I am closing the blog for sailing and I will get back to my sailing chair somewhere in October but in the meantime I will like to offer you all a last post that I hope will contribute for a better information about boat safety stability and the RCD class A certification. Something that gives food for thought and eventually contributes to the implementation of a bluewater RCD certification class.
When the RCD was being drafted they invited naval architects and naval engineers from several nations to contribute with studies and thoughts regarding yacht stability, including safety and final stability.
The general consensus regarding an offshore boat, what the Americans call a bluewater boat, was that it should have an AVS of about 130º and the correspondent final stability.
There is logic in what regards that number. There are studies that indicate that the average time for a boat with an AVS of 120º to re-right itself on the same sea conditions that lead to an inversion is between one and two minutes.
But things are not that simple because if the boat is inverted by a rough much bigger wave, that time can be much, much bigger and if one is on the cockpit at the steering wheel, even if attached to the boat, things can get ugly even with a one minute period.
So, taking all that discussion into consideration ,what is the AVS that is considered minimum to a class A boat to pass the RCD certification? Yes, the 130º value was considered but it is a maximum value that comes in a formula, this one: 130 - 0,002m) but always ≥ 100°, being m the boat mass
That means that a yacht with a small mass, a small boat, will need an AVS not very far from 130º but it also means that on a considerably bigger boat the AVS can be close to 100º.
And, of course, increasing ballast to better the AVS creates more RM and more efforts on the boat hull and structure making the boat more expensive.
The result is that mass production builders tend to follow the RCD minimum requirements in what regards AVS opting for one slightly above the one that formula gives as minimum. More expensive boats usually opt for having a higher AVS (having a bigger B/D) one considerably above the minimum that is determined by the rule for Class A, normally near 120º but rarely 130º or over 130º.
Sadly, due to lack of awareness of the public, some of the brands that make more expensive boats, like Wauquiez (Pilot Saloon) or Amel have abandoned that practice and come close to the one followed by the cheaper mass production boats, having an AVS not far from the minimum that is required for the Class A certification of the boat.
More expensive boats are made to a budget too and some are choosing to spend all the extra money on an even more luxurious interior and I would say with very good market results, but misleading sailors that assume that those boats have a better stability and seaworthiness than mass market boats.
Some examples to make clear the variation of the minimum AVS required (for certification on class A) with the boat mass : A Pogo 30 will need a 124.4º AVS, an Oceanis 41.1 an AVS of 114.3, an Oceanis 46.1 needs a 108.8 AVS and an Amel 50 needs only one AVS bigger than 100º (these values are approximate).
|Elan Impression 444|
This will allow Amel to say that an AVS of 110º (or so) is vastly above what is demanded for the certification of the boat as Class A, giving the impression that the yacht has a very good final stability and a very good AVS when in reality it is an average one and a poor one for a bluewater boat, if we take as measure those 130º that were considered on those studies as the value indicated for a bluewater boat.
And if we look at the B/D of the two Oceanis, both the smaller and the bigger one, we will see that with similar keels and not very different drafts (2.19, 2.35m), the smaller one needs a B/D of 29% while the bigger one only needs 25% to be slightly above the required AVS for the class A certification, meaning that the smaller boat has probably an AVS superior to the bigger one and I say probably because for good reason brands like Beneteau or Amel don’t make public the stability data of their boats.
|Hallberg Rassy 44|
Note that the same stability safety reasons that lead to consider necessary an AVS not far away from 130º on smaller yachts are not less valid on bigger ones even if RCD gives a different impression.
The only thing that changes is the bigger risk of being rolled by a wave on a smaller yacht (because the overall stability is smaller) meaning that a bigger boat will need a bigger breaking wave to be rolled. Not so much in what regards a knockdown by a bigger gust of wind or by one of the increasingly more frequent weird meteorological phenomena, since the sail area is proportional to the boat mass.
But I do know of several 40 to 45 (some bigger) yachts inverted by waves, numerous cases of yachts that were knocked down and a considerable number that were abandoned after several knockdowns.
I am sure that some will be asking themselves what has the AVS to do with knock downs? Or even with an inverted boat?
A lot, because a high AVS implies a big safety stability, meaning that not only the boat would be much harder to knock down as it will recover much faster from one and that is vitally important, because when a boat lies flat on the water has very few remaining positive stability and it will be at the mercy of the next wave.
That’s why most boats that are rolled are not by a single wave but by a set of two: the first one takes out the boat stability (knocking it out) the second one rolls the yacht.
An higher AVS means also that the area under the positive stability (on a RM curve) is much bigger, several times bigger, than the one over the negative stability, meaning that a much smaller wave than the one that had it inverted, can bring the boat back.
If the positive area of the curve is 4 times bigger, it means that for bringing the boat back, from an inverted position, you will need a wave 4 times smaller than the one that inverted it. If the positive area is only two times bigger that means that you will need, to re-right the boat, a wave half the size the one that inverted it and that, if the boat was inverted by a rough wave, it can be a problem.
|Najad 450 cc|
While a small 33 ft light yacht (that to be certified as class A has to have a better AVS than the one of a bigger boat) will recover very quickly from a knock down, the typical 45 mass production yacht (that can be certified with a much lower AVS) will take much more time and in some cases the water on the sails can even prevent it to right up on any reasonable period of time, leaving it exposed to the waves with little stability remaining.
