Sailing boats' Stability, STIX and Old Ratios

Discussion in 'Stability' started by Guillermo, Sep 3, 2006.

  1. Guillermo
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    Guillermo Ingeniero Naval

    Yes, the limit. But be careful: This estimated AVS may be pretty far away from the real thing. Could even be less. Only with a proper calculation its value may be trustable.
    And you have to take into account the downflooding angle. This usually is lower than AVS, and when reached we can no longer talk about intact stability, as we are doing here. Flooding has to be considered into calculations, and that's a more complicated world.
    Further on: What gives you a better idea of the ability of a boat to resist a knock down is the amount of area under the GZ (uprighting arms) curve up to 90º, rather than AVS. Or, easier, the value of GZ at 90º. The higher the better. If I were you I'd try to get all this info from the designer, as Vega suggested.

    If the negative part of the area of the GZ curve is big enough, yes, it will have a tendency to remain turtle. If the area is low, waves, or even inertia, probably will right her up in a few moments. To judge all this is why you need a proper GZ curve from the designer or an NA.

    The capsize safety factor was conceived after the Fastnet Race disaster. Is asumed that values under 2 are desirable for ocean going boats, although there is a lot of controversy about the trustableness of this. But it gives a clue. The lower under 2, the better.
    Motion comfort ratio was stablished by designer Ted Brewer to give a clue about the quality of the transverse motions of a sailing boat. It serves to compare a boat against other of similar size and type. A better clue to the quality of motions is the natural rolling period of a boat in seconds, T. A value similar to the breath of the boat in meters , even a little big bigger, is desirable. If T is too low, motion becomes uncomfortable, tiring and even risky (This is very important in fishing boats, where you need what is called a good 'platform stability', for the sake of the workers!). T depends greatly on the beam of the ship, so the bigger the more uncomfortable movements, generally speaking (But you cannot make B as low as to threat stability! It's a matter of balance)
    You can measure T very easily in your boat: Make her roll freely in calm waters at not very big angles (shifting people from side to side and then stoptping them at the middle, i.e.) and measure 10 complete full rollings, from one side to the other and back, and then divide the total period of time by 10 and you'll get an accurate measurement of the natural rolling period for that particular load condition. Be sure that nothing impedes the boat to roll freely (warps, etc).
  2. Guillermo
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    Guillermo Ingeniero Naval

    Generally speaking increasing weights aloft affect negatively the stability, but to judge what you mention, the curves are needed once again!
  3. luso
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    luso New Member

    This is the reply a got from Carof:
    Dear Ms or Mr (?)

    I am very sorry, but Lex Eterna was designed in 1973... more than 33 years... And I have not the values you asked for. In France Lex is approved for 1st category : without any limit (not limited)... No problem for the vanishing angle, here that is never asked for such a boat !!!

    Her weight is excessive! 23T is too much!

    The weight empty is about 15T, and the displacement maxi is about 19T.

    Maxi 20T...


    "Cordiales salutations" G caroff
    Not much unfortunately....
    This issue of not knowing the stability really bothers me. When we bought the boat it was empty inside just the bare hull so I measured everything including the caverns and the thickness of the metal hull. I also have the weight every thing that is going in the boat. Would all those measurements and number help in finding the curve? You have helped me alot already, I feel a bit embarraced asking all these questions, but these are some of the things I would like to know and I have tried to find out but to no avail. Could I hire your services?
  4. Guillermo
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    Guillermo Ingeniero Naval

    I've sent you a private message.
  5. Vega
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    Vega Senior Member

    Yes, and I was saying that I disagree. I mean you can obviously have your own opinion in what regards a boat for your own use but I don't think you should generalize.

    Or putting that disagreement in the words of another member:

    What sailors call comfort varies widely. What is comfortable to you can be just boring and devoid of any sailing pleasure to a sportier cruising sailor, and I am not talking of racing, but in making an Atlantic crossing in 15 days instead of 23.

    So, I guess that regarding safety, STIX is a good indicator, and the comfort you need is a very personal thing, and varies as much as the different kind of cruising boats that are suitable to do Ocean passages, from the very heavy boats with small rigs that you favor to the very fast cruising racers.

