Should I fear steel tanks?

Hi all
I am new to the forum and Sadly, between trawlers.
In an effort to rectify this situation I am negotiating on a 1998 N46. As it turns out, this boat has steel tanks (pre Hull 68) and I am wondering how concerned I should be.
In an effort to be proactive can anyone offer any information on dealing with corroded tanks on this model?
Can they be removed through the salon floor and cockpit door (after removing the engine of course)?
Are there any reliable methods of repairing/treating the tanks in place?
What would a ball park figure be to replace the tanks on this boat?

Thanks in advance.

If the tanks stay dry on the out side you will not have an issue on the outside. If you keep the water out of your diesel then you will not have an issue on the inside.

If the tanks constantly come in contact with bilge water you have a serious problem. If rain leaks find their way to your tank you have a problem.

If rain water can find its way into your deck fills you will have a problem. If your fuel supplier has water issues you have a problem.

If none of this applies to your situation then you have no problem.

Although I generally agree with the above, I would say if the tanks are built out of mild steel, it's not a matter of "if", it's a matter of "when". You can kick the "when" further down the road with proper design/inspections and maintenance . . . . but it's still a "when" IMHO. . . . Don't ask me why I know . . . .

slowgoesit said:

I would say if the tanks are built out of mild steel, it's not a matter of "if", it's a matter of "when".

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Thanks for the replies.
I was thinking the same, it's just that I cannot find any info on a tank refit for that boat. I saw on another post for a different boat, that it was proposed to cut an access into the hull and that has spooked me.
I would think that with four 250 Gal tanks, they would come out through the salon floor but I cannot confirm this so far. With Covid, I am unable to get on the boat to do any measuring.
Cross border boat shopping is pretty complicated right now.

tiltrider1 said:

If the tanks stay dry on the out side you will not have an issue on the outside. If you keep the water out of your diesel then you will not have an issue on the inside.

If the tanks constantly come in contact with bilge water you have a serious problem. If rain leaks find their way to your tank you have a problem.

If rain water can find its way into your deck fills you will have a problem. If your fuel supplier has water issues you have a problem.

If none of this applies to your situation then you have no problem.

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I agree, otherwise there would not be 50 year old steel tanks not leaking fuel.

Thanks Folks.

That makes me feel better about pursuing the boat.
I'll be sure to direct the surveyor to pay close attention to current tank condition including paying extra for an internal inspection if possible.

I'd just like to know what im in for in a worst case scenario down the road.

Unlike most boat buyers, one of the first things I look at is how can I replace the fuel tanks. Why, cause I had to replace the tanks on my last boat.

My current boat has all the fuel above the bilge and center line far from any possible deck or hull seam leaks. The fills are protected from rain water and the tanks are stainless steel. Even so, I have already mapped out the replacement process should I ever need it.

Non of this helped me when the washer/dryer went bad. There is always something.

Tiltrider has said it well.

On our DF's 15 year old steel tanks I'm not worried one bit. Why not? Assuming the builder followed DeFever's design criteria (always updated by Art as time passed) they are visible, not subject to standing water and any deck fill leaks easily noted. All four tanks have sloped bottoms to two separate drain fittings and clean out ports. Clean fuel from a busy safe supplier has been possible as we don't cruise the busy off FL islands, Mexico or Asia.

I've seen first hand aluminum and FRP tanks go bad due to faulty design and shoddy builders. If on the N46 you can see the tanks and they look good it then comes down to bad fuel potential and lack of use. The hours, sit times and cruising locales can be looked at for additional hints as to any future issues.

My steel tanks were built in 1942. Still fine. Keeping water out of the diesel, keeps them from corroding.

If you have access ports, drain the tanks, clean as necessary, use a tank coating approved for diesel and the tanks will last the life of the boat.

There are exactly three kinds of steel tanks which will not cause you any worry;(for a few years anyway)

1) Brand new tanks
2) Tanks which have always been free of water both inside and outside
3) Freshly rebuilt tanks

If your tanks do not fall into one of these categories, better be worried or investigate further.

pete

Tanks on my Willard 36 lasted about 45 years. Demise was due to water through the deck fittings.

I have a mountain cabin with a 250 gal propane tank. No way in hell is something that size coming out of a N46 in one piece. No way is it going in as a single tank either. ER is tight on these boats. But thisbisnt exclusive to a Nordhavn. If you want a boat with transocean range, it's going to have big tanks that are not easily replaced.

If you are worried about steel tanks, don't look at boats with steel tanks. There's nombre lunch here. You were likely attracted to this boat due to price. There's a reason the price was attractive.

