Multihull Collision Survivability

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

  1. redreuben
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    redreuben redreuben

    Buzz,
    I thought about this awhile ago, couple of things, if its rough and blowing a likely scenario given a capsize, chances are the boat will turn turtle while you are organising the safety of all the crew, after all that would be your first priority. In which case you may not have enough volume without flooding the float and then some.

    My other thought involved maybe using an inflatable dinghy or other inflatable bag and winching it down to the top of the mast using a halyard, assuming one can be retrieved.
    Just initial thought, far from fait accompli !
     
  2. redreuben
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    redreuben redreuben

    Oh, a sealed and buoyant mast can't be a bad thing.
     
  3. boradicus
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    boradicus Senior Member

    Great Idea

    That sounds like a great idea. Unfortunately, I am not to experienced with working with copper tubing. I replaced the rusted water canister in my travel trailer with copper tubing, and ended up JB Welding it due to a fitting leak. I have have better success with fixing swamp cooler, however. My only concern is that something could happen to loosen a fitting if the capsize is violent. Would it be easier to run some kind of rubber tubing like what is used on scuba gear instead? I don't know what that stuff is made of, but if it is inside the mast, I would think it would be free from UV degradation, and the only thing you would need to worry with would be heat inside the mast - possibly.

    On the other hand, you could use a tripwire to trigger a CO2 canister for rapid inflation, and rig the whole thing with a removable harness that you could slacken and unhook once the boat is righted. Then when you are at port you could refit and replace it, or if you have spares on board and feel like climbing the mast at sea.. But I've never done that and it sounds scary!!!
     
  4. boradicus
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    boradicus Senior Member

    Buzzman - Actually, the more I think about it, you probably would want something to release the mast cap too, and have it spring loaded for release - sort of like a car trunk release. If that is the case, then maybe running some wires up through the hollow of the mast to mechanically release it would work out. I like your scuba tank idea because it seems you could just run a compressor on it to re-shrink it again, and even if you didn't get the mast cap back on right you could probably depressurize the air bag enough to get it out of the way so you could sail in to a nearby port.. Just some thoughts!
     
    Last edited: Jun 8, 2013
  5. buzzman
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    buzzman Senior Member

    R
    Farriers idea was that it would automatically fitre as the mast passed 80 deg, so by the time the mast hit the water it would be inflated and thus "prevent capsize"....ie: prevent 'full inversion'.....I consider 90 deg and more to be a capsize, or, let's face it, if it doesn't pop back up it's capsized, otherwise it's just a 'klnockdown' as the mono boys call it.

    But that requires electronics, and a mercury switch, and lots of 'it could go wrong' stuff....

    My idea was KISS principle.

    The bag on top of the masthead would have the inflateable bag attached to it, so that as it was vacuum closed, it would close back on itself.

    Alternatively, have it in a tube with a 'pull through' attached so that it could be manually deflated at deck level and pulled back down into its tube, which might or might not be inside the mast....

    Copper tube is simple to use. Order the correct length from the plumbing supplies place (and lengths are usually 6m so would need brazing to get onger mast lengths) and reqest the fittings be attached, which requires flaring the tupbe end after placing the fitting over the pipe.....then you only have to feed it down the mast....

    And it would only need a plumber...and they are a dime a dozen, and relatively cheap for short easy jobs.....

    The other fittings with the armored tube are pre-fitted - you buy it with the fittings pre-fitted. So only a spanner needed. Ditto the air cylinder - which is probably cheaper to have filled with compressed air or even nitrogen than CO2.

    Compressed air dive shops can supply...nitrogen is the gas that puts bubbles in beer....most towns have a supplier of gases. Ask the local plumber who he gets his Oxy and Acetylene from - they'll be able to supply nitrogen and CO2, and probably air as well.

    It has to be cheap and re-usable and refillable cost-effectively, or it won't get used.

    I doubt rubber or plastic tubing would be any good inside the mast, but plumbers these days use a type of semi-hard plastic tube for gas lines, so that would be OK, but requires a special tool to place fittings on it, so you'd have to find a plumber who has that special tool and is licensed for gas fitting.

    The real difficulty should be the bag, but I reckon it could be made from fabric that is simply coated - not necessarily the heavy vinyl, but it would need to be seam-welded - which is not something that's easy to do DIY.

    I once had a guy repair the polyethylene (or whatever) plastic number plate/headlight surround on my trailbike, and he used "plastic welding" which involved heating a stick of plastic with a commercial hot-air gun until it melted and then smoothing it with a tool like putty knife.

    Not a soldering iron! That just burns the plastic, it's too hot!

    But I suspect one of those plastic bag sealers, that's like a hot clamp might be able to me made to work....

