Dewatering Fiberglass - Vacuum

[Miss Gradenko]

Picture a 15' sailboat with a hull made of a sandwich (from the outside in) of gelcoat (relatively impermeable), resin-impregnated fiberglass, stiff closed-cell foam, then more resin-impregnated fiberglass (which you see looking into the boat).

The foam is not one piece, but a grid of 2" x 4" pieces of foam on a flexible backing, like bathroom tiles. When flat and particularly when shaped around corners, there are voids between the foam pieces.

Rain falls on the boat. Small voids in the fiberglass layer allow water to penetrate the sandwich. Osmosis draws the water throughout the sandwich, leaving water in the voids.

Voila. A hull that weighed 240 pounds when built weighs 350 after just a few years.

We're going to try pulling a (deep) vacuum on a small part of the inside of the hull, after drilling some holes through the glass. If we can maintain a solid vacuum the water should flash to vapor and be collected in our cold trap (so we don't go through gallons of pump oil).

But is there a better/easier way?

THE QUESTION

What if we vacuum bagged the entire hull, which wouldn't be hard, and then pulled a partial vacuum? We wouldn't seek to reach the boiling point of the water (or, well, crush the boat), but to increase the rate of evaporation.

What is the relationship between (just) pressure and evaporation rate, and then among those and airflow? There will be some airflow, of course, through the bag material and our (presumably) crappy sealing job. We could regulate higher airflow into the bag.

And temperature...the boat will be in a shack where we could probably maintain the temperature at 16oC or so (this is Toronto, in November), and we could put a small heater inside the bagged boat.

In short, what conditions would maximize the evap rate, as a practical matter?

I've picked through a dozen threads on evaporation v pressure, but honestly they're beyond my basic physics. I'm pretty sure I never advanced through Poynting's Correction or any of the higher math.

Any help would be much appreciated. FWIW you'll be helping a non-profit community club that just wants our boats to be lighter, faster and to last longer.

Cheers...Ralph

Edited to add: For years we've tried putting a dehumidifier and space heater in a bagged boat, for months on end, to little or no avail. It may be that the attractive force between the water and the sandwich demands a flash to liberate the water reasonably quickly?

 

[Danger]

Welcome to PF, Ralph. I can't help you with any formulae, but I have an idea that might or might not be useful. Since I don't know how often you'll have to do this, or how much coin is at stake, I obviously also don't know how much you're willing to spend. My thought is to drill only two holes in the hull, at opposite ends. Attach a desiccant canister (maybe silica beads) to one hole and force air through the other one. The sort of air filtration units available for machine shop pneumatic systems would perhaps be suitable. I have no idea of whether or not that will work, but it might be worth investigating.
The two things that really puzzle me about this are why anyone would need a sailboat in Hogtown, and why someone named Ralph has the title "Miss". Are they related? If so, would I want to know?

edit: In checking over this post, I noticed an inconsistency in my logic. If you're forcing air through the hull, the desiccant at the exit is irrelevant; any transported moisture would simply disperse into the ambient air. The desiccant should be in the input line. Oops...

2nd edit: Are you the owner of the boat, or a manufacturer thereof? If the latter, I would suggest adding a flexible waterproof membrane to link the foam pieces.

 

[Miss Gradenko]

Thanks, Danger. That's a good suggestion, and those are fair questions.

Dessicant: I looked at using a dessicant canister but concluded that if we're trying to remove maybe 5-10 gallons of water it would be impractical to do it one pint (or so) at a time. I assume there's no such thing as a "self-draining dessicator" because the dessicant needs to be replenished. And any such thing would cost way more than our little budget could bear.

Sailing in Hogtown: As you seem to know of our little town (aka "The Big Smoke", or {until recently} "The City With the World's Biggest Prick") you will know it's on a very big and often windy lake. Apart from the many gin-and-tonic yachts, the fleet of boats I race regularly has 50 entries in our weekly races, making it (kid you not) the largest regular one-design race series in North America.

The sailing club owns the boats and is a ten minute drive from where I live.

Miss Gradenko: Is a holdover handle from a politics website I used to frequent, where I was sometimes letting my feelings show. Best I can reckon, my only latency is my mild liberalism, which got quite a workout facing down members of the then-nascent Tea Party. Don't get me started on that, particularly if you believe anti-matter has only been around for 6000 years or so.

Cheers...Ralph

A good flight is any flight you walk away from. It's a great flight if you can use the plane again.

