Multihull Collision Survivability

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

  1. cavalier mk2
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    cavalier mk2 Senior Member

    Those serious guys in the control towers have been taking a lot of naps lately....I don't think you'll make much progress advocating steel construction for multihulls or airplanes for the same reasons......No matter what kind of boat floats your fancy it is prudent to reduce speed in reduced visibility and shoal conditions regardless of construction. Most materials have their uses and place in boat construction, stainless steel can make good hardware;)
     
  2. CatBuilder

    CatBuilder Previous Member

    I think Rayaldridge makes an excellent point about F=MA.

    First, the F of a catamaran hitting a container at 15 knots is pretty similar to the F of a lead keel monohull hitting the same container at about 7 knots. Since the boats are constructed the same way (out of the same fiberglass materials), I would expect similar damage, since the forces are equivalent, given that the catamaran will weigh about half what the monohull weighs.

    Of course, steel is a nice, malleable metal that will dent in nicely before breaking apart. That's its main advantage here. Multihulls can't be made from steel because they can't function with all the excess weight (or M).

    I think the 3 earlier pictures of the boats with bow incidents are misleading. They are only pictures of boats that have crashed, and one that rode up a dock. I've seen probably about a dozen standard FRP monohulls ride up and over a dock. There is no damage, because that type of thing happens on a daily basis. The dock simply sinks a bit and you ride up and over, then back down again if you didn't go too far up. In all the times I've seen this, I have never seen damage to any fiberglass boat that rode up on the dock.

    Now what about those other two boats from the pictures? Who knows? We have no indication of what happened to them.

    My point is, those 3 pictures are bad examples because they are not at all similar or known situations leading to the damage (or no damage with the dock ride up). The pictures lead one down an emotional path to think that the one that rode up on the dock is stronger, when we have no factual idea if that is true.

    Also, materials science says that FRP can certainly be made as strong as steel, however it will always be more brittle and less malleable.

    There is a spectrum to take note of here between the various types of boats being discussed.

    The heavy steel boat will have far more F when it hits something, but it's malleable, so it absorbs that F into a big dent. The light fiberglass boat will have less F, but is brittle, so it may crack and shatter.

    This is true of all boats, nothing to do with multihulls, really.

    The only big difference with a multihull is that it is much lighter in weight (but not in construction) than its monohull cousins simply because it isn't dragging 10,000lbs of lead around with it. That's the only difference.

    "Light" means many things. In the case of a multihull it means no ballast, but similar layups to a monohull. This makes the boat weigh less, but the quality of construction is no different. A key point being missed, I think.

    Lastly, boats are all trade offs, as we all know. If you are into steel as a material, you are into being able to dent it and not worry. By default, you are also going to be a monohull. If you are into multihulls, you are, by default, going to be glass, wood/epoxy or aluminum. You are then into a brittle boat that can shatter on impact, but you design the boat with watertight bulkheads to deal with that issue.

    With the multi, you plow into something, blow a hole in your bow and keep on sailing.

    Repairs on that blown out bow are not a big deal either. Get out the grinder, some glass and the epoxy (and microballoons and silica). You can fix up a smashed bow over a long weekend of work.

    Same end result as the malleable steel boat, only difference being the multi is just faster, lighter and less prone to rust.

    PS: Truth be told, if I was willing to sacrifice speed, comfort and business opportunities to go with a monohull, I'd probably go steel as well. Just saying this so you realize my post isn't anti-steel.
     
  3. MastMonkey
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    MastMonkey Junior Member

    In your first post you asked if it was possible for a multihull to survive a severe collision and carry on sailing. Boats with reserve foam buoyancy, both multi and mono hull, can take severe damage and not sink. Collision bulkheads provide an even greater margin of safety. And they can continue sailing. I misunderstood your desires though with this design. My understanding now is that you are more interested in preventing damage not surviving damage. Inflatables are good for this, but my experience is only in rafting. We regularly run into rocks and across the bottom without damage. But you still have the problem that if it loses air, it loses all buoyancy. Compartmentalizing helps, but a sharp reef or container overgrown with barnacles would likely rip it into many unrepairable pieces. In such a situation as being stuck on a reef with one of the inflatables failing your main hull would immediately be subject to damage. You are still going to want reserve flotation in the main hull.

    All boat use momentum to a certain extent. You could add weight to the main hull, but you have to be sure to add the extra buoyancy needed to support it. This complicates the boat even more. You might have trouble tacking with such an arrangement, but my biggest concern would be trying to work against an opposing sea. The lack of intertia would lead to every wave slamming into you, brining you to a dead halt. That is my feeling anyway. At the size of boat you are considering I would also think that inflatables wouldn't have the necessary rigidity. The same reason they would be beneficial in a collision, would be a hindrance in performance. I could imagine the inflatable hull constantly bending and flexing. Like on a zodiac you would probably have to add some structural elements to provide rigidity.

