Plastic inbuilt fuel tanks

[billfisher]

Last shot as I can see your mind is set.
When the air expands it will push the fuel up the breather and out.

That's exactly what you said before.

It's not air it's fuel vapour mixed with some air. You have seen what happens when you leave a closed fuel tank out in the sun? It's the same effect - having liquid fuel in the breathers causes a vapour lock.

And why would i disbelieve something I have seen with my own eyes/ why is it so hard to take it on board?

[billfisher]

thanks mate , yes I did. I see what your saying and probably agree Now , issue is it’s hard to not do that when filling up. And always like it well full as reduces condensation issues.

If your hatch is readily removable it's not too hard to do it visually. I might take a little bit of trial and error at first. I doubt that leaving a small air space will cause any condensation problems. Some say that's a bit of a myth in any case. If you are getting significant water contamination it is probably due to some other effect.

[Achjimmy]

If your hatch is readily removable it's not too hard to do it visually. I might take a little bit of trial and error at first. I doubt that leaving a small air space will cause any condensation problems. Some say that's a bit of a myth in any case. If you are getting significant water contamination it is probably due to some other effect.

it’s definitely not a myth. I was trained as a pilot , we always kept tanks full to reduce condensation in saying that there was no evidence of it in this tank

[billfisher]

it’s definitely not a myth. I was trained as a pilot , we always kept tanks full to reduce condensation in saying that there was no evidence of it in this tank

Well this is food for thought and certainly makes a lot of sense:

https://www.yachtsurvey.com/myth_of_...fuel_tanks.htm

PS: I had a lot of problems with water in fuel. Once I finally got over the condensation theory I was able to find and fix the cause.

[blacklab]

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[blacklab]

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[scottar]

I have seen one American boat - belongs to a mate who lives across the road, where by design, the tank will vent in but not out. The boat is kept under a black fitted cover and during it's first summer, he noted the tank top would expand - it's stainless . He contacted the manufacturer and was advised this was normal and was to comply with the USA pollution law requirements. If your breathers are of a similar design it could explain what is happening. If they are effectively an open hose that allows pressure to equalize in either direction, any fuel in a breather line will simply be expelled by the pressure generated as the fuel vapour in the tank expands - unless of course there is some other sort of blockage. Some vessels incorporate an expansion tank in their breather lines to prevent this occurring.

I have found the easiest way with my own boat is to store (and preferably fill as well) the vessel with a bow down attitude when tanks are full. With the way my breathers are installed the rear one basically leaves the tank vertically whereas the hose for the front one travels the length of the tank before going upwards - thus potentially creating a "belly" when stored bow up in the front line that will hold fuel and the rear breather being below the fuel level where it exits the tank if the vessel is stored bow up. By storing bow down it is physically impossible for the breather that is positioned where any gas bubble will be located to be holding any fuel - unless the tank is overfilled.