Angle of Vanishing stability. [Archive]

Mychael

08-19-2006, 08:41 PM

I know what it relates to but I don't know the numbers for what is considered a good figure or average or poor (in degrees).
Reading articles on new sailboats I have seen the comment made that (some) designers/builders are reluctent to quote the AVS figure as it's not particularly high/good.
Is there a minimum standard that all commercially boats must meet?
Which boat or brand of boat is noted for being exceptionally good in it's AVS rating?
This is all in the context of sailboats.

Mychael

Hunter25

08-20-2006, 01:32 AM

110 degrees for near shore racers and 120 degrees for ocean racers are generally accepted standards. A few races require these be met, but production boats do not follow this unless designed to compete. The EU has attempted to address this problem with requirements, but I am not sure it makes better boats, just heavier ones. There is some difficulty in presenting stability information for a particular yacht. The numbers do not necessarily represent a true indication of a boats stability. Designers have recently begun to use different formulas, which seem to change form year to year, that draw a better picture for a perspective owner to understand. There are others here that can help, much more then I can.

Wynand N

08-20-2006, 03:30 AM

I usually aim at 130 - 135 degrees for my cruising sailboats, with a lot of initial form stability rather than ballast stability...

Alik

08-20-2006, 04:12 AM

Requirements towards AVS are listed in ISO12215-2. My opinion they are a bit liberal, one needs 130-140 degrees for good 40'-50' ocean cruiser.

Mychael

08-20-2006, 04:34 AM

From what I've read in magazines and post so far here is that ideally you would want at least the 130 degrees in a boat. so do you think that some new boat manafacturers are trading off good sailing and handling for comfort and "motor home" level of interior fitout?
As an example (and I have no bias to any particular boat) I have just read a test report on the Hunter 45cc. The auther of the article concluded with a "Likes & Dislikes" summary at the end. One of the dislikes was "Lowish angle of vanishing stability". In this case what I would consider to be a very low 109 degrees.
We are not talking cheap boats either with price being high six figure sums. Is this an "unfortunate "trend now in all new boat designs? More creature comforts but less seaworthiness?

Mychael

Wynand N

08-20-2006, 07:08 AM

In this case what I would consider to be a very low 109 degrees.
We are not talking cheap boats either with price being high six figure sums. Is this an "unfortunate "trend now in all new boat designs? More creature comforts but less seaworthiness?Mychael

There are two types of cruising sailboats;
1.) The ones that go sailing the big blue
2.) and the marina queens, some of which may be taken out for a day sail once in a while.

Unfortunately, only about 10% or so sailboats ever built, will cross an ocean or at least do an extended cruise.
The others are just there to impress the Jones's and friends - nice places for bikini clad ladies to catch a tan and have a nice whisky at sunset....;)

Guillermo

08-20-2006, 05:52 PM

...The others are just there to impress the Jones's and friends - nice places for bikini clad ladies to catch a tan and have a nice whisky at sunset....;)
Yeap...! And they crowd marinas, ask for an unreasonable level of services at the pontoons and 5 stars restaurants and the like ashore, etc., making prices rise so much...! :(

Mychael

08-20-2006, 06:29 PM

The other thing I note with a lot of the modern designs is that although they might be marketed as "blue water" cruisers, it seems to me that they make the modern cockpit too exposed for the colder/wetter climates. No place in the cockpit to "hide" from spray etc. Unless of course you then go for bimini's and dodgers and the like.
Personally I prefer a cockpit which is deeper and has a highish front to snuggle behind.