 

[Miss Gradenko]

Re edit #1: Wouldn't the dessicant be on the exhaust side? Oh wait, I see what you're saying. I guess it amounts to "be sure to blow dry air through the hull".

Re edit #2: No, I just use the boats, owned by our club, for the grand cost of $400 per year. (FYI, we attract lots of engineer/physics types who join us in going out of our minds trying to make them go fast.) The builder only recently started to fill the voids in the foam, before laying-up the inner glass layer, with some kind of paste to avoid (ptp) this very problem.

 

[Danger]

Re edit #1: Wouldn't the dessicant be on the exhaust side? Oh wait, I see what you're saying. I guess it amounts to "be sure to blow dry air through the hull".

That is exactly the thought process that hit me, resulting in the edit. I had posted the first part of my idea before thinking it through.
Your mention of "often windy" was exactly why I wondered about sailing on Lake Ontario. I've always thought of it being too choppy for comfortable boating.
I had no idea of the volume of water that you are dealing with; I was thinking maybe half a litre or so. Still, I think that desiccated air injection might be the best approach. To compensate for the amount of water involved, perhaps the boat should be hung by the stern in a vertical position (if space allows). Inject the air from the top and let both it and gravity assist in the moisture drain through the bottom. Again, just an amateur suggestion.
I had no idea that those Tea Party morons had any presence on the intelligent side of the border, and the thought disgusts me.

 

[DaleSwanson]

Regarding desiccant, a good, cheap, bulk source for desiccant is http://thefiringline.com/forums/showthread.php?t=236317". Not all litter is silica gel based though, so make sure you buy one that is. Even if you go with your original idea, it might be worth tossing some in the bag with the hull. The internet seems to agree that silica gel can absorb 40% of its weight in water. A 25 lbs box should then do just over a gallon, and then need to be recharged.

 

[Danger]

Cool idea about the cat litter, Dale. Most that I'm familiar with is made of clay rather than silica, but even that stuff has great absorbency. It's always my first choice for oil or antifreeze spills. If you are suggesting putting it inside the hull, though, I foresee one possible problem. The stuff expands as it absorbs, so you might just end up with huge clumps of unremovable mud stuck in there.

 

[Cleonis]

Voila. A hull that weighed 240 pounds when built weighs 350 after just a few years.

Man, that's a seriously waterlogged hull.

If I understand you correctly this is a boat design with all parts removable. If I understand you correctly, for winter storage the boat can be stripped back to just the hull, allowing you to compare the current weight to when it was first built. (And you find that in a couple of years the hull gains a whopping 110 pounds.)

Obviously this would not occur if the outer layers would be perfectly impermeable, but equally obviously, if a perfectly impermeable coat would exist you would be using that.

I wonder, is it possible to use a highly permeable gelcoat on the inside of the hull?
As I see it, the problem is that the water is trapped in between the layers. You describe that storage in a heated place does little to nothing to make the water migrate out again. What if you can make it easier for water vapor to escape? Ideally the inside coat would have to be a material with the properties of goretex. Some general remarks about drying:
As long as water vapor can escape you can dry things at any temperature (preferably above freezing, but then again, ice does evaporate). Clothes hung out to dry will dry because the gentlest of a breeze transports water vapor away. (Inside the house no breeze hence longer drying time.)

When water vapor is moved away more of the remaining water turns to vapor. The only thing that stops water from vaporizing is if the surrounding air is saturated with that water's vapor. Replace the air with unsaturated air and evaporation continues. A desiccant may offer a little speedup, but I don't expect much. For that hull I think the biggest issue is the fact that the water is trapped in the first place.

I wonder, what if you drill tiny holes in the inside coat, and treat the surface with a strong water repellent, to try and mimick the properties of goretex. Maybe the water repellent is effective enough that during sailing on the lake liquid water won't find its way down the holes, but that in storage water vapor has a way of escaping. (Risky move though; it's only effective if thousends of holes are made, and if it doesn't work as hoped then the problem is worse.)

Using just two holes, one for letting in air to the inbetween layer, and one hole to apply suction, I am doubtful. The motion of air is unlikely to spread everywhere. Probably most of the air flow will be along the shortest path, leaving most of the hull without any air motion.

 

[Miss Gradenko]

Man, you guys rock.

I wake up and find my thread flooded like I posted a Trig Birther rant.

Thanks for that.

I have comments on all of the above, including a survey of various techniques we've tried in past years, but got to run for now.

I hope you'll check back after I post so we can get to ground on this.

One thing I don't see is whether pressure has any effect on evaporation, and if so how much. We bought a 5cfm vacuum pump (used, for $200) so we can move the pressure around pretty well, and with some airflow.

