Welded floor - bung out when storing it?

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    Posted: 13 Nov 2019 at 1:04pm
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Our current boat has a fully sealed welded floor. It has a separate bung from the bilge one, but seeing as water should never be entering that space, we tend to just leave it in all the time.

Now that we're getting into some warmer weather, we went to take the boat out the other day and I noticed the floor was very slightly bulging. On removing the bung there was a noticeable hiss as the pressure was released. However, being Wellington, a couple of days later it was bloody cold and when we took the boat out the floor was bowed inwards this time.

What's the convention here? Should we be leaving the bung out all the time (other than when it's in the water obviously) or is it fine for this expansion/contraction to go on?
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Post Options Post Options   Likes (1) Likes(1)   Quote Redfinger Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 2019 at 2:01pm
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You should leave the "ballast" bung loose (only when storing obviously) to allow air to enter buoyancy areas - as ali gets hot and cold it not only contracts and expands as you have seen it can allow condensation - the air flow will dry it out inside the compartment. Remember to tighten up again when using boat.

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Post Options Post Options   Likes (0) Likes(0)   Quote Big -Dave Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 2019 at 3:01pm
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Is the bung internal wondered if fiyting a sintered filter might be the answer?
It could breathe, but resist flow of water.
If it is the only point of ventilation, the compartment would never be able to fill with water.

Otherwise, bear in mind, on a hot day in the sun, the floor can get pretty hot, then dropped in 20 degree water, significant expansion/contraction could occur.
you can't fix an idiot with duct tape, but it does muffle them for a while...
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Post Options Post Options   Likes (0) Likes(0)   Quote waynorth Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 2019 at 3:24pm
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It's not just the change in atmospheric temperature Rozboon - the temperature-induced pressure difference also occurs each time you put the boat in to cold water on a hot day, and vice versa. 

The pressure variance is proportional to the change in absolute temperature, so if you are looking at a 15 degree Celsius difference from say 25 degree air to 10 degree water, that becomes 298 absolute down to 283 absolute. That's only about 5%, so the pressure change would be +/- 0.05 bar, or +/- 0.7psi, which is pretty insignificant when you consider these sealed buoyancy tanks are pressure tested at the factory to ensure they are airtight. If the boat is sitting in the sun and out of the breeze though, the alloy can become skin-burning hot - maybe 60 degrees C - which could see a pressure change of more like 15%.

I have the same sort of sealed underfloor tanks, and only remove the bungs perhaps once a year, just to check that no water has got in. 

It can sound a bit dramatic as the hot/cold air rushes in/out past the bung. Seeing a noticeable bulge or dip in the floor sounds a bit odd though - might be worth a quick call to the manufacturer in case it has become detached from the deck stringers.
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Post Options Post Options   Likes (0) Likes(0)   Quote MATTOO Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 2019 at 4:35pm
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Waynorth is onto it.

Your concerns would be if salt water got in there.
Hidden enviroment and will require maintenance.
Don't want to go down that path.

Expansion from heat, if land stored for a while, nothing wrong with cracking the vents open if you can remember to close prior to leaving home.

Other than that, Waynorth is correct in his assessment, expansion not entirely relevant if the boat is well made and not to thin walled on its materials as severe expansion or retraction can over many years weaken weld points.

Most unlikely but most manufacturers will have built in this equation originally.

I say most as in life we have leaky buildings don't we.
Depends on competency and mindset of manufacturer.
Just cruising in my now sweetas pimped out Southern 755 HT0!
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