twice the mean wind pressure shall not be greater than the maximum righting moment at any heel angle.
....

 

I believe that the inspiration comes mostly from the Open 60, boats, the ones that are used in the Vendee Globe (solo race around the world without stops or outside assistance). The Safran, the boat that is going to be Skippered by Marc Guillemot in the next Vendee, is also a Verdier (and Van Peteghem , Lauriot Prévost) design.

After some major problems (some years ago) these boats have now severe safety rules. Minimum AVS is 127.5º and that is not bad even for a cruising boat.

If the Lansart has at least this AVS (and probably it will be even better) and is a strong boat, as it seems to be, it will be alright, even in a capsize....at least it will not be worst than the vast majority of ocean cruising boats that are in the market. Many have a lower AVS.

I will post a paper about how, taken in account previous accidents, these boats (0pen 60) have been made stronger, with new rule alterations, concerning safety (interesting paper).

http://www.sailing.org/meetings/2002november/papers/OC_03.pdf#search="vendee globe stability avs"

http://www.marcguillemot.com/en/index.php?2006/06/02/35-safran-open-60-meets-the-scrutinizers

 

Now let's swing the opposite side, on this STIX thing.
Following the wake of my last post, we see no boat under 3000 kg displacement may qualify for Category A under the RCD.
Means this Patricia Newman's "Gannet", a Macwester Rowan (6,8 m, 3100 kg), which is inscribed for the Jester Challenge, can be categorized as being an ocean going boat? Displacement seems to be 3100 kg, but probably Mmsc is lower than 3000.... Is Patricia worried at all about this? (Most probably she's not! )

Is Roger Taylor similarly worried about his junk rigged Corribee 21, "Mingming"...? ( )

As the UK is a more civilized country than Spain, most probably Patricia and Robert will be at the line in 2010, gods willing. Authorities in Spain wouldn't let them go, because although being old boats not subject to RCD, they wouldn't be comfortable with the thing and they would ask for a RCD certification by a Notified Body. Most probably Gannet and Mingming would not quailfy...

 

The RCD has no bearing on the use to which a boat may be put. It is specifically stated that this is the case in the introduction to the directive:

"The directive does not include any navigation or usage rules and there is no link between the design categories and any such rules..."

The RCD is only relevant when the boat is first placed in the market. If we in the marine industry are unclear as to the scope of the directive and use it in reference to matters to which it does not pertain, we can not then complain when the bureaucrats seek to similarly misuse it and control our freedoms to use our boats in anyway we see fit. If Spain has already started to control your boating by using the RCD, then complain to the Commission in Brussels because it is illegal to do so.

Similarly, cross referencing the RCD with MCA regulations for commercial vessels again muddies the water. The MCA does have a legal mandate to control vessels, but only those in commercial use, so don't give them any encouragement to extend their control to pleasure boats.

There is more at stake here than a few degrees of stability or a little down-flooding. Some of us want to preserve a world where people have to right to sail the Atlantic in their 21 foot boats if they should so wish, or whatever. The world would be a poorer place without the Mini TransAt, Jester Challenge, OSTAR, AZAB, etc and we should not contribute to the pressures the organisers of these events feel by our misinterpretation of any regulations that do exist.

The RCD is a fact of life. As a free trade agreement it has simplified the work of a few large companies that sell boats throughout Europe. Whether it has contributed anything significant to the safety of the boating public is debatable. Better education would still seem to be a better course than increased bureaucratic control. But the RCD could mutate into something really horrible that would impact on all aspects of our sailing lives. So stop prodding it with sticks and leave it alone.

 

"The world would be a poorer place without the Mini TransAt, Jester Challenge, OSTAR, AZAB, etc"

I totally agree with your feelings Crag, but unluckily RCD is already ruling our lives (as users) and not only manufacturers' or designer's.

