new steel diesel tanks for a mobo, stainless steel or mild steel?

[vas]

hello

a friend with a 35yo Princess removed his leaking diesel tanks (after removing the engines...)
Wants to built two new ones, slightly smaller.
Old ones were mild steel 2mm thick
his options are:

• mild steel 2.5mm
• stainless steel 2.5mm (304 he said)

options are based on what material can be easily found locally and what the fabricator is happy to work with.

12sqm of sheet is going to be close to 2k euro for the ss one, I guess around 500euro for the mild steel. Not adding labour on that.
He's not planning to change the boat and wants something that will age nicely with him and serve him for the next 15-20yrs.

should he go for mild steel?
that's my suggestion at least!

opinions?

cheers

V.

 

[Freebee]

mild steel goes rusty
304 stainless goes rusty- seems mild steel is good and mild steel will last a good number of years... i would recommend building a small sump with a drain in the design to remove water.

 

[Tranona]

Neither. Welded plastic is the way to go - the type made by Tek Tanks. There are fabricators of this type of tank in Greece. I had a holding tank made in Corfu but I know of others who have had them made around Athens. More expensive than mild steel but generally cheaper than stainless.

Big downside with metal is corrosion, general with mild steel and in the welds in stainless. Plastic, no corrosion problems and easy to make to custom design.

 

[vas]

thanks for the comments guys,

shape is a bit odd, they are "standing" on either side of the hull outside the engines, so outer side is slanted following the hull, inside is vertical, back and forth triangular/trapezoid shaped.
also needs a couple of baffles each to hold it together making inspection holes more difficult.

he's planning to have a sump/day tank in the middle between the engines and lower than the tanks to empty carp.
Not sure how easy it's going to get something built locally out of plastic.
Used to have a couple of rotomoulding companies in Volos, both gone.

I'll pass the info to Manos.

V.

 

[TernVI]

A quick google suggests plastic tanks might have a finite life of 'something over 10 years'.
Stainless might corrode on the welds, but you can get an old welded ss tank mended.

If smaller tanks are the way to go, you might get something off the shelf way cheaper than made to measure. In which case material might be something to compromise on?

 

[PetiteFleur]

Ready made plastic tanks are available from Tek Tanks, Plastimo, Osculato, ASAP Supplies, Vetus etc. They never rust but whether they have one to suit your installation, you'll have to check. Cheaper than made to measure Tek Tanks.

 

[jamie N]

Steel's OK with an anode inside. This taken inside an ETAP potable water tank after 20 years or so.



Edit: misread that they're for a water tank, and not a diesel tank.
In water, these are standard issue I believe.

 

[pvb]

Stainless might corrode on the welds, but you can get an old welded ss tank mended.

Not a cheap exercise if you've got to remove the engines first to get at the tanks!

 

[vas]

well, ready made tanks are out of the question I'm afraid due to shape. He'll probably get 300lt in 6 tanks on the sides compared to the 1200lt he currently has - not acceptable.
So custom work it definitely is.
Both tanks failed (unsurprisingly) at the bottom where all the carp was gathering together with water (which I strongly believe comes from the flush mounted stupid intakes on all princesses!)

V.

 

[northwind]

Given that the tanks in it are probably the originals and they did 35yrs, mild steel will definitely suffice for the next 20yrs, if you design a way of getting the water out they will be fine.

 

[TernVI]

Steel's OK with an anode inside. This taken inside an ETAP potable water tank after 20 years or so.View attachment 112943

Anodes will work in water, no bloody good in a diesel tank.
Whether water is 'potable' with bits of zinc anode in it might be debatable?

 

[TernVI]

Given that the tanks in it are probably the originals and they did 35yrs, mild steel will definitely suffice for the next 20yrs, if you design a way of getting the water out they will be fine.

That's 35 years of mostly old skool red diesel, it could be the last 5 years of modern diesel that saw them off?

 

[Plum]

hello

a friend with a 35yo Princess removed his leaking diesel tanks (after removing the engines...)
Wants to built two new ones, slightly smaller.
Old ones were mild steel 2mm thick
his options are:

• mild steel 2.5mm
• stainless steel 2.5mm (304 he said)

options are based on what material can be easily found locally and what the fabricator is happy to work with.

