Best material for fuel tank

[AngusMcDoon]

Has anybody done any scientific testing on the amount of condensation that actually collects in a tank?

The amount of 'fresh air' that enters a tank in 24 hours is tiny.

Yes, the late David Pascoe, a boat surveyor. According to him it's all a myth...

The Myth of Condensation in Fuel Tanks: Need a Full Fuel Tanks to Store or Lay-up Your Boat? - Boat Maintenance - by David Pascoe

A summary is that the air in a fuel tank can hold too little water vapour to condense out any significant amount, there is little air change & the most likely cause of water in diesel is a leaky filler cap or poor storage by the supplier.

 

[geem]

It's very difficult for a tank, moulded as part of the hull, to have a sump with a drain valve.

Mine is built over the keel. It has the sump formed by the keel shape. Obviously no drain valve but with a bonze 1 inch opening in the 600mm access lid, it is easy enough to inspec the tank and draw off any crud periodically with my electric fuel pump and filter. I also have an endoscope that I drop in there occasionally. It's all very easy being accessible through the saloon floor, directly over the tank.
Thr tank sides are cored with marine ply. This adds to the insulation effect. We don't seem to suffer any condensation as a result. Thr tank walls are about 1" thick

 

[geem]

Yes, the late David Pascoe, a boat surveyor. According to him it's all a myth...

The Myth of Condensation in Fuel Tanks: Need a Full Fuel Tanks to Store or Lay-up Your Boat? - Boat Maintenance - by David Pascoe

A summary is that the air in a fuel tank can hold too little water vapour to condense out any significant amount, there is little air change & the most likely cause of water in diesel is a leaky filler cap or poor storage by the supplier.

It's possible. The relative humidity in the tank will respond to the external humidity levels. Vapour pressure is one of those surprisingly powerful forces. If the RH of the air in the tank is high but then cools over night, the moisture will condense on the side of the tank. If the tank warms up and the RH drops, vapour pressure will try and reestablish parity between the RH in the tank and outside. This won't happen often as the tank tends to by hidden away but tanks in lockers where the locker is heated by the sun will likely be more susceptible to this. Also small tanks with less fuel will have less thermal mass so will be heated and cooled more easily.
Our tank is under the floor so the temperature is stable and a function of sea water temperature rather than air temperature so there is far less heating a cooling of the space surrounding the tank therefore less opportunity for a cool surface to develop for warm air to condense on

 

[Refueler]

Fine but impossible on most boats - and many who have that from the early days of GRP boat building might not agree with you. Not good to extrapolate from one example.

The Searider / Sunrider boats as I have - the 25ft Motor Sailer ... in Bilge keel form - in the 70's it was seen various boats used the keels as tanks, The SR 25 had one as fuel - other as FW. But like many other boats - owners installed tanks and stopped using the keels.
Personally - having seen GRP keel tanks opened up - its the last form of tank I would use.

Stainless ? Fine - I see no problem there apart from weight and making sure who welds it up makes good job. My 1986 Conq38 has St.St tank and no prob.
My 1973 MoBo has St.St tank in each side coaming of cockpit - no problem.
Mild Steel ? My 75ltr tank on SR25 is mild steel - actually its a truck tank modified with a angled lower side to fit into bunk space. Boat is ~1975 and that tank shows no signs of corrosion or fault.
Galvanized tanks seem to be less now than they used to be.
Plastic tanks ? Why not ?

They all work ...

 

[[70521]]

Yes, the late David Pascoe, a boat surveyor. According to him it's all a myth...

The Myth of Condensation in Fuel Tanks: Need a Full Fuel Tanks to Store or Lay-up Your Boat? - Boat Maintenance - by David Pascoe

A summary is that the air in a fuel tank can hold too little water vapour to condense out any significant amount, there is little air change & the most likely cause of water in diesel is a leaky filler cap or poor storage by the supplier.

Thanks @AngusMcDoon good to see I'm not the only one who questions the group think.

 

[Refueler]

Yes, the late David Pascoe, a boat surveyor. According to him it's all a myth...

The Myth of Condensation in Fuel Tanks: Need a Full Fuel Tanks to Store or Lay-up Your Boat? - Boat Maintenance - by David Pascoe

A summary is that the air in a fuel tank can hold too little water vapour to condense out any significant amount, there is little air change & the most likely cause of water in diesel is a leaky filler cap or poor storage by the supplier.

I will argue against his conclusion. Having spent years testing - owning / running PetroChem labs.

Let me ask you this : How much 'water' do you think Micro Bio contamination of Fuel requires ? It seems that people expect that visible layer or amount of water is required. Not true.
Diesel Fuel contains water in decimals of % ... enough to maintain Bio colonies. That water can be suspended or separated.

The tiny bit of condensation any fuel tank has - is MORE than enough to add to that ....

 

[[70521]]

I will argue against his conclusion. Having spent years testing - owning / running PetroChem labs.