    Regarding STIX I have shown that some cruiser-racers have a superior STIX number than some boats that are regarded as more suitable to cruising. Things are not always what they seem.

    I guess that I would chose one in between those two, but it is just my personal choice and I would not disapprove of any other choice, providing that the boat has the seaworthiness that it is needed and suits the sailors that will crew it ( the comfort and pleasure of sailing).

    Sorry for the delay in answering.

    I don’t know if I can have the stability curves of some of those boats, because if I will have them, they will not come from the internet and I will have to search.

    Modern well designed boats (as almost all big production sailing boats) have a relatively high downflooding angle (more than 100º).

    I will try to help you, but I don’t understand why you want to perform an approximate STIX calculation for those boats, if I have given you the exact STIX number, according to ISO 12217.

    I will post a curious comparison of several GH and RM curves and STIX numbers of several very different sailboats, among them a very fast ocean cruiser racer (IMS WASA 41) with a high STIX number. I hope it helps, if not, please tell me what you want exactly and I will see what I can find.

    Regarding the Bavaria 37, I agree that intact stability is very important, as well as the downflooding angle.

    I don’t know what the downflooding angle of the Bavaria 37 is, but judging by similar boats I would say that it is more than 100º. Probably, between 105º and 115º. But you provide a STIX of 39 for a 110º downflooding. I don’t believe it. I don’t think that boat has a Stix superior to 34, probably even lower.

    About this, there is no doubt; Bavaria has become the biggest boat manufacturer:p .


    Attached Files:

  6. Vega
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    Vega Senior Member

    About “Things (that) are not always what they seem”:

    For example, normally the most common opinion is that older and heavier boats were always safer and more seaworthy boats, with better overall stability, if compared with newer models.

    Let’s look at the Swan 44 MK2 (1996), an actualization of the MK1 (1989) designed by Frers 17 years ago.

    Swan 44 -Displacement 11 T; STIX 38

    And to the Swan 45, also a Frers boat but a 2002 Design:

    Swan 45 – Displacement 8.9T; STIX 57

    Another example : older, heavier and narrow boats are always more seaworthy than lighter, modern and beamier boats (and I chose as an example of the heavier boat one that is renowned by its seaworthiness, the Contessa 32).

    Contessa 32 – Displacement 4.3T Beam 2.9m STIX 33

    Elan 31 - Displacement 3.8T Beam 3.2 STIX 34

    Etap 32 -Displacement 3.9T Beam 3.4 STIX 36

    Finally it is good to see that some boats are what they seem. Let’s look at some examples of boats not bigger than 40ft, that will pass with flying colors the STIX 40 barrier:

    Regina 38 STIX 46; Najad 38 STIX 43; Malo 49.6; Southerly 110 STIX 55; Island Packet 380 STIX 55 ; Fisher 37 STIX 43 ; SEAQUEST PRIMA 38 STIX 52.

    You can have your pick, taking into account your sailing style and the comfort you need. Or, looking the other way around, the amount of comfort that you will be willing to trade for speed and sail pleasure.

    Here you have boats for all tastes, the ones that Guillermo likes, slow, heavy and very comfortable, like the Fisher 37 or the Island Packet 380, or the middle weights that I prefer, that go from the heavier Regina to the lighter Malo (that one would be my pick) to an Extreme cruiser racer, the Prima 38, not clearly for my age….but if I were 25 years younger, I would be very tempted to cruise and race with that boat. ;)
  7. Vega
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    Vega Senior Member

    Guillermo I have been preparing a reference STIX list, but it is a never ending thing, so I will post it incomplete and I will add more data when I have time to search. If someone posts more data I will post it here as well. This will permit us to have an alphabetical data bank on STIX and AVS (all data is according with ISO 12217).


