Ferrocement to strengthen / waterproof inside of steel hull

Discussion in 'Boat Design' started by Beginner123, Jul 16, 2013.

  1. Beginner123
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    Beginner123 Junior Member

    Hi everyone,

    Starting of I want to say I'm a complete newbie. Any advice or feedback would be really appreciated.

    My question is regarding whether it is possible and how to strength the inside of a steel hull with ferrocement.

    First some background info boat:

    Recently, I saw an old freshwater sailboat (missing original mast) with a steel flat-bottomed hull for sale. It as originally build in beginning 1900's and converted into a houseboat by adding a wooden hut on top in 1950.

    Some specs:
    - Length: 15.85 m
    - Width: 3.30 m
    - Design: Designed for traversing the Dutch big lake called IJsselmeer. Model is called an "IJsselaak" in Dutch.

    It is selling for an inexpensive price and, needless to say, needs a lot of repairs. My biggest concern is the hull itself: how much of the steel plates has rusted away? Is this actually safe to travel in? :confused:
    The immediate answer would be to hire a surveyor, but prices are high for a student like me. I've planned a visit on 25 July, so I'll start off with that.

    Having said that, I've had a lot of correspondence so far with the current owner and got a pretty detailed picture. So as not to bore any people, I've summarised the important bits below:

    - there has been some serious corrosion; near the water line there are spots were several millimetres have corroded away
    - rainwater has entered the inside of the hull, causing rust there too (see second image)
    - not having seen the boat yet and given that it's lying in the water, I don't know what the state is of the outside of the hull

    Past maintenance / conditions:
    - above the waterline the hull has been regularly tarred
    - below the waterline the boat has received no maintenance in the last 50 years, apart from tarring a bit below the waterline on backboard /starboard by making the boat slant to left/right using ballast
    - the boat has been lying in oxygen-rich freshwater (luckily no salt) for the last 50 years
    - it has been lived in continuously during that time:
    - the good thing is that I can assume small maintenance has been done and the boat has remained warm so that ice has not had as much of an effect in winters​
    - the bad thing? No zinc-anodes have been attached and this could have speeded up rusting combined with electricity usage ​

    I'm now considering various scenarios of how bad it could be and required maintenance:
    - only some rust under water line (blissful fantasy):
    Weld additional layer of steel plating on top of rusted places or just cut out and replace

    - the hull is on the verge of breaking apart and repairing it is financially not doable:
    In this case I could sell it off as scrap iron. However, this is not something I want to consider because it would be a waste of a boat that has it's own history. I'd rather not buy it in the first place.

    - there is considerable rust everywhere and steel plates have become too weak / thin:
    Here I've been researching several options (my current state of knowledge about boats can be mostly traced back to last week's reading ;) ).

    The most common method seems to be to sandblast the surface and weld an additional layer of steel plates to the hull (in Dutch: "dubbelen", in English, wouldn't have a clue). This comes with its own problems as moisture can get between the two layers, causing rusting from the inside out. Having e-mailed questions to several shipyards in the area, I was told it would cost at least 20,000 euro to do this. This is not within my budget, and at this price point you could seriously consider buying another boat in a better state.

    Looking at photos of the inside of the hull (there's 50 cm space between the wooden living construction and the bottom of the hull), I came up with another idea. This was, after sandblasting the steel clean, to pour some kind of plastic between the steel trusses. Epoxy resin, perhaps strengthened with glass fibre, was the first option I came across but the material is too expensive especially for these dimensions.


    Then I found out about ferrocementing and this seemed like a suitable candidate to me to provide additional strength / leak protection to the hull. It is cheap and, though I guess you need to do a lot of research about cement/chickenwire type, thickness, etc., it can be performed by low-skilled labour (i.e. me), is slightly flexible to avoid cracks and reasonably strong relative to weight. The ferrocement could be applied between each rectangular gap between the beams/truss, which would act as a sort of mould. I'd have to make sure to limit the weight so that there is not too much load on the steel below and the boat can still travel over the water at an acceptable speed. I think I will need to do it as professionally as I can to avoid complications down the road.

