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Thread: Unsinkable? How, why?

  1. #1
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    Default Unsinkable? How, why?

    Ok not to sound naive but we all talk about cats unsinkable etc and i am wondering why.
    Our own cat has crash compartments forward so if these were puntured no doubt wed be ok. But a major holing behind these i doubt our pumps or many others could keep up. So major holing our bows may be poking out or she'd turn turtle then maybe float upside down, great for S&R but hopefully wed be in dinghy w ditchbag and epirb flares etc. Then again they may not. Is ita the fact that the hulls would trap enough air that it would overcome the lack of ballast cats have? So maybe its a physics question im struggling with. Yes i have had "beachcats" that would not sink so please be gentle, this isnt the other "site"

  2. #2
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    Default Re: Unsinkable? How, why?

    many cats are designed so that the actual cat structure is buoyant,
    Tjis video explains is an explanation by one builder of what happens when their cat is holed all the way along a hull.

    About 8 minutes in you get the info:-
    BORKED
    Safe Sailing
    Paul
    Blog: www.suliere.com

  3. #3
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    Default Re: Unsinkable? How, why?

    ---------all boats will sink-capsize-burn-explode plus be subject to sabotage........remember the unsinkable Titanic

  4. #4

    Default Re: Unsinkable? How, why?

    not to mention Molly Brown.
    Tropical island life in the Devil's Triangle.
    http://2gringos.blogspot.com/

  5. #5

    Default Re: Unsinkable? How, why?

    Not all cats are unsinkable. But cats built from modern lightweight materials should be.

    As an example, the materials to build my boat - just the bare shell, took 3 double pallets, stacked 1 metre high.

    So 2.4 x 1.2 x 3 = 8.63m3.

    So the materials for just the bare shell would displace more than 8 1/2 tonnes of water. Add in furniture etc, and it would go well beyond that.

    The boat launched at 4.8 tonnes. And we were partly loaded at launch - full water tanks, quite a lot of food, bedding clothing was laready aboard. Fully loaded we're still probably below 6 tonnes.

    So, simply, the boat cannot sink. Even if all of the 50+ sealed compartments were holed, the boat should still float.
    Last edited by 44C; 2nd March 2013 at 09:14 PM.

  6. #6

    Default Re: Unsinkable? How, why?

    Quote Originally Posted by HappyEnding View Post
    Is ita the fact that the hulls would trap enough air that it would overcome the lack of ballast cats have?
    Not sure I undertand this question. Ballast is what would make you sink. It's because we don't carry several tonnes of lead or cast iron, that multihulls can stay afloat when holed. Some monohulls, having lost their keels, have also proven to be unsinkable.

    Modern cat's shoudln't need to rely on trapped air to keep them afloat. There SHOULD be sufficient bouyancy in their construction to keep them afloat. Although that's not always the case.

    There are also some monohulls that are designed to float when seriously holed. They generally employ very thick foam coring in their construction, to give them enough bouyancy to support their ballast. This costs some interior space, but would give additional peace of mind in return.

    Its simply a matter of weight vs volume of the materials used in construction. If the total density of the boat is less than that of water, it can't sink in water.

  7. #7

    Default Re: Unsinkable? How, why?

    Quote Originally Posted by 44C View Post
    Not sure I undertand this question. Ballast is what would make you sink. It's because we don't carry several tonnes of lead or cast iron, that multihulls can stay afloat when holed. Some monohulls, having lost their keels, have also proven to be unsinkable.

    Modern cat's shoudln't need to rely on trapped air to keep them afloat. There SHOULD be sufficient bouyancy in their construction to keep them afloat. Although that's not always the case.

    There are also some monohulls that are designed to float when seriously holed. They generally employ very thick foam coring in their construction, to give them enough bouyancy to support their ballast. This costs some interior space, but would give additional peace of mind in return.

    Its simply a matter of weight vs volume of the materials used in construction. If the total density of the boat is less than that of water, it can't sink in water.
    Maybe monos should look into auto ejecting keels? I mean it does make sense

  8. #8
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    Default Re: Unsinkable? How, why?

    The sad fact is that many large production catamarans can sink. That is because they have engines, generators, air conditioning units etc. The weight of those items can often be nearly as much as the keel on a low ballast ratio monohull. Furthermore many try to utilize every bit of interior space for accommodation, so don't fit watertight compartments.

