Multihull Collision Survivability

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

  1. rayaldridge
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    rayaldridge Senior Member

    I have to say, this thread has influenced my thinking about the boat I'm building. It's a folding 23 foot cat, built of ply. Originally. the first full bulkhead was 6 feet aft of the bows. I decided to put a collision bulkhead 3 feet aft of the bows. The bow compartment will be accessible via a deck plate, for inspection and repair access. Anyway, any bow damage would have to extend through two watertight bulkheads to reach the cabin, and the aft 4 feet is also a separate compartment.

    My center deck is two inches of foam, for upside down flotation, should I capsize. My beams are above the decks, and are sealed, so she should float pretty high upside down.
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Why not make that unknown into a fact? Surely better than assuming it is ok?

    It is a simple calculation to prove to yourself whether it does or does not actually contribute.
  3. HakimKlunker
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    HakimKlunker Andreas der Juengere

    What I like with your idea - Daiquiri pointed it out - is that an inflatable cannot be filled with heavy stuff. A multi in my eyes is superiour only when it is kept light in weight. Even then in very light winds multihulls often do not perform better than a monohull.
    The downside will be that despite having lots of square meters available, one will have to compromise with luxury living conditions.
    When it comes to comparison with RIB's: They have a solid bottom, haven't they?
    If the float is entirely inflatable, I would expext some flexing under way, which will reduce performance. If the respective owner can tolerate that, it will be ok because no two people have the same preferrences.
    But still I think of the risk of collisions. there are not only containers and whales out there. Especially close to river mouths or harbours you may also encounter trees with sharp broken branches, bits and pieces with nails in it, and so on. Even if a repair is relatively uncomplicated: doing this all the while will be quite frustrating - and, as one friend said: it takes away precious drinking time :)
  4. Skint For Life
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    Skint For Life Junior Member

    HAKIM, Thankyou for thinking that concept over :D:D:D I hear you regarding requiring some structure to stop the hull from flexing, I've thought this over and designed a few solutions in my head, I've just got to get them to paper.

    Also regarding ribs having solid structures, check out this rib which is fully inflatable: It looks to me to be a great design.

    I think with using non stretch materials and some form of inflatable skeleton that a desirable hull form and rigidity should hopefully be possible.

    Regarding losing luxury living and space, this mis-conception of the concept confuses me. If the boat is designed from scratch allowing for not using the outer hulls for living area then where is the issue? If the central accomodation hull is built the correct size then everything else is built accordingly then I guess it's kind of like building a mono the correct size for living in, or tri with no storage in the amas. To my mind it means you get the space you want plus a bigger boat for the money, many people think a bigger boat means a safer blue water boat.

    Regarding the idea of constantly patching the inflatable, I'm not sure how much of a reality that would be, I've seen alot of RIBs round here with not alot of patching, a RIB we have used alot for fishing and diving hasn't needed a patch. I've discussed an "airbag" release of pressure idea to try and stop the hulls from being puntured earlier in this thread.

    I've started up a thread to discuss the inflatable concept here:

    Please post further discussion on that thread so we can leave this one to discuss the original topic :D
  5. HakimKlunker
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    HakimKlunker Andreas der Juengere

    S'qse me - I was not aware of the other thread.
    We perhaps have a confusion of terms here:
    RIB normally stands for Rigid Inflatable boat, right?

    So, as here we talk safety,
    I would say that the soft concept will work quite ok when it comes to
    -call them- blunt collisions.
    Anything sufficiently sharp may cause a deflation of segments or overall.
    For the most endangered zones, a protoctive rigid part can perhaps assist. In my imagination that can be a steel tube following the stems and keel lines - or a VERY slim rigid cover. But that would have an effect on your transportability goals. The most likely collision will be in the forward section and the protection does not need to go all the way aft, then.
    This will not entirely protect, but at least reduce the risk - without spoiling the other advantages too much. Perhaps a lengthwise bending is reduced.

    From the repair point of view: The procedure is simple, yes.
    I just imagine the following scenario: Heavy (and I mean: heavy) weather and for some reason close to reef or s/thing with a collision and deflation following.
    That would certainly de-stabilize the shape in a way that it comes close to being unable to further manoeuvre.
    With the reduced buoyancy there also will be reduced stability. That moment I would say that one will be unable to carry out any repair and the situation must be handled as it is.

    (On the lighter side: also the life-raft is an inflatable ;))

    In some way I would also expect the tube to be integrated in the flow of forces. With this now distorted or interrupted, other parts such as the beams are maybe under a higher load. I think it is a good idea to reflect this in the design phase to create a sufficiently carrying structure even with the float ineffective.

    You see: I have concerns. On the other hand they all hinge around deflation. Some parts of the world's oceans are less polluted and less rocky close to the surface. In those regions the risk certainly is lower and your concept will work well.

