condensation in sealed "water tight" hulls

Discussion in 'Boat Design' started by jedkins, Dec 29, 2006.

  1. PAR
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    PAR Yacht Designer/Builder

    Poida, I'm quite familiar with Archimedes, but was attempting to offer some floatation to an over sight.
     
  2. epoxyman
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    epoxyman Junior Member

    Concepts: H2O Diffusion through "solid" layups, (vs osmosis) and how to minimize or respond to this process....

    Although it might seem surprising, water can actually migrate through composite layups. The possibility of this diminishes as the number of layers increases, and vacuum bagging helps to reduce the tendency. This problem is much more noticable in situations where efforts are made to reduce weight. Lower weight structures have fewer layers of cloth, and the use of stronger materials than typical E-glass can also allow reducing the number of layers. As well lower weight layups try to use the minimum amount of resin. Even before the layup becomes "dry" or resin starved, it becomes more porous to water diffusion on a molecular level, however.

    It is well known that water can penetrate layups osmotically and produce blistering on hulls. What I am talking about here, however, is greater than osmotic penetration; it is the actual migration of water molecules through the matrix, which is a type of diffusion.

    I happen to know something about this since for quite a number of years I used to make high end custom carbon fiber composite white water kayak paddles. Optimum weight, strength, and hydrodynamic shape called for a hollow core, but in the early years I had quite a number returned for water in the core. Extensive experimentation and consultation with others in the field lead to the conclusion of water "penetration", or diffusion through the layup.

    I attacked this problem from many angles. In order to eliminate or reduce the chance of micro airbubles allowing a passage for water, resin and very slow hardener were mixed using a slowly rotating container, rotating a couple rpm until the resin was absolutely clear. Moulding pressures were gradually increased using (using compression systems such that the critical areas were subject to 20-30 psi - approx a ton of pressure on the paddle mold), comparing to water entry. Increased pressure helped but did not eliminate the problem.

    In this regards it is worth remembering that passage though a solid membrane (layup) depends on the partial pressures of the material on either side of the layup. For those without the chemistry background this is not osmosis (which when water is drawn though a layup in order to equalize the water / dissolved substance concentration on either side of a membrane. One may also note that osmosis may actually generate a physical pressure on one side of a layup as a part of this process,which can cause layups to separate. The "diffusion" process where by molecules of water pass through a seemingly solid layer cannot cause a pressure buildup on either side of the membrane. Water molecules will slowly pass through the layup (if it is permeable as light layups are) as long as the water molecule concentration is lower on the inside than the outside. Increased water pressure (spray, water movement), will also increase diffusion rate (although not the rate of an osmotic process.)

    Basically, with whitewater paddles only subjected to immersion intermittently), the "break point" (of noticable water penetration) was at three layers of well saturated 5 oz carbon fiber; at this point water would eventually migrate but to a minimal extent. Below it would accumulate and above it would extrude faster than intrude as long as the paddles were stored dry, and so people would not typically notice a problem. However, to essentially eliminate returns, a filled core had to be resorted to. As I will note below this does not truly eliminate the problem, but reduces it to a non significant level for paddles which a usually stored dry (allowing reverse diffusion as long as water quantities are microscopic.)

    Regarding filling with foam one might certainly wonder... Would this not just mask the problem, allowing a heavy water soaked core which weighs as much a solid water but without sloshing around?

    If a non porous, non absorbing substance is infused into a hollow hull, then water will still penetrate a light layup. However, it will only fill the small voids found here and there, although eventual diffusion into the foam as well may occur. Thus filling the inside is worthwhile and using a pour in non porous foam that adheres to he walls will substantially reduce the problem.

    Barrier paint may help as well. The multilayer antiosmotic penetration paints are better at fully preventing water molecule pentration.

    But if you are talking about a fully sealed hull with no way to enter an drain water you are needing 100% and permanent prevention of the entry of water molecules and given what I am talking about this is going to be unlikely unless you are using a very heavy layup or materials not normally used in lightweight boatbuilding.