Let me ask you this : How much 'water' do you think Micro Bio contamination of Fuel requires ? It seems that people expect that visible layer or amount of water is required. Not true.
Diesel Fuel contains water in decimals of % ... enough to maintain Bio colonies. That water can be suspended or separated.

The tiny bit of condensation any fuel tank has - is MORE than enough to add to that ....

Not sure what that that has to do with the original post, condensation in tanks.

I fully understand the argument about the fuel bug living in the junction between diesel and water, but think that condensation is the least of our worries. I rate poor seals in the fillet cap, whoever thought that putting then on a horizontal deck near the water line needs to be asked why and crap fuel from suppliers.

After a dose of the bug - from a small marina with a tiny red fuel turn over I only get white from the local filling station en route to the boat. Before it goes in the tank it goes through a Mr Filter funnel and I'm gob smacked by the amount of crude it traps.

While seen as rather OCD by my sailing friends I've not had a problem on five years.

The tank has been emptied and cleaned once since my bug incident, due again next winter, and was clean.

 

[geem]

I will argue against his conclusion. Having spent years testing - owning / running PetroChem labs.

Let me ask you this : How much 'water' do you think Micro Bio contamination of Fuel requires ? It seems that people expect that visible layer or amount of water is required. Not true.
Diesel Fuel contains water in decimals of % ... enough to maintain Bio colonies. That water can be suspended or separated.

The tiny bit of condensation any fuel tank has - is MORE than enough to add to that ....

Our fuel filler has an o ring fitted. In addition I use silicon grease to seal the threads every time I fill up. I am pretty certain I don't get water ingress via the filler.
I think there is a lot to be said for keeping fuel tanks full if you are in a climate that sees a diurnal pattern of warm days, high humidity and cool nights. A perfect recipe for condensation. A full tank will create more thermal mass and reduce the temperature swing that the tank sides see by virtue of the mass of fuel. In addition, less exposed tank material means less surface area for condensation to occur. The top of the tank will likely be the source of condensation for most tanks as it is hard to keep the tank totally full so there will always be an air gap above the fuel. There may be a good argument for insulation for the top half of the tank to reduce the risk of condensation such that the tank stays warmer over night.

 

[Refueler]

Not sure what that that has to do with the original post, condensation in tanks.

I fully understand the argument about the fuel bug living in the junction between diesel and water, but think that condensation is the least of your worries. I rate poor seals in the fillet cap, whoever thought that putting then on a horizontal deck near the water line needs to be asked why and crap fuel from suppliers.

After a dose of the bug - from a small marina with a tiny red fuel turn over I only get white from the local filling station en route to the boat. Before it goes in the tank it goes through a Mr Filter funnel and I'm gob smacked by the amount of crude it traps.

While seen as rather OCD by my sailing friends I've not had a problem on five years.

The tank has been emptied and cleaned once since my bug incident, due again next winter, and was clean.

Given you opened the can of works with your post ... I find it interesting that you question my reply to post by you about amounts of moisture.

 

[vyv_cox]

Has anybody done any scientific testing on the amount of condensation that actually collects in a tank?

The amount of 'fresh air' that enters a tank in 24 hours is tiny.

Yes, in addition to the one mentioned in another post, a US tester known as Maine Sail has had a metal tank set up ashore in very changeable weather, well below zero in winter, hot in summer, for 13 years (last time I looked). Not a single drop of water has been collected in all that time.

 

[geem]

Yes, in addition to the one mentioned in another post, a US tester known as Maine Sail has had a metal tank set up ashore in very changeable weather, well below zero in winter, hot in summer, for 13 years (last time I looked). Not a single drop of water has been collected in all that time.

Was the tank full of fuel, half full or empty? It would be good to know more detail. Once water enters a tank with diesel in it. It will sink below the diesel so it can't evaporate. Its a trick used in commercial feed and expansion tanks where it is not possible to have a constant supply of make up water available to maintain the level. You add oil to the water and it sits on top of the water stopping evaporation.

 

[Tranona]

The Searider / Sunrider boats as I have - the 25ft Motor Sailer ... in Bilge keel form - in the 70's it was seen various boats used the keels as tanks, The SR 25 had one as fuel - other as FW. But like many other boats - owners installed tanks and stopped using the keels.
Personally - having seen GRP keel tanks opened up - its the last form of tank I would use.

Yes, fairly common in those early days - Kingfisher and Seadogs other examples but quickly dropped partly because hull and keel shapes changed and partly from contamination and leak issues. Flexible tanks were also in vogue for a short period and often used as inserts for built in tanks that had failed.

 

[[70521]]

Given you opened the can of works with your post ... I find it interesting that you question my reply to post by you about amounts of moisture.

Simples, it went off at a tangent. The OP was talking about condensation in metal tanks not the fuel bug.