    BENETEAU 50 AVS 109 STIX 41

    BAVARIA 42 STIX 36



    CATALINA 34 MK11 STIX 36

    CATALINA 42 40 AVS 105

    COMET 36 STIX 32.5 AVS 127

    CONTESSA 32 STIX 33 AVS 155

    CYCLADES 39 STIX 38 AVS 126

    CYCLADES 43 STIX 36 AVS 121

    DISCOVERY 55 STIX 37 AVS 122

    ELAN 37 STIX 40 AVS 136

    ELAN 40 STIX 37 AVS 128

    ELAN 44 STIX AVS 124

    ETAP 32 STIX 36 AVS 122

    ETAP 34 STIX 37 AVS 123

    ETAP 37 STIX 42 AVS 123

    FIRST 36.7 STIX 34 AVS 126

    FIRST 40.7 STIX 37 AVS 126

    FIRST 42.7 STIX 35 AVS 118

    FIRST 44.7 STIX 41 AVS 130

    FIRST 47.7 STIX 46 AVS 123

    FISHER 34 STIX 33 AVS 180

    FISHER 37 STIX 43 AVS 180

    HANSE 371 STIX 35 AVS 122

    HANSE 411 STIX 33 AVS 128

    HANSE 461 STIX 35

    HANSE 531 STIX 39


    HOD 35 STIX 41 AVS 140

    HUNTER CHANNEL 31 (single keel) STIX 32 AVS 130

    HUNTER CHANNEL 31 (twin keel) STIX 33 AVS 130









    J12O STIX 43 AVS 127

    J100 STIX 37 AVS 126

    J125 STIX 42 AVS 131

    J133 STIX 46 AVS 130

    J145 STIX 52 AVS 139

    J160 STIX 57 AVS 118

    J42 STIX 45

    J46 STIX 48 AVS 127

    MALO 45 STIX 62

    MISTERY 35 STIX 32 AVS 156

    NAJAD 380 STIX 43 AVS 128

    OCEANIS 343 STIX 34 AVS 134

    OCEANIS 351 STIX 35 AVS 125

    OCEANIS 361 STIX 32 AVS 120

    OCEANIS 36CC STIX 35 AVS 126

    OCEANIS 373 STIX 36 AVS 132

    OCEANIS 381 STIX 38 AVS 130

    OCEANIS 393 STIX 43

    OCEANIS 411 STIX 37 AVS 114

    OCEANIS 423 STIX 38 AVS 119

    OCEANIS 44CC STIX 34 AVS 110

    OCEANIS 461 STIX 37 AVS 115

    OCEANIS 473 STIX 48 AVS 119

    OCEANIS 523 STIX 46 AVS 111


    SEAQUEST 32 STIX 37 AVS 132

    SEAQUEST 36 STIX 46 AVS 142


    SOUTHERLY 110 STIX 55 AVS 151

    SOUTHERLY 115 STIX 51 AVS 150


    SOUTHERLY 135 STIX 54 AVS 139




    SUN ODYSSEY 40.3 STIX 36/38


    SUN ODYSSEY 52.2 STIX 50

    SUNFAST 35 STIX 33 AVS 127

    SUNFAST 37 STIX 32

    SWAN 40 STIX 36 AVS 112

    SWAN 44 STIX 38 AVS 123

    SWAN 45 STIX 57 AVS 134

    SWAN 46 STIX 53 AVS 128

    SWAN 48 CR/R STIX 49 AVS 135

    SWAN 56 R STIX 50 AVS 127

    SWAN 56 R/C STIX 55 AVS 124

    SWAN 57 RS STIX 53 AVS 132

    SWAN 60 R STIX 68 AVS 128

    SWAN 60 R/C STIX 70 AVS 122

    SWAN 601 STIX 84 AVS 153

    SWAN 62 STIX 63 AVS 122

    SWAN 68 R STIX 76 AVS 127

    SWAN 70 STIX 82 AVS 133

    SWAN 75 R STIX 81 AVS 121

    SWAN 80 R STIX 80 AVS 138

    SWAN 82 R STIX 88 AVS 127

    VANCOUVER 28 STIX 36 AVS 170



    WARRIOR 40 STIX 47 AVS 133


    X YACHTS 37 STIX 35 AVS 123

    X YACHTS 40 STIX 37 AVS 113

    X YACHTS IMX 40 STIX 47 AVS 132

    X YACHTS 43 STIX 40 AVS 119

    X YACHT IMX 45 STIX 46 AVS 124

    X YACHT 46 STIX 44 AVS 123

    X YACHT 562 STIX 57 AVS 122

    X YACHT 612 STIX 51 AVS 110

    X YACHT 70 STIX 73 AVS 126

    X YACHT 73 STIX 84 AVS 121
  8. Guillermo
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    Guillermo Ingeniero Naval

    Excellent posts Paulo, thanks!

