Multihull Capsize Prevention <split>

Discussion in 'Multihulls' started by MikeJohns, Jun 23, 2011.

  1. Corley
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    Corley epoxy coated

    A lot of those trimarans that broached were early designs. As Gary has said there were problems with the rigs that the boats carried and also mast positioning (which tended to be too far forward). Low buoyancy floats and hullshapes which were not optimised exacerbated the problems and increased the tendency to broach. It was all part of the learning process that led to modern multihulls.
     
  2. MikeJohns
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    MikeJohns Senior Member

    We'd have to consider displacment here and separate out ultralights from sensible cruising designs.

    The broach induced capsize is a danger for both types but more so for a Trimaran. Like a monohull In a beam on breaking sea the breaking crest hits the main hull and pushes the boat above it's centre of mass . There is a dynamic overturning moment ( called an overturning couple). While a cat can actually have a dynamic restoring moment from the same wave impact.
    Also as the the lee float doesn't have the volume that the lee hull of a cat has and the Tri assumes a more severe angle from the same wave.

    It's interesting that an intact but inverted Tri makes a better survival platform than an inverted cat and importantly for the future it lends itself to possibilities of self recovery options so even a larger boat could be self righted after the storm.
     
  3. MikeJohns
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    MikeJohns Senior Member

    Again this is diatribe, please look it up and avoid it in future discussion. You resort to insult in lieu of discussion.
    Why won't you engage me in the points I raise ?

    Keh ?
    Lets not forget what we are actually discussing here, it's your premise that "No Catamaran has ever been inverted by waves" it's a very misleading statement.

    It's as silly as imagining a majority agreement with your premise. Where in this thread has any one of these people agreed with you on this? Quote them .

    I notice you avoid every sensible line of discussion. I was still waiting to discuss with you as to whether we could include a storm jib in recent wave induced inversions. I wanted to debate why a storm jib would not only be insignificant but would even reduce the inversion risk.

    Without qualifying statements generalisations only lead to misunderstanding and even worse a reinforcement of prejudice. Neither have any place in developing safer boats or understanding their pitfalls.
     
  4. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    I'm having trouble intuitively understanding this. What do you mean by "pushes the boat above it's centre of mass"? It seems to me that trimarans have a lower center of gravity than a similar displacement catamaran. Wouldn't that tend to result in increased stability? (I understand the disadvantage to a trimaran of lower-buoyancy floats)

    Are you saying that trimarans are easier to self-recover? Or that because you can survive more easily in an inverted tri than in an inverted cat, you may be given an opportunity to re-right the tri, whereas you might not have that opportunity from an inverted cat because you might not survive?
     
  5. Gary Baigent
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    Gary Baigent Senior Member

    Presumably, MJ, (and please stop your pompous lecturing) presumably your storm jib is carried without a reefed main? Is that correct? To weather or running off? If the latter, (with no main) bad lee helm guarantees a potential disaster. And if beating lee helm will also be another disaster waiting to happen.
    Sailing back from winning an Auckland/Tauranga in a 17.5m racing catamaran, we came broad reaching under reefed main plus jib, through the infamous overfalls off Cape Colville, near Channel Island. A hard Northeasterly blowing with a large swell running, funneling between Cape Barrier and Colville - and Chris Barker, recently back from a Whitbread and eying (as we all were) these monsters rearing up astern, noted a particularly large, potential broaching version coming and quickly winched the headsail completely flat - just before we were picked up in white water and surfed at very high speed. He didn't want that headsail to do anything except blow the bows off, wanted it flat and blanketed behind the main. There was no hint of broaching but if the headsail had been trimmed free for offwind sailing, chances were very much that we would have rounded up on the big breaker. Headsails on their own are a crapology solution for reducing sail area in high wind and sea conditions.
     
  6. MikeJohns
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    MikeJohns Senior Member

    You mean could I please stop responding politely and intelligently to insult inuendo and diatribe ! How would you propose the debate should be conducted then?

    If someone says you are wrong because you disagree with their prejudiced view it doesn't cut it as a valid argument. We should be able to have a civil discussion without receiving rudeness for no other reason than disagreeing. Either there is another valid defendable viewpoint or there isn't.

    Naval architecture isn't a belief system, it's a science. That fact seems to escape some posters here. Also this is an open forum, for it to work well we should debate sensibly and clearly with logic rather than feelings and insults.

    So if you say something misleading like your post about mono-hulls don't expect that it will be unchallenged.
     
  7. MikeJohns
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    MikeJohns Senior Member

    There is a dynamic effect with anything you push off the centre of mass, jab anything on your desk off the centre line and watch what happens. It rotates.