    I'm trying to be realistic about the state the boat is in and how much work needs to be done. Yet, if in the slightest of chances, I'd be able to pull this off it would be a dream come true.

    Has anyone here got some experience in the use of ferrocement who could give me some advise on this?

    Below some questions I have:
    - do you think ferrocementing would work?
    - any alternatives?
    - what materials would work the best? I want it to last as long as possible.
    I'm reading stuff like pure silica sand, high-quality hydraulic Portland cement with puzzolane, etc.
    - how many layers of chickenwire and what concrete thickness would be suitable in this case?
    - How can I maximise the bond between cement and the hull and chickenwire? Would applying epoxy resin to the hull or chickenwire in advance help in bonding?
    - should the chickenwire be sanded? Should I watch out for galvanised wire?
    - any idea how much the material would cost per square metre?
    - any problems down the road I need to keep in mind?

    Please give some reasoning, so I can figure out the best direction to take.

    Attached Files:

    Last edited: Jul 16, 2013
  2. messabout
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    messabout Senior Member

    That is a charming old boat.

    Don't buy it. It will cost more to rebuild than it would cost for a boat that is in good shape. You mentioned that reality above.

    Ferro on the inside may very well react with the steel skin and framing, speeding the demise of the ferrous materials.. You must consider the additional weight of any such addition as well.
  3. PAR
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    PAR Yacht Designer/Builder

    Ditto Messabout's comments plus the weight and likely condensation issues that will invariably occur between the two materials. The best protection for alloy is paint, not a water absorbing material such as concrete.
  4. Nick.K
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    Nick.K Senior Member

    Hello Beginner.
    You seem to have read a lot in a week!
    You don't say what you want to use the boat for, it is obviously different to try to keep a house-boat afloat than to have a seaworthy craft.
    Filling the bottom of the boat with ferrocement as it is would not protect or reinforce the steel as the structure is covered in thick rust scale which is porous. water will pass through the scale and corrosion will continue underneath. If a hole opens up in the plate, the water will just seep up around the cement.
    It is not difficult to learn to weld (at least at a basic but sufficient level). I started welding about two and a half years ago and since then have re-decked and replaced a lot of hull plate on a rusty project hull. Yes, it does take a lot of time and effort and it would probably be quicker to build a new boat; but no, if you can do the work yourself, it does not cost a lot. Plate, welding consumables and equipment are relatively cheap. Why not consider doing some steel repairs yourself? A lot would depend on what you want to use the boat for. If you want a seaworthy hull, my advice is forget it and move on.
  5. Beginner123
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    Beginner123 Junior Member

    Thanks for replies

    Hi messabout, PAR and Nick.K,

    Thanks for your quick replies. I'm surprised to see three comments in only a few hours time.

    @messabout & PAR
    Good to get a heads-up about possible reaction between the steel and the cement and build-up of moisture between the ferrocement and steel hull. It's never nice to hear that your ideas won't work out but it's better to get a reality-check before you do something foolish.

    I do want to ask two questions though (perhaps I'm just stubborn):
    - what makes the steel hull of the ship different from the steel used to reinforce the FC? I.e. why would the cement speed up the oxidation of the hull but not, assumedly, that of the steel rods, wires and messing commonly used inside of it? From what I've read before the alkalinity of the cement supposedly helps maintain the steel. This could very well be wrong though and I'd rather be on the safe side.

    - would it be possible in any way to make the bond between the steel and cement so strong as to minimise gaps that moisture can travel into or is this a lost cause?

    Weight is a big issue and I don't have the engineering background to determine how a multitude of slabs bedded on a steel platform will act. As indicated before, if I don't know, I'd rather make the safer decision.

    @ Nick.K
    Thanks, I guess I have my style of obsessive reading.:)

    Yeah, I hadn't said what I intended to do with it. I don't want to try out going to the sea with the vessel. Unless I'd have a death wish or just want to wreck the ship for the sheer heck of it, I wouldn't want to enter turbulent water. I think the boat was probably not designed for that either: its flat-bottomed hull makes it less stable in waves.

