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Old 06-12-2009, 10:39   #1
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Stainless vs Plastic Diesel Fuel Tanks

I am just about to re engine my 41' 6" Westerly Oceanlord from a 43HP Volvo 2003T to a Volvo D2 55 or a Yanmar 4JH4E. (don't want get into that argument just now!!).
This gives me the opportunity to replace the old mild steel 200 litre tank with either a Stainless Steel or a Tek Tank Polyethylene tank. The advantage of the polyethylene tank appears to be lack of condensation, which seems logical. The only downside that I can think of is that, as the tank must have the same with outside dimensions as the old one, the wall thickness will be much thicker with plastic v stainless, therefore less internal volume (about 5 litres or 1 - 2 hours running time).
Do any of you old salts have any perspectives on the choice of material, good or bad?

Cheers all!
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Old 06-12-2009, 10:47   #2
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I'd go plastic...
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Old 06-12-2009, 11:25   #3
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Although on the surface the perfect corrosion resistance of plastic appears to make it the right choice, it is the wrong choice. Even a small fire in an engine room that compromises the tank changes a small. containable fire into a raging infero fuelled by 200 gallons of diesel. I wouldn't even consider thermoplastics for fuel tanks. You might also want to check with your insurer on this matter - I rather suspect they would be, shall we say, displeased. Best to use stainless (or even mild) steel, but given this opportunity to replace the tank(s), make sure the new tanks have a dimple (at least 2 inches diameter by 1/2 inch deep near the center of the tank floor with a threaded connection and a place to put a drain cock. This will let you tap off any water that accumulates (plus sludge) and almost completely prevent the corrosion and algal groth that results from free water on the bottom of the tank.
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Old 06-12-2009, 11:26   #4
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. . . not an expert, but I've read that stainless is not advised for fuel tanks because when stainless tanks fail, they can fail "catastrophically" (which I guess just means all fuel will be in your bilge). I'll be interested in seeing the more qualified responses to your question.
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Old 06-12-2009, 11:32   #5
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Most tank failures are catastrophic, unless a pinhole leak, and they all leave you with a bilge full of fuel. Been there -- wasn't fun when a seam in the port tank split (only "a bit") and left me with 40 gallons in the bilge. The $100+ in lost fuel was not the worst part. Took 18 months to get the stench of diesel out of the boat. Stainless does not fail any more than other materials as long as you keep chlorides (salt water) away and don't let it stand in salt water.
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Old 06-12-2009, 12:02   #6
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Go aluminum! Just have one built to fit. They will hold up to minor condensation corrosion and when you look in the inspection port with a flashlight you can see the bottom and what's there.

e.g. Aluminum Fuel Tanks, Marine Fuel Tanks, Marine Fabricating, Aluminum Boats, NJ
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Old 06-12-2009, 12:48   #7
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Thanks guys!
I must admit that I had not thought about the fire risk. The larger engine will mean that it is even closer to the fuel tank than it is already. I will check with my insurers tomorrow and feed back to this thread.
My current tank is mild steel, and there is a small sump that is welded in, rather than 'dimpled' in (I like that idea). There is at least one report on the Westerly Owners website of this having failed, although it was not catastrophic, just weeping.
If I go for a Stainless or Aluminium, I would always aim to 'top off' the tank whenever possible (as I do at the moment), and certainly over the winter (the most damp time of year on the west coast of Scotland......No, it's always damp here!).
I was also aiming to install a decent Racor filter system as well, with a sight glass, so I can monitor the water/contamination issue.
Any further advice/experience gladly accepted!
Cheers,
BM
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Old 06-12-2009, 13:26   #8
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One note. If your tank is not against the hull (air space around it) then as long as you keep the boat interior air warmer then the outside air the tank will not build up condensation.

It's the opposite of a glass of ice water. The cold side will attract the moisture from the warmer side, so keep the outside of the tank warmer or the same as the inside.
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Old 06-12-2009, 13:28   #9
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Just an additional consideration: I've run into a different issue with keeping tanks topped off, which I've always liked to do. We are under sail 99.9% of the time and run the engine so little that in 12 months it took a whole 2.5 gallons to top of a 40 gallon tank. The engine is run only to leave and enter a slip because I refuse to motor into headwinds - I stubbornly tack. The issue then is fuel turnover because at this rate it takes 16 years to turn over one tank (and 32 years if both were filled) !!! NOT GOOD. So I am going to install a 10 gallon working tank (for a 50 HP engine no less) so at least I can turn it over every 3 or 4 years. The two 40 gallon tanks will remain empty and dry until that kind of fuel inventory is actually needed. Your circumstances may be different, but 80 gallons of fuel is a bit more than I need.
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Old 06-12-2009, 13:56   #10
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My boat has an aluminum tank in the bilge. Its predecessor was exposed to salt water and had pinhole leaks along the welds. This was noticed on survey, so I had the tank replaced as part of the boat purchase. The surveyor recommended I have the new tank Rhino-lined ( Rhino Linings ) to prevent any salt water that may be in the bilge from coming into contact with the aluminum in the future. (I have a normal stuffing box at the moment so the bilge is always a little wet - although with fresh water now.) I did that (cost about $400), and it looks like it should do its job well.