    Does anyone have any ideas on the sealing and/or the fabric...??
     
  6. buzzman
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    buzzman Senior Member

    Another thought I had for smaller boats, and this was much more in the DIY range, was to use one of those small bottles that cafes use to hold whipped cream, which are charged with those tiny little cylinders of nitrogen.

    I found oout by googling that you can get those same cylinders in a larger size and in CO2....

    The cream-whip cylinder thingy is readily available form home shops and online, and can be had in different sizes, but I'm not at all sure about the volume of gas you could get into the cylinder and at what pressure....

    Does anyone know more about such things and could rule that idea in or out...?
     
  7. boradicus
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    boradicus Senior Member

    Had not heard of the refillable whip cream cans before, but sounds somewhat feasible. Maybe you could look into how hot air balloonists make their balloons for some cues as to how to seam the fabric? I'm not sure, actually. In diving, aren't there inflatable balloons used to carry stuff to the surface so the diver doesn't have to make a trip back up and use up his/her air? I mean with wreck salvaging, and treasure hunting and that sort of thing. Would those balloons be reusable too? Maybe you could ask some divers?

    I don't know how much it would cost, but if it was cheap enough you could go the disposable inflatable air bag route from the automotive industry. The only thing would be how to trigger the inflation and then collapse the bag after, but I believe they are meant for rapid inflation and then deflate gradually on their own so as not to suffocate. You would need to know the rate of deflation, and compensate for the corresponding displacement pressures underwater to figure what your window of time for righting would be to see if this route could even work properly. Probably it is best and simplest to go with your manual inflation method instead... Hope this helps. :?:
     
  8. cavalier mk2
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    cavalier mk2 Senior Member

    One thing you guys need to plan on is the mast floating downwind of the boat. The leeway then tends to force it under especially as the sails start countering the drift as they submerge. A righting system that can function without the mast is still needed as many boats lose the rig.
     
  9. buzzman
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    buzzman Senior Member

    Yeah, the refillable cream whip cans are used in restaurants and cafes. they screw apart on a tight thread and have a rubber O-ring seal.

    there is a one way valve on the side of the top half onto which the gas canisters are screwed - as they seat home thay automatically discharge, and multiple canisters can be discharged, but I'm not sure how many, before the pressur einside equals the canister itself, but we used to get two or three of the small canisters in there, and that with half a litre of cream inside as well.

    The top of the containter also featrures a trigger-operated nozzle, enabling the contents to escape, hence why I was thinking it could be used to fill a masthead airbag.

    As the largest of these containers is about $20, and the refill cylinders around $5 each, it; looked like it might be a cost effective DIY solution. You'd just have to ensure a good supply of canisters...!

    If the float was large enough it would support the masthead against drift, I imagine, but the need to recover without the mast is a good point....

    Hence my original idea of the spinnaker pole and water-filled para anchor, levered out from the side of the uppermost hull.

    Fill the lowermost hull and sink it below the surface....with mast held above the surface by the airbag....and the leverage of the para anchor full of water....and it might just be enough to re-right even a cat, certainly a tri should come over.

    So, anyone got any ideas about fabric for the airbag?

    I'm not sure of the compression of the CO2 in the little canisters, will have to check that, but according to Mr Google, an aluminum scuba tank holds 80 cubic feet of air when compressed to 3000 psi. The difference in weight is about 6 lbs.

    According to Mr Google, paintball guns use CO2 at 800psi
    http://finewhip.cart.net.au/store/88g-2.html

    But I don't have the math to calculate what volume that would be once expanded, nor what pressure the airbag would need to be, and consequently what compressed volume would be required.

    Does anyone who knows how to use a calculator know how to calculate this figure? Any clues?

    What pressure would CO2 need to be at to keep an airbag inflated at sea level?

    I must admit, physics is not my sport..... ;)
     
  10. buzzman
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    buzzman Senior Member

    Found this slightly cheaper 48L premade bag...
    http://www.intensitysails.com/flbaforop.html

    The basic idea is to have the airbag folded, and with a couple of velcro straps lightly holding it in place, attached to the top of the masthead to a plate that it parallel with the deck, mounted above the normal masthead fittings.

    The airline to it would run down inside the mast to the pressure bottle/cylinder, which could either be manually or battery operated (don't favour the latter, but it would need to be mercury switched if set to deploy automatically upon heeling beyond 80deg).
     
  11. buzzman
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    buzzman Senior Member

    OK, did some surfing...found this...seems this is the math required...only I don't know how to use a calculator.....can anyone assist?