    These are just my impressions though. My personal feeling is that the risk of collision is not great enough to justify designing the entire boat around surviving one without damage. Especially since it is possible to survive a collision adequately and safely enough to make port for repairs. Accident statistics show a very clear trend. The majority of boating accidents occur because of operator error. Things like failing to keep an adequate watch, making mistakes in navigation, and not having necessary safety gear.

    I was wrong about something in my original post. Repairs to small punctures can be made as good as new in some materials. Check out a material called Hypalon. It is easily repairable and has excellent chemical and UV resistance.
     
  4. Alex.A
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    Alex.A Senior Member

    Search te Pookie on this site - or google it - or michael Schacht's proafile website - not the yahoo one. Dont remember the program - discovery channel - power mono but 16'-20'. After ramming a submerged container several times they had to ram it nearly flat out, directly on the corner, to sink it.
    think car again - impact absorbtion and redirection of energy = crash box and watertight bulkhead.
    With a proa tho' - you can try and sail with the other bow as much as possible.
    As to shape - an interesting area - i'd go with a gentle curve.
     
  5. Dean Smith

    Dean Smith Previous Member

    ok mate sorry
    perhaps mike can go to the thread on that large tank
    Boatbuilders thing differently and it looks like those who make the living this way are way outnumbered:))
     
  6. Boat Design Net Moderator
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    Boat Design Net Moderator Moderator

  7. MikeJohns
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    MikeJohns Senior Member


    Alloy is a good choice for a catamaran if you want toughness. I presume you made an error in suggesting alloy is brittle. Steel is not a sensible option for any light weight boat of the sizes we are talking.

    As for F=m.a that's not applicable. That's the force required to accelerate something not the energy available in an impact which is (m.v^2)/2

    So if your for example the cat was traveling at twice the speed and has half the mass and both boats run into an immovable object, then the relative amounts of energy that have to be absorbed by the impact will be:

    Cat ((1/2)X(2^2))/2=1
    Mono(1X1)/2=1/2

    Speed has far more of an input than mass. Then it's all about how you absorb that energy; lifting the boat is the best way (riding up) but if you are into brittle failure material the damage can be very extensive if it fails to ride up.

    Metals yield and then deform, absorbing the impact energy. Brittle failure tends to be more catastrophic which I was illustrating with the pics I posted.
     
  8. rayaldridge
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    rayaldridge Senior Member

    Mike is correct and I was mistaken about the equation-- apologies. I was apparently thinking of inertial mass.

    However, I think that equation for kinetic energy is accurate only if the object hits squarely, in a way that entirely stops the boat. If the boat hits the object and glances off or rises over it, the deceleration is much less violent, and I think that a light boat is more apt to glance off of, or rise over, an obstruction than a heavy boat. A bicycle, even if traveling rapidly, is more likely to glance off of an obstruction than a truck traveling at half the speed.

    There's another aspect we haven't discussed. If, say, a catamaran hits a shipping container on the port bow, the starboard hull will swing around, absorbing some of the energy over a longer cycle.

    Another factor is the narrowness and sharpness of multihull bows. Because these bows have less crosssectional area, and the angles are far sharper, they are more likely to bounce off of obstructions. I once sailed a Wharram cat through a stump field on a dark night, and bounced off a few stumps, without damage.

    It was dumb.
     
  9. CatBuilder

    CatBuilder Previous Member

    Whoops. I'm working 70 hours a week doing grunt work (building a boat), so I failed to even think about which force equation we were using and just took the one in the thread at face value.

    Pretty embarrassing mistake for someone with a Physics background and career history at NASA.

    Velocity goes as the square. My mistake.
     
  10. catsketcher
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    catsketcher Senior Member

    Real world examples

    I know of a few cats and monos that have hit stuff offshore. So will list below

    -D Flawless - Crowther Supershockwave hit a whale (they think) at mid teens speed. Totally wiped out the boat and crew were in the liferaft in minutes. Sure she was going fast but she was built awfully light. The original Shockwaves were 200gm woven glass either side of foam. I hit one in my tri once (I was on starboard and it was a drifter) and the glass didn't crack it just smoothly deformed. Crowther didn't have watertight collision bulkheads in his cats. My verdict - uni around board case, strong (400-600gm) laminate min, and collision bulkheads mandatory

    - Windrider - Bailey's get their cat stuck on a reef. They get the natives to pull it up on the reef and spend 6 months fixing it. They sail away and live happily ever after. Good thing the boat was light to pull over reef with low tech equipment and light enough to float off with rudimentary equipment when tide was high months later.

    http://www.theage.com.au/national/perth-family-shipwrecked-in-pacific-cyclone-20100330-rbin.html

    - Hans Christian 42 - lovely mono from my bay. Mistook light in Lousiades. Ended up on reef and couldn't get boat off even with help of other yachties. Lost everything with boat stuck on reef and had to leave as a wreck. So a stout mono is structurally okay for a while but its weight makes it a target for the next weather and tide to start destroying it.

