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Old 18-11-2013, 20:34   #1
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Unsinkable

True or false...

Some cats are unsinkable.

What would make this so? I could see how construction techniques could accomplish this. Has anyone ever built in or possibly stored say empty one gallon water jugs (whatever) and used these for flotation? What have you heard about this subject and what is your general opinion?

Space is not a problem for me. I prefer stripped down to only necessities and I have plenty room unless I am only going for a short day cruise with guests. Survivability is paramount.
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Old 18-11-2013, 21:33   #2
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Re: Unsinkable

Can be true. Very seriously worth thinking about if you want to go offshore. Positive buoyancy is hard to come by in a monohull which derives a goodly portion of it's stability from having a big lead weight hanging off the bottom of the boat.

With a multihull made from a lighter than water material such as wood/epoxy composite it's basically a given. Maybe not so much with a solid fiberglass cattlemaran with tons of junk onboard, but still doable. It's nice that multihull construction tends to feature a lot of compartmented spaces so a glass or aluminum multi still has a chance.

Keep in mind that a mutlihull full of holes and floating half submerged and upside down is none the less still floating, and as such is still in the game as far as being a survival platform.

Now consider your options in a rubber life-or-death raft.
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Old 18-11-2013, 21:59   #3
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Re: Unsinkable

Nothing is unsinkable. But some boats can be much harder to sink than others. As stated above, many cats will float upside down, but enough damage could get them to sink too.
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Old 18-11-2013, 22:19   #4
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Re: Unsinkable

The people of Atlantis thought their Island was unsinkable.

I once had a Ranger 20 often called the Unsinkable Molly Brown. I filled it with water one day sailing with too much sail in too much wind. It did not sink.














However it did float upside down with the bow about 6' under water....
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Old 18-11-2013, 22:51   #5
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Re: Unsinkable

Rip a hull open when it makes a 6"x24" gash from a shipping container. Then have that hull sink into the water. Then have the rigging stress of the the submerged hull and the rig hammering away at the other hull.

If you hit a shipping container, plow into a cliff (it happens), drag and get holed on the rocks, or get riddled by a 3" deck gun I don't really think it matters what kind of hull you have (or how many).

As a guy that was in the US Navy, if they could find a way to make weapons platforms unsinkable they would have implemented it a long time ago.

If you want something tough buy a steel boat with collision bulkheads and an inner and outer hull. With that weight it will help to have a reactor onboard.
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Old 18-11-2013, 22:59   #6
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Re: Unsinkable

I just read an article about Jim Antrim trimaran that pitchpoled and went upside down.
Upside down, it was still very much above the waterline, and they stayed on it until rescued. It floated upside down for 13 months all the way across the pacific.

Aotea Pitchpoles in the 1995 Double Handed Farallones Race by Jim Antrim

It is a simply question of weight and bouyancy whether something will sink or not.
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Old 18-11-2013, 23:16   #7
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Rebel, That makes sense. I guess. Read moxie by Phil weld. Or survive the savage sea. Those adventures give a different perspective about hull make up and sinking. Of course with your navy experience you are probably right or righter or less confused then the other point of view you were thinking about.
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Old 18-11-2013, 23:33   #8
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Re: Unsinkable

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Originally Posted by sabray View Post
Rebel, That makes sense. I guess. Read moxie by Phil weld. Or survive the savage sea. Those adventures give a different perspective about hull make up and sinking. Of course with your navy experience you are probably right or righter or less confused then the other point of view you were thinking about.
I don't think it's possible to argue that two hulls aren't inherently more "float-able" than one, but it's just too much of a stretch for me to take that to being "unsinkable". There is a weird conception amongst some catamaran owners that if it "sinks" they'll just climb on top of their turtled craft, eat a sandwich, and wait for a chopper.
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Old 18-11-2013, 23:45   #9
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Re: Unsinkable

Usinkable?
Yes, I think there are catamarans that are unsinkable when filled with water by floating objects or holes.
Depends on the design and how many water tight compartments are installed and the total damage vs flotation.
Indestructable?
As Eric says, none can survive some of the things he mentions eg 3" deck gun, long time being pounded on rocks etc though it really has little to do with flotation in deep water.
re. Contact with a shipping container, log or debris, It's reasonable to think one would have more chance of staying afloat with two [or three] hulls with inherent flotation than any number of hulls with none. The extent of the damage and built in floatation being the major variables here .
Modern multihull design lends itself to numerous areas where positive flotation chambers can be utilised, the degree to which it is or isn't utilised is subject to someone's choice. ie builders or owner add ons etc.
In answer to your other question, I believe there are quite a number of cruisers using sealed plastic bottles of one kind or another stowed away in these areas.
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Old 19-11-2013, 00:09   #10
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Re: Unsinkable

Quote:
Originally Posted by rebel heart View Post
I don't think it's possible to argue that two hulls aren't inherently more "float-able" than one, but it's just too much of a stretch for me to take that to being "unsinkable". There is a weird conception amongst some catamaran owners that if it "sinks" they'll just climb on top of their turtled craft, eat a sandwich, and wait for a chopper.

