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Minimising Condensation |
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Martin&Rene
Vice Admiral Joined: 06 December 2009 Location: United Kingdom Status: Offline Points: 833 |
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Posted: 11 April 2021 at 20:46 |
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Unfortunately, I had started to move an interesting thread
by Steve on the 300/301 thread about his work on the fore-cabin woodwork into a discussion on
reducing condensation in the bedrooms, so I thought it would be better if I
continued here. Compared to other
forums, the one thing I like about the MyHanse forum is the way it sticks to
the useful discussions on original topic most of the time. Also I have decided to add in the article
that I wrote for a magazine on minimising condensation. When we first started sailing the yacht, we quickly found
out the problem of moisture underneath the mattress. We first started using Dry Mat underlay, but
found it made little difference. We then
found a product, called, I think, Ventair which was looked like a synthetic
sheet of horse-hair, 20mm thick with a coating on both sides. With this, we still got condensation on the
bed base and on the bottom layer of the matting, but not on the mattress. Unfortunately, I do not think it is still
available. The closest equivalent is https://www.mastercoversltd.co.uk/product/airmat-under-mattress-ventilation/
but I have no experience of this product. Anyway, I decided we were approaching the problem the wrong
way. Forget ventilation; I have seen
condensation on the outside of windows in Houston, USA, because the shops have
set the air conditioning low, but of course there is no lack of ventilation outside. Warm air on cold surface = condensation So, let’s get rid of the cold surface. Hence the idea of putting insulation UNDER
the base of the bed on the bilge side. The product I used is Recticel Eurothane Insulation Board
which is used in insulating buildings.
Any building supplier will have an equivalent product. We used https://www.jewson.co.uk/p/recticel-eurothane-gp-pir-insulation-board-2400-x-1200-x-25mm-IRESR025 I used the 25mm board which is sufficient for job, but if I
were to do it again I think I would actually use 50mm where it is possible. In a wet climate like Scotland, the problem on many standard
yachts is the fact that there are no hatches you can open when it is
raining. You can deduce from some
figures later on that sleeping in a confined space will generate overall
condensation. On our yacht we use the
back cabins as single beds. As we have a
cockpit tent, Rene, in the starboard cabin, can have the hatch open into the
cockpit. In the port cabin, I just leave
the door open. On the few occasions that
we have guests onboard, they have the fore-cabin. It would be better if they left the door
open, but of course they don’t. However,
a side benefit of stowing our dinghy upside down on the foredeck is that they
can have the forehatch open. A few years ago, I wrote a general article on minimising
condensation and it was published in a UK mag a couple of years ago, so I
thought I may as well place it here. Keeping Your Yacht Dry and
Smelling Fresh When we went looking for our first yacht, some boats never
made our short list. The reason, they
smelt damp, mouldy, unloved and uncared for.
We certainly did not want our yacht to end up like that, particularly as
we do not just sail in the summer months. Where does the Water Come From?
The table shows the “absolute humidity” in g/m3 (upper
line) and the relevant “dew point temperature” in 0C (lower line)
for certain air temperatures as a function of “relative humidity”. Example: For air at a temperature of 200C
and a relative humidity of 80%, the absolute humidity is 13.8 grams of water
per cu.m. of air and the relevant temperature at which dew will form is 160C. Reference: Abstract of table from, http://www.tis-gdv.de/tis_e/misc/klima.htm Firstly, the air that surrounds us has moisture in it. From the table we can see that at 80%
humidity at 200C, then 1 cu.m. of air will contain 13.8g of water
and for now we will simply take 1g water = 1ml. We each produce moisture, typically 400ml/day (17ml/hr) in
our breath and 400ml/day in perspiration (sailing in the UK?). Burning 100g of butane for cooking produces 155 ml of
water. So, the gas cooker may be
producing around 20 ml per day. Cooking can produce a large amount of moisture. Boiling a pan of pasta can easily produce
200-300 ml of water vapour. A typical small diesel heater has an output of up to 140m3/hr
and if the air it is pulling in from the outside is at 10oC and 80%
humidity then this air would contain 7.5ml of water/m3 and so we
could have over 1,000 ml water entering the saloon per hour. We all know how much heavier our sailing clothes are wet, so
having them in the boat to dry off, could easily introduce 100-200g (ml) of
water per item. Let us put those numbers into perspective. If the internal volume of the whole yacht,
above lockers, is 25 m3, then if the air has a humidity level of 80%
at 20oC, then the air will contain around 345ml of water. 2 people breathing during sleeping could add
in another 270 ml. By the time the
temperature has dropped to 10oC in the morning, then that same
volume of air can only contain 235 ml of water.
So, we have 345ml plus 270ml of water and yet the air can now only hold
235ml in the morning and so 380ml will have condensed out. What Can We do?