Mychael

Guillermo

08-21-2006, 03:59 PM

.... they make the modern cockpit too exposed for the colder/wetter climates. No place in the cockpit to "hide" from spray etc. Unless of course you then go for bimini's and dodgers and the like.
Personally I prefer a cockpit which is deeper and has a highish front to snuggle behind.
Mychael

For colder/wetter climates nothing like a good pilothouse motorsailer, in my opinion.;)

lazeyjack

08-21-2006, 09:56 PM

for CE cert there needs min, and it is a lot higher than 120, , in the Bay of Biscay a well known brand capsized and did not right, the BOT or whatever , body they have in UK, Investigated and found the stability wanting, I can't remember the brand, but its been well discussed
RECENTLY 2-3 years or so back, the angle was increased, a lot, for the Sydney Hobart
I im for 132 plus, at arrival, and 138 at departure, that is loaded, full fuel, water stores etc

MikeJohns

08-22-2006, 02:55 AM

Lazyjack, it was a Benetau 390.

Big problem is getting true stability info out of many designers, stability should be worst case for your design whether that is half tanks full tanks or no tanks. Also the extra gear that gets affixed tends to very quickly reduce the quoted stability of the light boats. Stability should also be run for a few tons of shipped water flowing around inside and all these figures made available to the purchaser. All we see is a nice smoothed curve without ref to what conditions apply and some of the glitzy curves are very poor, I know they will be a lot worse in real use.

Light Yachts particularly need something akin to stability booklets. This essential info on every commercial vessel is lacking in the yachting world but the skipper should be aware that if a radome, davits solar cells and 5 extra water/fuel cans on deck are added that the stability may reduce by 10 degrees on his light 40 footer.

lazeyjack

08-22-2006, 03:18 AM

true, but seem to member there was another brand,
Fortunately very few sail in knock down turn im over stuff
BUT take top designers like Finot and open 60, a la, Isa Autissier knock down, , stabilty way up there but stall overturned, and sat there upside down , flat decks, , that was a shock, got a call 3am in the morning from race headquarters, "Isa has capsised" Hell Mike I,m not a navel architect, or designer, but I do know that 90% of sailing yachts out there would be dangerous, to say the least in full on , full fetch, full gale sailing condition, not to even consider storm condidtions

Mychael

08-22-2006, 04:21 AM

In general aviation aircraft (of all sizes) the flight manual must include weight and balance tables so that the pilot knows how much weight he can put fore and aft and still remain within the aircrafts safe operating Centre of gravity.
I wonder how long(if ever) it will be before boat designers become legally obliged to provide the same sort of thing for their boats.
I beleive manafacturers of all kinds of boats must be made to meet a certain design standard in the way car makers have for years.

Mychael

lazeyjack

08-22-2006, 04:28 AM

yes BUT
WE can be totally overegulated!! not good, one of the reasons I will leave this land for good

Crag Cay

08-22-2006, 05:20 AM

Light Yachts particularly need something akin to stability booklets.

To single out light displacement boats is a little misleading. There are lots of heavy diplacement yachts out there with unproven ultimate stabilty angles.

I often see home built steel boats in particular where the builder says something along the lines of:

'we're going blue water cruising so what with coral reefs /containers/ icebergs, etc, we've decided to go with steel, but I've gone up a size in plate guage to make it bullet proof as well as adding two full size steel waterproof bulkheads, but as we'll live aboard year round, we've gone with the pilot house version, which is steel as well as I cant weld aluminium outdoors, and do you like the combined boom gallows / dinghy davits /radar arch / outboard storage / wind generator and solar panel mount I built on the transom, which can also double as a lifting point if we have be craned out in Ushuaia........and we went with over sized glavanised rigging as I don't trust stainless ......yeah, I know we did want the 42 footer, but in the end I think a beamy 34 footer will be big enough, especially as I've welded up those deck boxes for the dive gear. .....and yes, these 36 inch high fully welded hand rails are far more secure than a couple of wire life lines......... you're right, it is shallow draft, because it's important to find somewhere shallow to 'over winter'..... after all we don't want big bergs moving in and crushing us. .... yes the twin keels will be handy so we can dry out and do the bottom paint..... I'm really proud of the bow roller / asymetric spinnaker fitting as we can store both 45 pound CQR's there even when under way...... we were going to use lead balast but this guy in the club said glavanised steel punchings in chicken **** was nearly as dense if you factored in his IQ......'