Although it didn't apply in my case, I don't believe aluminum would fail "catastrophically" in the sense that you'd notice a small corrosion related leak first, that would gradually get worse. I've read that stainless will not give warning of the problem in advance, so you'll likely just end up with a rupture (although I'm hoping the experts here can confirm/deny this). This may be a distinction without a difference if you're coastal cruising because either way, you'll need to get on the problem quickly. If you're far from home however, maybe it'd be nice to have a little advanced warning.

My other choice was fiberglass (the factory switched to that due to widespread corrosion problems with aluminum tanks in the bilge), but I'm a little paranoid that if they start introducing ethanol (or similar) in some form into diesel in the future, it may cause delamination problems as has been such a nightmare with older gasoline powerboats with fiberglass tanks.

If I had the option to go with a reputable plastic tank, I probably would have done that as they're doing with many powerboat gasoline tanks these days. I think there's enough of a track record there to suggest they're very reliable with gasoline - not sure about diesel however. I guess there's pros / cons with each option, just like everything else on a boat.
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Old 06-12-2009, 14:19   #11
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Maybe this a good place to ask this. I have aluminum tanks about 10 years old that are coated in some amber film of some sort that I can't identify. They recently suffered a "near drowning" at the boatyard after a new throughhull placement and the water came up to almost the top. They said they washed them down but how critical is it to rinse them. Should I flood the bilge again with fresh water this time or just let things dry and stay that way. They are well above the little water that does stay in the bottom of the bilge and I can't see much of the sides or bottom to tell much.

How much damage might have been done? Is there anything I should do or be worried about.

Thanks,

Jim
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Old 06-12-2009, 14:24   #12
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i read that the plastic fuel tanks actually expand when they are full .. meaning you have to allow for this with the installation.
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Old 06-12-2009, 15:15   #13
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Originally Posted by jkleins View Post
Maybe this a good place to ask this. I have aluminum tanks about 10 years old that are coated in some amber film of some sort that I can't identify. They recently suffered a "near drowning" at the boatyard after a new throughhull placement and the water came up to almost the top. They said they washed them down but how critical is it to rinse them. Should I flood the bilge again with fresh water this time or just let things dry and stay that way. They are well above the little water that does stay in the bottom of the bilge and I can't see much of the sides or bottom to tell much.

How much damage might have been done? Is there anything I should do or be worried about.

Thanks,

Jim
Good question Jim.

I have replaced or repaired 4 aluminum tank installations so far..2 in a Gill net boat. One in a Ski boat and one in a cuddy cabin cruiser and Our Sea Ray.

All of them were pin hole leaks from electrolysis...salt trapped up against the tanks where there was a pressure point of contact not allowing for a drying.

In the case of the gill netter both tanks were replaced with new as the money saved to repair the existing was just not worth it...Both tanks were badly eroded in the 3 logitudal bedding strips where the tanks rested on strips of 3" wide rubber chafe protection across their whole bottoms.

All 3 other boats had similarly caused issues..Id go with Plastic or SS if it were me....leaning toward SS.
I do not believe all SS tank failures or even a predominate amount of them have been total catastrophic but will start to seep from a crack or crevice corrosion as well...and FWIW plastic will condensate as well just maybe less so...your boat is plastic and it definitely condensates right?..

There may be statistics out there stating they fail catastrophically the most but I'd like to see that verified in an Insurance study somewhere before I will accept it as fact or for that matter what percentage worth of weight it really is to that side of the argument...like is it .01% or something?...whoopee! if so.

Sorry to here of your predicament I'm in the same boat...Hard to fine quality yards anymore isn't it..
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Old 06-12-2009, 15:53   #14
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Chloride ions can cause stainless pitting as well as stress corrosion cracking. The catastrophic failures were probably caused by cracking, which would naturally make itself know suddenly and without warning. Aluminum is also subject to corrosion by chlorides. And chlorides are the other half of the sodium in salt in seawater. Neither metal is 100% forever in a marine environment. For that you need Monel (and lots of money). But if you keep the metal free of salt water and preferrably dry throughout, 30 years out of even a mild steel tank is quite achievable. Corosion and cracking don't result from a one-time exposure to salt water. It takes time on the scale of years. For my money, I'm probably going with aluminum but I'm keepin away water, both in an out. And keeping the engine room warmer than the ambient environment will certainly help.
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Old 06-12-2009, 17:41   #15
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Another consideration with plastic tanks is that it is difficult (mostly not possible) to install baffles in them. This is OK with small tanks, but not larger ones.

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