Cheers...Ralph

Michael: Explain it to me like I'm 8 or something.
Oscar: [lemonade stand explanation for surplus funds]
Michael: OK, explain it like I'm 5.

 

[Cleonis]

We bought a 5cfm vacuum pump (used, for $200) so we can move the pressure around pretty well, and with some airflow.

Cheers...Ralph

I gather from a previous message that the manufacturer has improved the building process, filling any gaps, so in new hulls there shouldn't be any voids for water to collect in.

As for the waterlogged hulls.
It seems to me getting air to move is the key thing.
What about creating suction at one point, and multiple holes for air to flow inbetween the layers? The inlet holes can be opened and closed in sequence, to maximize the chance that over the course of a drying treatment all parts of the hull get a period of air flow.

Perhaps it is possible to monitor the weight continuously. If you actually see a weight drop from day to day you know you're on to something. Conversely, if you see no change of weight for days on end you can decide to pull the plug.

 

[DaleSwanson]

Cool idea about the cat litter, Dale. Most that I'm familiar with is made of clay rather than silica, but even that stuff has great absorbency. It's always my first choice for oil or antifreeze spills. If you are suggesting putting it inside the hull, though, I foresee one possible problem. The stuff expands as it absorbs, so you might just end up with huge clumps of unremovable mud stuck in there.

No, I just meant he could put it inside the bag he was proposing to create a bubble of reduced pressure in his first post. As far as clay vs crystal litter, I agree that clay is much more common, but it is also not rechargeable (as far as I know). Once it becomes saturated it just has to be thrown away. Silica gel, on the other hand, can just be heated in an oven and all the water evaporates out (obviously, this isn't usually done due to what the gel is normally absorbing as kitty litter, but it can be done in the case of just water).

After posting that and doing a few more searches though, it seems that litter that is made up of only silica isn't that common. Most of the 25 lbs boxes I saw that mentioned crystal (in the litter world crystal = silica gel), are just clay with silica added. I'm not sure how large and cheap a container of pure silica gel you can get. The link I posted before quoted $5 for 4 lbs at Petco.

 

[sophiecentaur]

I have a GRP sailing cruiser and have read a lot of forum posts (Westerly Owners Association and others) about this topic. The 'Hot VAC' system is used successfully for drying out hulls with the dreaded fibreglass 'osmosis' You heat an area of the hull with IR heaters and apply a large 'sucker' to the surface with a vacuum unit. It's a long job. You keep moving the site of the sucker around the hull. It's electrical energy hungry but it works.

 

[Miss Gradenko]

OK, there's a lot to cover. Please bear with me, and thanks again for your help.

Hot Vac: Yup, Sophie, our Plan A is to try to achieve the poor man's Hot Vac. Apply a deep vacuum to a portion of the hull, heat it up and flash off the water. But if our inline cold trap isn't efficient then we'll end up fouling gallons of vacuum pump oil at $30/gallon. As of today I have all the pieces to give Plan A a go this weekend, and will report.

But Plan B, the driver of this thread, is to put the whole hull in a vacuum bag and try to dewater the sandwich through evaporation.

Hence my question whether a partial vacuum will speed evaporation. I just don't know.

Ingress: The water gets into the sandwich through voids in the sandwich. The most problematic void is at the inside back of the boat where the floor meets the vertical transom. This seam flexes and invariably cracks, and because the boats are stored outside during the summer with the bow elevated, rainwater pools right there.

Other voids are where there are through-hull fittings. We do our best to silicone them, but they are probably contributors.

I should mention that the dewatering is only Step 1 of our experiment. If it works - if significant water weight is lost - then Step 2 will be sealing up as many of these ingress points as possible so that Step 1 is not for naught.

Sandwich Channels: One problem with blowing air through the sandwich, end-to-end or otherwise, is that although there are lots of voids there may not be enough! That is, when the sandwich is laid-up (no vacuum is used), the resin will seep into some of the voids and seal them from adjoining ones. So we can't rely on the passages down the cut lines of the foam being free of blockages.

Dessicant: For dessicant to work it has to be in contact with the water in its liquid or vapor state. We can't contact the water in its liquid state, because it's trapped in the sandwich. And if we're liberating it from the sandwich through vaporization (by way of evaporation or boiling/flashing) then the job is already done and it's just a question of evacuating the vapor. We don't need to collect the water, though it would be interesting to see just how much we get out. I think I've got this right...

Monitoring Weight: Yup, we borrowed an electronic scale and will be working on the boat as it hangs underneath from a sling.

