Fuel tank leak

MapisM said:
Ok, so you tested an 800 liters tank by filling 3/4 of it with air pressurized at 0.2 bar for ten minutes.
You previously said that during that timeframe you could see the gauge dropping slowly.
Just guessing a bit now, did it drop to what, 0.18 bar? If the drop would have been even less, you would have struggled to see it on the gauge, so maybe it was even more - but let's stick to 0.18, for sake of reasoning.

A bit of math (didn't do it myself - easier to use a Boyle's law online calculator! :giggle: ) tells us that a pressure reduction from 0.2 to 0.18 bar with a total volume of 600 litres implies a leak of exactly 10 litres of air.
Now, let's leave aside the fact that I'd be worried by a leaking tank regardless of how "big" the hole is, if nothing else because there's only one way the hole can develop over time, and it's not in the direction of getting smaller.
Fact is, the only conclusion you can draw from your test is that there's a 1 liter/minute leak.
Or more, if the pressure actually dropped to less than 0.18 bar - and I have a funny feeling that it could well have been the case.
Personally, I'm not sure I'd dismiss that as "not big".

Mind, this is all theoretical of course, and even assuming that the leak is exactly at 1/4 of the total tank height, I guess that after filling up, you never found 600 liters of fuel (i.e. 3/4 of tank) in the bilge after 10 hours - which is what would happen with a constant leak of 1 liter/minute.
But that's because the real world leak rate depends also on other considerations: first and foremost, actual fuel pressure at 1/4 tank (even if it depends on how high the tank is) is certainly much lower than 0.2 bar. Besides, I guess it's a bit easier for air to find its way out compared to fuel.
Then again, whether it takes one minute or one day for one liter of fuel to leak out of the tank, it's something that needs attention anyway!

Sorry to sound a bit scaremongering, but I had to bang my head against the wall on this matter before, and I know how misleading these tests can be.
Good luck!
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Thanks P, all good stuff. In fact after ten minutes my pressure gauge read zero...but there was still a lot of pressure when released from the tank. That is why I didn’t trust my pressure gauge but by then I decided to add fuel to see if the leak restarts. So after five days I pumped out of the bilge a quarter of a litre of dirty watery looking fuel. Before I was getting half a litre a day of clean golden diesel.
Anyhow I added another ten litres today and Wednesday I will check again but I have told the wife to expect the worse
 
Well, of course the gauge might as well be inaccurate, but it's inherently tricky to use air for these tests anyway, due to its compressibility.
In my previous example, if the pressure drop was from 0.2 bar to zero (rather than just to 0.18 bar), the air leaked from the tank should have been a whopping 100 liters, go figure!
In fact, I'm not surprised to hear that you still felt some residual air pressure, even after the gauge didn't sense any.

One alternative you might try, before pressurizing again the tank, is blowing into it a lot of the special "smoke" used for testing.
It's a sort of vapour, dense and very visible (it also comes in different colours!), but which disappear after some minutes without any dirt and/or residues.
That doesn't make the test more reliable in terms of pressure variations obviously, but with a bit of luck it could help spotting the area, if not the exact point, where the leak is.
And if nothing else, some of that smoke coming out from the tank into the e/room would be the final proof that there is indeed a leak, beyond any reasonable doubt!
 
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I would have removed all the pipes to and from the engine / geny and used bungs .
Also ( it’s not leaking from theses ) bunged the filler and vent at the top of the tank .
This drop recorded by MapishM’s eloquent post ^^ could easily be a duff “ clamp “ = introduction of erroneous diagnosis.How do you know it isn’t?

Also liberally talked up suspects be them pipes or parts of the vis tank while it’s still leaking .

Without sight glasses you still don’t know if the tank is or ever was dropping .I mean consider this …….it’s a duff filler pipe always was and in the melee of filling up fuels gotten under the tank into the Beny core and as I said and Bouba confirmed there was a bit when it was rocking about in the water but when on the hard static seeped back into the ER emerged under the motor .
How do you know it isn’t without actually observing/ talking the filler apparatus while filling .
Eg a split where the filler pipe fits either neck of tank inlet or neck of deck filler ? Or plane duff pipage split - pin holed on a bend .
Could be years of fuel in the core that’s just now been discovered.

