Force water out hole with air pressure

Discussion in 'Boat Design' started by Shearwater, Jan 29, 2007.

  1. Shearwater
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    Shearwater Junior Member

    A compartment on a boat sealed so that it's airtight could be pressurized to prevent ingress of water and even force out water that covers a hole in the hull. The air pressure would also assist in repair by forcing the sealant and patch into the hole. Very little air pressure could save a small craft. One pound of air pressure can displace 2.25 feet of water. For example an Ericson 35 has an 11.25 foot beam. A horizontal cross section a few feet above the keel, I estimate would be about 2/3 x 35 x 11.25 or 262 square feet. 262 x 64 = 16800 lbs of buoyancy for a one foot cross section. The boat's displacement is 13000 lbs so only 0.77 feet need to be displaced. This would require only 0.34 lbs of air pressure to barely float the boat. And this doesn’t take into account the boat's natural floatation, which would make the actual air pressure required even less. You would need to use more air pressure to keep the water out in deeper parts of the boat.

    How much internal air pressure can a boat safely withstand?
     
  2. Poida
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    Poida Senior Member

    Depends on hull construction.

    The problem to me is cost, where is the compressed air coming from?

    Wouldn't filling the hulls with foam be cheaper over the long run?

    Or simpler to insure your boat.

    Blowing in a sealant like in tyre repairs, most sealants don't stick to water.

    And pressure may not be the problem, it may be volume. The bigger the hole in your hull the larger the air volume required to keep it out.

    However no idea is a silly idea, it's just I'm not convinced of the practicality.

    Poida
     
  3. Robert Gainer
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    Robert Gainer Designer/Builder

    Standard practice in the salvage business is to use low pressure air to force out water if the construction of the ship and the location of the damage permits it. But be careful because it's pressure times surface area of the deck so its possible to blow the deck off a ship this way if you use too much pressure. In fact I saw a barge have the deck removed this way some years ago in Upper Nyack, New York when an employee of the barge owner decided to do the company a favor and rise a sunken barge instead of having a salvage company come in and do the job. It was something to see when the deck went up and the barge stayed behind.
    All the best,
    Robert Gainer

    PS,
    Poida,
    Volume is not a problem with a rotary screw compressor. Some of our compressors weighed over a ton and we put them on board using cranes mounted on the salvage tugs.
     
  4. jehardiman
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    jehardiman Senior Member

    Another thing to remember is that the transient dynamics of the volumeritic mass flows can easily achieve differiential pressures of 10-100 times the normal DPs. I.e. the instant you crack open a 3/4 " 100 psi air line to blow water out of a totally flooded compartment with a 3/4" drain line, you pressurize the whole compartment to 100 psi.... Which generally has determential effects on compartment integrity.
     
  5. Shearwater
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    Shearwater Junior Member

    The river rafters have low pressure, high volume air pumps with pressure gauges, hand and battery operated, that would be perfect for the task:

    http://nrsweb.resultspage.com/boating/Pressure Gauge

    Raising a sunken boat with air has special problems. As the boat comes up the pressure differential between the inside and outside increases dramaically as the water pressure decreases. And just like a scuba diver holding his breath as he ascends, the boat can burst. But while still on the surface the pressure can be more easily controlled and kept low. Under 1 pound of pressure would suffice to save a small craft. My guess is that any boat worth it's salt could easily take 5 pounds of air pressure. For perspective: a toy balloon blown up by mouth takes 2 lbs of pressure, a can of tennis balls is at 12 lbs of pressure, and PVC pipe can take 125 lbs of pressure.

    The more ways a boat can be kept afoat the safer it is. A boat designed and built to be fully air tight and able to take ample air pressure is inexpensive life insurance. Most boats could be made air tight with a few modifications, and some caulking and weather stripping. Getting into the life raft (often the death raft imo) is a terrible option.
     
  6. Robert Gainer
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    Robert Gainer Designer/Builder

    Jehardiman,
    If all you have to deal with is a 3/4 inch opening you would pump instead of bleeding air into the compartment. A pump is a much quicker way to de-water a compartment. Air is used with large breaches in the bottom or lower parts of the sides in ships just to make a bubble large enough to buy some time and keep the ship up while you make other arrangements. You can’t raise a ship that is entirely underwater this way and air bags or barges with slings would be used instead.
    All the best,
    Robert Gainer
     
  7. jehardiman
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    jehardiman Senior Member

    Yeah, I know... just pointing out one of the commoly overlooked reasons you don't use air to dewater. Realistically, using air to dewater has all sorts of problems. From a damage control; "keep it from sinking"; point of view, emergency float bags make much more sense for a yacht.

    http://www.turtlepac.com/yachtdetails.htm

    Now for a "someone shot a bunch of holes in me" patrol craft, a fully integrated closed cell foam cored structure makes much more sense.

    http://www.safeboats.com/default/collar.html

    (Note: I have no commerical association with either company and recommend doing design research before selection of any product)
     
  8. Poida
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    Poida Senior Member

    Robert
    In my opinion you seemed to be changing the subject.
    My interpretation of the original post was to have a hull pressurised while sailing to prevent the ingress of water due to some sort of collision.

    Salvaging a vessel with a bloody great barge with a screw compressor on board is a totally different subject unless you are suggesting that when you go sailing you tow a barge with a compressor behind you.

    However you are agreeing with me in saying that volume is required.