    Two other issues:

    Regarding the condensation idea: It's my opinion that this is not the source of the water in the situation you are describing (although it certainly does occur.) This is because when the unit is first made there is no open entry for water or air. Thus what is in there might amount to an ounce or two of solid water when the temperature drops 20 or 30 degrees but this water will then re-evaporate when the temperature returns to what it was when the unit was made. Without some other way of molecular water entry it would be a closed system containing no more water then was in the air when the unit was made.

    In most full sized boatbuilding: The number of layers is such that diffusion through the full thickness of the layup is essentially zero. With the really lightweight layups there is a foam layer sandwich which reduces (but does not entirely eliminate) diffusion, but usually these boats are dry sailed and / or have opening into the hulls (which people methodically open to let water dry out.)

    A final note is that you only need to have relatively thinner areas here and there for a surprising amount of water to diffuse through. Thus the overall layup might be fairly water resistant but in join areas, seams etc., if there is a more permeable area that is wet all or much of the time, then water will get in.

    Thus my sense is that really the best option is to have an inspection port that actually does seal adequately. There are a number of ways to do this. Even if you can't get 100% seal it's beside the point (unless you are going transatlantic!), as periodically you suck out any acculumated water. By the way evne if you only have a very small diameter port you can drain water out of this with the smallest size wet/vac with a 3/4 inch hose attached to the intake. It will suck up water from several feet quite nicely and leave nary a drop.

    Sorry if this is long, hope it helps!:)
     
  3. Raggi_Thor
    Joined: Jan 2004
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    Raggi_Thor Nav.arch/Designer/Builder

    I think that was very informative, and well explained.
    Thanks :)
     
    Last edited: Jan 5, 2007
  4. Man Overboard
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    Man Overboard Tom Fugate

    I second that on Epoxymans post; well stated and speaking from a background of experience, this is good information to know.
     
  5. LP
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    LP Flying Boatman

    I don't know if this would be applicable, but I'll put out there anyways.

    Certain steel tube framed aircraft are pressurized with nitrogen gas (inert). The frame has an attached pressure gauge. If the gauge looses pressure, then there has been a breach(damage) in the system and inspection is required.

    Granted, water in amas is a far cry from a pressurized steel aircraft frame, but a fully sealed, watertight (airtight?) ama will more than likely pressurize on it's own if heated above the temperature at which it was sealed. Also, the reverse would be true of a colder temperature. A reduced pressure is also more condusive to consensation.

    By maintaining a positive pressure inside the ama, would it not limit moisture migration into the amas. The question then becomes, what is the minimum pressure value and at what temperature? Using that as a baseline, what type of pressures would be expected at maximum expected temperatures.

    I've probably errored in these figures, but here goes.

    At -30 F and 5 PSI as a baseline, I get 7 PSI at 140 F.

    I find it hard to believe there is only a 2 PSI increase over theat temperature so maybe someone would jump in with the proper use of PV=nrT.

    I used P1*T2/T1=P2 since everything else would be constant.

    Provided the pressures don't become excessive, a positive pressure ama system could be maintained simply by incorporating a valve stem and monitoring with a sensitive pressure gauge.
     
  6. epoxyman
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    epoxyman Junior Member

    No, actually. It seems like it should but actually it won’t. It sounds like a good idea, though. The reason it won’t work, however, will take some explaining. Bear with me...

    The reason is that the driving force for diffusion across a thin wall with some permeability is the “partial pressure(s)” of the substance(s) on either side. And, in a mixed solution (air and water), the partial pressures of the substances (air and water) operate (relatively) independently, within the ranges you’re suggesting. It doesn't seem like it should be so but stick with me.

    The partial pressure can be thought of (very roughly) as a measure of the amount of a substance in a given volume. Air partial pressure can be thought of as a measure of how many air molecules there are in a given volume. Water, of course, can be dissolved in air can be at a higher partial pressure (of water) the air will feel more humid, and at a partial pressure of water the air will feel drier.