    - Twiggy trimaran - Glenn is silly and decides to go in reef passage by GPS. Problem is the chart wasn't done in GPS and reef passage is out. Puts hole in boat. Motors tri into lagoon and fixes it up quickly.

    - My friend (in Fiji) gets call from Euro cat owner. His cat is on reef and hulls are holed. Jeff goes out with crew and drums and lifts cat onto beach, patches hulls with rough ply and sail back to boatyard for proper job.

    So if you are on a cat you need to give your boat as much assistance as possible. Divide the accommodation, install watertight compartments and put a keel shoe on for good measure. Then when the tide is high you can probably get your boat off yourself. A heavy mono will stand a much bigger chance of staying on the reef.

    As for kinetic energy and such it is hard to be exact. A cat has a sharp (read strong) pointed bow. Anyone who has hit anything with a multi knows they can do damage with their thin bows. They will probably hit something with their sacrificial stem rather than their forward sections. As for the energy to stop them as Ray says the CG is not behind the bow that is stopped so the negative acceleration of the boat as a whole is reduced. This is why beach cats bear away when the nosedive.

    Also most cats stay upright giving a log or the like something rather solid (the stem) to hit. A heeled mono gives large flattish area to hit with as well which will not be as strong as the stem.

    So what else can you do? Be like a car designer and instead of going massive go crumple zone. On Kankama I have cedar noses (like Oldsailor says) then 1.2 metres back is a watertight bulkhead. The container would have to rip through a lot of cedar and glass to get to that bulkhead but if we were going fast enough it may. Then we lose about 200litres of bouyancy. No worries. Then another 1.2 metres back is another bulkhead. Its going to take a huge velocity and a massive container to rip that much boat apart and still there will be no water in the bilge.

    Maybe the container hits the board. The string unis laid up forward won't break so bang goes the board. No prob. Pick up the pieces (I have done this on my tri) and put them together later. Usually when going fast you will not have boards down - broad reaching and running we pull ours up. (Most cruising is reaching and running)Rudders are kick up and pop up well (usually fish traps at night).

    You can always design to suit the loads. Most cruisers will never sail at consistently more than 12 knots. You can always ask your designer to draw you a bow (with bulkheads, nose cone and heavy laminate) to cope with this situation (Container to one bow at 12 knots). Its just a load condition like any other - unis pointing fore and aft inside and out. Is the worry of hitting something enough to compromise your choice of boat? It shouldn't be. Don't be like D Flawless. Be strong and put bulkheads in. Make your board cases bulletproof and have fun on a cat or tri. In all my times sailing on the East coast I have never hit anything more than a 2x4 motoring at night up a river or the occasional bit of bottom with the board. I wouldn't want to go offshore on lots of cats without full watertight bulkheads after the security of having them. You have a great sense of security like others get from a tough material. I have a tough cat. Mike likes steel - I like cats. Each to their own.

    cheers

    Phil
     
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  11. CatBuilder

    CatBuilder Previous Member

    I feel good about my cat's construction after reading this.

    1150g skins inside and out.
    25mm, 6lb core cell
    Extra keel reinforcing
    Foam crash bow
    3 watertight bulkheads in each hull
     
  12. rayaldridge
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    rayaldridge Senior Member

    That was an excellent and information-rich post, Phil!
     
  13. Skint For Life
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    Skint For Life Junior Member

    Wow! There have been so many posts it's hard to answer everyone, but I'll try.

    I'll start off by saying I think I have confused the issue by asking questions in my first post and then posting a possible solution to the problem. Perhaps we should keep this thread just on the topic of "Multihull Collision Survivability"

    As such I have created a new thread to discuss blue water cruising multihull designs that fit NZ trailer requirements. Here is the link: http://www.boatdesign.net/forums/mu...ater-cruising-multihull-38114.html#post463280

    CAVALIER "post 19" Thanks for that, I understand where you are coming from with riding up lowering the force to be absorbed at one moment in time. I think kick up, shear off or no underwater appendages seems like a great idea. Re: chambers, I have thought about this alot. I have even thought of inflatable bags that hang off of the hull skins so that even if the main structure is ripped open the independant bags should still keep it afloat. Thanks for your comments about the hulls being replaceable parts like tyres on a car, my thoughts exactly :D

