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View Full Version : Water/Condensation in fuel tanks -To fill or not to fill



Mad Fish
07-07-2016, 05:56 PM
That is the question. In my current boat I have an underfloor 160 ltr tank in my (Cruise Craft) I always keep it full at all times, filling it up after every trip leaving no room in the top of the tank for condensation to form between fishing trips. In 10 years I have never had any issue with water in my fuel, never had any in the fuel filter. Most of my trips are around 100 to 150k return.
At the end of the year I am upgrading the boat with the new boat holding 450 ltrs in two underfloor tanks (Noosa Cat). Carrying 450 ltrs of fuel around when I will probably only use 150 to 200 ltrs for most of my trips is my issue - I would like to keep them full at all times but the extra weight is likely to affect my fuel consumption. If I only keep them half full I am concerned with condensation building up in the top of the tanks.
I know that the fuel filters are there to filter the water out but a friend of my has had all sorts of issues with water in the fuel. What's your thoughts or experience.
Cheers John 8-)

PixieAU
07-07-2016, 05:59 PM
I thought you weren't meant to leave the tank full.

scottar
07-07-2016, 06:23 PM
I have never bothered in the boats I have used / owned and I have never had a problem or had water in the separators either. Have had stainless, alloy and plastic tanks. The boat is kept indoors - not sure whether this makes a difference. My preference is to leave them empty so as to add as much fresh fuel as possible just before use.

OldManRiver
07-07-2016, 06:27 PM
I have a coastal cat with 2 underfloor tanks that hold 200 litres each. I refill every second trip. Change filters every 100 hours. No problems.

Cheers

bluefin59
07-07-2016, 06:28 PM
I am the same as scottar above in 12 yrs I never fill my plastic tank before going home preferring to put fresh in each trip and never had a problem ,but again my boat is kept in a garage as well ...Matt

Mad Fish
07-07-2016, 06:30 PM
The boat will be out in the weather covered with a tarp at the moment so heating up during the day and cooling down at night I fish every week or two so the fuel stays fresh.

Moonlighter
07-07-2016, 07:07 PM
Keeping it outside in the weather definitely makes a difference. The temp variations each day are greater outside.

A part filled tank creates a situation where the vapour in the tank expands and contracts far more than the liquid fuel component as the temperature changes. Thus the part filled tank effectively "breathes in and out" far, far more than a full tank. Hence the opportunity for condensation to happen is created. And, at the same time, the fuel loses its more volatile components as they evaporate more readily too.

Which is why BP and other fuel companies recommend keeping any fuel storage containers that are not sealed completely (this includes boat tanks that have breathers) full of fuel: To both avoid condensation build up and evaporative loss of fuel.

I recall reading the BP document that shocked me about how much fuel can be lost from a part-full tank due to this evaporation. In fact last year, i left my boat with 60 litres in the 100 litre tank, in the garage, for about 4 months while we were doing a house reno. Couldnt get it out!

Now I have a NMEA2000 network on the boat and my fuel data is very, very accurate so i know exactly how much fuel was in the tank. I filled up on the way to the ramp and was shocked to find that it took 11 litres more fuel than expected to fill up and there can only be one explaination- evaporative loss.

So that is why I keep my tank full, and so do all of my fishing mates on their boats.

Gon Fishun
07-07-2016, 07:08 PM
Throw in a cup full of metho every 2nd trip.

Chimo
07-07-2016, 07:14 PM
I keep mine full and use a Mr Funnel equivalent to filter all that goes into it.

Mad Fish
07-07-2016, 07:27 PM
Keeping the 450 ltrs on board do you think it will effect the consumption much in the scheme of things?

Mad Fish
07-07-2016, 07:58 PM
Thanks for all of the feedback

juggernaut
07-07-2016, 07:59 PM
Most of the info I've read on the matter says to keep them full. But yes its a conundrum whether to leave room for fresh fuel as Scottar mentions.

Can you tape over the breathers when you're not using it?