Drilling Holes: On either Plan A (deep vacuum) or Plan B (partial vacuum evaporation) we will drill holes from the inside of the hull through the resin-impregnated fiberglass into the foam layer. We'll try to put the holes into the voids between the foam pieces, because the closed-cell foam is dry (of course) and we want to access as much of the channel matrix (see "Sandwich Channels" above) as possible.

Past Efforts: This has been a problem for 20 years. With temperature, vapor pressure and airflow being the most important variables (I think), we have kept hulls heated in a garage for months, wrapped them (with voids for airflow) with a heater and/or dehumidifier inside for months, in each case with and without holes drilled into the sandwich. Little or no water was lost, but Ontario Hydro gained nicely.

Gravity: Hanging the hull from one end could be done, but I doubt this would work as I'm presuming it's the attractive force between the water and the components of the sandwich that's distributing the water throughout the hull and making it hard to get out. Gravity would help, of course, but how much? If only a little, the hang time could be longer than...a Toronto winter.

Interior Parts: Cleonis asks whether the interior parts are removable. They are really not. Attached to the hull are sealed side tanks and a bow tank. These provide a place to sit, and add buoyancy when we flip (see "Comfortable Boating" below).

This is among the reasons why Plan A is not the best, because we can't access all of the hull from the inside without tearing out these tanks. Maybe the "sandwich channels" will allow the vacuum to penetrate these inaccessible areas, which would be great so long as the back pressure isn't so much as to kill the vacuum. We have a moisture meter and will be measuring lots of stations as the experiment unfolds.

We could pull the deep vacuum from the outside of the hull (like the Hot Vac folks), but this would require that all the gel-coat be stripped off and then later replaced, which is a big undertaking (particularly considering these hulls get polished to 2000 grit or more).

Comfortable Boating: Danger, I respectfully chuckled when you suggested Lake Ontario is too choppy for comfortable sailing in such a small boat. At risk of sounding needlessly macho, the club's boats are indeed used for teaching and pleasure sailing in sheltered bays, but when raced are pushed to their max. Bashing through (or, faster, planing over) 5-foot waves in 25 knots or more is a balls-out, ovaries-out, thrill. And doing it NASCAR-style, 4 or 5 boats abreast, is a ton of fun.

So the racing is quite competitive. 1/10th of a knot slower means I just lost the 2-mile race by a lot. (Trust me, I know all too well.) And heavier boats are slower, which is why the water is a problem (it also causes accelerated degradation, but that's a secondary concern).

Photos: If I'd known my evaporation question would have spawned such creative replies I would have added some pictures from the beginning.

Here's a pic of three Albacores planing down a deep reach in 20 knots or so (I'm in the middle, in CAN 8016):

[PLAIN]http://sailtoronto.com/images/theclub_1.jpg


And here's a pic of the inside stern of an Albacore hull (note the side tanks):

[PLAIN]http://albacore.ca/caa_files/images/Rigging_Project_02_CAN8050-1.jpg


I trust you'll agree that's enough for now. I look forward to your thoughts, and thanks again.

Cheers...Ralph

 

[Cleonis]

Hence my question whether a partial vacuum will speed evaporation. I just don't know.
Cheers...Ralph

As I said before, the hard part is getting any air to move in the sandwich.

Heat (very expensive) will speed up any drying process because hot air absorbs much more water vapor than cold air does, and I suppose that in general any material that absorbs water will absorb more of it when hot.

Lowering air pressure may have the paradoxical effect of slowing down the drying process because the thinnner air can carry less water vapor. My hunch is that since the water is so badly trapped you are just not going to get any convection. And lowering pressure will work if and only if you can get significant convection.

I get the impression that the only way for the water to come out is to come out the way it came in: by migrating through the marginally permeable coatings.
By aggressively drying the top surface of the coating water from deeper layers of the coating can migrate to the top surface (allowing water from the sandwiched layer to migrate outward etc.). I guess that if (a section of) the hull is heated this migration can proceed faster.

I suppose that in big money racing the boats are lifted out of the water all the time, so that the hulls spend very little time in actual contact with water. (That sounds seriously weird to me: boats that need to be kept away from water.)

 

[Miss Gradenko]

Thanks, Cleonis.

I grasp the seeming paradox, but if nature abhors a vacuum then won't the water evaporate more quickly to fill that vacuum?

Put simply, if the inner hull surface temp is 16oC and the air pressure is lowered by 20%, to 24 inches, will that pressure change materially accelerate evaporation, presuming sufficient airflow to clear the boundary layer? Or does pressure only make a material difference as the boiling point is approached?

