Multihull Collision Survivability

[Skint For Life]

Hi all,

I've searched the forums and can't find anything on this.

Watching a tv program the other night about a cruising cat hitting a reef and getting stuck on it and sinking made me think about the robustness of a multihull.

I mean I've heard of boats colliding with reefs, whales, semi-submerged shipping containers, floating logs etc. The big blue wobbly thing contains enough solid objects that I think it's worth discussing what happens when the light weight foam filled multi hits something hard with it's superior speed compared to a mono.

Is it possible for a multi to survive a serious collision at pace and carry on sailing? No hull puncture? Or is this just the domain of steel hulled monos?

I get that some foam cored boats float once a big hole is smashed in them, but is it possible to make a multi that is not a big heavy steel dog of a thing survive hitting a shipping container? Perhaps an underwater hull design that allows the multi to ride up over the object it hits?

Perhaps the occurance of the above makes it not worthy of discussion? More likely to get struck by lightning? I sure think it would be nice to know your boat could survive such an event

[Skint For Life]

Please just laugh this post off it is stupid, I've had a litre of energy drink and can't sleep.

In kungfu I was taught if the object your hitting is hard hit it with something soft, if the object your hitting is soft hit it with something hard. So palm strikes (soft) to the head (hard) and punches (hard) to the stomach (soft)

It makes sense, if you punch someone in the head you can easily break your hand........your probably wondering how this relates to the subject.

Well I was thinking, say for a sailing multihull boat of any given boat size you had to pick any boat material to crash into a shipping container at 40KPH

If you choose steel hulls, it probably won't sail well.
If you choose grp/foam hulls, it will probably smash the bows on impact.
Plywood, much the same.

I think I would choose inflatable hulls, the reason being as the hard object (container) is hit by the soft object (inflatable hull) the hull deforms to the shape of the hard object, absorbing and storing the energy to bounce back into shape moments later. Inflatable hulls are light so the vessel going at 40KPH has less momentum/inertia? Than the steel equivilent. The lighter it is the quicker it is going to stop ramming itself into the hard object.

I think inflatables are underated, I have found them in powerboat applications to be very see worthy, a RIB for example compared to the same size and engine equipped aluminium pontoon boat is quicker, more nimble, can carry more payload etc. If inflatable hulls are good enough for the coastguard to rescue you from the most dire situations in big seas then they are good enough for me.

The smartkat is a cool idea but I think there could be far more potential for the concept.

I have drawn up a rough sketch of a boat that may make you think and or laugh.

I'm not sure whether or not it's a cat or tri, hence the name. The outer hulls are inflatable, the main hull is above the water except for the rudder, centerboard and when required outboard. All the accomodation is in the main hull, the main hull is very low to the water to keep the COG low, as such it has a wave deflecting design. The outer hulls are inflatable and have many independant sealed compartments.

In a cruising type size I'd guess some rough dimensions of
main hull: 12m long 2.5m beam standing height shower and galley.
outer hulls: 16m long 1.8m wide at widest point
LOA 16m BOA 10m

I think if such an ugly design could get to 40KPH and then hit a container the result may be the hull/hulls deform alot, if they ride up the object you could expect the centerboard and rudder to be shorn off. If only the leeward hull were to hit I think the hull would deform alot, round up on the impact point, depending on heal and wind pressure it could produce a pitchpole. If only the windward hull were to hit it could possibly ride over the object, or again round up and pitchpole.

I'd be really interested to hear what people think of this wacky concept.

[rayaldridge]

I think the problem with inflatables might be that a shipping container has some sharp corners. If you hit it hard enough, you might tear the fabric.

Good multihulls consider the problem of hull damage by compartmentalizing. A collision bulkhead is designed to allow the bow to be torn open without flooding the rest of the hull.

[daiquiri]

Another problem with inflatables is that their internal volume can hold just air, and cannot be used for anything else. So all the accomodations and cargo holds have to go to the main hull, and that's not very space-efficient. Sailboats intended for coastal and off-shore cruising need all the internal space they can get, so I don't see this idea as applicable for them.
However, I see that your drawing shows a kind of small multihull raceboat, which could be ok for this solution, imho.

[cavalier mk2]

I advocated inflatable bow sections on the racers to better cope with damage such as sodebo sustained. Perhaps spare sections could be taken in case of deflation though attaching a section at sea wouldn't be easy. Still a fix a flat approach seems less race ending than the current sacrificial foam section.

[CatBuilder]

From my understanding and experience with multihulls (specifically catamarans): Replying in red inline...