I understand the state members in the EU have full power to enforce whatever rules within their own waters, just not contradicting EU's general normative. That's what's happening in Spain and other EU countries (I know about Portugal and France, but maybe others). Here we have quite a restricting normative and to go to sea you must fully license yourself and your boat to do so (Whatever the size, except auxiliaries under 2,5 m) and sailing or operation zones are asigned based in the Design Category and level of safety equipment carried aboard. So, at least in our country we have already the problem on our necks.

Maybe not so in the UK, as you have a boaters' strong opposition to new and upsetting rules (Because of your powerful associations like the RYA) and it seem you are being able to delay some things. But down here that's what we have.

I think we have no enough power or strength, whatever we do in these forums, to influence the future of the RCD at all. Lucky we were if we could! But this is a Boatdesign place, so I think it's the right place to discuss this kind of matters, with as deep a technical level as possible, trying to get knowledgeable people's diverse points of view and most welcome technical contributions. At the end, that's what these Forums are for!

Talking about this, I'd greatly appreciate NA's, designers and the lot of other knowledgeable people around here, to post their contributions on this STIX thing, discussing particular cases known to them or their general knowledge on the matter. As many tech info as we can share and discuss, supported on workable data, the more useful Boatdesign forums will be.

Cheers

Footnote: Crag, have a look again at my post #38, as I have tried to clarify its meaning.

 

Mini transat boats are designed and built to EU category B since 1998.

 

Correct me if I'm wrong, but the two sides of the coin I see are this:

Mainland Europe tends to "protect people from themselves" by legislating and creating oversight. This creates branches of bureacracy who's purpose it is to make us safer through regulation.

Mainland America takes a more conservative stance of leaving people to their own devices to a certain extent. If you have the money, you surely can buy just enough bike or car or boat to get yourself into a lot of trouble. And you don't have to wear a helmet on that bike either, and your lisence only cost you $30 at the DMV. This is by no means the case everywhere here, nor does it seem it will be that way forever.

It's really a debate as to which is better. Given a more lax stance, you start hearing about accidental deaths and a poignant story will come out, everyone will be up in arms for more laws and regulation. On the other side, you may obtain a better safety record with more regulation, you exclude the squids and hooligans, but for many(including myself) the price of government involvement and exhaustive rule making(and the resultant enforcement and penalization) is too high to pay!

 

The US coastguard has the power to prohibit any vessel putting to sea if they feel it may endanger life. It also dictates the safety equipment you must carry and States requires all boats to be regestered.

No such regulations exist in most of Europe.

The really 'crazy' trans atlantic crossings to and from Europe (eg 10 foot long boats, pedelos, etc) either start from Canada or go to West Indies because such thigs are not allowed in the US.

The lack of helmet wearing on motorbikes is evidence of the power of lobby groups rather than some innate 'right of freedom'.

 

Are you sure? Their designed displacement is a mere 1050 kg (As per http://www.dixdesign.com/didimini.htm), so not reaching the mandatory 1500 kg.
Boats intended only for racing are not covered by the RCD rules....

 

Yes, but the AMA appeals to the "right to freedom" when they make their pitches. It's all about freedom...blah blah blah...my personal freedom. My personal freedom to encroach on your freedoms!

I'll talk motorbikes since that's what I know best. A person in America can go out and buy a motorcycle of any displacement and capability regardless of their personal capabilities. All that's required to be legal is to take a $5 written test and a skills test to become motorcycle certified on their already $30 lisence(yes, I did complain about the $30 when I paid it too). In Europe their is a strict tiered lisencing system based off engine CC and peak HP. Additionally, it costs incomparably more to become lisenced in Europe and more to insure(counter-intuitive though that is).

Safety data will show that riding in America is more dangerous due to the rarity of personal protective gear such as helmet, suit, gloves, boots, armor(it's not "cool" ) as well as the comparitively broad granted privilege to the public to drive. That being said, would I trade that for Europe's over regulation? Probably not.

But you're right, Crag- I have considerably less of a case to prove that tendency in boats.