Many of these designs have quite tight 'radius chines' (18 inches) and shallowish deadrise so feel 'stiff' when sailling, especially as they are often under canvassed for their final displacement, but I suspect they have no better ultimate stability than many of the high volume (interior and production numbers!) cruisers that they so readily sneer at.

MikeJohns

08-22-2006, 06:19 AM

Great diatribe but you missed the point.

I was talking about designers published stability curves and the effect that typical additions that you'll find on any cruiser have on that stability .

A light boats COG very quickly moves up with typical additional gear and stores. In comparison heavy boats can take a much greater load before the COG shifts significantly. Just a simple relationship that anyone who has completed a few weights and moments calcs could tell you.

Your character with his additions might just survive in a heavy displacement craft , but in a light weight the same character is doomed.......

Crag Cay

08-22-2006, 07:06 AM

ust a simple relationship that anyone who has completed a few weights and moments calcs could tell you.

Thanks for the patronising comment. However, displacement and CoG are only part of the picture when it comes to Limits of Vanishing Stability. The breakthrough in lifeboat design in the 70's was when we fully grasped this.

My 'diatribe' was purely a warning to those who may draw comfort from having heavy displacement - it may mask problems in stability, but it wil never correct them.

Modelling only tells you so much. I think it would be very interesting to take some popular boats of typical design down to Poole, load them with weights to compensate for crew, half tanks, cabin sole awash and plenty of loose gear below, and use the crane to flip them upside down. If it's the only way to really prove designs for the RNLI, why not 'blue water' pleasure boats as well. It would make great publicity material. Ah..... if they came up.

MikeJohns

08-22-2006, 07:55 AM

Where do you draw the line between patronising and educational?
If you are not a novice then I guess it is patronising but by your post you did not appear that knowledgable sorry.

Weights and moments calcs are often a real eye-opener for the novice designer and you get a very good feel for just how the COG and COB relate. With light boats it's alarming to see just how much the COG shifts with a bit of weight on deck ....then back into the design spiral.

Be sure not to confuse dynamic stability with static vanishing stability. The mathematical models are very accurate at predicting statics.

Guillermo

08-22-2006, 04:59 PM

...I wonder how long(if ever) it will be before boat designers become legally obliged to provide the same sort of thing for their boats....
Well, they are. At least for professional ones.

Guillermo

08-22-2006, 05:07 PM

.... If it's the only way to really prove designs for the RNLI, why not 'blue water' pleasure boats as well....
Selfrighting test is mandatory for life saving boats and it tests not only the selfrighting ability, which can be quite accurately demonstrated by calculus only, but also, and no less important, watertightness, structural integrity and the ability of engines and equipment to survive the inverted position. Very specifically the ability of engines going iddle when inverted, without stopping or over-revving.

Mychael

08-22-2006, 05:27 PM

It would be good if every yacht designer gave a claer and open report on the AVS of a particular design. The more information available for a buyer the better he is able to make and informed and (correct for him) choice.

Mychael

Guillermo

08-22-2006, 05:46 PM

...The more information available for a buyer the better he is able to make and informed and (correct for him) choice...
I'm not so sure. I deal with professional boat owners and can tell you most (And I mean MOST) of them ever look at the stability booklets.
And from what I know about the recreational market, MOST owners don't even know what are you talking about.

hiracer

08-22-2006, 06:37 PM

And from what I know about the recreational market, MOST owners don't even know what are you talking about.

Agreed, but many of us are capable of some level of discernment, if only the information were available. It's not. And that's a problem in itself. Let's not confuse problems.

If various AVSs depending on load conditions were available for each boat model, eventually the recreational market, or some segment of it, would get the point about COG. Cocktail discussions would be different. It's the chicken-and-the-egg thing. In this case, I think the information is needed first, before buyers will slowly acclimate themselves to it.