    Personally, I believe that technology cannot replace good seamanship.
    It will be relatively easy to plan voyages that avoid the most dangerous situations. As a result, this looks like a plus for your concept (when it comes to safety aspects)
    However: see the other thread
  6. waikikin
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    waikikin Senior Member

    Nice point Ad Hoc, I remember working on a Lock Crowther designed cat that had an inverted waterline marked on the lines plan- assuming of coarse built to plan & had inverted bouyancy tanks, foam filled bows, compartments fore & aft, pull out & hinged bunks for inverted living/sleeping, plumbing access to fresh water tankage whilst inverted, escape/access hatches, slip resistant coatings & ring bolts to underwing. Another 40 foot cat had inverted bouyancy tanks installed that the electrician found very handy for instalation of downlights & associated wiring & holes !!
  7. redreuben
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    redreuben redreuben


    I don't know if that inspires confidence ! . . .or not !

  8. tspeer
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    tspeer Senior Member

    I disagree. I only have one experience with an overturned multihull at sea (a 31' trimaran), and the conditions weren't extreme. However, what we observed was the boat turned broadside to the waves and the outer hulls acted as breakwaters for the main hull - only occasionally did splashes reach the main hull. The rig acted like a sea anchor to add a lot of damping. The motion was very easy - much smoother than would have been the case in a life raft or dinghy. It was quite easy to stay on top of the overturned hull, and we passed our tethers around the extended daggerboard to make sure we stayed there.

    I think a cruising cat would be a very stable platform when capsized. The bottoms of the hulls would be quite far out of the water and providing a lot of shelter from the wind and waves. I'm not sure where the best place would provide the most shelter - backs against the lee side of the windward hull (waves passing overhead?), backs against the windward side of the lee hull (water that tops the windward hull falling short of the leeward hull?), or on top of the leeward hull (more exposed to the wind, but maybe less splashing).

    Jim Brown used to maintain that the best approach when capsized was to tie an inflated liferaft between the hulls. The boat provides a much bigger target to see than the raft, and it would act to shelter the raft and moderate the motion. The raft itself would provide cover and the safety of an enclosure. Best of both worlds.
  9. tspeer
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    tspeer Senior Member

    This is a much under-appreciated reason to have an escape hatch. It's not so much for getting out of the boat, but for getting back into the boat. (Getting out from under is not that difficult, even with an inflated PFD. One simply pushes off the overturned hull or net and walks hand-over-hand to the edge.) In addition, the escape hatch vents the pressure surges from the waves that otherwise can make the inside of a hull intolerable.

    The other lesson to be learned is, "If you ain't got it on you, you ain't got it." You can't depend on being able to go back into the boat after a capsize. And the wave action in concert with an open companionway is incredibly efficient at flushing things out of the cabin. That EPIRB or hand-held VHF may not be there when you do get back inside. Instead, when knarly weather arrives, I believe it is better to distribute the EPIRB and handheld VHF to crew members to carry in the pockets of their foul weather gear. Ideally, everyone on the boat would have a means of electronic signaling. Then, whether the emergency is a crew overboard or a capsize, the people in the water have a means of calling for help and reporting their position, and you aren't depending on being able to get back into the boat.
  10. catsketcher
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    catsketcher Senior Member

    You can always use the axe

    Gday Tom

    I never capsized my 31 ft tri but we always had an axe tied onto the outboard mount. The original Twiggy capsized twice and they cut a hole to get in an out.
    Cats have a slight problem in that if you put a hole in the bottom of the boat the hull will settle and you probably don't want that.

    On Tony Grainger's site back in the mid 90s there was case of a cat capsizing in the Med. One crew member was outside. The others were inside. After a few hours the inside crew went quiet - they had asphyxiated. (NOTE - I have not verified this story independently)

    For our boat I still have an axe near the outboard mount. As we have fore and aft collision bulkheads as well as vertical ones I would bung a hole near the hull bottom (now the top) up the front - about 3.5 metres aft from the bow. We could make two - one for each hull and have plenty of room to sleep and get warmish. When conditions are better we can go inside and get stuff - that is the hard part - I wouldn't want to violate the airlock so I wouldn't want to put a big hole in the rest of the hull. It would have to be a swim or chop a hole in an underwater area close to the surface that is easy to get through. For my part Kankama has water tanks that have valves on the breathers and netting was (I took it off when we stopped cruising) on the cupboard fronts. I deck painted the underwing which meant I didn't have to fair it.

    When Rose Noelle capsized they lost all their water hence the breather valves.

    I probably should have a better calamity bag near the motor nacelle. It is in the cockpit - ready to use and accessible from on top and underneath. I should probably put some water and food (fruit cake is good I heard from a guy who capsized on St Therese in 1988) and you wait until things get easier before crawling around inside.

    It's do-able but many cats aren't set up for a flip. Tris are much easier.


  11. MikeJohns
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    MikeJohns Senior Member

    We probably have different ideas of rough conditions!