    Shearwater only mentioned pressure in his post and yes, at a draft of say 1metre only a pressure of 30 kilopascals would be required to equal the pressure of the incomming water. However he didn't mention volume and I was merely pointing out that volume has to be taken into consideration. You can have a little tiny compressor using bugger all power to produce a high pressure but you would need a lot of power to produce a large volume at atmospheric pressure.

    The main problem I can see, is:
    1. That you can not determine the size of hole are likely to get in your hull as the result of a collission.
    2. Shearwater asked if the hull would take the pressure, as I said it would depend on the structure of the hull.
    3. After the collission and the hull has been holed, it's structure would then be weakened, any air pressure in ther hull could blow the hole open wider and increase the damage.

    Take a bag full of plastic shopping bags with you. If you get a hole, jump over the side with a handfull of them and jamb them in the hole. I know if I get a plastic bag stuck in the inlet of my pool pump it stops the water going in.

    Kindest regards
    Poida
     
  9. Johtaja
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    Johtaja Junior Member

    Won’t the pressure inside the hull be released into the atmosphere as the hull I breached?
     
  10. Shearwater
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    Shearwater Junior Member

    Poida

    I meant only to pressurize the boat after a hull breach. The volume of air needed in the above example of the Ericson 35 is quite modest. To pressurize the whole boat to 1 lb of air pressure requires 122 cubic feet of air to be pumped in. If you only pressurized one water/air tight compartment it would be much less. The size of the hole makes no difference in the amount of air needed to stop and reverse the inflow of water. The very low pressure (1 pound) involved would not cause any further damage to the boat in my opinion, although at some higher pressure it would. Using pressure would be very inexpensive, requiring only minor modifications on many boats, some caulking, and an air pump.

    A very accurate air gauge for air pressures below 2 pounds per square inch can be made with some tubing and a jar for when you're outside the boat. Fill a 1 quart jar 1/3 full of water. Run a 1/4" tube from the boat's interior thru the lid into the airspace of the jar. Run a second 1/4" tube from the bottom of jar, thru the lid, and extend vertically for 5 feet. Make all jar connections airtight. The level of water in the veritcal tube will give the air pressure in feet of water (2.25 feet = 1 pound).

    If you're inside the boat when it's pressurized, just stick a tube connected to the outside into any open water. The level the water rises to gives the pressure.

    The plastic shopping bag idea for plugging a hole sounds excellent.

    jehardiman I also like having float bags as a last resort. The float bags, though, clog the interior space and will not stop the inflow of water until the boat is quite full of water.

    1 kilopascal x 0.33456 = feet of water .... 9.8 kilopascals = 3.28 feet of water or 1 meter

    http://www.scottecatalog.com/ScottT...39e6b458093de0f58525683900593153?OpenDocument
     
  11. ted655
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    ted655 Senior Member

    Anyone who has or is getting their water supply from a well knows that sooner or later, the pressure tank will waterlog.
    A pressure tank on a well is exactly what is being discussed here, ONLY with a more effeciant cylindracal shape, without the agravation of sloshing action.
    I've installed em in every concievable position and configuration. Even the "bladder" types will lose the air charge and fill up.
    I'll stay with foam, bilge pumps and the "whale". My luck would see to it the compressor broke the same hour the air charge was lost in the storm that suddenly came up. Oh dear!:eek:
     
  12. Poida
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    Poida Senior Member

    Sorry Shearwater I cannot agree with you. Sometimes I find to explain what I mean, requires extreme exageration, so here goes.

    You have a calm river flowing along, and the pressure of the water is about 5psi. Stand in the middle of the river with a 1" air hose in your hand being pressurised by a 100psi compressor.

    Now face the end of the hose upstream turn on the compressor and try and stop the flow of water with the air hose.

    This is the difference between pressure and volume.

    If you get a hole in your hull, the amount of volume you need will depend on the size of the hole.

    As Johtaja said the air will most probably vent to atmosphere anyway especially since the hole will probably caused by a floating object and occur on the waterline.

    Bye

    Poida
     
  13. timgoz
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    timgoz Senior Member

    I can't see how the proposed system to prevent a holed boat from sinking would be either practical or desireable in any way in the present.

    It's always good to keep the brain greased though. Discussions such as these keep the mind working, and possibly lead to future inovation.

    Take care.

    TGoz
     

  14. Shearwater
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    Shearwater Junior Member

    Poida You're mostly right about a hole on the waterline. But even at the waterline there is enough volume above it to float the boat. Water would rise in the boat until the hole is covered and at that point air pressure could prevent further inflow, keeping the boat on the surface but half flooded. Plus at any point the air pressure would help in patching the hole by forcing material into it.

    The boats Moviestar and Hugo Boss in recent around the world races with leakage right at the keel were very savable. Pressurize the boat. Dive down and attach lines to the keel bulb and lash the broken canting keel to the deck.

    I don't know of a commercial sealant that sticks well to underwater surfaces but I had experience with a natural one, crude oil. Sailing my FJ off Santa Barbara a few weeks after the infamous oil spill we had a good day of sailing with no oil in sight. Heading back we make a shortcut downwind across a large kelp field. And there was the oil. Unable to put the center board down because of the kelp we ended up mired in crude. It stuck very well to the very wet bottom of the boat. The very thick aged crude would work very well in patching underwater holes.

    Here're some possibilities from the web: JB Weld - Water Weld, Weld-on epoxy putty stick, Somay Rubberkote 1047, Somay #874 Leak Roof Sealer, Stockholm Tar, Sikaflex - Sika Grout 212HP, Marine Tex FlexSet, and polysulfide Life Caulk by BoatLife.
     
    Last edited: Feb 5, 2007
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