condensation in sealed "water tight" hulls

[jedkins]

Is it possible for condensation to form in sealed air and water tight hulls? I am thinking specifically of beach cat hulls and trimaram amas.

I am building a 20' trimaran and would like to completely seal the ama hulls whereby sea or rain water cannot enter. However, I am concerned that from experience with beach cats such as Hobies that there always seems to be some water within the hulls. I 've had similar experience with ama hulls. The hulls seem to be sealed but there it is - a little bit of water - is there any possibility its condensation or must it always be from a leak? Thanks for any advice

[FAST FRED]

If you fill the interior with foam it might stay dry , although the foam has weight .

FF

[jedkins]

Unfortunately I dont want the extra weight but I wonder what causes the water to form in the first place...?
If you fill the interior with foam it might stay dry , although the foam has weight .

FF

[PAR]

Temperature and humidly changes causes moisture in the air to condense and evaporate repeatedly each day. You could try an inert gas like argon, but keeping it truly vapor tight would prove troublesome.

[longliner45]

polyuethane foam will absorb moister after time,,and need periodic replacement

[Frosty]

If its sealed then how do you know there is water in there? I guess you must have an access hatch!!!!!! Probably that leaks a bit, especially as the day cools and the air inside contracts.

A little bit of water is not normaly a problem is it? unless you are building in steel.

[jedkins]

Yes, That's why I dont a hatch - they always seem to leak. !!(actually a beckton plate) . Water left in the bilges of a wood boat will cause a problem eventually as it obviously causes rot over time. And in Amas, because of the compartmentized bulkhead arrangement water is often difficult to spot unless many plates are installed in deck. Which is a catch 22 as they tend to leak.!

Getting back to my question - Does condensation occur in seal hulls like Amas ? I wonder if anyone can give me a answer to that specific question?

If its sealed then how do you know there is water in there? I guess you must have an access hatch!!!!!! Probably that leaks a bit, especially as the day cools and the air inside contracts.

A little bit of water is not normaly a problem is it? unless you are building in steel.

[Raggi_Thor]

If you fill the hull with cold dry air and manage to make it not only water tight, but diffusin tight, then no water will condensate as long as the temperature is allways higher than it was when you sealed it...

But this is theory, I think it's impossible to make it "diffusion tight".

[tom28571]

If you fill the hull with cold dry air and manage to make it not only water tight, but diffusin tight, then no water will condensate as long as the temperature is allways higher than it was when you sealed it...

But this is theory, I think it's impossible to make it "diffusion tight".

This is pretty much true. An enclosed volume in a humid environment that has repeaded daily temperature cycles will "pump" moisture vapor through the walls unless the walls are 100% impermeable. It may take many years but will happen.

That is the theory. I have had no problem with air tanks built of plywood and well coated with epoxy over decades of use.

[jfblouin]

I'm building a boat with 24 air and water tight compartments (I assume). The biggest one is .25 m3 (9 CuFT). They are sealed with Epoxy. Somes sections have fiberglass also. But I live in Canada and temperature vary for -35C to +35C (-30F to +95F). With this calculator http://www.cactus2000.de/uk/unit/masshum.shtml I calculate that if I seal the compartment with 50% humidity at 15C (60F) I capture 6.4 gr (less than .25 oz) of water. If temperature drop to -35C (-30F) 6.2gr of water (or 6.2 ml) will condensate in compartment. If I can seal at 0 C (32F) I capture only 2.4 gr of water so only 2.2 gr can condensate at -35C.

It is not a lot of water.

The compartment need to be really water and air sealled because if new humid air can enter in the compartment at each temperature variation (by variation of air volume) or each variation in air pressure, a lot of water can be accumulate in compartment.

[Man Overboard]

If you manage to get a perfectly tight container, you can still get water in the container by way of vapor pressure. To give an example of how powerful vapor pressure is; we have all seen cars that have rust that has formed underneath the paint, causing the paint to bubble. This is not necessarily due to rain, but vapor pressure within the air itself which forces minute molecules of water through the paint where it collects in high concentrations in the pores of the mettle substrate. (As a vapor) With changes in temperature the vapor will condensate into water. You may be able to get around the problem by slightly pressurizing the compartment. A gauge and a location on the compartment for filling with air would do. If you are building with a composite material, than epoxy is the best at reducing to a minimum the effects of vapor pressure.

[jedkins]

Thanks for your comments. I live in the tropics where the day and night temperature rarely deviates between 27-34c. I am not sure abt our sea temperatures but the water is never very "cold" . However, Humidity is always very high. My amas are ply and are complety sealed with epoxy/glass. I guess I have a sealed area of abt 40 cubic feet. I will try the calcultor you mentioned to see if I can work out how much water I might capture.

I'm building a boat with 24 air and water tight compartments (I assume). The biggest one is .25 m3 (9 CuFT). They are sealed with Epoxy. Somes sections have fiberglass also. But I live in Canada and temperature vary for -35C to +35C (-30F to +95F). With this calculator http://www.cactus2000.de/uk/unit/masshum.shtml I calculate that if I seal the compartment with 50% humidity at 15C (60F) I capture 6.4 gr (less than .25 oz) of water. If temperature drop to -35C (-30F) 6.2gr of water (or 6.2 ml) will condensate in compartment. If I can seal at 0 C (32F) I capture only 2.4 gr of water so only 2.2 gr can condensate at -35C.