    CATBUILDER "post 20" Thanks for sharing that experience. I'd like to point out to all here that I would feel safer on a boat knowing it could withstand impact with little to no damage compared to a boat that everyone has to be alert on. Obviously good practices should be followed, but we are all human at the end of the day and we are not perfect. Following on from this I think we should all admit that although the modern cruising multi is a great thing it is not perfect, I'm not knocking them, I'm merely saying EVERYTHING can be improved upon. Otherwise why would so many people waste their time designing and inventing.
    "post 32" Thank you for this :D, there are amatuers (me included) here who would just take those picture at face value without your input. "If you are into multihulls, you are, by default, going to be glass, wood/epoxy or aluminum. You are then into a brittle boat that can shatter on impact..." I propose Inflatables be added to that list :D light weight and they are not brittle and they do deform, I think it's the best of both worlds :D

    MIKE JOHNS "post 21&23" Thankyou for those pictures, scary stuff! That Cat sitting there smashed to bits with the guy standing looking at it is exactly the sort of outcome I would like to avoid. "post 27" "Collisions are a fact of life for boats and they are relatively common." This is the sort of stuff I want to know, does anyone have reports on this to show the relative occurances? It seems to me like some people are saying collisions are fact of life and others are saying it's not a big deal don't allow for it. I invite all to share their experiences or lack of experiences of collisions at sea.

    RAYALDRIDGE "post 38" "There's another aspect we haven't discussed. If, say, a catamaran hits a shipping container on the port bow, the starboard hull will swing around, absorbing some of the energy over a longer cycle.

    Another factor is the narrowness and sharpness of multihull bows. Because these bows have less crosssectional area, and the angles are far sharper, they are more likely to bounce off of obstructions." Interesting points ray, I have thought of these also. Along the same lines I thought, what happens if the daggerboard/s are down when the impact is on one bow only? Does the boat still rotate about the point of impact? Or do the daggers keep it tracking true and damage the bow more?

    DEAN SMITH "post 26" Thank you for sharing your collision experiences :D

    OLDSAILOR "post 29" Thank you for letting us know what you use in your boat to help with collisions :D

    MAST MONKEY "post 33" You are correct, I have been misleading, asking two or more questions in one. I guess it was the overall idea I was getting at. I asked about surviving the damage of impact, I thought that even better would be a boat that didn't get damaged on impact, hence the diversion. As I have written above I have started a new thread for my concepts so that this thread can be left for the discussion of "Multihull Collision Survivability" whether it be ultimate survivability of the craft, crew or survivability through no damage on impact.
    You make very good observations and points on inflatables and the prospect of being stuck on a reef, thankyou for this :D Added bouyancy to the main hull is duly noted :D Regarding tacking, I was under the impression that a multi with the weight mostly in the middled tacked better than a multi with the weight in the extremities. Why is it you think the concept may tack poorly? Feel free to answer these concept specific questions on the other thread :D Regarding rigidity, I have considered combating this with an internal inflatable skeleton inflated to a high pressure, using non-stretch materials to stop deformation. I have also considered the possibility that some lack of rigidity may be a good thing, in big swells/waves with the boat riding down the face of a wave a normal multi can sometimes submerge the bows, with the concept I proposed possibly the hulls would flex upwards and not submerge anywhere near as much (check out the other thread for a rough picture of this theory) I have also considered the possibility of rigid stuctures between the beams that the inflatable hulls attach to, I believe they would stiffen the structure and also be out of the way enough as to not adversly effect the impact absorbing qualities of the inflatable hulls. Regarding not designing the boat around this alone, I had other parameters too, again, in the new link all is revealled :D Thankyou for your update about Hypalon :D

    Thanks to all for sharing their experience and ideas :D:D:D
     
  14. MikeJohns
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    MikeJohns Senior Member

    Phil not so much likes, rather hard engineering facts.
    In my experience people love their boat until something goes wrong and then they change their opinion in a few minutes.

    The reason I like metals is the large factor of safety inherent in collision. Catastrophic failure is very unforgiving and you are trusting to luck as to just how that occurs and whether the vessel is recoverable afterwards .

    Alloy cats can be very light if they are well designed they just need more skill to design and build. Alloy offers an alternative to brittle failure and very low FOS when it comes to damage. I like alloy too!
     

  15. CatBuilder

    CatBuilder Previous Member

    Alloy (aluminum, right?) would be the last material I'd feel safe using for a catamaran.

    Why? Metal fatigue.

    The main strength beams must be carefully examined with expensive equipment as they age.

    I'd prefer a well engineered FRP catamaran.
     
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