    It will however, need to travel slowly for around 75 km through inland waters (meaning the hull will need to be pretty much flawless). After that I intend to live in it. I have come across several places where I could moor the boat. Right now it is floating and there's no leakage but I don't want to risk leaks during the voyage or in later years.

    Some more newbie questions:
    - Would removing the surface rust with a sandblaster (or more likely, now I think about the fact that it's inside a ship - a sanding machine or attachment on compressor) mitigate the issue of water seeping between the two surfaces or are you talking about the complete hull?
    - Would the concrete mixture be too weak to close off the space between the frames from water in case there is a leak under one of the "slabs"?

    Welding is starting to sound more doable, when you say it like that. :)
    I've never done anything bigger than soldering with a soldering iron, but perhaps I can learn it.
    I had another idea where I'd cut steel plate to the size of the space between the framing and then weld it to the frames. Is this more reasonable?
    I'm also wondering whether it be good to put epoxy glue, tar or oil between the steel cut-outs and the the hull to prevent moisture in-between affecting them.

    On an internet shop I found 1 m2 steel with 5 mm thickness for 112 euros (http://www.ijzershop.nl/301-staalplaat) . I guess I can get it a lot cheaper in bulk from a wholesaler or from a private seller with no use for it but a back-envelope-calculation:
    cost = (area bottom + area sides )*112 euro per m2

    = (15.85*3.3+2*1.5*(15.85 + 3.3))*112
    = 12,292.56 euro
    Which would mean I'd be back where I started.
    Say it would be half the price though in real life. I think then it might be doable to fix this ship. How much does it cost per m2 where you live?

    The issue is that iron ore prices have risen so much in the past years and will probably take some time to revert closer to its historical (inflation-adjusted) mean. By that time my ship would be worth a whole lot less. :-(

    Now I've got one more issue (the questions keep coming).
    How would I actually get to see / test how bad the hull is when it is lying in the water? Taking it out would be costly in which case I'd almost have to buy it in the first place already.
  6. JSL
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    JSL Senior Member

    Looks like good money after bad. Get it properly surveyed and then decide
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Talk about iron ore prices got me thinking, rust should be worth something these days, as basically that is what iron ore is ! (haematite)
  8. Nick.K
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    Nick.K Senior Member

    Cement does preserve steel very effectively. Recently I was working on a steel trawler where we replaced most of the bottom plate aft of the engine room. The centre had been filled with cement. Under the cement the steel was like new but just to the side of it the 8mm hull was pinholed in places with only the exterior paint holding out the water. In your case, you wouldn't be putting cement on top of steel but on top of rust. It would be almost impossible to remove that type of rust effectively by hand, and blasting would pretty much destroy your interior. Don't underestimate the work, time and dirt that is involved in an old steel boat. After the trawler job it took fully two weeks for my hands to get clean again.
    I think you should get an ultrasound survey, and also see if you can find some guys locally to talk to who work repairing these type of boats. Older or retired boatyard workers have a lifetime of practical experience, many love to pass on their knowledge and could probably tell you whether she is likely to sink or swim and the most effective way to repair her.
  9. MikeJohns
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    MikeJohns Senior Member

    Cement with a pozzolanic aditive is used to good effect. The pozzolan make the cement waterproof. Alkaline conditions passivate steel and stop it corroding. The cement coating should be on clean steel either bright or even pref with a slight surface rust , not on thick rust and never over paint, the paint goes over the cement.

    Coating steel vessels with ferro cement is no different to building with ferro cement except you weld studs to the hull and attach mesh to the studs and then plaster on one side. Vessels I've seen this done to have been a success but I'm not convinced that simply repairing the hull is a simpler cheaper option !

    There are several steel wrecks around that have remained intact under the sea mainly because of the cemented bilges preserving the central girder and bilge plate.