    As I'm interested in more deeply analize STIX through its numbers and its implications, I think we could focuse on a couple of boats (to compare them, precisely because things may not be what they seem), for which we could get as many detailed info as possible.

    Maybe we could use those Swan boats you mention: 44 MK2 and 45 . Do you have detailed info on them, including GZ curves?

    Also very interesting should be to compare the old 44 from Sparkman & Stephens with the 44 Mk2 from Frers.

    I have begun to gather info on those boats, but unluckily the info, even at Nautors' pages is not good enough. I'm going to digitalize images from Nautor's pages and convert them to CAD (I'll do that at the office, next Monday), to be able to get some of the needed (but not publicised) measurements as accurately as possible.

    If you do not have GZ curves for those models (44, 44 MK & 45) we may try to get them from Nautor, for the Mmsc load condition.

    By the way: I've never stated I favour very heavy under-rigged boats. Maybe my posts caused you that misunderstanding, but I never said that. What I've stated (And still maintain) is that in my opinion heavier boats are generally speaking more inherently safe for all around short handed ocean cruising than light boats. Even STIX favour higher displacements to this end, its value influencing FDL, FBD, FKR, FIR and FWM factors (wow!), as has been already seen in previous posts in this thread and can easily be screened for in the "Determination of Monohull Stability Index (STIX)" page from Eliasson I've posted also here.

    Being LBS =(Lb+2Lwl)/3 , we get, for the Swan 45 a 12,657 figure and, for the 44 MKII, 11,507. That's quite a difference, greatly influencing the STIX calculation, which's factors just correct up and down the 7+2,25*LBS figure, what we may call the 'basic' STIX number for a boat.

    The Swan 45 has only 43 cm more in Lh than the 44 MK II, but much more Lwl (12,07 m vs. 10,56 m). We can see here the great advantage STIX gives to bigger flotation lengths against hull lengths, favouring plumb bows against longer overhangs (I think this is one of the reasons for this trend in many modern designs). As I've stated before, boats may be not safer because of this, but in fact less, one of my concerns.

    More: In this Swans' case, the 7+2,25*LBS values are 35,48 for the 45 and 32,89 for the MK II. If we compare the STIX value posted by Vega for the MK II, we find the several Factors correcting the 'basic' STIX make for an increase of 15,5% (coming from 32,89 to 38). This effect alone would bring the 45's "basic" STIX of 35,48 up to 40,99. And, if we add the effect of, let's say a 5º increase in Dfl (let's do this just for the sake of discussion), just adding some freeboard or deck volume, we can get a further 3% increase (As we found it happenes with the Bavaria 37, when we increase Dfl from 105º to 110º). We can get, just because of these two effects combined, an STIX of 42,3 which is already bigger then the 38 figure for the 44 MK II (It is still far away from the stated 57 value, you may say, and you'd be right, so that's why we must find where the differences are and why).

    But, even not being these effects (If they are at all) the main cause for these Swans' STIX difference, through this exercise we can realize again how just using a higher Lwl/Lh ratio combined with a higher Dfl angle (this last being maybe only a matter of not watertight openings positions on deck, or also bigger deck volumes, not hull changes) can lead to very significative variations in STIX. So it may easily happen, or at least we can suspect this, that a boat with a higher STIX number than other it is not inherently more seaworthy than the one with lower STIX (As I said before, a higher Dfl is safer than a lower one from the point of view of downflooding, for sure, but this only doesn't make a boat more seaworthy: A too high freeboard also rises weights and increases windage, i.e. All of us know many boats around there with high topsides which do not stand up precisely because of their seaworthiness....)

