Diesel Tank Lagging

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I have been giving the condensation in diesel tank problem a little thought over the end of year celebrations and wondered if anybody has 'lagged their tank'?

I'm in the fortunate position of having a tank that is easily accessible, being tucked away behind the aft cabin in the transom. With the large amounts of polystyrene about at this time of year I might be tempted to build a structure to insulate the tank and reduce that naughty condensation.
 
Stemar said:
Before going to all that trouble, I'd be tempted to try and keep the tank dry inside by having the breather air go through a jar of silica gel. Bung the gel in the microwave or oven for a bit now and then to refresh it.
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Getting to the breather tube is a little more troublesome as it requires the removal of the tank!

I am not convinced that there is much, if any, fresh air that gains access to the tank during the winter months as I seal my breather with electric tape. During the summer months it is different as fuel is being used and replenished via the filler cap.

The lagging is more about keeping the contents of the tank at a constant temperature to discourage condensation over the year.
 
john_morris_uk said:
How much of problem are you trying to solve?
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It is preventative...

Looking to do some high latitude sailing in the future. As condensation issue are seen as the temperature difference in air and the amount of liquid it can hold, my train of thought is to reduce the changes of temperature in the tank as I can't control the temperature of the transom space.
 
Sandy said:
It is preventative...

Looking to do some high latitude sailing in the future. As condensation issue are seen as the temperature difference in air and the amount of liquid it can hold, my train of thought is to reduce the changes of temperature in the tank as I can't control the temperature of the transom space.
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Understood, but it assumes there’s moisture in the air in the tank. I saw some calculations somewhere once that says it’s negligible.

Edit typo sorted
 
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There have been theoretical studies suggesting that this is a non-problem. Over and above that, Mainesail, a highly respected yachting investigator, conducted a practical investigation that resulted in no condensed atmospheric moisture after 13 years! Google will find many references to his work.
It is far more important to ensure that the filler cap is water-tight.
 
My 220 litre fuel tanks have dirt/water sumps. I check them religiously, especially after rough passages, and have never had a measurable amount of water.
 
vyv_cox said:
There have been theoretical studies suggesting that this is a non-problem. Over and above that, Mainesail, a highly respected yachting investigator, conducted a practical investigation that resulted in no condensed atmospheric moisture after 13 years! Google will find many references to his work.
It is far more important to ensure that the filler cap is water-tight.
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Yes. I did a worst-case calculation on a half full 100litre diesel tank and came up with 5 millilitres of water per year. In reality, I have never seen any water in my water-seperator nor in the bottom of the tank when I do my annual sampling. I check the filler cap seal frequently.
 
Yes - I have lagged my diesel tank and it does reduce condensation inside. My tank is stainless, about 50 litres. I used an old yoga mat my wife donated! It is foam rubber, about 6 mm thick and was just long enough to fold around the back, top and front sides. The two sides of the tank are close up against the bulkheads, so no insulation is required. I used to get water trapped in the sedimenter, but now just drain off about a teaspoonful from the bottom of the tank at the end of the season.
 
Like several others, I have doubts about condensing water vapour in fuel tanks amounting to a significant problem. But leaving that question aside, I also do not see how insulation of the tank will help in the long run.
I imagine that air flow in and out of a fuel tank at rest will be very small indeed. If this is correct, any air that is sucked in as fuel is consumed will stay there until the tank is filled up again and the air pushed out. So if motoring on a hot, humid day, some air with high moisture content will likely enter the tank.
When the engine is shut down and day turns to night, the tank and its content will cool. Insulation may slow the temperature drop but cannot stop the tank from reaching ambient temperature – unless it is heated in some way. When the temperature has dropped sufficiently, condensation of the water vapour in the air will take place. If not on the tank wall, then on the fuel surface.
 
neil_s said:
Yes - I have lagged my diesel tank and it does reduce condensation inside. My tank is stainless, about 50 litres. I used an old yoga mat my wife donated! It is foam rubber, about 6 mm thick and was just long enough to fold around the back, top and front sides. The two sides of the tank are close up against the bulkheads, so no insulation is required. I used to get water trapped in the sedimenter, but now just drain off about a teaspoonful from the bottom of the tank at the end of the season.
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I think you're in a dream world, or experiencing the placebo effect. 6mm of foam left for weeks in winter will soon reach an equilibrium temperature and have no effect on condensation.
 
penberth3 said:
I think you're in a dream world, or experiencing the placebo effect. 6mm of foam left for weeks in winter will soon reach an equilibrium temperature and have no effect on condensation.
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I can only say it works for me - and I don't bother filling up the tank at the end of the season.
 
neil_s said:
I can only say it works for me - and I don't bother filling up the tank at the end of the season.
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Its one of those items that's very difficult to prove works or not.

Its like a Rope Cutter - unless you actually see the rope IN the cutter - you will never know if it works or not.

'Sweat' in a tank is same as in an engine ... its due to changes of temperature IN and around it ... not just a drop in temperature.
 
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