    There's a good example here , Joseph Conrads only command, the Barque "Otago" the remains are exposed at low tide here in Hobart. "James Craig" was another one here able to be reflaoated and rebuilt largely because of the cemented bilges.

    cement gets a bad name because it gets applied over paint or stopped in the middle of a large flexy panel ..
  10. waikikin
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    waikikin Senior Member

    The "James Craig" has a cement coating to the bilges after rebuilding, around 1.5"-2" thick & fairs through the limbers etc.
  11. hambamble
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    hambamble Junior Member

    Correct me if i'm wrong, but my understanding was that the James Craig had a cement coating so that she was seaworthy to go from Tasmania to Sydney where she was restored to full glory by the SHF. I thought that all traces were removed in place of the traditional iron hull construction during restoration. I thought they did it as a "cheap and nasty" way of getting her afloat so that she would survive the trip up to sydney.
  12. Beginner123
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    Beginner123 Junior Member


    Wow, lots of replies.

    @ JSL
    Your comment about throwing good money after bad is similar to what I've been hearing from multiple people. Guess this means I will have to be ultracareful and only buy the boat if the hull is reasonably intact.

    As I mentioned in my first comment, hiring a surveyor is pretty damn expensive, but perhaps it is unavoidable. Going off this site - http://www.blonkjachtexpertise.nl/t...xpert-taxaties-boten-zeiljachten-motorjachten - an ultrasonic hull survey would cost 560 euros, or about 6 days of working for me. That's a lot of money if I'm not even sure if I'm going to buy it in the first place.

    Perhaps I'll be able to get some connection in the area through friends who would be able to take on this work. Alternatively, maybe I can borrow an ultrasonic gauge from a friend and just do it myself.

    Then we arrive at my next problem, is it even possible to take ultrasonic measurements inside the keel when the boat is still floating in the water? Would the higher density of the water interfere with the measurements or is this a non-issue? Or does this depend on how advanced the gauge is?

    @ Mr Efficiency.
    Totally agree the rust should be worth something.
    I wonder though whether this would be considered by the scrap metal broker, or if they just relieve you of the rust for free. :)

    @ Nick.K
    Your comment about the cement preserving steel with clashes with that of messabout and PAR. To be quite honest, I'm more inclined to believe you because of the actual experience mentioned.

    Okay, I'm getting it now.
    A thin layer of hard rust would help the cement bond but a thick layer of loose rust on the inside of the hull would not bond well with the concrete.
    And depending on how thick the steel plating still is, sandblasting might actually create holes in the hull.

    For my opinion about / questions regarding the surveying see bottom reply @ JSL.

    I've found a great do-it-yourself shipyard about 10 km away in Rotterdam called Scheepshelling Koningspoort, where I could do the repairs / get them done (http://www.havenmuseum.nl/index.php?pid=42, in Dutch). The nice thing is that you can hire assistance. It is possible for me to get contacts of welders and other tradesmen in the area who could work off the books. I've called them up and there are some places free after 4 November.

    @ MikeJohns
    Good to know for what the pozzolane is used for.
    I had just copied the word from another forum without having a clue as to what it is or its function.

    Nice ships. Guess it would be useful to do some research about their renovation because of the similar circumstances (rusted old ship needs to be able to travel on water again).

    @ Waikkin
    Ok, 1.5"-2". Any idea if this would be suitable for this boat or would say 1" be more suitable for a smaller freshwater boat.

    Side point: nice to hear some Australian(s). I've lived there for a number of years.
  13. Beginner123
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    Beginner123 Junior Member

    @ Hambamble

    Ok, interesting.
  14. waikikin
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    waikikin Senior Member

    Hambamble, you are correct in that a lining of cementious material was applied to the interior of "James Craig" for the Tassie to Sydney journey, it was removed with much effort to make way for furthers repairs, here & there you can see some chisel marks on the original plating from the removal process. After repairs to the structure & plating a lining material has been applied, I believe it is a cement compound with some modifiers for flexibility/adhesion, it's quite neatly installed to up around the bilge stringer .

  15. MikeJohns
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    MikeJohns Senior Member

    She was cement washed and plastered inside the lower bilge originally and this kept a lot of the plate in quite good condition. This was common with vessels of her vintage. Cement and plaster are often used as emergency repair material on ships they patch weak areas quite effectively.
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