As for the marginally permeable coating, we're planning to drill holes through it to expose some of the voids (and maybe a lot of the voids, if the channels are open) to the air so that the relevant surface area is increased.

And in fact these boats are kept out of the water when not being sailed. The ingress is not through the outer gelcoat layer to any appreciable extent, but from the water migrating from voids on the inside surface of the hull.

I think you can rest comfortably in knowing that although water molecules are very small, such that few compounds are actually waterproof, the hulls of boats are, for all practical purposes, sufficiently water resistant.

Cheers...Ralph

 

[Cleonis]

Thanks, Cleonis.
I grasp the seeming paradox, but if nature abhors a vacuum then won't the water evaporate more quickly to fill that vacuum?

That's not how it works.

Let me make a comparison.
A Torricelli barometer:
http://www.ems.psu.edu/~nese/f4_2.htm

The vacuum that is drawn is a very good vacuum. In effect the Mercury does not vaporize at all. (The amount of Mercury vapor above the Mercury column is negligable.)

Mercury that is left to stand does vaporize, but the question is, what gives the fastest evaporation?
Fastest evaporation, I think, is when there is normal air pressure above the Mercury, and convection, continuously replacing the air, so that the air close to the Mercury is never saturated.
Slower, I think, is when a vacuum is maintained around the mercury. The fact that a vacuum is maintained will interfere with the necessity for convection. With little to no convection, you get an equilibrium with just as many mercury molecules returning to the liquid as are evaporating from it, leaving you with zero net evaporation.

Getting back to the case of water:
Think of evaporation of water into air as similar to dissolution of a substance into a liquid, such as dissolution of sugar in coffee.
Hot air can carry a larger percentage of water. In a sense one might say that more water can dissolve in hot air than in cold air. That is one of the reasons that heat speeds up drying.
In a case where you can maintain convection I actually expect that by increasing pressure you can increase drying capacity. (Generally a solution will saturate at a particular percentage. So I expect double pressure air to carry twice as much water vapor.)

Conversely, the extreme case of no air at all.
Water does evaporate into vacuum of course, but the vapor pressure of water is quite small. Certainly it is far less than atmospheric pressure.
So if you are in a situation where maintaining a degree of vacuum reduces the volume of convection then you lose drying capacity.

Anyway, by the looks of it isn't possible in the first place to get any convection where you really need it.

I get the impression that drying out the hull is as difficult as drying an egg. The egg's membrane is hardly permeable to water. There is very little you can do to speed up drying of an egg. Warmth and convection are your best shot. A vacuum (with correspondingly reduced convection), I don't see that helping you.

(I wonder: could it be that drying an egg is actually a good model for drying the hull? If so then you can set up small scale experiments.)

 

[Cleonis]

Put simply, if the inner hull surface temp is 16^oC and the air pressure is lowered by 20%, to 24 inches, will that pressure change materially accelerate evaporation, presuming sufficient airflow to clear the boundary layer? Or does pressure only make a material difference as the boiling point is approached?

I expect so.
http://en.wikipedia.org/wiki/File:Water_vapor_pressure_graph.jpg"
I expect that at 16^oC you are not going to come anywhere near the low pressure level for water to boil.

In other words, I don't see a higher evaporation rate at the achievable lower pressure.

 

[sophiecentaur]

@Cleonis:
Your model of evaporation is a bit shaky. Water, at any temperature, has a vapour pressure and this pressure is independent of the presence of other gas molecules. (Law of partial pressures) It is often stated that "hot air can carry more water than cold air". It certainly appears that way but the fact is just that the vapour pressure of water is greater at higher temperatures and it will evaporate quicker when hot. The effect of reducing the pressure on the outside of the membrane will be to increase the statistical probability of water molecules moving across the barrier.
Supplying heat is, of course, expensive but it's a compromise between temperature difference and time taken. Some insulation can help and, in the end, the limit of heat needed would only be enough to supply the latent heat of vaporisation.
The Hot Vac system does work well (expensively and time-consumingly) in removing water from within the fibreglass skin itself. Removing it from the inner layer of foam could be easier if some (temporary) holes could be made in the inner skin of the sandwich. But they would have to be filled / covered whilst the boat is being used. (Not hard to achieve)
btw, wouldn't covering the boat when out of the water help to reduce the effects of rain?