12sqm of sheet is going to be close to 2k euro for the ss one, I guess around 500euro for the mild steel. Not adding labour on that.
He's not planning to change the boat and wants something that will age nicely with him and serve him for the next 15-20yrs.

should he go for mild steel?
that's my suggestion at least!

opinions?

cheers

V.

I fitted a mild steel diesel tank and after 15 years, when I sold the boat, the inside was still perfect and looked like new (I fitted an inspection hatch). The big problem is corrosion from the outside but if you are confident in getting a good paint system and can be sure that paint film will not get scratched before/during fitting then you will certainly get the life expectancy you require. However, given that the engines need to be removed then someone in the future will have an exoensive problem so a buyer will probably want that cost deducted from the asking price. I would go for aluminium alloy. The correct alloy will not corrode. Many of the French built motorboats have aluminium alloy tanks. I have one now that is 21 years old and perfect inside and out.

Www.solocoastalsailing.co.uk

 

[jwfrary]

Mild steel well treated with the right paint system - hempel commercial paints make the epoxy based paint for tanks of all types. It’s a hempadure of some type - Fair cheaper and more reliable than doing it in stainless. Welding process is loads easier as well!

 

[jwfrary]

I fitted a mild steel diesel tank and after 15 years, when I sold the boat, the inside was still perfect and looked like new (I fitted an inspection hatch). The big problem is corrosion from the outside but if you are confident in getting a good paint system and can be sure that paint film will not get scratched before/during fitting then you will certainly get the life expectancy you require. However, given that the engines need to be removed then someone in the future will have an exoensive problem so a buyer will probably want that cost deducted from the asking price. I would go for aluminium alloy. The correct alloy will not corrode. Many of the French build motorboats have aluminium alloy tanks. I have one now that is 21 years old and perfect inside and out.

Www.solocoastalsailing.co.uk

Main problem with Ali is finding the right fabricator with the skill set to make a reliable weld. Not so easy!

 

[Momac]

Aluminium tanks are not immune to corrosion.

 

[Gary Fox]

Mild steel is better. It is easy and cheap to fabricate a custom design. It won't go rusty inside, and not on the outside if prevented.
Inspection hatches to get to all of the insides are 100% vital. 2.5mm seems a bit thin, but I assume it's a planing boat so weight sensitive.

 

[KompetentKrew]

Not sure how easy it's going to get something built locally out of plastic.

Tek Tanks say their custom tanks are made from welded HDPE - suspect you'll get more results if you use those keywords.

I believe that welded HDPE has applications for industrial liquid tankage - fish farms and chemical manufacture and suchlike - so fabricators should be found worldwide.

 

[oldgit]

Have had boats with stainless,aluminium and mild steel.
The boat with alloy fuel tank corroded after 20 years and had to have new bottom welded on ,cheap to fix, a real B******* to remove.
The boat with stainless fuel tank was 101% perfect after 40 years.It was a displacement boat, warnings that stainless can split along welds in bouncy planing boats.
Another boat around 40 years old, both water tanks eventually failed on lower weld seams. Replaced with plastic tanks from Ebay, much much cheaper than either stainless or mild steel AND a lot easier to fit back in the awkward spaces under coamings.

Present 30 year old boat has stainless water and mild steel fuel tanks.
Was very anal indeed about checking fuel tank condition when purchasing the the above boat , normally would not even have been on the radar.
It is of course impossible to inspect the entire tank .

A skipper who moors nearbye, had a leak in the mild steel tanks of a Princess 41.
He lost much of a boating season. Boat had to be put ashore and the saloon dismantled . One engine was completetly removed and the other shifted sideways to allow old tanks to be cut up for removal.
New tanks (mild steel)were fabricated. Various other "little jobs" had to be sorted during strip down.
About 2 months ashore in the end.
No idea of total bill but must have run into several thousands.

 

[vyv_cox]

My stainless steel fuel tank is 35 years old and in perfect condition. The answer is to use the correct material and the correct filler rod. The sheet metal should be 304L (where L means low carbon) and the filler 316L or 308. This avoids sensitisation, which is the reason welds fail in many stainless steel tanks.