 

[jwilson]

A well designed plastic tank with inspection hatch designed to be used and reseled, and even better a bottom drain point, would be ideal. Unfortunately the typical production boat rotomoulded tank doesn't have an access hatch, nor a drain point, and the top fttings are only held down with self-tappers and a gasket. As installed it works well, till you have to clean it out, and you will have to do this eventually. Then you can't do any more than poke about with a pipe to suck out crud, and when you redo the top fittings the self-tappers won't grip as well and you get diesel leaks. It's all fixable with work, but the standard ready-made fit-from-the top hatches don't seal well on a thin-walled flexible tank top.

 

[LittleSister]

Over the years all have proved to have had their little problems.

Over the years all have proved perfectly serviceable.

Moulding a fuel tank into the hull would appear to be a no brainer using the miracle of glassfibre and of course it saved the cost of custom fabricating and installing a cutom tank.

My current LM27 (a 1970s design with lots of clever thinking) has the (huge!) fuel tank moulded in to one side of and below the front of the cockpit (and the water tank is its mirror image on the other side). The hull there curves inwards down and aft, and forms the curved outer side, bottom and the back of the tank, while the inside (engine compartment bulkhead) and top (immediately under wooden floor of cockpit locker) are flat GRP panels.

More commonly found in motorboats the glassfibre fuel tank came and went .

I've read that the bio-ingredient in bio-petrol (but not that in diesel) attacks GRP, and this has been a very big issue in the USA, where there are very many older petrol mobos with GRP tanks. Perhaps this is (part of?) the reason GRP has fallen out of favour with mobo builders, as there's a chance/likelihood they'll be fitted with petrol engines.
(Note the bio-ingredient in bio-diesel is a completely different substance, and is compatible with GRP.)

 

[Wansworth]

A well designed plastic tank with inspection hatch designed to be used and reseled, and even better a bottom drain point, would be ideal. Unfortunately the typical production boat rotomoulded tank doesn't have an access hatch, nor a drain point, and the top fttings are only held down with self-tappers and a gasket. As installed it works well, till you have to clean it out, and you will have to do this eventually. Then you can't do any more than poke about with a pipe to suck out crud, and when you redo the top fittings the self-tappers won't grip as well and you get diesel leaks. It's all fixable with work, but the standard ready-made fit-from-the top hatches don't seal well on a thin-walled flexible tank top.

Considering how important a motor is theses days it would seem details like tank cleaning get little thought at the design stage.

 

[geem]

Over the years all have proved perfectly serviceable.



My current LM27 (a 1970s design with lots of clever thinking) has the (huge!) fuel tank moulded in to one side of and below the front of the cockpit (and the water tank is its mirror image on the other side). The hull there curves inwards down and aft, and forms the curved outer side, bottom and the back of the tank, while the inside (engine compartment bulkhead) and top (immediately under wooden floor of cockpit locker) are flat GRP panels.



I've read that the bio-ingredient in bio-petrol (but not that in diesel) attacks GRP, and this has been a very big issue in the USA, where there are very many older petrol mobos with GRP tanks. Perhaps this is (part of?) the reason GRP has fallen out of favour with mobo builders, as there's a chance/likelihood they'll be fitted with petrol engines.
(Note the bio-ingredient in bio-diesel is a completely different substance, and is compatible with GRP.)

Nice to see that clever design on your boat.
I suspect it is not more common to have built in tanks due to cost. It's labour intensive, but done well, it makes a fantastic tank with little to go wrong. The access lids on our tanks are huge. They are secured on cork/rubber gaskets. The lids themselves are moulded such that they are raised in centre giving them lots of strength and rigidity. They are 10mm thick solid grp and fixed down with a ring of 10mm bolts glassed in on the underside of the tank.

 

[Tranona]

Considering how important a motor is theses days it would seem details like tank cleaning get little thought at the design stage.

In reality though there are tens of thousands of boats with tanks as described and problems are rare. Better quality fuel and filtration covers it most of the time. Of course more adventurous folks going away from "civilisation" would consider fuel cleanliness more important and equip/maintain their boat accordingly

 

[LittleSister]

Considering how important a motor is theses days it would seem details like tank cleaning get little thought at the design stage.

We've gone in little more than a couple of generations from having a motor being considered a luxury, to fully accessible fuel tanks with drain sumps being considered highly desirable.

I can definitely see the advantage, but seriously there are zillions of boats out there without, mostly having no serious problems. It ain't a reason not to buy a second-hand boat.

For manufacturers (except at the money-no-object end of the market) providing such tanks as standard is one of very many 'nice-to-haves' that taken together would significantly increase the cost of the boat, which in turn would reduce sales, and hence viability and profits.

 

[Wansworth]

In reality though there are tens of thousands of boats with tanks as described and problems are rare. Better quality fuel and filtration covers it most of the time. Of course more adventurous folks going away from "civilisation" would consider fuel cleanliness more important and equip/maintain their boat accordingly

To some extent it’s a question of size,a 40 footer has more scope to position fuel tanks and access hatches and filters etc than a 26 footer