Mychael

08-22-2006, 10:17 PM

I agree with Hiracer. I learnt to ask questions about sailing as I went along. I also knew to ask a lot of questions before I ever even got started, I'm not any sort of expert but I know enough to know I need to know certain things.
Some of the information I'd like to have is just simply not available and it should be.
I also agree with Guillermo in that a lot of owners don't know what you are talking about. That's just the new "breed" of person and is the same with boats as it has been with motor cars like some drivers cannot even operate a manual car. But the education and information needs to be there for those that seek it.
To highlight this "non-knowledge" boater type. I was in a discussion with a "smelly boat" (power boat ) owner at a course I was doing. He had just purchased a new power boat (probablly around 20odd foot) with all the latest gizmo's. He was not into fishing, just "getting out in the boat" . In any case I had to explain to him what I owned and his reply to me was "I'll probably get a sailboat when I'm older and want to take it easy". He had no concept of what is actually involved in sailing. He was much offended when I replied that "yachties" say the same thing about buying a power boat when they get a bit too old for hauling up sails and stuff.

Mychael

M&M Ovenden

08-22-2006, 10:45 PM

I'm not too sure what to think of that godliness of the AVS. It doesn't mean enough to me on it's own. A boat could have great capability to right itself up, if it's tender and keeps rolling, how good is it?
If a boat is sold as having an AVS of 145 degrees, it doesn't seem enough to decide it will be a good ocean going boat as it also has to maintain a comfortable motion. Why are conversations on a boat's stability always led buy how good it is at coming back up and never by how good it is at staying up?
I understand the importance to withstand a knockdown but I find good stability info should say more about a boat.

Murielle

lazeyjack

08-22-2006, 11:00 PM

fair enough, stability can mean many things, at anchor, downwind, directional,
After you,ve been involved for donkeys years you kinda get a feel for how"stable" a boat is
How the lines are, how they will effect the boat upwind, downwind, in a quartering sea
I think what people are just saying here is, that a stiff boat beats a tender one? Take a look at my gallery and guess the AVS ? LOOKS TENDER? LOOKS high wooded, well try get the rail down!!

Mychael

08-23-2006, 04:31 AM

And to go the full circle of this thread.Just saying that really a lot more information should be made available by the designers/builders of boats.
Then those that want to know can know and those that can make sense of the data are able to make a good informed decision, the rest can make their choice from the glossy brochures.

Mychael

hiracer

08-23-2006, 11:25 AM

He was much offended when I replied that "yachties" say the same thing about buying a power boat when they get a bit too old for hauling up sails and stuff.

Mychael

Too funny; too true.

Guillermo

08-23-2006, 04:01 PM

...If various AVSs depending on load conditions were available for each boat model, eventually the recreational market, or some segment of it, would get the point about COG. Cocktail discussions would be different....
Of course I absolutely agree. But maybe some wouldn't be able to sell their boats.....:rolleyes:

Mychael

08-23-2006, 04:30 PM

Then it would be like cars, market competition. We'd benifit in the end as all designers would be forced to do it smarter/better to stay in the race.

Mychael

gggGuest

08-23-2006, 04:38 PM

Then it would be like cars, market competition. We'd benifit in the end as all designers would be forced to do it smarter/better

Or maybe all designs would merge into a characterless mediocrity, with no spark or interest, just a desperate chasing of the latest new fashion...

Mychael

08-23-2006, 05:09 PM

I don't believe that improved safety necessarily must equate to being bland. (Or Beige) as Billy Connoly would say.

Mychael

Guillermo

08-23-2006, 07:06 PM

To get the CE mark, manufacturers have to get the STIX number in the process, but only a few of them publicize it for their models. So the question is why stability info is usually hidden or not properly explained, being it almost always thoroughly studied by designers....

