Multihull Collision Survivability

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

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Hmmm..your two statements are incompatible.

    Firstly, I assume you travel, and by aeroplane?....built from aluminium and oh yes, has fatigue issues too. Or do you elect not to fly owing to the aforementioned?

    Incorrect, and you’re thinking of composites, they require expensive instruments to verify the structural integrity of the matrix/reinforcement etc. NDT of composites is more time consuming and also not 100% accurate too. But that is for a different thread.

    All it requires is just careful design. Knowing where the load paths are and the magnitude of the loads. Then as with any material, one designs accordingly.

    If you care to state your reasons why you hold these “beliefs” as such, I can explain further. But with such a gross carte blanche statement that is more prejudice than fact, it is hard to go into detail without knowing what you have been told (as fact when is in fact fiction) and what you don’t know.

    Care to elaborate?
     
  2. sabahcat
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    sabahcat Senior Member

    I think you may find they are 200gram kevlar with 400g woven for abrasion
    At least windswept and I believe the super shockwaves were.
    Pumpkin eater I believe was 400gsm on a 12mm divynicell core

    With DFlawless, Gavin said he believed he sustained damage during the earlier sail through Cyclone Bola.

    Below is my build showing watertight areas and collision B/h's

    [​IMG]

    16mm Kiri core
    600gsm d/b in and out
    3 layers of 600gram d/b up to 200mm above waterline
    3 layers around nosecone staggered back to first yellow b/h
     
  3. Bruce Woods
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    Bruce Woods Senior Member

  4. sabahcat
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    sabahcat Senior Member

    Have you got any other examples from this big sailing world of multi's tripping from collision?

    Still floating at least.

    No water tight bulkheads in a mono that's holed and what happens?..............................glub....................glub glub..............glub glub glub
     
  5. catsketcher
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    catsketcher Senior Member

    Follow up

    Sahbah - thanks for the info. I worked with Pete Kennedy who built Pumpkin eater. IIRC it was 10 ounce woven which is like 350gm not 200gm(I just found a converter). Nowadays we would use stitched which would be stronger.

    Bruce I am aware of the probs with watertight bulkheads. The Titanic is one example. I am surprised at the Spirited 380 doing this. She does have multiple compartments.

    http://au.news.yahoo.com/video/queensland/watch/23530682/248154/269/

    In this story Rod says that the container hit them on the hull side. This tends to throw water on much of my contention that you can make the bows stronger to assist. What I don't get is why the thing filled so quickly with a stern and bow compartment. Maybe the stern one was taken out along with the middle of the boat.

    cheers

    Phil
     
  6. MikeJohns
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    MikeJohns Senior Member

    I don't agree either.

    Boat building aluminium alloy can be visually inspected. Composites cannot be inspected very well at all.

    Ironically FRP is usually more fatigue prone than alloy even with new ideal, perfect cure controlled environment coupons.
    As Ad Hoc says your post is a bit of a contradiction. If it's well engineered it doesn't fatigue that's implicit in engineering design.
     
  7. MikeJohns
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    MikeJohns Senior Member

    The Spirited 380 flipping over from a collision with an unseen object is interesting. Is it a freak accident or the downside of overly light Cats with too high a CG maybe? Recently another cat much larger was simply blown over by an estimated 60 knot gust of wind and lost off Fiji.

    It's also significant that it apparently took 6 hours of repeated attempts to get back into the sunken inverted boat to retrieve the Epirb. I hope the boat is recovered I'd be very interested to see the impact. The whole bride deck was completely immersed too.

    I think if the weather was rougher you'd have no chance of staying with the upturned hull for long.
     

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    Last edited: May 21, 2011
  8. CatBuilder

    CatBuilder Previous Member

    I will elaborate on the issue of metal fatigue after the work day. I have examples, plus, as you all know, aircraft ate routinely imaged for metal fatigue and their parts replaced as the cracks show up on the scan.
     
  9. cavalier mk2
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    cavalier mk2 Senior Member

    Seems like a good idea to have positive buoyancy built into those tall cabins and flying bridges. Something more than a air chamber that can be breached like foam would have kept them upright. I agree about keeping the CG low as well as the cabin CE.
     
  10. marshmat
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    marshmat Senior Member

    I don't think comparing catamarans to jetliners is a fair argument.

    Weight is so critical to fuel consumption in the aircraft that it is cheaper to design the aircraft with low safety factors, and regularly test and replace certain parts, than it would be to design those parts for infinite life. (They'd be heavier, increasing the fuel burn, and a complete commercial aircraft has a finite lifespan that is fairly well known in advance.)

    Weight is important, but not so critical, in the catamaran. The cost of periodic NDT and repair, and the additional cost of engineering to low safety factors, can't be justified here.