ShaneC
07-07-2016, 09:39 PM
Keeping it full will affect your consumption but not as much as you think. My NC tanks hold 620L, I leave them empty after a big trip until the next mainly due to the weight on the trailer for big tows. Never had an issue but I keep an eye on the water separators and change them regularly. If you use the boat a bit I wouldnt sweat it mate.

snatchy
07-07-2016, 09:39 PM
Our boat has a 220L tank. Boat is kept outside under a tarp in north Queensland. Tanks usually left part empty. I have never ever had any water in the tank or filter (clear bowl at the bottom).

There is a drain at the very bottom of the fuel tank that allows easy draining - so I use it and drain the tank about 3 times a year and put the old remaining fuel in the wife's car. I prefer to keep the fuel fresh when going out offshore. I always put the first bit of the tank drain into a glass jar and there is never any water - never has been in 9 years. So that is about 27 times I guess i have drained the tank.

Same situation with my 4wd kept outside - never an issue with any water in either of the two fuel tanks or fuel filter, despite being kept outside in the tropics for 13 years with little use and usually partially filled tanks. Except the one occasion when i picked up litres of water and crap at a fuel station that required rebuilding the injection system $$$

I think the supposed condensation issue is not real in practice - the water comes from elsewhere. I really thought it was supposed to be a serious issue but i never see any water so my opinion has changed.

hainsofast
08-07-2016, 08:22 AM
My mechanic recommended a product called fuel doctor for this.

Gon Fishun
08-07-2016, 09:33 AM
The only way your going to get a load of water in your fuel tank is by misadventure. Bad fuel from servo, loose cap out in the rain, sabotage, etc etc. As far as condensation in your boats fuel tank, it would be negligible. What moisture does form would drop to the bottom of the tank and be sucked up by pick up and caught in the water trap. If it gets to carby it sits in the bottom then evaporates while sitting between trips. How many million cars, trucks, bikes, are running around without a water trap or a water problem in their fuel.

Fuel Doctor or a bit of metho for peace of mind.

juggernaut
08-07-2016, 09:58 AM
An interesting read.

http://www.yachtsurvey.com/myth_of_condensation_in_fuel_tanks.htm

gofishin
08-07-2016, 10:49 AM
An interesting read.

http://www.yachtsurvey.com/myth_of_condensation_in_fuel_tanks.htm

Yes, good article. Same link was also posted in a thread discussing all things fuel related approx 3 or so years ago. Went on for weeks/months or more, & at least a 1/2 dozen pages from memory.

For condensation to occur within a fuel tank, one physical (psychometric) property of the air in the air+fuel 'vapour mix' must be reached, and that is any surface that this mix is boarded by/contained within must reach the dew point of the air vapour within the tank. In other words, something needs to make that surface colder... like an air conditioner etc.

For a lot of Engineering reasons, for 99% of boats & their normal storage conditions, there is more chance of winning the Lotto than this condition being present/met.

Petrochemical Engineers are a smart bunch, but their storage situations are a lot different to what we boaters do/see etc. And, their thermodynamic (heat transfer etc) situation(s) are vastly different - and some of their theories have been around since 'old school thought'.

Been following this principle keeping my tanks as empty as possible for ~20+ yrs, tin and glass boats, stored inside plus outside. No problems, & the fuel top-ups required keeps the fuel as fresh as possible.
Cheers
Brendon

Moonlighter
08-07-2016, 11:52 AM
BP talked about fuel life and evaporation and also mentioned condensation. I knew if I looked hard enough I would find it!

Here is what they say about keeping fuel in tanks:

http://www.bp.com/content/dam/bp-country/en_au/media/fuel-news/petrol-life-vehicle-tanks.pdf

a short extract:

MAINTAINING FUEL IN EQUIPMENT TANKS
It is not possible to provide a foolproof strategy for engines that are used only intermittently, however the following principles help.
1) Always add some fresh fuel when the equipment is to be used if it has not been used for more than a week. This will provide additional volatile components and protect from cold start high revving detonation and piston damage.

2) Always keep the tank half full to stop water vapour from being sucked in and condensing.