So there are basically 3 main strategies to reducing the
problems of moisture on a yacht.; ·
Minimise the amount of water vapour introduced
to or present in the yacht ·
Minimise temperature variations. ·
Wherever possible keep surfaces above the
relevant dewpoint temperature. Cooking is one area where we can minimise moisture. ·
Boil water in kettles or in pans with the lid
on. ·
Use a pressure cooker and turn the heat down
when it has reached pressure, so that it does not produce too much steam. ·
Cook rice in a wide neck thermos flask. Heat 1 vol rice and 2 vols of water in a pan
until it boils and then pour all the contents into a preheated flask. Leave for 20-25 mins. This not only reduces the amount of steam you
produce, but also frees up a cooker ring. ·
Use fresh pasta, available in vacuum packed
bags, as it tastes better and needs less cooking. ·
Many yachts have ventilators above the cooker
but having them opened when it is windy is often not possible as the gas get
blown out. Having the heads door and
companionway hatch open may sometimes be an option. ·
Close all the doors to the bedrooms prior to
cooking so that warm moist air does not reach the colder ends of the boat where
it can then condense. Whilst many people see a cockpit tent as a luxury item, to
us it is a vital part of minimising condensation in a yacht. The cockpit tent should be simple and quick
and easy to put on and take off. We can
even leave the frame on. The initial advantage is that no matter which way the wind (and the rain) blows you can have the top hatch open when cooking so the cooking moisture goes out of the saloon Secondly, wet clothes and towels can be dried in the
tent. A common weather pattern in the UK
is to have a clear sunny morning that steadily clouds up. In the early morning warmth of the cockpit
tent, items can have dried before you even get up. You can also air and dry all the cushions and bedding. When we are about to leave the boat, we aim
to have all the bedding and mattresses in the cockpit tent for a period to air
off, before packing the bedding into sealed vacuum bags. If your yacht is on a mooring, this may be
best method of airing mattresses etc. The entry hatch covers are now protected from direct wind
currents and rain. We improve the
situation overnight by covering the horizontal and vertical hatches with sheets
of cloth or PVC. This reduces the
variations of temperature inside the yacht and condensation on the hatches and
cuts out the early Scottish sunrise. Aft cabins often have hatches opening into the cockpit and so these can now be left open inside the tent, even when it is raining. In contrast, many forward cabins have poor ventilation and so closing the door for privacy at night actually worsens the condensation problem. Insulation
Putting insulation into the boat needs to be done with care
to ensure you are not generating another problem. We must be careful that warm air is not
getting into a cold air space behind insulation resulting in condensation in
areas we cannot see. Moisture in the bedrooms and in
the mattresses, is a common problem.
Increasing the air flow under a mattress seems to us to be approaching the problem
from the wrong perspective. We sail in Scotland where the
seawater temperature varies between 8ºC and 14ºC. The area under the bed bases will be close to
these temperatures. We have stuck pieces
of 25mm PIR
insulation board on the underside of the bed bases and hatch covers. The result: in the aft cabins there is now
just one area of significant moisture, on the hatch cover of the batteries,
where we had not been able to place any insulation. We even managed to insulate over the water
tank.
Heating There are basically 3 approaches to living on the
yacht. The “outdoor living approach”
means wearing warm clothes, keeping the hatches open so that the internal
temperature of the yacht only varies by small amounts to that of the
outside. The “cool bedroom approach” is
to open the hatches and cool the yacht cabins down just before you retire for
the night. Finally, there is the
“comfortable approach”. We often have a mixed approach to heating if we are in a
marina. We may run the diesel heater for
an hour or so, as the warmth from the ducting heats much of the structure of
the boat, thus reducing the number of cool surfaces on which condensation could
occur. We may then switch to a small
electric fan heater. We often use a dehumidifier when the diesel heater is on and we have power available. In fact, when it is not too cold, just running the dehumidifier can keep the cabin warm. When we have not been on power on anchor or on a mooring and it has been cold at night, we would aim to run the dehumidifier when we return to our marina at the end of the trip. At the ends of the season, when the yacht is still in the water, we bring the shore power lead direct into the saloon and put a timer on there for a heater. (Yes, I know we have a galvanic isolator, but I like to do it this way.) The photo shows what we do when the yacht is ashore in winter, when the second timer feeds power back onto the yacht's battery chargers. When you leave the
yacht, leave everything lifted up to air off.
If you sail often, just air off one area after each trip. End of Season and Laying Up We live 250 miles from the boat and leave it ashore for 4-5
months over winter. We make sure the
inside of the yacht is as dry as possible before we leave it. Whilst we take all the bedding home, we have
to leave the mattresses and cushions, but these are always left propped up
whenever we leave the yacht. On the day
before we leave the yacht we will run a heater and dehumidifier in each of the
cabins to really dry out the mattresses.
We do not stay on the yacht when it is on its cradle and so we leave a
heater and dehumidifier running overnight.
The next day, we aim to do very little in the boat and no cooking. The yacht is shut up so that there is just
the gap between the vertical and horizontal companionway covers that is open
the air. We leave a greenhouse heater on
timer for around 6hrs overnight and a couple of the dehumidifying gel packs
near the electronics. The battery
charger is also on a timer for half an hour per day. To date, the boat has not felt stale when we
have returned to it and the moisture traps have been around half full. Whilst this approach works for us, yachts left in different environments may need a different approach. A yacht left in the South of East of England could well be subject to much higher variations in temperature over a 24hr period, so the yacht will tend to “breathe” more. In areas where mist or fog occurs regularly, then the air moving into the yacht at night may already supersaturated. So, under these conditions, sealing a yacht and using a decent desiccant humidifier may be the best solution. This could also be the most desirable solution in tropical areas, where the warm moist air is so conducive to the formation of mould. Relying on just ventilating the yacht is not the ideal solution and if that is the only option then any soft furnishing, books etc need to be taken off the yacht. So, if you want your yacht to remain smelling fresh, you and
your crew need to follow certain strategies through the whole of the year. |
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Martin&Rene Hanse 341 Dipper Wheel steering, 3 cabin layout, normally based in Scotland
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