It is not a lot of water.

The compartment need to be really water and air sealled because if new humid air can enter in the compartment at each temperature variation (by variation of air volume) or each variation in air pressure, a lot of water can be accumulate in compartment.

[Raggi_Thor]

Drive up in the mointains, fill the hulls with cool dry air (add a little preassure), seal it and drive home :-)

[jedkins]

No mountains or cool dry air here in Phuket Thailand.......Drive up in the mointains, fill the hulls with cool dry air (add a little preassure), seal it and drive home :-)

[Man Overboard]

You can make a home-made condensation trap by coiling up an airline hose and stuffing it into a bucket of ice water; after the line passes through the ice water, run it into a small pressure tank. Preferably one with a drain cock. If you can find an old junk truck in your area (one with air brakes) there will be a small tank just like what I describe on the truck somewhere. This setup will also give you dry air for any kind of spray applications you might be using during construction.

[SamSam]

If its sealed then how do you know there is water in there? I guess you must have an access hatch!!!!!! Probably that leaks a bit, especially as the day cools and the air inside contracts.

A little bit of water is not normaly a problem is it? unless you are building in steel.

You pick the boat up and shake it to hear if there is water in there. You got it right about the contracting air. If the amas or whatever are sitting in the sun, they get heated like an oven. Rain immediately cools them down and if you have a 'watertight' hatch with a rubber seal, water will sit in the channel between the hatch cover and and its frame and get sucked right past the rubber seal. Each time it happens water goes in but not out, so it adds up. It would be interesting to connect a vacuum gauge to an ama and see how much is developed with a hot hull and a cold rain. Sam

[jedkins]

That would be too complicated for me. I have noticed that condensation can be reduced in other boats by storing cushions and other absorbant materials in the hulls. I wonder if there is a purpose made material or product that acts in the saame way?. You can make a home-made condensation trap by coiling up an airline hose and stuffing it into a bucket of ice water; after the line passes through the ice water, run it into a small pressure tank. Preferably one with a drain cock. If you can find an old junk truck in your area (one with air brakes) there will be a small tank just like what I describe on the truck somewhere. This setup will also give you dry air for any kind of spray applications you might be using during construction.

[RAWRF]

Unless you have foam, you are always going to have moisture inside a sealed container, and the foam absorbs moisture eventually. I would put one way check valves in the amas and have float actuated bilge pumps as backup. I had this same concern with a pontoon boat once and I did the above. Those sealed pontoons can actually get a lot of water buildup pretty quickly, perhaps half a gallon a day under fluctuating temperatures.

[longliner45]

if you want the airtight compartments for safety,,in case the boat swamps or is damaged by reef,,,the bildge pumps will create another hole for water to enter through the (exaust or exiting water) ,,we glassed our airtight compartments and then blown polyurethan foam into them ,,they lasted a good 6 years and when we replaced the deck ,,we replaced the foam,,,or better yet if you can measure the compartments,,and make the foam the same size and glass it over completely,,and slid them into the compartments,,this will make sure they dont soak up any water.

[Frosty]

When the boats not in use open up the hatch and let the air dry it out. when you go out it-- shut them. Look at the big plywood speed boats at the Boat Lagoon, there hatches are open when not in use.


I think its a problem that no one has got around or worried too much about.

[longliner45]

Im sorry ,,,misread your question ,,your not talking airtight compartments ,,you mean water tight hull,,,,my bad,,,,,,,,longliner

[Frosty]

No Longliner I think air tight is what he wants. I was suggesting putting some hatches in so that he could dry them. Although I can see I didnt make that clear.

[Poida]

What about those crystals they put in packaging to absorbe moisture.
A hatch with a strainer attached to the lid, place in some water absorbing crystals and change them at frequent intervals.

Can any material be 100% water tight especially when it is submerged with weight on it forcing water through microscopic pores?

So it is impossible to completely water tight a compartment and absolutely impossible to get water out.

To my way of thinking it would be better to have a hatch that might let a little water in because it is easier to get it out.