    With the boat heeled already on the wave front the plunging breaker is pushing the centre hull and you could draw the inversion process superimposing the craft on the wave front. Effectively it’s like a cat with much less beam and with a lower volume in the leeward hull. Where the breaker pushes relative to the centre of mass is a dynamic couple, add to this the moment from resistance from a the more deeply immersed lee hull. There is also a lower roll inertia than there is on a Catamaran, this relates to impact energy and angle of heel.

    I’d need to sketch some diagrams to make this clearer.

    Some earlier Trimarans were very vulnerable from beam on breaking waves because they had insufficient volumes in the floats they just acted as a very effective tripping mechanism (similar to the detrimental leaving of keels down on a Cat ). It would help to a greater volume in m^3 than the vessels displacement in tonnes. Even then the mechanisms are quite different to a Cat.

    Note it doesn’t make one vessel safer than the other, it’s just understanding the vulnerabilities and the inevitable trade-offs.

    A trimaran offers a number of ways to re-right and effect self rescue if so designed. There are some ideas that look quite promising. Also A tri floats higher inverted and stays dryer inside making it easier to design for inversion survivability.
     
  8. MikeJohns
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    MikeJohns Senior Member

    I don’t think a blue water severe gale or storm is anything like the same paradigm as your 57 foot racing cat anecdote. Also what was the wind strength?



    In that tale sheeting in the headsail hard in was an option that worked since dropping the main wouldn’t have been an option.


    The flat headsail would blow the bow downwind if the boat rounded up. So you have the wrong scenario.

    I don’t agree that a jib alone will give any problems from lee helm when running it will help keep the bow pointed downwind and is a good option and later lends itself to towing a drogue, in fact I’d argue that having the main up would be foolhardy in storm force winds unless you were desperately trying to get to windward.

    What of the sensible advice that if unavoidable bad weather is forecast and you have the manpower then you should get the main and the roller furling sails off and stowed below as low as possible (along with all heavy items) to reduce weight aloft and windage. Then free flying and easily dropped storm sails are carried with finally the jib as the last resort sail until it is either dropped to lie to a drogue astern or parachute fwd if there is sea room.

    Just lying ahull as Catbuilder proposed as an invulnerable tactic is what we were actually discussing.

    I'd suggest flying a storm jib keeps you in control and moves the centre of effort forward from the drive from the rig which can be substantial in high wind with light boats. That would keep the boat from broaching not encourage it and let the autopilot keep some dignity to give the crew some respite.
     
  9. Silver Raven
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    Silver Raven Senior Member

    To Mike Johns & Ad Hok. You 2 really should -
    - - - print out all you've posted in this place & read it all - at one time
    - - - have a dispassionate look from a good distance - at what & how
    you've said your piece. Could do better ??? I think so !!!

    Then have a total rethink about - just how you are wording your comments - in the hard cold real light of day - in the big - world-wide picture & then probably try to be less - cinfrontational & much more communicative ?? Hey U 2 guys - What U think ?? Confrontation is distroying !!!

    The first words in your first sentence - - "I don't think a blue water" Yea - that's the problem we're having with U 2 - it's "I don't think" - you both are much to fast to 'draw & shoot' to be bothered to; read - stop - listen - think - then make a polite answer.

    Did you even take the time to read what what was written & what the conditions were & the qualified experience of those on board - I sincerely doubt it. Have you ever been in conditions like those described ?? I also sincerely doubt that - as well !!!

    The gross stupidity of quoting 1973 happenings as an important part of a rational discussion for todays multihulls is - quite frankly - beyond reason of the sailors that are out on the oceans doing the real sailing. Don't U 2 think?

    In another 'forum' in 'Boatdesign' someone said - Naval Architecture is a 'Mature Science' & my answer to you 2 as well as him was & is - - Gawd help us all. I'd wager that the last 1000 inovations/developments have been designed & carried out by - non Naval Architects. Yes/no ???

    Gary - mentions - as a way of informing all of us - of other possibilities - that - by the way - - HAVE happened - - Not you think have happened, or might - have happened - - BUT - did happen - & what your answer is - is that the action that was taken - by a person more experienced than you will ever be in all of your life & oh by the way 1 of the top 10 in the world - that the said action - was wrong . Gawd - what the heck would he know about how to sail a boat . YEA RIGHT - Therein lies your problem -

    I have a suggestion - since the 'Senior Moderator' requested us all to be polite - & I will obey his request - so since you two have a personal agenda to dissagree with most everybody in this 'forum' - why don't you go and start your own 'forum' heading ?? PLEASE. Most of us will not follow U 2 & you'll be happy there, I'm sure.

    Whether what you say is right or not - is not the point - it's your attitude that some - including me - find MOST offensive. Which I do.