We can conclude that the Class A certification, as a measure of the boat stability and seaworthiness, is useful in what regards 30/33 ft boats and gives information about a more or less seaworthy boat (for instance, separating the Oceanis 30.1, that does not have the stability to be a class A, from the Hanse 315, that has), but it will say nothing about the seaworthiness differences (in what concerns stability) between an Oceanis 41.1 and an Hallberg Rassy 412: they are both class A boats (by a big margin) even if they are very different in what regards final stability and AVS.
|Elan impression 394|
And since the RCD main objective is to certificate products providing the public with information regarding their safety and conditions where they are safe to be used, it is obvious that in what regards boats over 36ft it fails miserably since they are almost all class A boats, giving the impression that they are equally suited for offshore, bluewater sailing.
Of course, the ones that gain with this situation are big mass production builders that want to make boats adapted to most users, the ones that will never sail in bad weather, boats with a smaller stability and AVS but with a certification that will put them on the same class of boats designed to be much more seaworthy and designed to be sailed on more demanding weather.
That is clearly a commercial advantage but it neither allows transparency in what regards sailboat characteristics and the market nor provides consumers with accurate information. I have to say that sail magazines have done nothing in what regards making clear this distinction between yachts with different final stability, negative stability and AVS.
That distinction cannot be evaluated on a test sail, that is rarely done in bad weather, much less with a knock down or inverted boat, but that difference results very clear when comparing stability curves. Magazines used to publish them but mysteriously stopped doing that. And even when they published them they never commented, specially if they revealed a poor AVS, final stability or a big negative stability.
It is ridiculous that the higher seaworthiness certification is today achieved by almost all cruising boats over 30ft. Should not the consumer who wants a much more seaworthy boat than a 30ft have the right to clear information about that?
That will only be possible creating a new higher seaworthiness and stability specifications class, one that will allow consumers to distinguish between boats like an Oceanis 41.1 and Halberg Rassy 412 and that will separate boats that can be sailed offshore from the ones that are designed to be sailed offshore, bluewater boats.
There have been talks about that for a long time but I don’t see any will to pass that to the law since big boatbuilders are completely against that new class, for obvious reasons, that have nothing to do with transparency, information and public interest.
|Hunter Legend 50|
There are more factors on the RCD in what regards seaworthiness and stability, like the minimum energy at 90º, the downflooding angle and the STIX but they are secondary to AVS, final stability and negative stability. Sure, the minimum energy at 90º is very important but it is directly related to the AVS (since the types of hulls today are very similar), the downflooding angle is very important and should be considered but it is easily improved and does not depend on the basic stability of the sailboat while the STIX only gives a general idea of the boat overall stability, not final stability and it has a factor that can count very negatively without reason, I am referring to the boat sail area, as if the sails could not be easily reefed.
For the ones that think that I am exaggerating let me remind you that the IMOCA are self-rightening. We could say that they are designed to be sailed on the southern seas and the worst seas on the planet but that is not the case with the 40 class that are meant to be ocean racers.
On the 40 class box rule it is not only demanded that the boat pass the stability criteria to be approved as a class A boat but to ” proving that the boat is capable of righting itself from the broached position with empty ballast tanks.
It must be when heeled at 90 degrees … the boat in measurement trim is kept in this position with the aid of a strop passed around the mast at the level of the measurement band at the top point of the mast ... the load exerted on the strop must be a minimum of 235 kg and a maximum of 320 kg.”
This on a boat with a displacement of a bit more than 4000 kg and a mast of around 20m means a huge RM at 90º. On a cruising boat that weights 2, 3 or 4 times more that value should be 2, 3 or 4 times more. Those values for the 40 class boats are so big that give most of them the ability to be self-righting (using the water ballast).
These values of RM at 90º are not only measured on the water (and not on the paper) but also are several times bigger than the ones that result from the stability curves needed to certify a yacht in Class A.
Should not a cruising boat meant to be sailed bluewater have at least similar final stability values, the ones that warrant that the yacht is capable of righting itself quickly from a broached position? Should not consumers that want to buy a bluewater boat have accurate information about this? Is it not for that that the RCD exists?
Certainly they are not being informed now, since the RCD allows yachts to be certified in Class A with an AVS just a little bit over 100º and such a boat in sailing trim, with sails on the water, would not be able to right itself from a broached position on a short period of time, staying there, knocked down, almost without positive stability at the mercy of the waves.
A new RCD category for bluewater boats, against the will of mass production builders, will only happen if there is enough public pressure and for that sailors have to be informed and aware of what is going on.………………………………………………....
A request to you all, I am sailing and I will not post this on any sailing forum or on facebook groups. If you agree with what it is said please contribute to the discussion on this topic, not only here but on the internet sites where you discuss sailing information. You are welcome to share this post where you find it useful.
For the ones that want to follow my personal sail log I do that on my facebook page. I am already sailing and having fun even if it is yet a bit cold this year on Macedónia, North of Greece. You can still see the snow on the top of the mountains.