    Accommodation space air pockets are not included in inverted buoyancy calculations - although having one is obviously a bonus. Remember that such pockets can be emptied when upside down through open seacocks, watertank breathers etc.

    The latest ISO requirements for the RCD only insist on unsinkability for those multihulls likely to capsize. The ISO rules and RCD cannot cover the consequences that may arise from any damage. They only deal with "intact stability and buoyancy"

    So if you want to have a genuinely unsinkable boat then it is certainly something you should clarify with your boatbuilder or designer before purchasing. Don't assume that just because it is a multihull then it must be unsinkable.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com

  9. #9

    Default Re: Unsinkable? How, why?

    SMJ,

    I disagree. Any device for scuttling several tons of weight under heavy stress conditions would have to be a very complicated contraption. Think of the way the space shuttle got rid of its two booster rockets - explosives and probably a veeeery good electronic system to prevent malfunction. Try to locate something like that on a sailboat, and you will have an unreliable system which probably will cause more sunken boats than saved ones.

    Because that's the second problem - a monohull normally does not have enough buoyancy even without the keel to stay afloat, and with the keel missing the only stable position is with the mast pointing straight down. This may give the crew a short respite before the lack of fresh air will have them heading for a very difficult exit. And sooner or later the ship will go down.

    If you follow the cases where monohulls were lost, you may remember the one a couple of years ago where the keel snapped off accidentally and the ship went down so fast that only the two people on watch survived - the other two went to a wet grave. I don't remember the name, but the details remain with me.

    Another thought - when exactly do you scuttle your keel? Many monos roll and make it back upright still floating, still able to survive. Even if you have taken in water or are taking it in, your bilge pumps all rely on a known "down" position to work.

    No, I don't think that's a good idea - despite the fact that I don't have that problem.

    Oliver

  10. #10

    Default Re: Unsinkable? How, why?

    Quote Originally Posted by Oliver L. View Post
    SMJ,

    I disagree. Any device for scuttling several tons of weight under heavy stress conditions would have to be a very complicated contraption. Think of the way the space shuttle got rid of its two booster rockets - explosives and probably a veeeery good electronic system to prevent malfunction. Try to locate something like that on a sailboat, and you will have an unreliable system which probably will cause more sunken boats than saved ones.

    Because that's the second problem - a monohull normally does not have enough buoyancy even without the keel to stay afloat, and with the keel missing the only stable position is with the mast pointing straight down. This may give the crew a short respite before the lack of fresh air will have them heading for a very difficult exit. And sooner or later the ship will go down.

    If you follow the cases where monohulls were lost, you may remember the one a couple of years ago where the keel snapped off accidentally and the ship went down so fast that only the two people on watch survived - the other two went to a wet grave. I don't remember the name, but the details remain with me.

    Another thought - when exactly do you scuttle your keel? Many monos roll and make it back upright still floating, still able to survive. Even if you have taken in water or are taking it in, your bilge pumps all rely on a known "down" position to work.

    No, I don't think that's a good idea - despite the fact that I don't have that problem.

    Oliver
    My statement was meant to be a joke.

  11. #11

    Default Re: Unsinkable? How, why?

    OK, got me!

    Oliver

  12. #12
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    Default Re: Unsinkable? How, why?










    Thought I'd post a photo of a cat my wife and I found floating off Martinique in 2008. It's about as full of water as you could get.

    Cheers.
    Paul.

  13. #13
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    Default Re: Unsinkable? How, why?

    I believe that's the typical position a normal cruising cat will take after taking on water. I have added a lot of polystyrene to the ends of my cat.
    Roger

    ------------------
    I look to the future, because that's where I am going to spend the rest of my life - George Burns

  14. #14
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    Default Re: Unsinkable? How, why?

    svquintana wrote, "Thought I'd post a photo of a cat my wife and I found floating off Martinique in 2008. It's about as full of water as you could get."

    Paul,

    Good pictures; thanks for posting.

    Any idea what happened to cause that, and what ultimately was the fate of that cat?

    Marshall
    "People sail for fun and no one has yet convinced me that it's more fun to go slow than it is to go fast." -- Dick Newick

  15. #15
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    Default Re: Unsinkable? How, why?

    You're welcome, we often chuckle at the memory of this event.

    Insurance Fraud; that's what happened to the boat.

    The marine police met us in harbour, (we called them to find the owner) and sent a diver down to investigate. No damage, but hoses were cut on two thru-hulls, one in each hull, and all hatches were open.

