Multihull Collision Survivability

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

  1. Dean Smith

    Dean Smith Previous Member

    some nice posts, all very sensible
    I have owned a cat, I like em, I have chartered a larger cat, we had fun
    But I would never ever sail a light cat across oceans, surfing down waves in the middle of the night, being belted my force nine for days, give me a metal hull, so I can bounce off logs, whales, containers and --- cats
    it is not just this, it is the fact that you can never ever leave a cat to look after herself with full sail on, if the boat becomes overpowered when one is not on deck and there in nobody up there to bear off or dump the main, you can get into serious trouble, capsize and then it is no good calling home to mum
     
  2. oldsailor7
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    oldsailor7 Senior Member

    A metal hull is not going to save you in those circumstances. A multihull is not a Tank.
    Anything which could survive a collision with a floating container or similar, without damage, would have to be so heavy that it would not be a very viable Cat or Tri. :eek:
     
  3. Skint For Life
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    Skint For Life Junior Member

    CATBUILDER. Thanks for that, you are obviously correct. My mistake, it is just a scenario, but yes someone should be on watch. Does the person on watch always see the object before they hit it though? On the T.V program I saw the boy on watch only yelled out "REEF!" after they had hit it and the occupants had collided with the internal structures of the boat.

    WHITEPOINTER
    I have a friend who has a macboat (rotomoulded plastic?) that we go out on a fair bit. It seems to be a pretty robust boat. Although I think it's alot different to an inflatable, it does not deform and bounce back anything like an inflatable, It's a solid object so cannot be deflated and stored in a far smaller size. It's heavy compared to an inflatable. Possibly too heavy to be used to make a multi out of? Regarding a proper hull shape, I have considered this issue. I'll share more at the end of this post for all.

    MASALAI Thanks for that I'll look into it I've got 106 and 107 but not 108 yet.

    MAST MONKEY
    Thanks for your thoughtful input. Regarding foam: really? all of the same things? I would have thought that with the foam hull punctured water would still get in and destroy belongings? I understand you can sometimes sail them on, limp to the nearest port, then major repair work. It seems to me to be alot more of an ordeal than replacing a punctured inflatable hull with a new one and sailing on at full speed.

    Ok if repairs are not as easy as I'm guessing, is complete hull replacement a viable option? Regarding leakage, I would think that if such a crazy boat was to be built that an onboard compressor would be a must. So topping up the hull till making the next port would not be a huge problem. Regarding U.V. degradation, I'm thinking of the hulls as a replaceable part anyway, just a means to an end.

    Regarding momentum, does the multi rely on momentum? I was under the impression that momentum was only really a storage of energy. I've found on my previous beach cat that the only time I wished I had some momentum was when I was tacking. If momentum was required why does it have to be an issue? Couldn't this boat be made to be any weight? Sure the outer hulls are light, but the main hull could be made to be very heavy, because all the weight is in the main hull and all the really heavy stuff: engine, batteries etc. Will be down low I would have thought this would be ideal, don't people try and get the weight of their boats down low and in the center as much as possible?

    ALEX A
    Can you please give links for the proas? Also the TV program, I'd really like to see that, was it a multi? I've seen footage of a mono being rammed at full speed (mono full speed) onto rocks, it was interesting but I'd sure like to see the same with a multi at full speed. Regarding bow shape, I wonder the same although no-one has posted up saying they have the perfect bow design to ride up objects at pace.

    CAVALIER
    Are you suggesting we should be looking at designs that ride over solid objects in the ocean rather than ones that take the impact well? Can you please suggest some designs like this? One thought is if a multi rides up onto a reef, then it is stranded and will be destroyed, as happened in the TV program written about in my first post. I'm not saying I'm against the idea, just stating a possibility.

    OLDSAILOR Thanks for the tip, that sounds like a great idea. I guess what I'm proposing is similar except you don't have to build a grp ama in the first place and you can keep a spare ama rolled up on board ready to go.

    DEAN SMITH
    Thanks for that, I'm not proposing a light or a heavy cat, just a basic design idea. If the design was heavier would it make it safer? As you are aware metal hulls (thick enough to survive severe impact) for a multi are not really possible if sailing ability is to be retained. Hence the discussion and my subsequent concept. Many people like multis over monos, I don't want this to turn into a which is better discussion, I'd like to hear some good ideas on how to make multis even safer, better able to withstand collisions, maybe it's not possible to make a light multi robust or able to withstand a major impact without expensive hull repairs I just thought maybe the prospect of such a multi made it worthy of discussion.