3)Use a fuel that contains anti oxidants, metal deactivators and corrosion inhibitors to protect metal surfaces such as BP Ultimate

4) Using a hotter spark plug will help to reduce carbon deposits

Table below is from the same document. Likely that evaporative loss is bigger issue than condensation, it seems.

Fed
08-07-2016, 01:05 PM
Always keep the tank half full to stop water vapour from being sucked in and condensing.

So it should be half full only, empty & full are no good?
I would have thought half full would have only half stopped the air transfer.
What if the tank is twice as big or half the size, would you fill to three quarters or one quarter respectively?
Or maybe that should be one quar.... WTF!
Funniest thing I've seen for a long time.

I'm sure a lot of this stuff is just copied from the previous guy who also copied it or made it up, and then the myth perpetuates.

Moonlighter
08-07-2016, 02:27 PM
So it should be half full only, empty & full are no good?
I would have thought half full would have only half stopped the air transfer.
What if the tank is twice as big or half the size, would you fill to three quarters or one quarter respectively?
Or maybe that should be one quar.... WTF!
Funniest thing I've seen for a long time.

I'm sure a lot of this stuff is just copied from the previous guy who also copied it or made it up, and then the myth perpetuates.

Read the BP document linked in the post. Its a direct extract, no editing.

You can extract your foot anytime you like.

Fed
08-07-2016, 02:43 PM
You don't have to be a genius to see I quoted BP moonlighter, only a fool would think that quote was yours.
Your response begs the question though, do you believe the BP document?

Moonlighter
08-07-2016, 03:25 PM
The condensation issue may well be debatable. Even the engineers seem divided about it, with due respect to my Club mate and friend gofishin.

I trust BP's statements about the loss of volatiles and evaporation from part filled tanks, they know what happens to their fuel when stored, and my own experience backs what they say up.

And to me thats the bigger issue anyway when tanks arent kept full, loss of fuel and degradation of its quality. The evaporative loss rates can be quite extraordinary, especially in hot climates.

In the modern era of fuel data presented on smart displays or NMEA2000 networks, that can easily lead the skipper to think he's got more fuel on board than he really has. Had one guy on THT decrying the inaccuracy of the Suzuki engine interface system because he ran out of fuel when it said he had 15 gallons left. But in fact it turned out that for the past 6 -12 months he had never filled his tank to full, just done partial fills and input the amount added, and left the boat in the driveway always with less than 3/4 a tank for weeks at a time. Evaporation took care of the rest. Had he filled to the top at least occasionally and reset the network to full, he wouldnt have had a problem.

scottar
08-07-2016, 03:53 PM
In the modern era of fuel data presented on smart displays or NMEA2000 networks, that can easily lead the skipper to think he's got more fuel on board than he really has. Had one guy on THT decrying the inaccuracy of the Suzuki engine interface system because he ran out of fuel when it said he had 15 gallons left. But in fact it turned out that for the past 6 -12 months he had never filled his tank to full, just done partial fills and input the amount added, and left the boat in the driveway always with less than 3/4 a tank for weeks at a time. Evaporation took care of the rest. Had he filled to the top at least occasionally and reset the network to full, he wouldnt have had a problem.

Agree 100%. Relying on any measurement method other than a full tank prior to going out, sight gauges or a calibrated dip stick is going to end in tears and a tow at some stage IMO. Unless I am only going a very short distance and have a spare portable tank to cover any miscalculation on my part, my tanks are filled every time either on the way to the ramp or the night before.

Noelm
08-07-2016, 04:03 PM
Throw in a cup full of metho every 2nd trip.
The Metho thing has been myth busted many times, unless you near empty your tank, and add about 20l of Metho, then syphon the whole lot out, it does nothing, pour it into petrol and it just mixes instantly, can't possibly get to the bottom, mix with the water, then by magic, mix with the fuel.

gofishin
08-07-2016, 08:28 PM
Agree 100%. Relying on any measurement method other than a full tank prior to going out, sight gauges or a calibrated dip stick is going to end in tears and a tow at some stage IMO...There is another alternative Scotty, and it doesn't rely on a full tank every time... in fact never if you don't want/need to [emoji6].

scottar
09-07-2016, 12:19 AM
There is another alternative Scotty, and it doesn't rely on a full tank every time... in fact never if you don't want/need to [emoji6].
Do tell......

fisho64
09-07-2016, 02:21 AM
more important than anything, do NOT put any ethanol blend in it.
Especially if you are going to leave it standing.

gofishin
09-07-2016, 07:53 AM
Do tell......The first requirement is that your engine gauges/management system needs to provide fuel consumed, or your sounder/another system etc.