 

[Cleonis]

Water, at any temperature, has a vapour pressure and this pressure is independent of the presence of other gas molecules. (Law of partial pressures) It is often stated that "hot air can carry more water than cold air". It certainly appears that way but the fact is just that the vapour pressure of water is greater at higher temperatures and it will evaporate quicker when hot. The effect of reducing the pressure on the outside of the membrane will be to increase the statistical probability of water molecules moving across the barrier.

Yeah, you're right of course. I was too eager to simplify.

I know the dissolution analogy doesn't fly, so I disclaimed 'in a sense you might say that', but on second thoughts I shouldn't have presented that picture in the first place.

For gases the total pressure is the sum of the partial pressures of the constituents. The constituents coexist independently and exert partial pressures independently. As you state, water vapor has in and of itself a higher vapor pressure at higher temperature, hence in warmer air water vapor can be present at a higher percentage than in colder air.

I also need to correct the following error: I suggested that increasing air pressure will increase "water carrying capacity", but that's not the case. I got carried away with the dissolution analogy, I should have reasoned in terms of partial pressure and probability.

About Hot Vac:
Clearly the heat is the most important factor; everything goes faster when hot.
How about the Vac? Is it the (localized) lowering of pressure that is effective, or is it continuous evacuation? In other words, is maintaining convection over the treated area more important than lowering pressure over the treated area? I think so.

 

[Miss Gradenko]

@Sophie

chuckling again...yes you might think we should just cover the boats, but those covers are expensive (the club has 18 Albacores), and a nuisance when the boats get used so much...and because the mast is up we'd need the type of cover that passes around it, which doesn't (in my experience) really seal out rain that well...that said, people who own their own boats always use them, so you certainly have a point

@Cleonis
If I understand correctly, the deep vacuum is needed to accelerate the phase change thus liberating the water, and the improved convection from airflow through the sealed area is needed to clear the field...so the first is central and the second is limiting...I think

@everybody
Re our Plan B, could you give me some rough idea of how much faster the water in the hull's sandwich might evaporate if we bag the boat and pull a partial vacuum? It seems the vacuum necessary to significantly accelerate the evaporation is toward the high end, such that the hull would probably collapse.

But because Plan B would be so much simpler, I'd love to know if that's just wrong.

Cleonis wrote "at 16oC you are not going to come anywhere near the low pressure level for water to boil", but this approach is not trying to reach the boiling point; just to speed evaporation at a lower temp.

We can, of course, manipulate pressure, temperature and airflow fairly widely inside the sealed bag.

Cheers...Ralph

Edited to add: I see Cleonis also wrote "In a case where you can maintain convection I actually expect that by increasing pressure you can increase drying capacity. (Generally a solution will saturate at a particular percentage. So I expect double pressure air to carry twice as much water vapor.)"

What say, all? Presuming this is right, what does the pressure/evaporation curve look like? Ought our concern not be crushing the hull, but exploding it?

(If you haven't deduced, I'm finding this a lot of fun, and we're trying Plan A this afternoon.)

 

[sophiecentaur]

Yeah, you're right of course. I was too eager to simplify.

I know the dissolution analogy doesn't fly, so I disclaimed 'in a sense you might say that', but on second thoughts I shouldn't have presented that picture in the first place.

For gases the total pressure is the sum of the partial pressures of the constituents. The constituents coexist independently and exert partial pressures independently. As you state, water vapor has in and of itself a higher vapor pressure at higher temperature, hence in warmer air water vapor can be present at a higher percentage than in colder air.

I also need to correct the following error: I suggested that increasing air pressure will increase "water carrying capacity", but that's not the case. I got carried away with the dissolution analogy, I should have reasoned in terms of partial pressure and probability.

About Hot Vac:
Clearly the heat is the most important factor; everything goes faster when hot.
How about the Vac? Is it the (localized) lowering of pressure that is effective, or is it continuous evacuation? In other words, is maintaining convection over the treated area more important than lowering pressure over the treated area? I think so.

Glad you agree with me - I didn't fancy a 'pistols at dawn' on that subject .

Re the Hot Vac, I guess that the rate of diffusion of water through the skin will just be proportional to pressure difference - i.e. a combination of increasing the vapour pressure on one side and reducing the pressure on the other. But the initial evaporation of the water would require energy and the temperature inside the skin would presumably drop (lowering the VP) unless some latent heat were supplied.

As the filling will be pretty porous, I'd think the pressure on the inside side to be much the same everywhere but the water loss would probably be more in the warmer bits. To avoid re-condenstion, it would be best for the heat and vac areas to coincide.