    OLDSAILOR
    Thanks for the input, I'm sure we can all agree that a metal hull thick enough to withstand a major impact on a sailing multi is not viable. Like you said a multi is not a tank and there is no real point in making one a tank if you want to sail it. I will say however that I have to disagree in theory with your comment that anything that could survive a collision with a container or similar without damage would have to be heavy. In my proposed concept the theory is not to fight fire with fire (weight and steel vs weight and steel) It is to disperse energy, absorb the impact in a non-harmful way, I think the basic theory is sound. When a head hits an airbag the airbag takes the full impact of the hard head just as the hard steering wheel would, the difference being instead of a hard head and a hard steering wheel being damaged as an end result you have a hard head hitting an impact absorbing and dispersing cushion and at the end of it all you should hopefully have a head still in good shape and an airbag still in good shape.

    TO ALL Regarding making an inflatable hull with a shape that will still sail ok, I had a thought of a hull that has an internal inflatable skeleton, which is skinned and both the overall hull form is compartmentalised and so is the skeleton. Each section of the internal skeleton would be pumped up to a high pressure, giving good structure, as they pump up hard they would pull the outer skin taut. The big compartments of the hull would be inflated with a low pressure to add to form and give some bouyancy. If anyone has trouble visualising what I am saying, think of skin of frame building where the frame is an inflatable skeleton, the bulkheads are skinned to create airtight compartments and once the outer skin is on the hull is complete, to be inflated in sections. Also kite surfing kites have an inflatable skeleton that gives them structure. :D

    Airbag idea: Airbags are designed to inflate rapidly but also to let the air out rapidly if they didn't the would be to solid and would hurt the driver and most likely pop. By letting the air out they cushion the impact for the driver. If the above (inflatable skeleton) hull was to impact a solid object the pressure in the skeleton would increase rapidly as it deformed, if the pressure was not vented the skeleton would surely be cut or burst from pressure.

    If the skeleton (and hull sections) were to have pressure relief valves that dumped excess pressure to the atmosphere then theoretically as the boat rammed into the solid object, the hull would deform, the skeleton pressure would increase rapidly, excess pressure would escape and the hull would not cut or burst as easily as if it retained all of it's air. Once the impact is over, simply pump everthing back up to the correct levels.

    Thanks for all the ideas shared so far everyone, lets keep it going :D:D:D
     
    Last edited: May 15, 2011
  4. cavalier mk2
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    cavalier mk2 Senior Member

    I am suggesting that sledding over an object will increase the deceleration time lessening the forces involved. For this reason I like bow overhangs versus plum stems, if you sail into a sheer wall they won't make much difference but most objects are floating close to the water line-logs, containers etc....Stopping less abruptly is always better on the occupants who most likely won't be wearing seatbelts etc...Using an inflatable hull as an energy absorbing sled does the same thing.
    The cat you mentioned had keels, I recall a Wharram (great overhangs) that bounced over a reef into the water on the other side and was successfully extracted when things were calm, so having underwater appendages that don't hang you up can be important.
    I think replacing the whole hull at sea is asking a lot but any safe inflatable is divided into multiple air chambers so it shouldn't be necessary. Maybe having emergency inflatable chambers like Old Sailors acid bags in each section would let you keep air in a section even if the outer skin was torn.
    Sort of neat thinking of the hulls like car tires to be replaced but there will have to be significant manufacturing to bring the price down to reasonable levels. Mid ocean is a bad time to find out you need retreads when you've been stretching your budget....
     
  5. CatBuilder

    CatBuilder Previous Member

    Yes, I can attest to that fact from experience.

    I was on bow watch many years ago in the fog in Maine, USA. We were moving along through fog so thick you could barely see the bow from the stern and I heard the sound of waves breaking. YIKES!

    No sooner did I hear the sound than I yelled out "LAND!" to the helmsman (and navigator since it was his Dad's boat). No sooner did I yell it out then we hit the bottom since it was a keel boat. Everybody on board looked like on the old Star Treks when they crash around in the ship after they get shot or something. Of course, the navigator and helmsman had read the depth instead of the heading on the chart's compass rose. Guess he should have stayed sober.

    Anyway, yes... I can vouch for the fact that you don't always see what you hit. Especially if it's at night.
     
  6. MikeJohns
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    MikeJohns Senior Member

    Watertight subdivision has always been the preferred solution. That's the reason for collision bulkheads, crash boxes etc.