    U 2 put across the attitude - that your education - is something special & that makes you - something special & better than the rest of us.
    My personal answer to that is - and you may quote me - IN YOUR BLINKEN LUNCH BOX - Not in my life-time - not ever going to happen.
    U 2 are just 2 other people in this world & should have learned by now to express your opinions without shoving your attitude down my throat.

    PLEASE feel free - to start another subject heading - will be pleased to assist in any way we can, but for goodness sake - stop denying - the facts we put forward - as not true - especially - when thay have already happened - that really sucks & we don't want to listen to your continued diatribe. Many thanks, Ciao & enjoy your many sailing miles, jj
     
  10. HASYB
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    HASYB Senior Member

    Apart from telling each other how to behave and divide, I still think the tread progresses in understanding how to sail and act better on multihulls in big winds and/or high seas.
    With valuable contributions of experience and points of view.

    Cheers
     
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  11. Ad Hoc
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    Ad Hoc Naval Architect

    You’ll have to ask the original poster that, whom claims no multihulls have capsized from breaking waves. It is his assertion, no one else’s. Yet despite evidence to the contrary seems to have changed tack. I personally don’t give two hoots. I just found a reference that countered the claim. Either one accepts it or one does not. Whinging and moaning about it because it refuted the claim…why is that my fault?

    Your statement also smacks of blind faith. Since it is rather like saying the dinosaurs that were wiped out by a large asteroid that hit the earth 65m years ago, is not worthy of discussion of how and why simply because it was 65million years ago; thus ancient events why should we debate it. Utter nonsense!

    That is classic blinkered myopic view point. Thus you just wish to find an answer that suits your perception of reality. Not sure why really?

    Well I’m very sorry if you do indeed feel that way, none is intended. Since science has no ego nor emotion, nor “takes sides” per se, it is just a fact. How it is interrupted is out of the control of the messenger.

    Thus one can only conclude you feel intimidated by such responses?? I’m sorry you feel that way, which is why you have written such a reply as you have. I cannot be held responsible for how you misinterpret a posting that is down to you alone. You appear to have read something into a reply, where none exists. It was clear to me that MJ wanted to set limits for the debate, without such it becomes a pointless pissing contest of personal subjective opinions. (Which is has).

    Sadly this is an example.

    No one is shoving anything down your throat. Other than you’re own interpretation of how you have read a reply. I cannot control anyone’s paranoia any more than I can control the sea! Clearly you feel vexed that someone does not have the same opinion or point of view as you, and thus resort to emotive irrational debating rather than sticking to objective facts.
     
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  12. MikeJohns
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    MikeJohns Senior Member

    You are being very inventive. I didn't discount that persons ability, I disagreed with Gary's interpretation of that persons actions. And can you tell me what the windspeed was ? That's also a critical fact. Had they been running in a severe gale for 8 hours already it might be more meaningful.

    I wanted to present Catbuilder with modern cats flipped by waves that were not under sea anchor but under way. When under small amounts head-sail only. My argument being that they would naturally be so rigged.

    What you are really saying is that you don't like any criticism of a subject dear to your heart. Frankly I think the illogical and vitriolic responses are indicative of a deeper insecurity. there seems to be a fear that admitting a flaw such as " Cats can and have been flipped by waves alone might lead to a chink in hard and slowly won increase in public confidence.

    But this is a boat design forum for discussing boat design aka Naval Architecture. If a free discussion offends you the problem really is yours.


    Why? You have never actually asked me, which would have made it a factual. Instead you make something up, and use it as a criticism. If you can't engage in a decent technical discussion don't resort to personal attacks.
     
  13. HASYB
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    HASYB Senior Member

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  14. CatBuilder

    CatBuilder Previous Member

    Don't worry silver Raven. Not all of us are impressed by their "education." I have a Masters in Physics with a minor in Computer Science. My first job out of school was at NASA. Things I have worked on are in space right now as well as in the news regularly. Yes, I started sailing as a teenager. These guys with their certificate in naval architecture don't really threaten me when they cop "I'm smarter than people who actually have been aboard catamarans" attitudes.

    Like you said, all you have to do is read their posts. They are obviously inexperienced.

    In another thread, one of these "designers" didn't even know the forward beam at the bow on catamarans takes the forestay load. He thought the primary force on it was to hold the bows in place! Yikes! Never stopped to understand the geometry of the seagull strikers. Educated? Not in structures.

    So no worries. Not knowing something that basic about cats means you really don't need to listen to them.

    They probably aren't as educated as they claim to be.
     

  15. Gary Baigent
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    Gary Baigent Senior Member

    Definitely, the Goon Show.
    The wind had eased to 20 - from over a sustained 45-50 knots for the previous 24 hours - which built large swells in the passage. Anyone who has spent time around Cape Colville knows what the place can be like with tide running against the seas.
     
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