As a collateral commentary: I still have my ration of doubts about the usefulness and meaning of this STIX number. I find it's a gathering of a lot of sub-stix parameters seeming to make quite a lot of sense by themselves, but the final number is not so clarifying to me...May the mixing could mean in fact nothing and, even worse, be dangerous, by hiding some undesirable characteristics? Should the manufacturers/designers provide the sub-stix parameters one by one, instead of mixing them in a final figure? Has this been discussed before in these forums or any others?

Here an spreadsheet for the STIX calculation, provided by SailDesign:
http://www.boatdesign.net/forums/showthread.php?t=7572&highlight=stix

Interesting page to get info on these matters:
http://cruisingresources.com/Hull_Shape_and_Performance

Most interesting, not only about stability:
http://www.oossanen.nl/pubs/VOA_MY_CEM.pdf

Mychael

08-24-2006, 06:45 AM

Crag Cay

08-24-2006, 09:21 AM

Personaly I think the time must come when anything designed/produced and commercially sold must meet certain clearly defined safety criteria and make all the information available to buyers. Also these specifications must not just be on a designers say so but must be indepently tested before being marketed. You can still have something with racy handling, requiring skill to sail well, like a high performance car but it must also meet a certain standard in safety.

This is exactly the situation that exists in Europe, with the exception of some very specific boats or those used in very sheltered waters. The problem with this boat wasn't a lack of regulation, but a failure of the human implimentation of those rules.

There was also a manifest failure by the operator to exercise either reasonable competence or even common sence. It's possible that the outcome of this accident would have been no less tragic even if the boat had been Cat C compliant. However, a change in some of the other contributing factors would have prevented it altogether. The son admits that if they had listened to the Inshore Waters Forecast, they would never had set out.

The interesting thing about the MIAB website is it gives an idea of all the incidents that occur in the commercial boating world, despite every aspect of their boat's design and operation being fully controlled by legislation. The RYA, I believe, is correct in maintaining that we have enough legislation and controls, and it's the effective implimentation of these together with adequate training, that will ensure acceptable safety standards.

Tim B

08-24-2006, 12:01 PM

It is still down to the designer to supply the appropriate stability information to the client. Most will be happy to supply it. To be honest, the AVS is not perhaps as important as the AGZmax (angle of maximum righting arm). Which is typically about 70 degrees. AVS is a good safety number, but AGZmax really specifies the maximum operational heel angle.

I'm not sure if a designer/broker has to supply stability information by law, but it is a good idea to supply the owner with two copies of the owner's manual and stability book. One for home, and one for the boat. These should (ideally) be hard-bound. The owner's manual and stability book may not contain all the information in the designer's report (not necessary by law, but useful) but it should detail everything that the owner is likely to need. The designer should also have a copy of the owner's manual and stability book as well as his own notes and report which are available to the owner (for viewing only) at the owner's request.

My thoughts on the Bez 2 incident are well publised on the "Dinghy Tradgedy" thread. I believe it was passed for Class C on the basis that it would be adjusted to incorporate the extra bouyancy. Evidentally this didn't happen. and therefore it is between the owners, importers, builders and designers to argue about who is liable.

Tim B.

Guillermo

08-24-2006, 06:53 PM

A brief news article in "Practical boat Owner" (english magazine).

"Death Trap dinghy Owner to Sue"
Two people died in a Bez 2 dinghy.Due to capsize. Investigation into the incident shows that the boat should never have been given the RCD Cat.C. It was misleading as to the vessels capability as tests showed it had a propensity to invert and lacked buoyancy.

Personaly I think the time must come when anything designed/produced and commercially sold must meet certain clearly defined safety criteria and make all the information available to buyers. Also these specifications must not just be on a designers say so but must be indepently tested before being marketed. You can still have something with racy handling, requiring skill to sail well, like a high performance car but it must also meet a certain standard in safety.

Mychael

To get a CE marking is mandatory the intervention of a Notified Body to perform stability tests, in almost all cases, and this NB checks all stability calculations after testifying tests, so it's not only a designers say so. But, as happened with the Bez 2 (case already discussed in these forums) even NB's may make gross mistakes, to say the least, as well as owners.