    Compressed Gas and Air - Storage Volumes
    Calculate the storage volume of compressed air or gas

    Sponsored Links

    The storage volume for a compressed gas can be calculated using Boyle's Law:

    pa Va = pc Vc (1)

    where

    pa = atmospheric pressure (14.7 psia, 101.325 kPa)

    Va = volume of the gas at atmospheric pressure (cubic feet, cubic meter)

    pc = compressed pressure (psi, kPa)

    Vc = volume of the gas at compressed pressure (cubic feet, cubic meter)

    Volume of free gas in a Storage Volume

    The amount of free gas at atmospheric pressure in a given volume as a cylinder storage can be calculated my modifying (1) as:

    Va = pc Vc / pa (2)

    Gas can be stored in high-pressure cylinders ranging to 6000 psig (410 bar), normal-pressure cylinders ranging between 2000 and 2500 psig (140 and 175 bar) and low-pressure cylinders ranging up to 480 psig (34 bar).
    Example - Volume of Air in a Cylinder Storage

    The standard atmospheric air in a 250 cubic feet cylinder at 2000 psig (2014.7 psia) can be calculated as:

    Va = (2014.7 psia) (250 cu ft) / (14.7 psia)

    = 34264 (cu ft)
     
  12. redreuben
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    redreuben redreuben

    What about the Co2 cylinders from SodaStreams ?

    You can buy the for $5 from carboot sails.
     
  13. buzzman
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    buzzman Senior Member

    Yeah, they'd probvably work, I'd be guessing they'd be the same pressure, and are only $20 new....

    The criutical thing I don't know is how many of them it will take to provide the necessary 200lbs floatation (for a 27ft tri)...

    Any engineers out there can perform the maths?
     
  14. boradicus
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    boradicus Senior Member

    Okay, new idea here! I thought about your idea of intentionally filling one hull with water. Well, check this out. One reason that I thought this a bit risky for righting at sea, is that you are destabilizing your raft, and that just seems problematic, especially if people are wet, cold and experiencing a level of shock. But you need a raft anyway, right? Of course all of these things need to be accessible - so like a through-hull compartment could work - so put a through hull on both outer hulls so that in case one is wrecked, you have access to the one above the water.

    Now here is my idea. In both through hulls, have inside the compartment, a life raft AND a temporary inflatable hull. It doesn't need to be something that will last for months, so it can be light weight, as long as it is in a protected area where it is not exposed to too much heat - same for the life raft.

    Okay, so here goes - if the mast is a problem for righting, this should solve it. Recover you life raft first, and gain access to emergency chemical heating pouches to recover a bit from the hypothermia, it the sea is cold or if people are in shock. Then pull out the temporary inflatable hull, and canister - I am guessing you can use a diver tank - dunno about this part - but depends on the size of the yacht probably. Inflate slightly to expand it enough to lash it to the hull you are about to water-fill. Lash the air canister to the hull you are about to sink. Clip two lines to form a harness on either side of the hull you are about to sink from the underside and run them across and under the other hull. Open the valves in the hull, and rig a line to trigger your air canister in addition to the harness (do this at the same time you set your harness). when the hull starts to sink, come about to the other side and grab hold of your harness. When the hulls lie vertically in the water (hopefully), trigger the canister to inflate the temp hull. Maybe you could even have a light weight alloy brace to offset the temp hull at a 90 degree angle and to the inside so that it acts like a lever arm to lift the boat to the proper side.

    When the boat is righted, deflate the temporary hull so the boat will lie properly again.

    If you have hull damage, and your 90 degree brace arms have an adjustment on them to pull the inflatable hull in parallel to the original, then adjust the braces to bring the inflatable hull and the original hull to waterlevel, and you have your temporary hull alongside your damaged one to function as a float to keep above water. You might be able to sail to port in such a case if the damage is not too extensive. If it is extensive, then maybe having access to at least one of the righted hulls will increase the chances of survivability until such a time that rescuers arrive (yes, epirbs!!!).

    Do as anyone think this would stand a chance of actually working?
     

  15. buzzman
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    buzzman Senior Member

    No reason why not.

    There are a number of manufacturers of buoyancy products - mainly for salvage operators - that could be so used.

    And could be inflated via a manual double-acting foot/hand pump like those used to inflate rubber boats - cheaper and less to go wrong.

    Frankly, if you have a rubber dinghy (Metzeler, Zodiac, Avon etc) and deflate it for ocean passage, it could easily be strapped underneat the wings or bridgedeck, or strapped to the side of the filled hull, and then inflated to add buoancy and assist with the recovery.

    As an example, these heavy-duty float bags would work:
    http://www.doowin-dooflex.com/Products&Solutions/Inflatable-Buoyancy-Bags.html
     
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