The second is that a bit of mucking around is required to 'set up' the system, including some 'calibration mucking around' (draining tank & accurately measuring contents) a few times after every say 1000L thru your tank.

The third is that you need to keep a small 'boat notepad' (bible!) and religiously record fuel used every trip, as well as fuel filled (for every fill-up), regardless if you filled the tank to full or not.

If you are happy with this Scotty the system will provide you a very accurate indication of what is in your tank to the nearest litre normally. Yes, nearest litre, no BS.

If required, can provide more info later/maybe tomorrow, bit busy today.
Cheers

Spaniard_King
09-07-2016, 09:03 AM
The first requirement is that your engine gauges/management system needs to provide fuel consumed, or your sounder/another system etc.

The second is that a bit of mucking around is required to 'set up' the system, including some 'calibration mucking around' (draining tank & accurately measuring contents) a few times after every say 1000L thru your tank.

The third is that you need to keep a small 'boat notepad' (bible!) and religiously record fuel used every trip, as well as fuel filled (for every fill-up), regardless if you filled the tank to full or not.

If you are happy with this Scotty the system will provide you a very accurate indication of what is in your tank to the nearest litre normally. Yes, nearest litre, no BS.

If required, can provide more info later/maybe tomorrow, bit busy today.
Cheers

Not many NMEA 2000 systems are accurate to the nearest litre! Where do you get this information from ? How do you calibrate an NMEA 2000 fuel management system ?
Are you assuming that all systems can be calibrated as this is not the case!

Condensation in fuel tanks should not be an issue, if you have problems from water in the fuel it's more likely to be coming from the bowser or an external leak like the breathers in the wrong spot or rain coming in the filler cap. I see many fuel systems that have sat for years in the weather with no water in them.

Fed
09-07-2016, 01:53 PM
So you start with an unknown quantity of fuel due to evaporation.
gofishin
You enter a known quantity of fuel from the pump.
You enter the fuel used which may or may not be accurate.

Then the notepad (bible) magically tells you how much fuel is left down to the nearest litre?

That doesn't sound right to me, it must be a hell of a notepad.

Moonlighter
It all seems pointless if you have to continually fill up to get a fresh starting point.

FWIW and you can laugh if you want but I use a fuel gauge & it works just fine.
Come to think of it I've had countless cars over many many years with the old style fuel senders and they all worked fine too.

inveratta
09-07-2016, 01:54 PM
Not many NMEA 2000 systems are accurate to the nearest litre! Where do you get this information from ? How do you calibrate an NMEA 2000 fuel management system ?
Are you assuming that all systems can be calibrated as this is not the case!

Condensation in fuel tanks should not be an issue, if you have problems from water in the fuel it's more likely to be coming from the bowser or an external leak like the breathers in the wrong spot or rain coming in the filler cap. I see many fuel systems that have sat for years in the weather with no water in them.


reminds me of a warning from an old boat repairer.... plain old rainwater destroys more boats than almost anything else

scottar
09-07-2016, 05:26 PM
The first requirement is that your engine gauges/management system needs to provide fuel consumed, or your sounder/another system etc.

The second is that a bit of mucking around is required to 'set up' the system, including some 'calibration mucking around' (draining tank & accurately measuring contents) a few times after every say 1000L thru your tank.

The third is that you need to keep a small 'boat notepad' (bible!) and religiously record fuel used every trip, as well as fuel filled (for every fill-up), regardless if you filled the tank to full or not.

