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It has now been over a year since I set this up and I have now checked the tank on 7 separate occasions. To check it I remove the fuel sender and send some colored paper into the tank rubbing all the walls and tank floor I can get to. As of yet every single attempt at finding "condensation" has come up BONE DRY...

Does an empty tank condensate.....?? This one doesn't....

EDIT 6/25/14: Just checked it again last night and it is still bone dry. Not even a hint of moisture. I chose to check it yesterday because humidity levels have been in the 90%+ range and night time temps have been dipping to the high 40's 47F - 49F and day time barn temps reaching 118F!!! That is approximately a 70F swing with 80-90% humidity....

EDIT 5/18/14: About two months ago I set it on some 2" thick stone pavers as some surmised that the tank needs to remain cool as it would if in the belly of a boat at ocean temp. Up here in Maine, when hauled out for the winter, there is no "ocean temp" but I placed it on the slower to change temperature stone pavers anyway.. Results, STILL BONE DRY....

EDIT 10/22/15: Topic just came up on one of the sailing forums and it prompted me to go check the tank again. Just finished a very hot summer and now we are seeing temps into the 20's at night and 60's during the day. Tank interior is still 100% bone dry. Considering the tank has been here since March 31 2013 and today is October 22, 2015 I think it is safe to say an empty sailboat fuel tank will not "magically" fill with water.

I personally prefer to leave my tank empty some prefer full. You will have to decide what works best for your boat...

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Guest 13-Sep-2017 19:41
This condensation myth is driven by every one involved.

The boat owner do not want to think that the water is coming from a faulty O-Ring.

The boatbuilder does not want to admit that the breather could be the cause.

The fuel staion do not want to admitt they had they sold dirty fuel.

I have only had water once in 40 years of boating. The cause was a faulty O-Ring. I never fill up the tank if I do not have to.

Volvo Pentas recommondation is to store the boat with empty tanks as the shelf liffe for fame dirsel is only 2.6 month depending on temp.
Jim K 17-Feb-2017 18:20
I think Jon P-T has a good point. With the very high temperatures there is a good chance you're re-evaporating during the day any small amount that might have condensed at night. Fuel in the tank could act as a check valve, allowing condensation while preventing evaporation. I can't drain mine, so leave them completely full if I have to, but usually we use the boat and go someplace warm in the winter, better yet.
Guest 30-Oct-2015 14:32
I always left my tank empty and it has not any wáter.
if the tank is empty and condensate wáter, the wáter is going to evaporate.
Larry 02-Sep-2014 11:58
Alchemy has it exactly right!
Mike 17-May-2014 13:54
Also... Diesel is VERY hygroscopic. You're bringing in the water with the fuel in many cases. It then precipitates out of the fuel and settles to the bottom of the tank as condensate.
Guest 19-Apr-2014 17:18
I have a fuel polishing business and have pumped a lot of water out of tanks. I don't know where it comes from but there are limited sources. Splashing into the vent, dry rotted fuel cap O-rings and dry rotted sending unit gaskets are all sources on the boat. You can also get it from the source. Always try to visit busy marinas, if there has been a sever storm recently or an excessive high tide their tanks can get infiltrated as well.
Paul West
Guest 12-Apr-2014 16:51
Hi Maine,
One situation often found on boats but not modeled by your setup is to have the tank held at a temperature colder than the air via contact or proximity to the ground or water. I think this will make a big difference in the results.