Eric,
I just read this as I posted above.
I couldn't agree more, floatation potential can be so varied up or down, even by an owner, it becomes too subjective for such sweeping and emotive statements.
Cheers,
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Old 19-11-2013, 00:12   #11
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Re: Unsinkable

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Originally Posted by maytrix View Post
Nothing is unsinkable. But some boats can be much harder to sink than others. As stated above, many cats will float upside down, but enough damage could get them to sink too.
It may be possible to destroy one, but a lot of multihulls actually are unsinkable. They are built with foam cores, and the hulls themselves actually weigh less than the water they displaceme. Much like a block of polystrene foam the only way to sink them would be to chain a block of lead to them. It has little or nothing to do with water tight bulkheads, or compartments.

There is also the reality that because floatations is dispersed among three hulls even if one were critically damaged there is still a significant chance of having one that is serviceable. On a tri this could possibly end with the boat still being able to limp home, even if slowly.
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Old 19-11-2013, 00:43   #12
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Re: Unsinkable

I plan to put 1-gal jugs roped together in my bow compartment, which is sealed from the rest of the hull, but has a deck hatch. So water tight, but if damaged, it would fill with water, so the jugs will keep it bouyant even if water gets in. Must bunch them, or they will float out of any hole big enough. I've been saving up the empty 1-gal chlorine jugs from my swimming pool maintenance. These are pretty solid containers with rubber sealed lids.

I mentioned in another thread, was thinking of actually filling them with water to make the boat heavier and sit lower in the water in a storm, so it doesn't blow away so easily, as it will be a relatively lightweight boat. Or not a bad idea to use them to hold fresh water that become empty as you use them. A big tank itself could be holed.
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Old 19-11-2013, 01:05   #13
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Re: Unsinkable

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Originally Posted by maytrix View Post
Nothing is unsinkable. But some boats can be much harder to sink than others. As stated above, many cats will float upside down, but enough damage could get them to sink too.
Nothing? There are ROCKS that float!

There's no excuse for building a cat that CAN sink. They should be built from lightweight materials. Hulls and decks should be cored. There should be many sealed compartments, but they really shouldn't be necessary to stay afloat.

The materials to build my basic hull came on 3 double pallets stacked 1 metre high. That's 3 x 1.2 x 2.4m. which equals 8.64 m3. The boat weighed 4.8 tonnes at launch. And there was far more material than the basic hull deck kit contained.

So the fact is, this boat CAN'T sink.

There are plenty of boats that can't sink. The range of Boston Whaler powerboats for instance. Almost any sailing dinghy. Most cored construction monohulls wouldn't sink if they didn't have the keel dragging them down. And there are some (The ETAP range for instance) that use extra thick cores so they have enough positive buoyancy that they can support their keels even when flooded.

I don't understand why so many people find this concept so difficult to understand. If the average density of the material the boat is built from is less than that of water, the boat simply cannot sink.
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Old 19-11-2013, 01:11   #14
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Re: Unsinkable

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Originally Posted by nimblemotors View Post
I plan to put 1-gal jugs roped together in my bow compartment, which is sealed from the rest of the hull, but has a deck hatch. So water tight, but if damaged, it would fill with water, so the jugs will keep it bouyant even if water gets in. Must bunch them, or they will float out of any hole big enough. I've been saving up the empty 1-gal chlorine jugs from my swimming pool maintenance. These are pretty solid containers with rubber sealed lids.

I mentioned in another thread, was thinking of actually filling them with water to make the boat heavier and sit lower in the water in a storm, so it doesn't blow away so easily, as it will be a relatively lightweight boat. Or not a bad idea to use them to hold fresh water that become empty as you use them. A big tank itself could be holed.
Figure each jug is good for about 8lbs of flotation. But they probably don't stack very well. You may want to consider something that can achieve better density, say ping pong balls in a net bag, or packing peanuts.
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Old 19-11-2013, 01:59   #15
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Re: Unsinkable

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Originally Posted by 44'cruisingcat View Post
Nothing? There are ROCKS that float!

There's no excuse for building a cat that CAN sink. They should be built from lightweight materials. Hulls and decks should be cored. There should be many sealed compartments, but they really shouldn't be necessary to stay afloat.


The materials to build my basic hull came on 3 double pallets stacked 1 metre high. That's 3 x 1.2 x 2.4m. which equals 8.64 m3. The boat weighed 4.8 tonnes at launch. And there was far more material than the basic hull deck kit contained.


So the fact is, this boat CAN'T sink.

There are plenty of boats that can't sink. The range of Boston Whaler powerboats for instance. Almost any sailing dinghy. Most cored construction monohulls wouldn't sink if they didn't have the keel dragging them down. And there are some (The ETAP range for instance) that use extra thick cores so they have enough positive buoyancy that they can support their keels even when flooded.

I don't understand why so many people find this concept so difficult to understand. If the average density of the material the boat is built from is less than that of water, the boat simply cannot sink.
44C has explained it very well.

1 cubic meter displaces 1000kg or tonne of water. So 1 cubic meter of foam or balsa core will float 1 tonne minus the weight of the cubic meter of materials. A designer/builder should know the buoyancy of the materials used in a build and any of the cored lightweight cats should have no issues.

Additionally watertight compartments ( 1 cubic meter = 1000kg buoyancy) will assist.

Very heavy cats may not float as high as the lightweight cored vessels but should not sink as long as the weight is not heavier than the sum of the positive buoyancy of the core materials used and watertight compartments.

Sums should be done by the builder of actual weight including cruising load A - the positive buoyancy of the materials B = sink or float - simple

I would not be puting the life of my family at sea relying on plastic water jugs for buoyancy.
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