sarabande
Well-Known Member
Brought back recently a boat from the continent, where she had been static in the marina for a few months. As part of the due diligence before departing, we flushed the freshwater tanks (1 x flexible Crewsaver mounted high serving as a storage tank to the 1 x flexible Plastimo, used as the day tank to the fresh water services) .
Back in the UK, I removed the cabin sole and inspected the bilge where the Plastimo (lower) tank is located; it was evident that the bilge had filled with a mixture of rain water and diesel in the past, as there was a 'tide mark' around the turn of the bilge. Both tanks are in 'envelopes' to prevent chafe, and the tank in the bilge was slippery with the oil/water mixture.
I took out both tanks, and cleaned them. The Crewsaver, which had not been immersed in the gungy bilge, was fine. However, I could see that the inner bladder of the Plastimo, normally a clear translucent white, was discoloured; so, I opened the outer envelope, and took out the inner bladder.
There were two areas of quite different coloration and flexibility, with dark patches of what looked like seaweed attached to the inner wall. Serious and aggressive cleaning failed to remove the dark patches, so the bladder was declared unfit for service, and cut up.
Additional investigative work at the filler cap revealed that the original O-ring - perhaps 15 years old ? - had started to crumble and was no longer acting as an effective seal. Over the months that the boat had been unused in the marina, and possibly prior to that, it seems likely that sea and rainwater had washed into the tank through the defective filler cap, and contaminated the water tanks. It wasn't just rainwater, but also faecal droppings from birds, and general shore dust from the adjacent land !
So our poor old tank was acting as a primaeval soup with biologially active water on the inside, and decaying diesel and water on the outside. The black bits on the inside were probably slime moulds, which at the best of times are not exactly known as sterile adjuvants to a freshwater tank.
The pictures show a section of the tank cut up for inspection, and clearly show the translucent white parts which were at the top of the tank and neither in contact with the 'soup' nor the external oily water; the plastic retains its flexibility. The brown parts are the areas in contact with the bilge water and/or the inner contamination; the material here is hardened and almost brittle.
Lessons learned;-
1 don't position a water filler cap so that it sits on the deck adjacent to the toe rail, where sea and rain water gather.
2 check that the O-ring is capable of providing a proper seal against ingress of water.
3 don't allow plastic water tanks to be in contact with oily water.
4 make sure that you can see the inner part of water tanks to check the contents for cleanliness
(5) design the cabin floor so that it can be taken up in sections, and not in one long length requiring two strong men and much cursing.
Following on from another thread, I will also be giving consideration to placing some support material (not carpet in case it gets wet) under the tanks to prevent abrasion and immersion in even a bit of the bilge water.
Cost of new O-ring ? Less than 1 p from a box of them in the workshop.
Cost of new tank liner ? About £50
Cost of time? About 6 hours
I have an unused in-line polypropylene wound waterfilter with a clear plastic bowl. For future fillings, I am going to use this between the hose and the filler cap, to ensure that what goes into the tanks is clear of any sediment or general gunge. And Milton tablets (VicS's recommendation, as against the bleach I used to use
)
I only mention brand names for the sake of accuracy. I do not think that any flexible tank, plastic or PVC, would survive being immersed in hydrocarbons and seawater, and being filled with gull poo soup.
Back in the UK, I removed the cabin sole and inspected the bilge where the Plastimo (lower) tank is located; it was evident that the bilge had filled with a mixture of rain water and diesel in the past, as there was a 'tide mark' around the turn of the bilge. Both tanks are in 'envelopes' to prevent chafe, and the tank in the bilge was slippery with the oil/water mixture.
I took out both tanks, and cleaned them. The Crewsaver, which had not been immersed in the gungy bilge, was fine. However, I could see that the inner bladder of the Plastimo, normally a clear translucent white, was discoloured; so, I opened the outer envelope, and took out the inner bladder.
There were two areas of quite different coloration and flexibility, with dark patches of what looked like seaweed attached to the inner wall. Serious and aggressive cleaning failed to remove the dark patches, so the bladder was declared unfit for service, and cut up.
Additional investigative work at the filler cap revealed that the original O-ring - perhaps 15 years old ? - had started to crumble and was no longer acting as an effective seal. Over the months that the boat had been unused in the marina, and possibly prior to that, it seems likely that sea and rainwater had washed into the tank through the defective filler cap, and contaminated the water tanks. It wasn't just rainwater, but also faecal droppings from birds, and general shore dust from the adjacent land !
So our poor old tank was acting as a primaeval soup with biologially active water on the inside, and decaying diesel and water on the outside. The black bits on the inside were probably slime moulds, which at the best of times are not exactly known as sterile adjuvants to a freshwater tank.
The pictures show a section of the tank cut up for inspection, and clearly show the translucent white parts which were at the top of the tank and neither in contact with the 'soup' nor the external oily water; the plastic retains its flexibility. The brown parts are the areas in contact with the bilge water and/or the inner contamination; the material here is hardened and almost brittle.
Lessons learned;-
1 don't position a water filler cap so that it sits on the deck adjacent to the toe rail, where sea and rain water gather.
2 check that the O-ring is capable of providing a proper seal against ingress of water.
3 don't allow plastic water tanks to be in contact with oily water.
4 make sure that you can see the inner part of water tanks to check the contents for cleanliness
(5) design the cabin floor so that it can be taken up in sections, and not in one long length requiring two strong men and much cursing.
Following on from another thread, I will also be giving consideration to placing some support material (not carpet in case it gets wet) under the tanks to prevent abrasion and immersion in even a bit of the bilge water.
Cost of new O-ring ? Less than 1 p from a box of them in the workshop.
Cost of new tank liner ? About £50
Cost of time? About 6 hours
I have an unused in-line polypropylene wound waterfilter with a clear plastic bowl. For future fillings, I am going to use this between the hose and the filler cap, to ensure that what goes into the tanks is clear of any sediment or general gunge. And Milton tablets (VicS's recommendation, as against the bleach I used to use
)I only mention brand names for the sake of accuracy. I do not think that any flexible tank, plastic or PVC, would survive being immersed in hydrocarbons and seawater, and being filled with gull poo soup.
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