    If there’s more of it (anything) per cubic foot on one side than the other, either as liquid or gas, and the substance (water) can actually cross the wall to some extent, then it will do so (diffusion.) As I've noted previously this is not the same as Osmosis.

    The counter intuitive thing is that the partial pressures of two entities (water and air) don’t really interact that much. That is to say, as you put more water into the air, it does not increase the air pressure in terms of the number of air molecules per cubic foot. Conversely, if you force more air molecules in (or pressurize the hull) it won’t stop water molecules from entering, either. (With Osmosis the relative concentrations of the solvent and solute are make all the difference in the world, but that is a different issue.)

    By the way, air can also dissolve into water and as the amount of dissolved air goes up it does not increase the water volume either. Interestingly, hot air can dissolve more water molecules into a given volume (before the molecules coalesce and fall out a bilge water), and yet cold water can dissolve more air molecules in (which is why fish die when the water is too hot -- they asphyxiate.)

    But back to the issue at hand, the most important thing to know is that what seems like such a solid strong thing as water or air pressure, is really 99.99999% empty space. If you took a cubic centimeter of air and expanded it and everything a zillion (that’s right, a zillion) times, so that it was a big as an aircraft hanger, you would see air molecules about the size of little ping pong balls. There’d only be a few of them in the whole place and they’d be zooming around here and there (REAL fast). The cumulative effect of them banging against the side of the hanger as they zoom around is what makes air pressure, by the way.

    Now, air molecules will only want to have a certain number of them per unit of space; as the temperature goes up they zoom faster and want more elbow room and as it chills down they slow down and relax. (Pressure in a gas changes with temperature.) So there’s a whole lot of empty real estate around and water molecules that sneak in through the wall (diffusion) can go freely into all this free space. What’s interesting is that at normal ranges, the air and water molecules don’t seem to mind or interfere with each other a whole lot. It’s like the air molecules only seem to care how many air molecules there are (before they push on the sides of the hanger wanting more room), and the water molecules only seem to care how many water molecules there are.

    So what happens is that regardless of how many air molecules there are (unless you compress the space to the point of liquifying the air), the water molecules just keep entering and sharing all that space with the air molecules. Eventually, when enough water molecules have come in and they see too many other WATER molecules around (and especially if the temperpature gets colder) then some of the water molecules get tired so to speak and fall to the floor and run together (condensation, making bilge water.)

    Thus, you can add more air molecules (within reason) without taking up the space needed for water molecules because it’s all mostly empty space anyway and that’s the way the air and water molecules act. That’s why pumping air or argon, for that matter, into an Ama won’t stop water molecules from diffusing through a thin laminate into what they (the water molecules) see as empty space and virgin territory, far less crowded (in terms of water molecules) than out in the seawater. And that is (a simple version) of the theory.

    This can be shown more concretely (as they do in introductory chemistry lab) or you can do it at home. Perk up a fresh pot of coffee and make sure it’s real hot, (or better yet put a small pot of water on the stove and bring it to not quite boiling and hold it there.) Mark or observe the water level. Now start to spoon in sugar, one spoonful at a time, making sure that each spoonfull is fully dissolved. Keeping the water hot, spoon in sugar until it is won’t dissolve any more. You will be AMAZED how much sugar that liquid will sop up, WITHOUT raising the water level at all! The volume of the water intially, plus the volume of the sugar initially, will be MUCH greater than the volume of the hot sugary water. This makes the whole thing easier to grasp.

    So unless you fill the core with something solid that really does physically prevent water entry (like high density hydrophobic expanding foam) or make the walls impermeable (heavy) you will most likely still get water coming in. Gas pressure won’t do it.

    Now who said the stuff you learned in school never gets used?
     
