Multihull Collision Survivability

Discussion in 'Multihulls' started by Skint For Life, May 12, 2011.

  1. MikeJohns
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    MikeJohns Senior Member


    Hard to predict not impossible at all, but these days also easy to measure. Strain gauges and accelerometers have given a lot of insight into the design of offshore structures and vessels. Modern class rules that cover cats take these into account so you'd be well advised to follow their lead.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    That used to be case may years ago, not now though. Advances in mathematics assisted greatly.

    Excellent ground breaking work was undertaken by the likes of Neumann, Bretschnieder and of course Pierson-Moskowitz that lead to spectrum presentation of the sea decades ago. Thus for given wind and fetch, waves and their energy and amplitudes can be calculated with a high degree of accuracy.

    So, the sea, from an engineering perspective is no longer considered, impossible. (We call them RAOs now).

    From the above leads us in to the field of seakeeping. That is to say, the dynamic response of vessels in a seaway, or, the ability to go to sea and successfully and importantly safely execute its SOR/ mission, in a given sea state.

    Seakeeping directly relates to the sea spectrum, noted above, since what size wave? And analysis of ship motions via Green’s Theorem or more complex hydro-elasticity/strip theory etc etc all play a major role in providing data that can be used to “design” and reduce the “impossible” to “known’s”.

    It of course can be argued that these “known’s” are not absolutes nor 100% correct, which is true, of any statistical representation. But, no more or less than that used by the aircraft industry. Since many aspects of loading for an aircraft, such as turbulence is identical in analysis to a ship in waves. It is all about mitigating the risk.

    So, we can design with a high degree of certainty of what loads a vessel is likely to experience and hence how to we mitigate the loads to a “safe” design that is “fit for purpose”.

    I beg to differ. The reason why is that most of these smaller boats are ostensibly built or have been built by amateurs. In the sense that the boats have not been analysed nor designed with “fit for purpose” in mind. Simply because they don’t know how, or too proud to ask for professional help. It is a case of “she’ll be alright”..and “looks good enough to me” and the classic “I’ve been building boats for years, you can’t tell me”. All resulting in the same...

    It is this nonchalant attitude that breeds others to think..hey I can do that too..and the end result is a failure. Simply because the boat has not been designed and even perhaps built properly. Due process to proper analysis and understanding of what the boat is designed for where and how..and then the correct building methods and welders/platers to fabricate is lacking. Without which then equates to that impression as you have supported….small cats built in ally are no good. This is an incorrect in my opinion. There are many excellent small cats about. Mostly in the commerical market..ie Class rules and legisaltion, not "i know what I am doing..don't question me" attitude.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    The loads on a boat are just as “predictable”.

    As for fatigue, you may not be aware of BS8118. This is the bible for fatigue deisgn, in any engineering discipline. Here, an extract, is showing the different Class of joint.

    BS8118 Class.jpg

    This then relates to what is called a FAT Class and here are endless curves of FAT class joints with their respective fatigue limits, all of which are very well documented. It is not just the aircraft industry that has good fatigue curves! I could go on..but it’ll take over the thread.

    FAT Curves.jpg

    Commercial boats are inspected regularly. And most have proper inspection and maintenance staff for such. Which goes back to my point…. all of which occurs daily in the commercial/professional filed, does not occur in the leisure/amateur field. Hence the misconception that small ally cats are bad..
     
  4. oldsailor7
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    oldsailor7 Senior Member

    We seem to have drifted away from the title of this thread.
    It is Not-- "How do we design for prevention of structural failure".
    It is-- "How do we design for Multihull Collision Survivability.

    They are not the same thing. :D
     
  5. peterchech
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    peterchech Senior Member

    Yeah it seems a boat that won't sink isn't so great if it just barely floats above the surface capsized. Trying to hang on to it in rough weather might just mean getting bashed against it...

    no one has mentioned "escape hatches" yet- or at least a saw that is accessible in the event...
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Well, im affraid that's where you're wrong.

    The orginal posts asks:

    To understand about "hull puncture" is a debate/discussion about structure. The shape, or hull form plays a minor role. The buoyancy aspect comes into play after hull breaching. If the hull is not punctured, it is a simple intact stabilty issue.

    If you drive a nail into wood, this is a much easier task than trying to drive the same nail into say steel. One will enter or "puncture" the material the other won't. Ergo, structure and the material of choice is directly relevant to the posters question.
     
  7. oldsailor7
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    oldsailor7 Senior Member

    Sorry Ad Hok.
    You are clutching at straws. :rolleyes:
     
  8. sabahcat
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    sabahcat Senior Member

    Havent thought this out have you :rolleyes:
     
  9. peterchech
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    peterchech Senior Member

    Enlighten me
     
  10. MikeJohns
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    MikeJohns Senior Member

    Thats' the crux isn't it?

    Build it out of Alloy, use crash bulkheads rake the stem. It would be hard to get any better for this paradigm.
    Otherwise wood is quite good in collision providing it's thick enough. Relatively thin composite shells are never a good option for collision. It's all about toughness of the material (toughness as an engineering term) and energy absorption and failure modes of the material.
     
  11. sabahcat
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    sabahcat Senior Member

    Obviously you consider it better for the vessel to sink like a stone underneath you and for you to be bobbing around by yourself in a vast ocean, after all, if that happens at least you eliminate all possibility of getting banged around
    or being easily spotted
    or staying out of the water reducing munchies attack and hypothermia
    and you have also lost possible food and water supply

    I'm sure there are many other advantages to having a floating vessel remaining and staying with it
     
  12. sabahcat
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    sabahcat Senior Member

    Really needs to be 50ft plus for this to be viable due to weight/performance issues
     
  13. peterchech
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    peterchech Senior Member

    "obviously"... You assume too much. My point was that if possible the hull should float high. You're very defensive man...:rolleyes:
     
  14. mikereed100
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    mikereed100 Junior Member


    I recall seeing photos of a catamaran build, French maybe, in which the hulls were done in plywood single chine with foam blocks laminated to the bottom, about 30-40cm deep then shaped. The result was a round bottom hull with flat foors and solid foam below the waterline. It seemed a brilliant way to build a boat to me. The build would be arguably simpler and faster and although you would gain some weight and lose some usable volume in the bilges you get an unsinkable boat that can weather an impact with a container, reef, what have you, without catastrophic damage. Wish I could remember where I saw it. Now where did I put my glasses....

    Mike
     

  15. cavalier mk2
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    cavalier mk2 Senior Member

    It is also important to have buoyancy high up in case of capsize as the low stuff won't be helping then. In fact if all the bilge is used up you have less inverted headroom.
     
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