    Large cats that have turned up afloat with lost crew, one a year or two back had the remains of someones attempt to tie himself or the life raft to the prop. All 3 crew were lost, none found. Perhaps some decent fixing points should be provided underneath too and a hatch with harnesses and survival suits and EPIRB and all the rest.

    The entire underside of the bridge deck is immersed in this recent event I posted the pics of. and the mast has apparently collapsed too which adds a fair amount of danger.
    A hatch in the underside of the bridge deck would help you dive more safely but the chance of finding anything would be down to luck. I'd like emergency access into a sealed compartment in one hull for real survivability.
  12. catsketcher
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    catsketcher Senior Member

    IIRC correctly the attachment points on liferafts are designed to break away if loaded highly - this is to stop them being dragged down by a sinking boat. Also looking at the video of the capsized Chris White cat with a guy in a dinghy it looks awfully nasty in a dinghy and would be pretty bad in a liferaft too webbed between a cats hulls. I know I would be axing a hole in the compartment if it was me there.

    For quick rescue in warm conditions I would think hanging on would be fine. For offshore voyaging I think that you need somewhere to get out of the water, keep watch, and stay safe. It has to be somewhere high and this means on the bottom of the boat. It will hurt but you will have to chop into the bottom of the hulls. Hatches in the bridgedeck or cabin side are no good for living - access fine - but you have to be outside and dry to survive.

    This is what they did on Rose Noelle, Gulf Streamer, Twiggy, Triton, Meridian and many others - all tris. The crew of Rose Noelle were 119 days capsized - we thought they were dead but the boat brought them back because even upside down it provided them somewhere warm and safe to live. (John Glennie called them death rafts and it may have been good that his crew did not have one to get into - they weren't always getting on well) I love Jim Brown but I would rather use the method used by people who had success in coming back from a long term capsize - chop into the bottom. A Searunner would have a great dry spot on the bottom of the cockpit - no need for a raft. Getting cat owners to think properly about capsize habitation has always been a hard sell. Tris have good inverted flotation by virtue of their type - cats need clever design.


  13. Bruce Woods
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    Bruce Woods Senior Member

    With an axe?

    Has anyone here actually tried hacking a hole in a strip plank cedar hull panel with an axe?

    Whilst building our latest, 600db/15mm cedar/ 400db, I took to a superfluous cutout section of the hull with an axe, as an experiment, without much luck. I would hate to using a hatchet whilst clinging to the whole upside-down shooting match trying to get a hole cut big enough to get trough.

    So what I'm saying is maybe a small axe is ok for foam or ply, but I'm not sure about strip-plank or balsa or kevlar.
    I've got a narrow blade saw and an auger arrangement stowed just in case.

  14. rob denney
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    rob denney Senior Member

    Axe fans: Lay up a piece of hull, then in the middle of a dark, wet night try and chop a hole in it with your trusty axe. Do it in your dinghy for a bit of realistic rocking and rolling.

    The first time I capsized, we had an axe. Managed to chop a hole in 6mm ply with fglass on one side and get the flares and epirb out. It would have been hard work to make it big enough to get inside, and the sharp edges would have torn the wet weather gear. Also impossible to close it afterwards, so stuff kept floating out.

    After chopping the hole, I gave the axe to my crew, without realising he was in a state of shock. Couple of minutes later I asked him to pass me the axe. "What axe?" He had dropped it over the side.

    We were rescued by the Irish navy who, after trying to sink the boat by ramming it, sent across a couple of huge stokers in a RIB with a serious, double bladed fire axe. One of these guys stood up and chopped at the keel, which was half a dozen layers of cloth, no core. The axe bounced off, and very nearly threw him backwards out of the RIB. We radio'ed him and suggested he try the side of the hull (12mm Airex, 400 glass each side). He did, but was only able to make splits, not holes.

    The boat was eventually sunk after half the crew had shot it full of semi automatic small arms fire and 50 rounds from the AA gun.

    On a boat going out of easy rescue distance from shore, or overnighting, anything less than a proper opening/closing hatch well above the submerged waterline is a death waiting to happen.


  15. catsketcher
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    catsketcher Senior Member

    That's really interesting Rob. I will have to amend the axe thing - I know I can cut through my hull as I have done many times before putting in hull fittings, board cases etc. Probably better to use a saw to get through it.

    My problem with a permanent access hatch is that it will be too low for keeping dry. You have capsized and I have not (crossed fingers) but Ian and Cathy cut a hole in Twiggy low and then did another up high (inverted), same with Rose Noelle. The guys on Meridian somehow cut their way into a Horstman with a belt buckle. The story of the axe reminds me of Lock's story of the same thing with Glucometer in the Round Britain. I should have remembered. Ian and Cathy used an axe on a ply boat - saw is better on composite.

    I need to source a tough saw - start at the keel where there is lots of curve and go side to side from there. What am I thinking? Keep the thing up the right way first!


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