    P.P.S.: I would like to encourage other people (especially designers) to share their info and opinions on these matters with us (This is becoming a Vega-Guillermo conversation only, which is not rich enough).
  9. fcfc
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    fcfc Senior Member

  10. Guillermo
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    Guillermo Ingeniero Naval

    Already knew it, but thanks a lot anyhow. It's a very interesting paper about a very interesting program for parametric first stages of design estimative. Probably it will become a powerful tool in the future, although for the time being it seem to serve only for a certain style of boats (finned bulbed keels, masthead rigs and single skin fiberglass)
  11. Vega
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    Vega Senior Member

    Hello Fcfc,

    Nice paper, but about a program for sailboat design: “The CEM as presented here, is a first small step, on the way to a powerful tool in sailing yacht design”.

    STIX has nothing to do with Yacht design, quite the opposite: it is an evaluation for an existing Yacht about its stability and seaworthiness. It is intended not for professional, but for the common user, just to give him a correct idea of the seaworthiness of a given boat.

    It is in that sense that RYA says that: “just the same way it is mandatory for the fuel consumption of all new cars to be published…that stability information should also be available to a buyer of a boat”.

    STIX is only an easily understandable index (but found through a complicated calculation that takes into account a lot of different stability factors) that is, in the RYA opinion (and mine), the most sophisticated stability screening tool available.

    Certainly very superior to the old ones, that in many cases were very misleading and were not unified. I mean, you needed to take a look at several to have an idea of the boat, and many times a wrong idea, mainly with modern deep drafted bulbed yachts.

    Don’t get me wrong, it is not a perfect tool, (only the best until now).

    Personally I take a look at the STIX for having a general idea, but my evaluation is done independently by the analysis of the RM curve and done by a close examination of the boat (and I mean not a virtual one), with answers about the way it is built and the kind of materials that are used. Special incidence in the way the keel is fixed to the boat, about the rudder and about the hatches and all the “glass” surfaces and the way they are secured to the boat. Not that these are the only important points, but if special care is taken with these ones, there is a good possibility that all the others are well taken care of.

    But for understanding a RM curve correctly you have to know a lot more.

    STIX is just a number and its interpretation could not be simpler: The bigger, the better.:p


    Thanks Guillermo:) ,

    I agree with you in what concerns the misleading importance of the LOA and LWL of the two Swan boats, regarding STIX. Not on what concerns higher freeboard, regarding the 45, compared with the 44. The 45 is a cruiser-racer, a winner in many oceanic races, and a boat with low freeboard.

    For what you want, I think the best will be a comparison between the Swan 45 and the 46, both very recent boats, but also very different ones: one a racer, with some cruising in mind, the other a pure cruising boat.

    I have my proper suspicions about the way STIX can mislead, when comparing very different boats, but I don’t want to influence you. We can see about that when you perform that comparison.

    What do you think of the two boats that I think will fit your criteria to this comparison?

    The racer has a higher STIX(57) and the cruiser a lower one (53) and it is a much heavier boat.

    I think that I have the GZ (or RM curve) of both boats.

    Attached Files:

    • 45.JPG
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    • 46.JPG
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  12. Guillermo
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    Guillermo Ingeniero Naval

    those two boats may serve. I'd appreciate if you provide the data you have for them.
    Also I would like to study boats with their STIX between 32 and 40. if you have data for such boats from your list, it would be a great help.

    About the STIX not being used in the design stage, I'm afraid it's not like that. You produce a rule and inmediately all designers will try to stretch it as fully as possible. As a matter of fact, what I'm interested in, is analizing the ways STIX can be stretched to produce boats fulfilling the different categories requirements, being the 'A' the most significative, as well as finding the incongruencies and weaknesses that, as any other human system, it has.

    From a marketing point of view, I don't see a boats' manufacturer telling to potential buyers: "don't worry, the boat is 'B' categorized, but having a STIX of 31, it's almost an 'A' one". Probably his designer will do the trick and make that boat go into A category, from the very beginning. Don't you think so?

    If you read carefully the CEM (which is a designing tool) paper (see at the end of point 4.2 there), you'll realize it estimates the STIX number, among many other things, as it is an important data right from the very early stages of design.