@Miss Gradenko: As you say - most owners take care of their boats better than they would treat club boats. If the club wouldn't want to spend out on covers then it's unlikely they'd shell out for hot vac either!
You'll just have to save your pennies and get one of your own - or just get so good at helming that the weight of the boat becomes a secondary issue! I know well just how it is possible to spend infinite amounts of cash on a boat.

Did you ever consider putting a small bung at the very lowest point of the hull when stored, so a pool doesn't form?

Having read your long post in a bit more detail, it seems there are probably significant air channels within the skin. I think you could get some joy by using a vacuum cleaner, (rather than a hi vacuum pump) joined to the hull at one end (stern) and a hole at a high point near the bow and running it for many hours. A high volume of circulating air could well solve the problem without applying heat. This calls for an experiment, I think.

Having, we hope, solved the problem, you will then need to remove or reduce the main cause on water ingress.

 

[Cleonis]

Cleonis wrote "at 16oC you are not going to come anywhere near the low pressure level for water to boil", but this approach is not trying to reach the boiling point; just to speed evaporation at a lower temp.

Yeah, but you see: lowering pressure is no substitute for raising the temperature.


Let's say you have small scale experimental setup where any water vapor is transported away immediately, so that you can assess actual evaporation rate. You can lower the ambient pressure but I think evaporation rate will stay pretty much the same, until you reach the boiling point, where there'll be a sudden increase.

As Sophiecentaur pointed out, this sort of thing is about probability distribution. The water molecules have a range of velocities, statistically a tiny percentage will have sufficient velocity to escape the fluid.
Air at normal atmospheric pressure is very thin already. If memory serves me: the avarage distance between the molecules is in the order of ten times the size of the molecules. So water molecules that are candidates for escaping the fluid don't have to muscle their way into the air, getting out (of the liquid) is the hard part.

Increasing temperature will have a significant effect, because it raises the percentage of water molecules with "escape velocity".
Reducing the surrounding air pressure to "make more room" won't do much, as there's room to spare anyway.

When there's insufficient convection then many water molecules will plunge back in the liquid. Key to increasing net evaporation rate is to transport any water vapor away. (But of course the problem that's stymying all of your plans is that you're pretty much barred from getting convection where you need it.)

 

[sophiecentaur]

I concur with all the above, cleonis.
I would just point out that, in this particular (very practical) case, there may be enough through-flow of air possible in the region between the skins to achieve drying with 'free, dry air' (there- I lapsed into the vernacular myself!) that is at ambient temperature. That will almost certainly be the cheapest way forward. Any extra heating would speed the process but there would be less lowering of temperature because of fresh air being available all the time..
Looking again at my 'lapse' I wonder whether 'forced drying' is actually made more effective by the presence of other gas molecules- I think it must be. i.e. the situation may not be explainable by just talking in terms of vapour pressure.

 

[Miss Gradenko]

Cleonis wrote:

"You can lower the ambient pressure but I think evaporation rate will stay pretty much the same, until you reach the boiling point, where there'll be a sudden increase."

So our Plan B - bagging the hull and trying to increase the evaporation rate through decreased ambient pressure - looks like a non-starter.

As for Plan A, we successfully boiled water out of a small part of the sandwich on Sunday.

We put the boat in the sail shack, cut off the top layer of glass/resin to expose about 4 square inches of foam, sealed a piece of vacuum plastic over the area with a piece of polyester fabric inside (to maintain evacuation channels), inserted the 1/4" vacuum hose and sealed the entry with silicone.

We managed to pull a vacuum of 27" or so, and heated the area (from the other side of the hull) with a propane heater until it was too hot to touch.

As expected, the water started boiling out like crazy, and quickly condensed as it was expelled from the pump's exhaust (we haven't built our inline cold trap yet). The polyester was soaked, and in total we may have drawn out half a pint or more in just a few minutes.

Not a bad start.

Next step is to get a larger vacuum hose to take better advantage of our 5cfm pump and try to maintain a higher vacuum in light of the backflow (the pump is rated at 25 microns and pulled over 29" when tested in a simple vacuum bag). And more outflow may help the vacuum penetrate further through the sandwich, along the cut lines in the foam.

I'm waiting for a call back from the manufacturer to see where we reach diminishing returns in terms of the hose diameter - they offer 1/8" increments up to 3/4".

Thanks, all. Will let you know when we get some more results.

Cheers...Ralph

@Sophie

Yes, we've tried to install bungs there for a while, but the club is concerned the sailors being taught won't remember to put them in the hole. I think this is silly, because we could tie neon tape to them and paint a big red arrow pointing at the hole, and in any event they would figure out the omission pretty quickly as their feet got wet.