    Lightweight construction has never stood up well to collisions of any sort.
    The alloy boat at around 100 tons has suffered less damage than the small light boats. It's common for small steel boats with raked stems to show little more than paint damage. It's all in the hardness ( abrasion resistance) once the material starts to crush all the energy goes into shearing.

    Ultimately its down to what you hit, and where you hit it .
     

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  7. daiquiri
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    daiquiri Engineering and Design

    It looks like this boat was smilling, but I guess it actually wasn't:

    [​IMG]

    :p
     
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  8. MikeJohns
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    MikeJohns Senior Member

    The boat owner gets a new boat from the insurance company he's smiling:)

    Here's another broken dream, the reality of light weight construction is that there is little reserve strength for collisions and groundings.
     

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  9. cavalier mk2
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    cavalier mk2 Senior Member

    I wonder how many steel hull ,won't float if badly damaged sailors, fly on airplanes, non of which are designed to survive mid air collisions..... a multihull's light weight is a significant safety factor when combined with shoal draft and positive flotation. It is better to step off a boat driven up the beach than get rolled out in the surf or sink before the beach is in sight....
     
  10. rayaldridge
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    rayaldridge Senior Member

    Here's a factor I've always wondered about. It seems to me that heavy construction carries a penalty in collisions-- that of inertia. A light boat will stop with the dissipation of much less energy at the point of impact than a heavy boat. A keel boat, moving with the inertia of thousands of pounds of lead, must have to be enormously strong to stop the boat without crushing at the point of impact.

    As cavalier pointed out, some damage may be averted if the boat can ride over or glance off an obstruction. The heavy boat will be less likely to do this, because of inertia, which resists change of direction.

    Or so I theorize, though I could be completely wrong.
     
  11. Dean Smith

    Dean Smith Previous Member

    try telling that to my wife, she managed to belt every rock and bombi in Fiji in 1981 when we sailed there in our steel yacht Also try telling that to the owner of a charter fishing boat whose relief skipper went up on a brick at 15 knots in the middle of the night, alloy boat, pushed the chine flat way up, never split the weld, pushed the panel between the outboard and inboard hull girder up so far it had to split, but he limped to port with pumps going and we cut the bottom damage out and in one day it was fixed. HellI sound like Brent:)) funny how people with lightweight cats, not that al boats are at all heavy, are always trying to defend the integrity of such
    Some nice pics mike, dalquiri you often make me smile
    I am having a prob coming to grips with STEM pic, usually there is a big alloy flat bar right there, and they do not dent like that, but then that is a very small pic
     
  12. MikeJohns
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    MikeJohns Senior Member

    :rolleyes::rolleyes::rolleyes:
    Aircraft don't tend to encounter large heavy objects that float around in the air. Birds and weather balloons are reasonably predictable and we go to great lengths of organisation to get airliners flying on different paths. It's called traffic control and it's taken very seriously.

    Collisions are a fact of life for boats and they are relatively common.

    You mention steel boats; they have even commonly survived being hit by ships, that's the other end of the spectrum.

    One of the scariest things I've seen at sea was a large heavy barnacle encrusted log floating almost submerged, it was massive. That's where riding up would help dissipate the collision energy. Friends leaving Belgium for the Canary Isles very nearly ran into a container in the English Channel.
     
  13. MikeJohns
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    MikeJohns Senior Member

    The heavily built boat is much more likely to get away with no significant damage. Look at the 3 pics I posted, one vessel is(approx) 100 tons one is 3 tons and one is one ton. Play mix and match with the levels of minor damage to major destruction.
     
  14. oldsailor7
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    oldsailor7 Senior Member

    Steady on Guys. We are talking Multihulls here not welded steel plate ships.
    I found a Western Red Cedar 4 X 4 suitably rounded made a good stem on all my sailing Trimarans. Served me well in certain encounters. Dents easily, but is just as easily repaired.
    Multihulls need to be strong but Light, otherwise their performance and sea keeping qualities are degraded.
    Alertness, navigation, proper seamanship and good windward ability need to be continually addressed in order to avoid these sometimes trying circumstances. :eek:
     

  15. rayaldridge
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    rayaldridge Senior Member

    I don't think that "heaviness of construction" is the reason steel boats are able to survive collisions better than multihulls, or for that matter, monohull yachts built of wood or composite. I think it's the ductility of the material-- it's able to deform without rupturing.

    Though steel, with its inferior weight to stiffness ratio, is unsuitable for multihulls, there are aluminum multihulls, and these are probably better able to withstand collisions that would rupture boats built of wood or composite.

    Force equals mass times acceleration. Multihulls may be moving faster, but they have less mass.
     
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