If you are happy with this Scotty the system will provide you a very accurate indication of what is in your tank to the nearest litre normally. Yes, nearest litre, no BS.

If required, can provide more info later/maybe tomorrow, bit busy today.
Cheers

It will work as long as your fuel management gear is spot on, the bowser calibration is spot on and in your haste to get fishing you don't cock up the maths (not always the case - especially the maths::)). Eventually, small errors will catch up with you if you put yourself in the situation of needing the last drop of fuel in the tank. If you always make sure you leave a healthy safety margin and periodically "reset" by either filling or draining, then not a problem.

gofishin
09-07-2016, 05:32 PM
... to the nearest litre normally. Yes, nearest litre, no BS. For some reason I wrote 'litre' but meant to write 'percent', i.e within 'one percent'. Apologies. One litre error in over a thousand litres would be 'NASA good' wouldn't it.

Numerous checks over the last ~6 years (tweaking calibration in the 1.5yrs prior) have always been round the 1% out or less, mostly a little +ve, below stats a little -ve.

Last check after the total for all trips (for the period checked) from the yammy gauge was after 1024.2 L was consumed. The bible said I had 55.9 L in the tank, but the measured amount drained was 49L. i.e. I had 6.9 L less than I thought. 6.9/1024.2 = -0.7% error.

Mostly it is a plus %, but still less than 1.0 %. Always like to have more in the tank than the bible says.[emoji1]

Think the above '-' error is because during this particular period I put 10L in a small jerry when I added some fuel for one Peel trip (running the tank down), but it ended up in the mowers. Gotta add the right notes in the bible!

Before tweaking the calibration it was between 3-5% out (using more than the yammy gauge said).


Not many NMEA 2000 systems are accurate to the nearest litre! Where do you get this information from ? How do you calibrate an NMEA 2000 fuel management system ?
Are you assuming that all systems can be calibrated as this is not the case!...Stand-alone Yamaha Square Digital fuel gauge. Don't know if it is N2k or not. Yes,it can be calibrated Garry. Gotta delve deep into the manual, but it is there.

Yes, assumed all N2k fuel gauges can be calibrated. I am shocked that some can't! I know most guys wouldn't bother (to check/tweak calibration), but some would want to.
Cheers

Moonlighter
09-07-2016, 05:46 PM
Not many NMEA 2000 systems are accurate to the nearest litre! Where do you get this information from ? How do you calibrate an NMEA 2000 fuel management system ?
Are you assuming that all systems can be calibrated as this is not the case!

Condensation in fuel tanks should not be an issue, if you have problems from water in the fuel it's more likely to be coming from the bowser or an external leak like the breathers in the wrong spot or rain coming in the filler cap. I see many fuel systems that have sat for years in the weather with no water in them.

Well, maybe your Honda systems arent accurate the the mearest litre, but plenty of Suzukis, includincg mine, are.

That is a fact. Not an opinion.

When I fill up after a trip, I am never more than a litre out in normal circumstances, in the last 5 years. The only time has been when ive had the boat sitting for months with half a tank and have lost fuel due to evaporation, but once the tank is refilled and set to full, near 100% accuracy is again normal. Typically I am within +- 150-300ml accuracy. The biggest factor in my experience is making sure you fill up at the same petrol station and same bowser, as the tank angle is the biggest cause of variation.

And I set my mates DF300 network up, and his is the same, in fact I was remarking to Gofishin last night on the phone that at 1770 last year, we did 2 trips out to Fitzroy and beyond in his boat, which has a fuel tank just over 500 litres, and when we filled up, the system was within 200ml of accurate, and that was about a 270 litre fill! That wasnt by any means an abnormal result for him, we fish together regularly in his boat and its par for the course.

i have helped dozens, if not hundreds of people, set up their Suzuki N2K networks and most of them report similar accuracy after a couple of calibrations. If you dont know how to calibrate a Suzuki N2K system, I will be glad to give you a lesson if you call and ask. Its pretty simple.

gofishin
10-07-2016, 11:35 PM
So you start with an unknown quantity of fuel due to evaporation...Have never experienced noticeable loss due to what could be deemed evaporation Fed. If anything, with my depressingly little amount of boating over the last 3 or so years, with long periods of non-use, any evaporation should really show up with my system!

