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Moving tank vent - to where?

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Kox View Drop Down
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    Posted: 20 March 2023 at 16:28
I have had diesel pest in my fuel tank during wintertime. 

I cleaned the tank, filters and hoses from the tank to the engine. Today I put a finger in the filling hose from inside the tank, and discovered gunk there also. One shouldnít wonder, but I didnít think to check. 
Stupid me!

While Iím at replacing the hoses anyway, I would like to reroute the vent hose. Now it is placed on port side just aft of the spray hood. The hose is about 6.5m long, and it has kinks on it. I donít know if thatís the proper word, but diesel (and condensed water) will be trapped in the hose, since it is not ďfallingĒ all the way to the tank. 
When filling the tank diesel is spluttering out of the vent. 

I would like to mount the vent on the transom port side. However the outlet of my Webasto heater will be about 25cm below the vent. 

Any comments appreciated;)
Hanse 315, Ella, Hull# 640
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Kox View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kox Quote  Post ReplyReply Direct Link To This Post Posted: 22 March 2023 at 09:13
To answer my own question.

I called Webasto Denmark, and they did not see any problems in mounting the vent close to the heater exhaust.

I then went to a spare part supplier to get a new vent that I would then mount on the transom.
The guy I talked with, advised against a transom mounted vent. The reason being that since the fumes are being dragged in behind the boat when it is moving, and since the exhaust gases contain a lot of H2O, air with vaporized water will enter the tank to replace the diesel used by the engine (and in my case also the heater).
He claimed that's why the vent is normally placed somewhere on the side further forward.

So instead of moving the vent, I bought a 90 deg. vent, which should make it possible to route the vent hose with a constant "negative rise" to the tank.
Hanse 315, Ella, Hull# 640
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redlion View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote redlion Quote  Post ReplyReply Direct Link To This Post Posted: 22 March 2023 at 11:30
I too have had problems with water in my fuel tank for years and have to empty my fuel/water separator (Diesel Bug Treatment | Fuel Treatment & Additives | Fuel Guard (fuel-guard.co.uk) throughout the season. I estimate I take out maybe 1/4 to 1/2 pint of water per summer and the diesel always has a milky appearance. I will be interested to see your modification and hear if it improves matters. Thanks for posting.
It,s not over till the fat lady drowns
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Kox View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kox Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2023 at 14:53
Originally posted by redlion redlion wrote:

I too have had problems with water in my fuel tank for years and have to empty my fuel/water separator (Diesel Bug Treatment | Fuel Treatment & Additives | Fuel Guard (fuel-guard.co.uk) throughout the season. I estimate I take out maybe 1/4 to 1/2 pint of water per summer and the diesel always has a milky appearance. I will be interested to see your modification and hear if it improves matters. Thanks for posting.


I think the proper name for what i had is Diesel bug. i have attached a picture of the filter. My diesel tank had exactly the milky appearance you describe, so I think you might have it as well.

My modification is not going to result in less water in the tank than before. But it is going to ensure that no water can be trapped in the hoses.  
A little water will condense from the air when the temperature changes. That is inevitable. I am going to make sure that it runs to the tank by means of gravity by itself. 
From the tank it is either sucked away and taken out in the water separator filter or burnt in the engine. 
I plan to suck the bottom of the tank 2-3 times a year in the future. I bought a small 12V dieselpump with a capacity of 1,5l/minute, which should be so slow, that I can get most of the water out, without stirring up the tank.

Bear in mind, that the tank is not level, it is lower in the front, and the suction is about 1 cm above the bottom of the tank, this means that quite some water can be in the tank that will not be collected in the filter.

The diesel bug needs water and diesel to grow and it lives in the layer between water and diesel. So if you remove water, the bug can not live.

I am no expert, but I used quite some time and effort to get rid of it:

1. Emptied the tank and threw away the contaminated diesel. (Bye, bye 50l of diesel).

2. Cleaned the tank with fresh diesel. Emptied it again. Cleaned with brake cleaner. Cleaned with clean diesel again.

3. REplaced both filters.

4. Put 10l of fresh diesel in the tank and removed the return hose from the pump to the tank. 

5. Ran the engine and collected the 10l of diesel and threw it away.

6. Changed both filters again.

1.1 Only then I discovered by chance, that the hoses have the bug also. I'd recommend you to do it after 1. above.

7. Replace filling and vent hoses

8. Run the engine hard a couple of hours 

9. Replace the filters again after 10 hours or so.

10. Suck the tank at intervals.

11. Mumble "dieselpest!!" (diesel bug) in the future when my dad and co-owner suggests that we only replace the oilfilter every second time we replace the lubeoil :-) He also suggested we replace the water separator filter every 5 years. I'm soo glad I did it anyway!


Hanse 315, Ella, Hull# 640
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grumpydatagit View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote grumpydatagit Quote  Post ReplyReply Direct Link To This Post Posted: 24 March 2023 at 19:05
I  change the "O" rings on the deck mounted Fuel and Water fillers every other year as these let water in. It took me ages to find the source of salty water in my fresh water tanks !
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Kox View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kox Quote  Post ReplyReply Direct Link To This Post Posted: 31 March 2023 at 11:36
Originally posted by grumpydatagit grumpydatagit wrote:

I  change the "O" rings on the deck mounted Fuel and Water fillers every other year as these let water in. It took me ages to find the source of salty water in my fresh water tanks !

Do you, by chance, remember the O-ring size, Grumpy?
Hanse 315, Ella, Hull# 640
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grumpydatagit View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote grumpydatagit Quote  Post ReplyReply Direct Link To This Post Posted: 02 April 2023 at 14:27
This is the size I used

4mm Section 38mm Bore NITRILE 70 Rubber O-Rings

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