Boat fuel tank problem

Submitted: Thursday, Feb 02, 2006 at 08:53
ThreadID: 30343 Views:13752 Replies:9 FollowUps:0
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Hi, I'm hoping somebody has experienced and solved this problem before and can advise. It concerns a 6.5 metre fibreglass boat with a plastic underfloor fuel tank. It has a well sealed filler cap and the only other opening is two breather tubes that emerge high up in the outboard well. These have the usual top designed to prevent water access. And of course the other opening in the tank is the fuel pickup.
Last two times the boat has been used, each a few months since last used, there has been more than a litre of water in there. Only after a huge bun fight to remover the water was it possible to get under way.
The manufacturer has had a look and declares that water cannot get in. He doesn't understand what's causing it.
Any clues?
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Reply By: Nifty1 - Thursday, Feb 02, 2006 at 09:12

Thursday, Feb 02, 2006 at 09:12
Hi PJ,

I believe that with the normal expansion and contraction of a fuel tank due to diurnal variations (been wanting to use that word for ages) it will suck in moisture-laden air through the breather. When the tank cools at night, the moisture will condense and stay in the tank. It will only be a drop or two but it adds up over time. Problem is worse in humid climes, and where there is a big night/day temperature variation.

I even know how to get the water out! With the tank nearly full, pour in some methylated spirits and use the fuel or pump it out. The meths mixes with the water and it in turn mixes with the fuel. I'd use about half a litre in a 20 litre tank - not sure what the right amount is if you have a full litre of water though.

It's an old trick I used with the fuel tank on trailer sailers which didn't get a lot of motor use. We'd do a whole season on about $10 worth of fuel.
AnswerID: 152454

Reply By: Diesel Power - Thursday, Feb 02, 2006 at 09:13

Thursday, Feb 02, 2006 at 09:13
Most probably condensation, is the boat stored in a shed? or in the open? If you leave the boat idle for lengths at a time you will find condensation forms in the tank due to temperature fluctuations. If the fuel level is quite low this can make the problem worse.
I suggest
1. Fit a water separator if not already fitted.
2. Fill the tank before putting it away.

Unfortunately you may find suggestion 2 could lead to fuel going stale if the boat is left for extended periods but for a month or two it shouldnt be a problem.

Hope this helps

Cheers and Beers
AnswerID: 152455

Reply By: Michael B - Thursday, Feb 02, 2006 at 09:15

Thursday, Feb 02, 2006 at 09:15


Michael B (SA)
AnswerID: 152456

Reply By: pmacks - Thursday, Feb 02, 2006 at 10:03

Thursday, Feb 02, 2006 at 10:03
The problem is caursed by leaving your boat with nearly empty tanks and is a combination of condensation and moist air being sucked into the tank through the breather tube.
The only way to lessen the problem is to put the boat away with full tanks.
I have a boat with 150 lt underfloor tank and this is how my mechanic told me to overcome the problem.
AnswerID: 152465

Reply By: PJ73 - Thursday, Feb 02, 2006 at 10:38

Thursday, Feb 02, 2006 at 10:38
Thanks for the replies, with a combination of storing boat with full tanks and Nifty1's metho idea I think I'll manage a bit better now.

AnswerID: 152473

Reply By: Gerhardp1 - Thursday, Feb 02, 2006 at 14:55

Thursday, Feb 02, 2006 at 14:55
Nowadays you don't need to use metho any more - just fill up with the 10% Ethanol blended fuel (or if you like 50/50 with normal fuel for a 5% blend) and the water will be gone as well. Plus you have a higher octane fuel which might make your motor run better.
AnswerID: 152519

Reply By: Dazz78 - Thursday, Feb 02, 2006 at 17:02

Thursday, Feb 02, 2006 at 17:02
G'day PJ73

Not sure about using ethanol blended fuel. The BP depot where I buy my fuel recommends not to use ethanol blend for any marine use. Don't know the reasons but would be worth asking. I will ask again myself next time I get fuel.

AnswerID: 152546

Reply By: Member - Woodsy - Friday, Feb 03, 2006 at 08:23

Friday, Feb 03, 2006 at 08:23
Hi PJ73

The breathers on my underfloor tank are rolled into 2 full 360° vertical coils before they attach to the external breather caps.
This means that there is always a 'seal' of fuel held in the coils that prevents moisture from entering.

Well I think that is the theory.

Happy boating and
Happy 4 wheeling


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AnswerID: 152668

Reply By: geocacher (djcache) - Saturday, Feb 04, 2006 at 19:10

Saturday, Feb 04, 2006 at 19:10
Would fitting something like a filter in the breather line, as is often done in diff breathers not assist in these cases?

Something like a water trap like used on compressor air lines? They even self drain (some of them) and they're designed to get water out of humid air?

AnswerID: 152995

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