  13. Vega
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    Vega Senior Member

    That's not a problem to find data on boats with the stix between 32 and 40, and of course, you are right, for having a boat classified in class A I believe that some designers use some tricks.

    But as a marketing and information tool, the STIX is a failure (till know). I mean the average consumer doesn’t even know what it is and almost nobody in the boat industry uses the STIX to advertise the qualities of their boats. Fact is that nobody, except Malo and Southerly gives that information on their catalogs.

    It’s a shame, but to the manufacturers, STIX is only important to get their boats classified as Class A and as the requirements for that are very low, it is only on boats with less than 36ft that they have to “control” it on the design process.

    For what you want (information about those tricks) I would suggest that you got information on the new OVNI 345. All OVNIs, except this one, are designed by Philip Briand; this one is designed by Joubert-Nivelt and I have heard that one of the reasons is the “need” of having this boat approved as Class A.

    I don’t have the data on that boat, but if you ask that to the Alubat Spanish importers (Argonautica), they will get it for you (it will depend on the way you ask:p ).

    I will see what information I have on boats that are classified as Class A and obviously are close to the limit (light 34ft with low AVS and a big negative stability).;)

  14. fcfc
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    fcfc Senior Member

    Although ISO 12217 is an international rule, I think it is mandatory only in Europe (Recreational Craft Directive for CE mark). US, australian or new zealand boaters have either no rules, or their own sets of rules.

    On ABS rules, the only rule dealing with under 80 ft boats is ORY-37, which only deal with sailboat structure. Nothing for stability, nothing for small powerboats.

  15. Guillermo
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    Guillermo Ingeniero Naval

    Paulo has posted a new thread on the interesting new Class 40. See

    I bring that to here because I think it's a nice example of what shall not be done with the RCD. Saying those boats are capable for family cruising is out of the limits of the RCD. Those boats are clearly conceived for racing and this is out of the scope of the Directive.

    Taken from :
    "Owen Clarke Design also claim of their long coachroof design: "Designed for true offshore competitive performance but also as a dual role yacht in which the family can take off cruising, rolling off fast effortless miles to those holiday destinations. The characteristics of a solo race boat will make her a reliable and steady short handed fast cruiserwith the foot released a little of the accelerator and a cold beer released from the ice box!""

    Taken from
    "The safety of the boat and its crew is the inalienable responsibility of the sole owner, or its skipper, which must ensure that the yacht is in a good condition all over, perfectly seaworthy, and that it is manoeuvred by an experienced crew, which has taken the appropriate training and is physically capable of confronting the bad weather."

    This is getting absolutely eschizofrenic!

    From a technical point of view, it's interesting to note that under Class 40 rules is mandatory to fulfill requirements of ISO 12217:2002 for A category, but, on top of that, they demand the passing of a 90º test:

    "This test is aimed at proving that the boat is capable of righting itself from the broached position in it’s least favourable ballast configuration....
    When heeled at 90 degrees the loaded boat is kept in this position with the aid of a strop passed around the mast at the level of the band at the top point of the mast. The tension exerted on the strop must not be lower than 220 kg."

    Ability to recover from a knockdown is considered at the STIX by means of the Knock Down Recovery Factor FKR. So it seems there is a redundancy here. Do these Class 40 guys think STIX is not good enough...?

    I would like to bring here again one my first posts in this thread. I stated there some thoughts from Rolf Eliasson:

    "But it has to be noted that, since STIX gives no consideration to the importance of the negative areas, it is fully possible to draw a boat that's wide and flat with a low center of gravity, with a GZ curve that reaches to the sky quickly, and then drops again as quickly as it rised, just passing the minimum Avs requirement, and producing a negative area almost as big as the positive one. Such a boat could still receive a category A rating, which it clearly does not merit; it would be unsafe in the conditions outlined in that category."

    "...It's always possible to add more factors (To the STIX) without disturbing the end result, which should hover around a factor of 1. I'm thinking of Stability Loss due to Speed Factor, Transverse Inertia Factor, and GZ-area Ratio Factor (positive vs. negative areas), among others...."

    Attached Files:

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