The more volatile compounds 'evaporate first' we are told by the experts, and then what, the evaporation rate slows down with each 'stage' of volatiles being lost? Not sure, haven't delved into the theory.

What I would assume is that the rate of evaporation within an underfloor boat fuel tank is very very low. I mean really, how much can be lost through a meter or two of 12 or 16mm breather hose (or maybe sometimes two hoses)? Note: not saying it doesn't happen, I just have not noticed it!

One thing I would think is certain is that the overall amount of evaporation is proportional to the cumulative range of the thermal expansion & contraction cycles (changes in bulk fuel temp to max and min values), experienced within the tank.

A tin boat stored outside with a dark colour tin floor would suffer the worst (heat up and cool down fuel the most). A glass boat stored under an insulated roof in a ventilated/open wall shed etc, would suffer least.


It will work as long as your fuel management gear is spot on, the bowser calibration is spot on ...Bowser calibration is regulated by Federal Laws and is very tight Scottar, +/- 0.3%. Testing is by both periodic and random testing, though I am not sure of the testing frequency.

I am always more concerned (annoyed probably the better word) about where the bowser /nozzle is going to click off or cause me grief - a function of the boat up/down hill orientation at the servo, an unavoidable slight trap in the underfloor transverse section of my breather line, the tank design, and the wear-n-tear experienced with the cut-off functions of the typical ZVA nozzle.

Quite a few years ago, when fuel was over $1.50/L, I remember some articles regarding testing of over 1500 or 2000 servo pumps in VIC (or maybe NSW?). The media beat it up 'how people were being ripped off' etc (must have been quiet, no Kardashian news etc). Think they based it on a Commodore/Falcon size tank/fill-up (say 75L??), and one of the headlines was something like 'owners being robbed', but the end of the sentence was later confirmed as something like '... robbed of 2 or 3km per tank'. WT?

Think the worst pump/bowser tested provided around 230-250ml less than it said it did (based on the 75L, whatever the tank size was). i.e. just over the 0.3%. But, when you read the fine print from the testing body/organisation, more pumps were giving too much fuel than those short-supplying, and the cumulative totals were such that more fuel in total was supplied than should have been.

Getting more than ~85%-90% of the rated tank capacity is a pain in my boat (like many boats). This is much more of a concern to me (as a possible source of error) than any potential bowser calibration error. When I do want to fill completely, I always top off with jerries (after it starts spitting and pissing fuel down the side of the boat ☹️).


... Eventually, small errors will catch up with you if you put yourself in the situation of needing the last drop of fuel in the tank. If you always make sure you leave a healthy safety margin and periodically "reset" by either filling or draining, then not a problem.Hasn't been a problem for ~10 years and 2 boats with this method, although with the last boat there were a few differences.

Occasionally I may have second-guessed myself, when things didn't look right, but a few head clearing breaths, check the math etc - all good [emoji1].

I also cross-reference to the fuel gauge (digital, 10 bars) which I also recorded fuel qty 'for each bar' 3 or 4 times, from dead empty, with the boat at the same inclination as it floats in the water with 'average loading'. Yes, a gauge is only 'a guide', but a good one nonetheless in this case.

Completely drain it once per year, just before the service (or more often if I haven't used it for ages and want to blend old fuel). This to check/reset as you say, but also to make as sure as possible I have no water/crap in the tank. I drain thru the filter, then check clear bowl and replace filter at the service.

I guess people use a system what they're used to/what they are happy with. I know mine is very accurate, and I'm more than happy to keep using it. [emoji1]
Cheers

fisho64
11-07-2016, 08:40 PM
The ONLY time you have too much fuel on a boat, is when its on fire.

scottar
11-07-2016, 09:09 PM
The ONLY time you have too much fuel on a boat, is when its on fire.

LOL. Or when the DOT run you over the scales - only to find out your 500 litre tank being full puts you a few hundred Kg's over your vehicles tow weight.