BTW - when PAE redesigned fiberglass tanks, they also shifted location so the trim is better when full. Deeper inspection into the model changes and upgrades might be helpful

Peter

I have never been a fan of any tank system which uses standpipes (top-draw) instead of bottom-draw to supply fuel to engines. If the draw point is well down at the bottom of the tank, water and other gunk won't collect there unknown to the owner but rather end up in filters with water separators and water alarms if water levels get too high between draining. I got about an ounce out of my two Racor 500s the other day when I drained them for the first time in several years (bottom-draw tanks here). I assume that a vessel of the touted quality of an N46 would have drain ports for the tanks. My trawler had these, but no valves had been attached. One day I just added them, to full tanks. I prepped the valves with the properly threaded nipples and petroleum resistant teflon tape; pulled each drain port pipe plug; put my thumb over the hole; and then quickly threaded the valve in place. Lost about an ounce or two of fuel per tank. Valves were ball with locking device and a pipe cap threaded to them just as a secondary safety. I sold that 1972 boat with its original steel tanks in 2015.

Pete Meisinger said:

There are exactly three kinds of steel tanks which will not cause you any worry;(for a few years anyway)

1) Brand new tanks
2) Tanks which have always been free of water both inside and outside
3) Freshly rebuilt tanks
If your tanks do not fall into one of these categories, better be worried or investigate further.Pete

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One that doesn't fall into Pete's "safe" categories is the sewage holding tank. Urine is so corrosive that it will turn any metal--even 316 stainless--sewage holding tank into a colander within an average of about 10 years. This does not apply to gray water tanks.

--Peggie

rgano said:

I have never been a fan of any tank system which uses standpipes (top-draw) instead of bottom-draw to supply fuel to engines.

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Top draw tanks are actually the best, as a failure of fuel plumbing does not flood the bilge with fuel. Your complaint isn't really with the concept, but rather the execution. Bottom draw tanks frequently, even typically, have a fitting welded or bonded to the side, leaving perhaps 1/2" depth that cannot be drained.

If you replace the tanks in a boat, you have the opportunity to correct the sins committed by so many boat builders. Whether top or bottom draw, the tanks should have a bottom that slopes towards a distinct low point, and that low point should have a small sump lower still, from which the bottom fitting, or dip tube draws. All of the junk will collect there, and you can drain the tank completely but for the partial contents of that sump. In addition, there should be a bung directly above that sump, which allows sucking it out with a suction type oil change pump periodically. This will remove the junk and any remnants of fuel.

While you are at it, do not mount the tanks on their bottom surface, but on C or hat shaped channels. These will hold the bottom off the wet surface they will otherwise sit on allowing it to dry, and if they corrode it will not hole the tank.

Some thoughts, as I do corrosion work for industrial sites:

1. You can probably have then thickness tested with an electronic tester (Ultrasonic spot testing). Meters cost as little as a few hundred $ up to a few $1000. Not sure how accurate the cheap ones are, though.

2. You could clean, and passivate a steel tank if it's corroded on the inside. Fairly simple chemistry to get this done.

Lepke touched briefly on something that needs to be considered. If you have access to the entire inside of the tank through inspection ports that are large, and interior baffles do not prevent access to any inside surfaces, you can clean the inside of the tank, and apply a good quality tank sealant, formulated for diesel fuel, to the inside of the tank. Assuming that the tank is still structurally sound, and that pinholes or such occurred from inside out, that is a pretty much a permanent fix. If on the other hand, damage to a tank has occurred from outside in, it won't work unless you address the damage from the outside as well.

We looked at a 1984 Defever 48 whose aft tanks rusted through from the outside in probably 12 years because the lazarette hatches didn't seal correctly. Also made us concerned with the forward fuel tanks, leakage through the deck onto the tops of the fuel tanks. Area wasn't visible through casual inspection, and boat still had teak decks. We chose not to purchase the boat because of this. Good luck in whatever you decide!

Thank you all for the quick and informative responses. Lots to think about with regard to installation, inspection, and preventative measures.
Perhaps the fact that I cannot find any instances of tank replacement on these older boats, indicates that the design of the 57 steel tank N46s was a good one.
I’ll guess see what “survey says“ and go from there.

Comoshun3 said:

Thank you all for the quick and informative responses. Lots to think about with regard to installation, inspection, and preventative measures.
Perhaps the fact that I cannot find any instances of tank replacement on these older boats, indicates that the design of the 57 steel tank N46s was a good one.
I’ll guess see what “survey says“ and go from there.

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I have heard of a couple N46 steel tanks getting cleaned and internally coated in the Seattle area.

I believe one was a weld seam failure not rust through the sheet wall.

This appears to be the way to deal with N46 tanks as replacement would be a bugger as the tanks are really wedged into the ER.
HOLLYWOOD

DDW said:

.

If you replace the tanks in a boat, you have the opportunity to correct the sins committed by so many boat builders. Whether top or bottom draw, the tanks should have a bottom that slopes towards a distinct low point, and that low point should have a small sump lower still, from which the bottom fitting, or dip tube draws. All of the junk will collect there, and you can drain the tank completely but for the partial contents of that sump. In addition, there should be a bung directly above that sump, which allows sucking it out with a suction type oil change pump periodically. This will remove the junk and any remnants of fuel.

While you are at it, do not mount the tanks on their bottom surface, but on C or hat shaped channels. These will hold the bottom off the wet surface they will otherwise sit on allowing it to dry, and if they corrode it will not hole the tank.