  7. Man Overboard
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    Man Overboard Tom Fugate

    Lp,s solution is basically the same as mine, and I think the most likely to minimize the problem, as well as barrier paint, or anti osmotic paint as Epoxyman has suggested.
    I base this in part on Epoxymans statement;

    "Increased water pressure (spray, water movement), will also increase diffusion rate"

    I am not as well versed in chemistry as physics, this is more of a cross between the two, but since the process is dependent on fluid passing through micro voids in the layup, it stands to reason that it would not be possible if there was a gas slowly passing through in the opposite direction.

    A small inspection port makes since for a number of reasons, no reason not to have one.
     
  8. Man Overboard
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    Man Overboard Tom Fugate

    Ha, Ha... I noticed Epoxyman has posted before me, I stand corrected.
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    The concensus seems to be that a little moisture will get in no matter what even if the most impervious coating if used.

    If you want to get that small amount of moisture out flushing with dry air may work. This only needs small tubes that connect to a couple of vents that could be easily resealed.

    How to dry the air efficiently? In compressed air systems the moisture that condenses is removed by a drier. The simplest types use a dessicant; some dessicants can be regenerated for reuse. When the compressed air expands it gets even drier. It could be stored in a portable cylinder and left overnight to provide a trickle of dry air for several hours then no power is needed at the beach/dock/marina. Not sure how big a cylinder is needed ...

    Just leaving the dessicant in the hull may be an option but it will have to be changed from time to time and therefore accessed through a hatch which would be bigger and therefore harder to seal than the vents.

    I wouldn't bother, personally but then I don't live in the tropics; all I have to do is invert the boat and leave the hatches open during the winter ...

    The real issue is what harm is the water going to do? In a wood hull there is rot and in a resin hull there is fungus and there are chemical inhibitors that will help control these. In a steel hull there is rust and it is very difficult to prevent, but a fully closed steel hull can be made airtight and filled with dry nitrogen at a few pounds pressure. Then an occasional check with a pressure guage is all the mainenance you would need, at least for the inside.
     
  10. jehardiman
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    jehardiman Senior Member

    jedkins;

    There is also one further reason that you that you do not want a passive sealed compartment.....weight. A sealed compartment must resist the changes in atmospheric pressure and differential pressure. I've seen 1/2" steel skin plating and steel manhole covers dished because a compartment was sealed on a warm day and it got cold, creating a large differintal pressure due to the large volume of the void. It took a 6 foot handspike to pry the cover off. Additionally, the normal atmosphere fluctuates ~ 4" of Hg. That equates to ~ a 2 psi pressure differential. That is 288 lbs per square foot or the equlavent of an additional 4' of water head. To support these types of load the minimum skin must get thicker and and the compartment bulkheads/frames larger/more closely spaced, both of which add weight.
     
  11. LP
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    LP Flying Boatman

    Here's an idea.

    Use the natural properties of a pressure vessel to make a passive water pump to keep water out of your amas.

    Place a line/tube at the low point in the ama and port it to a protected location outside the ama. Every night the ama will "breath" in and morning the ama will "exhale" expelling any accumulated moisture from the ama.

    Just a thought.
     
  12. Frosty

    Frosty Previous Member

    Ive never heard such a fuss about a bit of water in a boat.

    The boat is surrounded by the stuff,-- whats this panick over a little tiny bit on the inside.

    Put a hatch in it and mop it out like a normal boat would be.
     
    1 person likes this.
  13. epoxyman
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    epoxyman Junior Member

    Fascinating idea! Very imaginative!
     
  14. Frosty

    Frosty Previous Member

    Amazing!!! I have both points given and taken for the above post.
     

  15. epoxyman
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    epoxyman Junior Member

    Yes, I notice that the 10 points I started with when joining this forum have been reduced to 5 and it would seem that you are the source of this. This is pretty disappointing to me. This thread is of concern to SOME people on a practical basis, it is of opportunity to understand chemical and physical properties and their application to boat design, and has given the chance for some imaginative thinking. Sad that you should find that offensive or objectionable in some way, and I hope that this is not typical of the members of this forum.:(
     
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