[monrosm@shrewsb]

It seems to me that people are suggesting very complex and most of them physical ways to stop water getting to your hulls.
Firstly let me explain why the condensation forms as many other people in their reply’s have partially done. This is as im sure you know due to a significant change in temperature on the inside of the hull than on the outside. Now there aren’t many practical ways to stop this change in temperature, in fact its almost impossible, without going to great lengths and expense.
Now, this condensation would not form if the water vapour was not inside your hull, so the question is 'How do I get rid of it', well most people would state the obvious, create a vacuum in the hulls.....however most people say this before they think about it. If you did this you remove the substance that makes your hulls float, which is of course...'Air' meaning one thing they wont float. Even replacing it with foam, still wouldn’t work (or in some substances, give very minimal flotation) as air in-between the foam pores make it float....so you need a better but probably far simpler solution.
If you visit this website 'http://www.keepsafe.ca/scavengers.shtml' you will find one possible solution to your problem. This is an oxygen absorber, firstly you might be questioning why this would help....Well look at the chemical composition of water....H20, now, if you remove one element from here....the Oxygen, it makes it impossible for the water to form. All you are left with is a hull full of H, Co2, N2, and other minor gasses in the air...This is all ok as i assume nothing is going to be doing any breathing in your hull. This stops the condensation problem, so if you hull is truly air water tight and has no waster residue when you seal it, it should stay like this. However if there is a leak, this oxygen absorber would only work until a certain volume of water is in the hull (depending on how much absorber you put in) at this point it becomes saturated and will not absorb any more Oxygen, for this I don’t have a very good solution, other than using Silica Gel, to absorb the moisture, although it doesn’t absorb much...
Hope this helps
Stefan Monro

[Frosty]

Oh dear-- oh dear-- oh dear

[Raggi_Thor]

It seems to me that people are suggesting very complex and most of them physical ways to stop water getting to your hulls....Stefan Monro

And your's is simple?
:-)

I think we have to be realistic,
you can keep the water out, but not the vapor in the air.
This is how modern rain jackets work, the are water tight but not air tight.

So make at least one small hatch for inspection and ventilation.
I think the cheap plastic inspection hatches is quite water tight.

[PAR]

Stefan, boats float, as a result of a long since dead Greek spilling some water out of his bath, which he calculated was equal to his weight. Granted, his water logged butt had considerable air in it, but even if he didn't, his shape would have still displaced sufficient water, to support his mass.

[Man Overboard]

Stefan

“If you did this you remove the substance that makes your hulls float, which is of course...'Air' meaning one thing they wont float”

I don’t mean to nick pick here, but because this is a boat design forum, I thought I might point out that air is not what causes objects to float. Objects float if the weight of fluid displaced is greater than the weight of the object. So, in fact, if you remove the air from a container, the container will weigh less, floating higher out of the water. (Less wetted surface area.)

Another example: My 100 cubic foot scuba tank weighs 40.2 lbs. When empty it has 3.5 lbs of positive buoyancy, so it floats. When filled with 100 cubic feet of air it has 3.9 lbs of negative buoyancy so it sinks (8” in diameter 26” long) The air adds 7.4 lbs.

[Poida]

Par
It would have only been equal to his weight if he was floating in the bath.
Archimedes actually had at the time of his famous bathing, realised how to calculate the purity of gold. Believe it or not.

Poida

[catsketcher]

Hello all,

When I wanted to learn a bit of boatbuilding I went to work (for free mind) for one of Brisbane's best multihull builders. He had built a 40 ft schooner tri (won lots of races) with sealed floats. He put water sensors in the floats and left them for years. This was in Brisbane, a somewhat similar climate to Thailand.

I was told by Robin Chamberlin (great designer down here) to put a small amount of fungicide in every compartment after sloshing the whole lot in oodles of epoxy. I had to cut into one of the compartments a while ago and was gratified to see the ply in good nick. The idea is that the copper napthalate forms a fog of fungicide so if any water is in the air (as it must be) than you get air laced with fungicide rather than straight water.

Build it well with no bolts or protuding steel inserts and cover it after you put the copper napthalate in is my call.

cheers

Phil Thompson

[PAR]

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

[epoxyman]

Yes, That's why I dont a hatch - they always seem to leak. !!(actually a beckton plate) . Water left in the bilges of a wood boat will cause a problem eventually as it obviously causes rot over time. And in Amas, because of the compartmentized bulkhead arrangement water is often difficult to spot unless many plates are installed in deck. Which is a catch 22 as they tend to leak.!

Getting back to my question - Does condensation occur in seal hulls like Amas ? I wonder if anyone can give me a answer to that specific question?

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!:)

[Raggi_Thor]

I think that was very informative, and well explained.
Thanks :-)

[Man Overboard]

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

[LP]

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.

[epoxyman]

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

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.


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?

[Man Overboard]

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.

[Man Overboard]

Ha, Ha... I noticed Epoxyman has posted before me, I stand corrected.

[ancient kayaker]

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.

[jehardiman]

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.

[LP]

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.

[Frosty]

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.

[epoxyman]

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.

Fascinating idea! Very imaginative!

[Frosty]

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.

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

[epoxyman]

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

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.:(

[ancient kayaker]

Jehardimans:- I did not appreciate the power of a small pressure differential over a large area. Maybe a kayak style hatch with a waterproof flexible sack would provide pressure relief. Getting a bit complex ...

Tropical fungii have an appetite for most non-metallic things and probably love fiberglass. I haven't been to Thailand although I've been close, surely it's a concern there?

About points, this has been interesting thread on an important topic with some really good ideas so far; hope it remains so.

My ancient fiberglass canoe has (hopefully) fully sealed bouyancy chambers; in view of the information on diffusion should I be worrying about this? It's a virtually universal practice on factory boats.