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Exactly how ours are done but without the bung.
Instead we have a ball valve at the bottom of the sump/sight glass - handle removed when not in use..

The other thing we have is no deck fillers - I believe leaking O rings are a major source of contaminant so our fillers are 3 ft up the cabin sides.

Glad that I've fiberglass tanks, I don't even think about them. Made a pair of fiberglass tanks for a friend with a TT that had rotten metal tanks. Not all that much work or expense, wouldn't even consider replacing metal tanks with metal.

I have one aluminum tank on my boat. Centerline and not down in the bilge. Do I have to worry about aluminum as much as I would with steel? I didn’t think I did.

I had an aluminum tank on a sailboat fail, corroded from the outside in. It was sitting on the hull and the lowest corner of the recess could trap water and was not iimbered. It lasted about 20 years that way. I cut the lower corner off and patched it, limbered the recess, and reinstalled it sitting on DriDeck. That way I could rinse around it to remove any accumulated salt and it would dry.

sean9c said:

Glad that I've fiberglass tanks, I don't even think about them. Made a pair of fiberglass tanks for a friend with a TT that had rotten metal tanks. Not all that much work or expense, wouldn't even consider replacing metal tanks with metal.

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Sean, did you find that some FRP products are better rated for diesel than others? When alcohol started being added to gasoline some FRP tank material proved unsuitable.

My previous boat had 50 year old steel tanks, no issues at all with them.

Tanks

I have no idea whether or not you can replace the fuel tanks without doing surgery on the decks, etc. In our case, when an aluminum tank failed we had to slice open the deck to R&R tanks.



But as a tank manufacturer, I do have a few thoughts/opinions on tank construction and design...


1. Virtually all marine tanks should be top fill and top draw. If a tank has a bottom outflow, it's another potential failure / leakage point.



2. By nature, you are operating in a damp, salty air environment. Steel is prone to corrosion with or without direct contact with water.



3. Attempting to internally coat the interior of a baffled tank rarely works well. Just too many nooks & crannies to ensure good, 100% coverage of coating. We discontinued that practice several years ago.


Ed

Tanks less than full get water inside

There is water in air. Condensation will happen inside of any tank as it does on the outside of a glass of ice water. The less full the tanks the more air and thus the more condensation. If a boat sits for years without moving it will accumulate water in the fuel tank.

For metal to rust, two things are required: water and air.

If the tanks are dry on the outside, they won't rust from the outside. If they're water free on the inside, then they won't rust from the inside.


Outside water is more likely to corrode than inside water, since there isn't much air inside a tank, particularly if the boat is stored with full tanks.


It's unusual for a motor vehicle to not have steel tanks and these tanks are exposed to water every time it rains. In 60 years of driving, how often have I had to replace a fuel tank because it corroded? Never.


That said, if the bottom of a steel tank is in bilge water, I wouldn't buy the boat.

cardude01 said:

I have one aluminum tank on my boat. Centerline and not down in the bilge. Do I have to worry about aluminum as much as I would with steel? I didn’t think I did.

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My almost 50 years old “black iron” (carbon steel) fuel tanks don’t leak and have no sign of rust.

The aluminum fuel tanks in my 20 year old Avon Seasport Jet tender did develope a small leak which I repaired with epoxy.

I think it depends on the tank getting wet or not.

Petroclean up above Seattle epoxied the inside of my steel tanks. They pumped the diesel out, saved it then cleaned it and put it
back in after they fixed the tanks. They cut 3 port hole in each tank, cleaned the tanks and then epoxied the inside about 4" up from the bottom. Better cost than pulling the engines to get the tanks out and replace them.

Bob M said:

For metal to rust, two things are required: water and air.

If the tanks are dry on the outside, they won't rust from the outside. If they're water free on the inside, then they won't rust from the inside.


Outside water is more likely to corrode than inside water, since there isn't much air inside a tank, particularly if the boat is stored with full tanks.


It's unusual for a motor vehicle to not have steel tanks and these tanks are exposed to water every time it rains. In 60 years of driving, how often have I had to replace a fuel tank because it corroded? Never.


That said, if the bottom of a steel tank is in bilge water, I wouldn't buy the boat.

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Bob, not really correct...



Marine fuel tanks live in a damp, salty environment. If you keep them "dry", which means zero humidity, they will last a long time for sure. I know of no boats that have a zero humidity environment in the engine room.



All tanks create water inside, as the previous poster explained. There is moisture in the air than condenses inside the tanks. That is why good boat owners run water filtration cartridges. Racor, Facet, Velcon, etc...


It has been common practice for cars & light trucks to have plastic fuel tanks for several decades now. Your gas powered car or truck easily accepts typical amounts of water in the fuel, and your plastic gas tank doesn't rust out. You have water in your car's plastic gas tank right now.


Diesel powered vehicles always have good water separator filtration. My Chevy pickup has a water in fuel sensor in case the filter cannot contain any more water.



My 68 year old Jaguar has a steel tank, which has always had minor rusting inside. At least for the 40 years I've owned it... At one point I had to have it resealed and coated internally to keep the fuel from leaking, and the rust from clogging the fuel filter.