Originally Posted by Skint For Life

Hi all,

I've searched the forums and can't find anything on this.

Watching a tv program the other night about a cruising cat hitting a reef and getting stuck on it and sinking made me think about the robustness of a multihull.

The situation of a multihull being lost on a reef is typically one of abrasion. The boat hits the reef, is stuck on the reef, and is constantly scraped along the reef, like a big piece of sand paper, which grinds through the bilge, through bulkheads, and eventually breaks the boat up. This would not happen with a steel boat, usually, but would happen to any fiberglass or ferrocement boat. Also, the multihull would have more of a tendency to get caught on the reef, rather than bouncing off because of its shallow draft and multiple hulls. A deep draft steel boat would probably hit its keel and get off. A catamaran might float right up and over the reef, getting stuck on top of it more easily.

I mean I've heard of boats colliding with reefs, whales, semi-submerged shipping containers, floating logs etc. The big blue wobbly thing contains enough solid objects that I think it's worth discussing what happens when the light weight foam filled multi hits something hard with it's superior speed compared to a mono.

It's going to take a huge hit and it's going to sustain damage,, unless somehow the hull rides up and over the obstruction. This is a slight possibility owing to the very shallow draft of the multihull. Even then, it's going to sustain severe damage. Properly designed multihulls are heavily reinforced in the bows for just this reason.

Is it possible for a multi to survive a serious collision at pace and carry on sailing? No hull puncture? Or is this just the domain of steel hulled monos?

Maybe. Maybe not. Same chances as any other fiberglass boat, except the multihull is going much faster in many cases. It all really depends on what you crash into and how the boat hits it. If you slam into the side of a steel ship with the bow of your multihull, you will break teh bow apart. That's not as big of a deal as you would imagine, because the bows on a properly designed multihull are designed to be able to break and not allow water into the boat. There is usually a watertight bulkhead aft of the bow, so no sinking would normally happen.

I get that some foam cored boats float once a big hole is smashed in them, but is it possible to make a multi that is not a big heavy steel dog of a thing survive hitting a shipping container? Perhaps an underwater hull design that allows the multi to ride up over the object it hits?

It doesn't have much to do with foam vs. balsa vs. solid glass. It all comes down to watertight bulkheads. Catamaran engine rooms are (if well designed) completely sealed off from the main area by a watertight bulkhead. There is also one or more of them up at the bow. This leaves 3 distinct areas in the catamaran hull that are cut off from each other. You could smash the bow off and still float fine. You could hit something so hard that your engine breaks out of the bottom of the hull, leaving a 1.5 meter hole in the bottom of the engine room and you'd be fine. You could break a hold in the middle section, but your chances would be a bit less there, since it's the biggest section. Supposedly, you'll be fine there too, as you have the bow section and the engine room still walled off from the main section, giving you flotation on either end of the flooded hull. That's how it works with multihulls.

Perhaps the occurance of the above makes it not worthy of discussion? More likely to get struck by lightning? I sure think it would be nice to know your boat could survive such an event
I'm actually more concerned with getting struck by lightning and with fire than sinking. IMO, each of those is more likely to cause fatalities than holing.

Lastly, there are some unlucky things that can happen:

You bash the watertight forward bulkhead so hard into something that it breaks the hull below it and tears it out. Then, you only have the engine room on that side keeping that hull from sinking. A slim margin, when you consider that on most catamarans, there is a deckhouse connecting both hulls and if the water gets high enough, it could make its way to the other hull.

[Skint For Life]

I apologize for not mentioning in the first posts that I know about multis using collision bulkheads and compartmentalization.

RAY

a shipping container has some sharp corners. If you hit it hard enough, you might tear the fabric

Yes true, but it seems to me that the repair would be alot less work than re-glassing a grp boat. Just fix the puncture and be on your way. If one hull was completely flat then the boat should float on it's main and other hull, allowing you to pull the injured hull up onto the tramp between the main and other for repairs.

DAIQUIRI

Another problem with inflatables is that their internal volume can hold just air, and cannot be used for anything else. So all the accomodations and cargo holds have to go to the main hull, and that's not very space-efficient

True, but if one were to accept this and design a main hull big enough for their needs and the inflatables designed to suit you end up with a boat that is essentially only one main hull to build, which to me seems easier than building a large cruising multi cat or tri. Surely building inflatable pontoons is far faster than making grp amas?