    We "rescued" the cat, and the owner approached us and said we were due a salvage of at least 40,000 euros, and promised us the world. We thought he was a bit off the mark (and his rocker) and settled with his lawyer for 2000 euros.

    The tow was only 4 miles and 2 hours of our time in flat water; the boat was valued at 60,000 - 80,000 euros. Salvage is generally calculated using time/energy, vessel value, risk and results.

    I suppose we could have gotten more, but we had business in St Croix to attend to.

    Cruising isn't always about making a buck, we planned to do it for nothing, but since the guy said his insurance would pay big bucks, we called his lawyer and settled for the first offer.

    The insurance company paid the owner our salvage but we never heard from him. Why they paid him and not us, is something we can't understand.

    His lawyer said he skipped town to France. No wonder why, as the marine police told us this was the third time this cat had been "rescued" in a year and they were somewhat suspicious.

    The boat was sold to a neighbor. He and the boat are well, last we heard.

    A boat's worst enemy is often it's owner.

    Cheers.
    Paul.
    Last edited by svquintana; 4th March 2013 at 02:16 AM.

  16. #16
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    Default Re: Unsinkable? How, why?

    2000 euros for 2 hours of your time in flat water?? How does that feel?

    I once found a powerboat run out of fuel 5 miles off the French coast, and towed him in. This was a little more effort, as I didn't have a working engine at the time, so I had to get a line to him under sail, and tow him under sail only. That worked OK - in fact hardly dented my speed. But I couldn't take him into port - I had a keel deeper than the sand bar allowed. I cast him off in shallow water, close to the port entrance, and suggested he called on the VHF to get a tow in. The fellow was outraged that I didn't take him the whole way. Pretty unpleasant fellow I thought - but I wouldn't have dreamed of claiming salvage.

    That same keel got me stuck on a sand bar outside Exmouth. A powerboat came over and asked if I'd like a tow. I thanked them for their offer. I threw them a line, and they then said it would be 100 for 5 minutes of their time. I told them I'd rather let the boat be wrecked on the sand bar and told them to F-off. I did get off with a bit of a struggle, and into the river as intended.

    I know the rules of salvage are pretty punishing for someone needing help, but really, can't we just help out other boaters in trouble? Isn't that a primary rule of sea-faring? Has it always got to be a matter of seeing how much you can make out of a situation?

  17. #17
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    Default Re: Unsinkable? How, why?

    Quote Originally Posted by pir8ped View Post
    I know the rules of salvage are pretty punishing for someone needing help, but really, can't we just help out other boaters in trouble? Isn't that a primary rule of sea-faring? Has it always got to be a matter of seeing how much you can make out of a situation?
    We have towed dozens of people in, though typicaly we were in our dive boat.
    We once pulled in 3 guys, who were on leave from Afghanistan, their boat wouldnt start, I took them the whopping 3/4 of a mile to the boat ramp. Seatow, wanted $400.00 to do it. all they had was $150, which I was offered but turned down, I mean these guys are active military!!!
    Latest rescue was 2 guys and a dog in their Kayak a Shrimpers wake had capsized, We joked with them about salvage rights on the Kayak, but they could keep the dog lol....

  18. #18
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    Default Re: Unsinkable? How, why?

    I won't get into a pissing contest with you.

    We've towed many boats off bars, reefs, rocks and the like. We've never asked for compensation, nor expected any. The owner probably felt that if we were paid, it would legitimize his claim. Please re-read my post, I suppose it's my fault for not putting it in bold and running it by my lawyer to see if I'm conveying the right message, but here's the part I'd like you to read.

    "Cruising isn't always about making a buck, WE PLANNED TO DO IT FOR NOTHING, but since the guy said his insurance would pay big bucks, we called his lawyer and settled for the first offer."

    I didn't read my post carefully enough to thwart the nitpickers, I didn't mean to write "always". Cruising isn't about making a buck, is how it should have read.

    Get over it!

    Paul.

  19. #19
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    Default Re: Unsinkable? How, why?

    Serves me right for not ending my post at "insurance fraud".
    Last edited by svquintana; 4th March 2013 at 04:18 PM.

  20. #20
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    Default Re: Unsinkable? How, why?

    Quote Originally Posted by svquintana View Post
    Serves me right for not ending my post at "insurance fraud".
    Don't worry. It seems that there is always one!

    Mike
    Nothing works on an old boat, except the skipper.

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