So far we know nothing no further on due to the unreliability of the “clamping “.
 
Portofino said:
I would have removed all the pipes to and from the engine / geny and used bungs .
Also ( it’s not leaking from theses ) bunged the filler and vent at the top of the tank .
This drop recorded by MapishM’s eloquent post ^^ could easily be a duff “ clamp “ = introduction of erroneous diagnosis.How do you know it isn’t?

Also liberally talked up suspects be them pipes or parts of the vis tank while it’s still leaking .

Without sight glasses you still don’t know if the tank is or ever was dropping .I mean consider this …….it’s a duff filler pipe always was and in the melee of filling up fuels gotten under the tank into the Beny core and as I said and Bouba confirmed there was a bit when it was rocking about in the water but when on the hard static seeped back into the ER emerged under the motor .
How do you know it isn’t without actually observing/ talking the filler apparatus while filling .
Eg a split where the filler pipe fits either neck of tank inlet or neck of deck filler ? Or plane duff pipage split - pin holed on a bend .
Could be years of fuel in the core that’s just now been discovered.

So far we know nothing no further on to the unreliability of the “clamping “.
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We had an engineer from Beneteau come look at it before New Years and he seemed to think along Porto’s line. Which is why I have been as deliberate as possible in searching for clues. Anyhow if there is diesel in the bilge tomorrow I am calling it a tank hole!!
 
How do we know and can eliminate it’s not just a previous overfill ?
I mean duff breather connection simply spilled it under the tank unknowingly and as said it’s been like this years and now manifest ( due to the blocking angle in the yard ) as a leak as it seeps out?
 
Portofino said:
How do we know and can eliminate it’s not just a previous overfill ?
I mean duff breather connection simply spilled it under the tank unknowingly and as said it’s been like this years and now manifest ( due to the blocking angle in the yard ) as a leak as it seeps out?
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Because I am trying to start the leak up again by filling the tank...
 
Bouba said:
We had an engineer from Beneteau come look at it before New Years and he seemed to think along Porto’s line. Which is why I have been as deliberate as possible in searching for clues. Anyhow if there is diesel in the bilge tomorrow I am calling it a tank hole!!
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Arh new info folks ……….” We had an engineer “ !
 
Bouba said:
Because I am trying to start the leak up again by filling the tank...
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But you need to chalk / talk pipe age and observe under the deck filler on top of the tank pipes while it’s filling .
Otherwise it just goes where it’s going now and seeps back and looks like a tank leak when it’s split fuel .
 
Any competent yard or mechanic is going to nod wisely as you regale him with your research and conclusions and then go away and do his own thing which will be in the absence of anything you haven't mentioned to remove the tank. You are wasting time and effort.
 
Bouba said:
Because I am trying to start the leak up again by filling the tank...
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If youre filling the tank up using the normal filler hole & pipe then if that does have a leak (as per portos hypothesis) you'll still get fuel in the bilge - but you wont know whether its the filler pipe or the tank with a hole
 
julians said:
If youre filling the tank up using the normal filler hole & pipe then if that does have a leak (as per portos hypothesis) you'll still get fuel in the bilge - but you wont know whether its the filler pipe or the tank with a hole
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I have checked all the pipes (including the filler) and all are dry (obviously there are a few places that are inaccessible but there is no sign in the surroundings of diesel). I suppose I could attach a long hose to the spout of my fuel can and bypass the filler pipe
 
Honestly, you could still be fannying around with home pressure tests and and adding small amounts of diesel in 12 months time and be no further forward. Give it to a professional, bite down on your gumshield and expect a suitable invoice. The sooner you get this fixed the sooner you can get back on the water and forget about it.
 
Aardee said:
Honestly, you could still be fannying around with home pressure tests and and adding small amounts of diesel in 12 months time and be no further forward. Give it to a professional, bite down on your gumshield and expect a suitable invoice. The sooner you get this fixed the sooner you can get back on the water and forget about it.
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Yes..tomorrow is the cutoff line
 
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