CAVALIER

I advocated inflatable bow sections on the racers to better cope with damage such as sodebo sustained. Perhaps spare sections could be taken in case of deflation though attaching a section at sea wouldn't be easy. Still a fix a flat approach seems less race ending than the current sacrificial foam section.

Yes that sounds cool. My guess is that a boat built as above would be lighter than the equiv built grp boat. I would think that for a cruising boat perhaps a complete deflated hull could be taken along and the weight penalty not be too severe. Re: attaching at sea, with a good system possibly involving lashing the hull to the beams or lashing the hull to a hull top plate structure that runs between the beams then inflating the hull I would think it shouldn't be too much of a drama, other than being a very wet job.

CATBUILDER Thank you for all that information, very informative.

Re: lightning, hehe the concept above has it's own rubber "gumboots" to help survive lightning

Thanks for putting it in perspective though it's nice to know the relative occurances of these issues.

TO ALL

I think the idea of having a boat that you know you can peel across the oceans as fast as you like without danger of collision and sinkage would be really cool. I understand water tight bulk heads and how awesome they are to keep a multi afloat.

Just imagine though, your on your cruising cat, your asleep in you bed, everything is on autopilot, almighty crash, thrown against bulkhead, bump your head hard? get out to find you've crashed into a container, it's sliced a nice big gash down a third of one hull and your taking on water. Your not exactly going to sail away from this, alot of your worldly possessions are in that hull getting destroyed. It's not an ideal situation, survivable yes, but no jumping for joy.

Now your on kungfu, asleep you awake to the feeling of being in a slingshot, you get up to find you've crashed into a container, you've rounded up on it, you drop sails, listen to air hiss out of the forward few compartments. Wait around long enough to be sure you still float ok, then go back to bed to patch or replace the hull in the morning.

Some of you may have noticed my dimensions allow the main hull to fit standard trailer regulations, the setup would be easy to demount and trailer because of the low weights of the outer hulls and the low volume of the outer hulls on the trailer when deflated. Some of you may have noticed the much longer outer hulls compared to the main, this is to achieve a much bigger boat for the money, also the forward couple of meters acts as a impact cushion before anything could hit your nice grp main hull. The ugly bumper boat of the 7seas

[CatBuilder]

I know it's just a scenario, but if someone is sleeping below with the auto pilot steering, they do deserve to die for putting others' lives at risk. Breaking that particular law has very serious safety consequences.

[whitepointer23]

a good compromise would be rotomoulded plastic hulls, you can retain a proper hull shape and they are almost indestructible.

[masalai]

Hi Skint,
Have a look at the current edition of 'Multihull World', #108 - May/June 2011, (published by Gordon & Gotch in Australia) and you will find an "infomercial" (page 134) near the back of the magazine in collision survivability of ATL products...

Their website is www.multihull.com.au

[MastMonkey]

Integrated foam flotation would do all of the same things. It is perfectly suited for a lightweight multi. Put enough off it in strategic locations around the boat and even with the bottoms ripped off you could float and sail with most of the boat out of the water. Watertight bulkheads add even more safety.

I like the inflatable idea, but I do not think inflatables are as easy to repair as you believe. Snagged the wrong way and you will end up with a long and impossible to adequately repair gash. And patches over small holes are not as strong or airtight as the original. Air will seep out and you will have to continually blow it back up. Many materials used to make and repair inflatables degrade in UV too.

One other concern I would have would be with the lack of weight. It would be easy to get thrown around because you could never build up much momentum.

[Alex.A]

Check out Harry proa - was/is using foam ends on the hulls and the Te pookie on Schachts proafile site - an inflatable proa of almost equal lengths. Great to deflate and transport - then pump up and sail. That said - inflatable materials wont have a long lifespan.
I also saw a tv program that tried to destroy a plywood+glass boat on a container and it wasn't as easy as you think - they had to ram it really hard directly on the corner!!
Perhaps the bow shape also plays a role? Plumb or curved.....

[cavalier mk2]

Over is better than into....For the North American West Coast, remember the tsunami debris will be headed this way.

[oldsailor7]

I have posted this story before, so I won't dwell on details.

In a plywood Tri we hit a deadhead at speed and punched a hole on the main hull.
The floats took up the floatation and we sailed on to our destination.

As a result of this experience, when I built my Buccaneer 24 we filled the floats with inflatable plastic acid bags just in case the floats got damaged in a similar accident.

Very successful and gave great peace of mind.

[Spiv]

Originally Posted by oldsailor7

inflatable plastic acid bags

Sounds interesting, could you give me a link to this idea please?