Angle of Vanishing stability.

Discussion in 'Stability' started by Mychael, Aug 19, 2006.

  1. Mychael
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    Mychael Mychael

    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
     
  2. Hunter25
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    Hunter25 Senior Member

    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.
     
  3. Wynand N
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    Wynand N Retired Steelboatbuilder

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

    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.
     
  5. Mychael
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    Mychael Mychael

    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
     
  6. Wynand N
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    Wynand N Retired Steelboatbuilder

    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....;)
     
  7. Guillermo
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    Guillermo Ingeniero Naval

    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...! :(
     
  8. Mychael
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    Mychael Mychael

    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
     
  9. Guillermo
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    Guillermo Ingeniero Naval

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

    lazeyjack Guest

    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
     
  11. MikeJohns
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    MikeJohns Senior Member

    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.
     
  12. lazeyjack

    lazeyjack Guest

    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
     
  13. Mychael
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    Mychael Mychael

    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
     
  14. lazeyjack

    lazeyjack Guest

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

  15. Crag Cay
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    Crag Cay Senior Member

    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.
     
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