    And the boat is subject to many hard-to-analyze load cases, such as collisions with fixed objects, which it has to withstand on its own. The collision scenario in the aircraft is dealt with by traffic control, computer intervention and pilot training; if a jetliner hits anything its own size- even at just five or ten knots- it will sustain damage and will be unusable until repaired.

    I would argue that the "proper" approach for the boat is to do a sufficiently thorough engineering analysis and structural design, with appropriate safety factors, that the structure will have effectively infinite life even accounting for fatigue.

    As for the collision survivability aspect in a cat or tri... Watertight bulkheads, of course. I like to see them arranged so that a breach of both the forefoot compartment and the one behind it will leave the boat floating, dry inside, and able to get to shore on its own. (This means a tri's outriggers should be compartmentalized too.) Likewise for a breach in the engine compartment (a bad through-hull, or a shaft failure, or a rudder failure...). And likewise for a full-speed strike to the daggerboard or rudders; the foils should break or pop up before the stress on the trunk or shaft is enough to cause a hull breach.

    Cored composite construction can be done so that, even with all compartments flooded, the boat doesn't sink. It may be a write-off, but at least it's a large, floating, highly visible SAR target. An aluminum multihull can easily be given enough small, sealed air-filled compartments that it'll remain floating even after capsize and severe damage to all hulls. I don't think there's any excuse for building a cruising multihull that is capable of sinking.
     
  11. sabahcat
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    sabahcat Senior Member

    The majority of sailing cats I have sailed on had ropes with knotted loops stretched under the bridgedeck from back beam to and around the front beam .
    The idea being that you will have a harness on if offshore and in the eventuality of capsize, you can then clip on to the loops on the underside.
     
  12. CatBuilder

    CatBuilder Previous Member

    Sorry. I can't find the information I had. I am clearly seeing an aluminum catamaran beam in my memory with stress cracks from years of sailing. The boat was seen maybe 7 years ago online in many places.

    Unfortunately, I can't find the information anymore, so I have nothing to back it up.

    I can't produce any facts, so I have to bow out. Too tired from a day of abrading the interior hull parts for extra reinforcements.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    With respect, you have missed the point. Who is comparing catamarans with jetliners?

    The comment was related to fatigue and that fatigue is so bad it renders the use of aluminium as a material of choice as never.

    Thus, just highlighting that other forms of transport also use aluminium which also have fatigue issues. Beyond that requires further elaboration to ones understanding behind such sentiments which I asked for.

    For professional engineers/designers, this is not an argument, it is fact. This is the methodology that is required for any design, beit, boat, car, plane….

    You can have the best design in the world, ever, but if the manufacturing is crap, then the boat shall fail. You can have the best fabricators/laminators in the world with the most stringent QA, but if the design is crap, it too will fail. The success of any boat is a blend of the two.

    Failure requires context, was it manufacturing or design? Then to address the why’s….so often failures are owing to one small event, such as designer not using a stress reliving hole, or the welder too lazy to do a weld return or the plater to bend a radius but pressed a radius and so on.

    Taking one isolated incident out of context and with no facts does no one any good. It creates false urban myths and does not advance peoples knowledge and understanding of the subjects that ultimately they wish to persue. If you have questions, just ask away...many here are happy to answer.

    Engineering is about facts, not prejudice nor anecdotes.
     
  14. oldsailor7
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    oldsailor7 Senior Member

    Aircraft structures are designed to flex within their elastic limits.
    Fatigue limits are a known factor and are used to determine the service life of the individual parts. Service schedules are thus determined.

    The random stresses applied by sea and wind are impossible to predict. Thus calculated static loadings are used multiplied by a presumed maximum.
    That is fine providing it does not result in an unnecessarily heavy boat.
    Lock Crowther used a factor of 6X. :cool:
    Even so continual inspection of stressed areas is essential.
     

  15. marshmat
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    marshmat Senior Member

    Y'know, I can't even remember how it came up now.
    My point was, essentially, about fatigue in aluminum- if, as is the case in an aircraft, you have very predictable load cases and a strict testing and inspection regime, it may be OK to say "this part will likely become fatigued if you use it for more than X flight hours, so replace it at 0.7*X hours". The assumptions underlying that idea- that you're inspecting regularly, and that you know all the load cases- are not valid on a boat, and the boat's structure should be designed so that fatigue never becomes an issue. If it's aluminum (as AdHoc knows better than any of us), that means controlling the load paths, eliminating stress concentrations, and taking proper care in manufacturing to ensure that there are no regions, even small ones, that have less than infinite life under all design loads.
    I think the reason we see so few successful alloy cats in the smaller sizes is that the engineering effort to do it properly is just too expensive for many yacht-scale applications. In larger sizes, it pays off.
     
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