 

[sophiecentaur]

If people can't remember bungs then they deserve to SINK!
I have done it my self and been very embarassed. It only happens slowly.

I'd be a bit wary of overheating the hull - boiling shouldn't be necessary if you are prepared to wait a while.

 

[Miss Gradenko]

Yes, Sophie, we're also concerned about heating the hull too much. We want to dewater the sandwich, not delaminate it.

That's why we're going to try a larger diameter vacuum hose (1/2", instead of 1/4"), in the hope of countering the backflow with increased outflow. If it works, we should be able to maintain a deeper vacuum, and that vacuum should penetrate further through the sandwich.

That deeper vacuum will mean we won't have to heat the hull as much to reach the boiling temperature where the water, now vaporized, is extracted quite nicely as predicted.

But, when you say "boiling shouldn't be necessary if you are prepared to wait a while", what do you have in mind?

I've been thinking the phase change is reducing the attractive force so as to permit the water vapor to flow out of the sandwich.

Are you thinking we might scrap the heating altogether and just use the system as a really strong vacuum cleaner? A Shop Vac on speed?

Our observations were that without the heating, little water was removed. Only when we heated the hull did the boiling start (we could hear it, and see it through the film) and condensed water vapor spewed from the vacuum pump exhaust.

Cheers...Ralph

 

[sophiecentaur]

My point is that you don't need to have washing on a washing line for it to dry (or even in a tumble dryer). If the hull belonged to me, I wouldn't want it heated as brutally as you have been doing to your club's boats. Your "propane heater" method scared me!

The only difference between boiling and evaporation is the rate - and that's just because the vapour pressure is equal to ambient pressure when the water boils. Under a low enough vacuum, of course, you will get boiling at much lower than 100C - but heat is needed to keep the temperature up (lost latent heat will cause temperature drop).
The details of the best method will depend upon the detailed nature of the filling. The more air it will allow to circulate, the lower temperature / higher flow / weaker vacuum you can use. Only you know that.
Your water "spewing out" was a good sign, though - as long as you weren't roasting the GRP at the same time. How much water did you actually extract, compared with how much you think is in there?

I just had an alternative whacky idea that you could put the boat on a turntable / centrifuge and 'spin dry' it! What price Health and Safety?

 

[Miss Gradenko]

I think that's well summarized, Sophie.

Our aim is to get the vacuum down to 28 inches or more. Then the boiling point will be 100oF or less. Our guess (we forgot to bring our meat/hull thermometers) was that we heated the small part of the hull to maybe 120oF or so, such that it boiled at 27 inches.

At that temperature we started to see a couple of blisters appearing about the size of a quarter, suggesting we were delaminating the hull. But I don't know the melting point of the polyester resin. FTR, the heater was as low as we could get it without extinguishing the flame.

We probably extracted about a pint of water trying just to prove the concept. The boat weighed maybe 250 lbs when new and now weighs 350, so if we say 10 pounds of that is extra material from repairs over the years, then that leaves 90 lbs or 9 gallons. Lotsa water.

This volume doesn't surprise us, because the needle on our moisture meter (which reads to 50%) is pinned all over the hull, particularly on the floor. Indeed, when we carve off the outer coat the exposed core is visibly wet.

So do you think that if we don't add heat at all...just use the vacuum pump as a very strong Shop Vac...it might suck the liquid water out? We can try that of course, but do you have a guess? The pump is continuous-duty, so with a proper cold trap we could let it run for hours without changing the oil.

Spin the boat! Like lettuce! Love it! You know sailors, so "whacky" is pretty relative. And as for H&S, we're fond of saying "do you want to sail safe, or sail fast?", usually just before we hit the water.

With a club full of McGyver wannabes, I'll bet we could find a way to do it.

No wonder YouTube has so much content.

Cheers...Ralph

ETA: I just remembered an earlier suggestion to hang the boat from its bow. That couldn't hurt, and might help, and would increase the wackiness factor considerably. We'd support the stern, of course. Sweet.

 

[sophiecentaur]

Ha ha.
That delaminating thing is something you might expect when the vapour pressure is above ambient and there happens to be a weak spot. Perhaps that's a good reason for not boiling. After all, a heavy boat is better than a boat with damned great holes in.

You could always spread the head out using a metal plate - or use a hot air gun instead and operate it at a reasonable distance from the grp.

Keep us informed. I only wish I could be there to see (he lied: distance lends enchantment, as they say).