Poured concrete for ballast in a sailing hull..a no no??

Discussion in 'Boat Design' started by Carr, Mar 9, 2009.

  1. Landlubber
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    Landlubber Senior Member

    Cement rendering steel and concrete as ballast in boats are two entirely separate issues......
     
  2. MikeJohns
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    MikeJohns Senior Member


    It has to do with the stiffness, a thin layer flexes along with the hull plate and frames as they move whilst a big mass is rigid and a gap opens and closes along the margins. Water enters this gap and once rust starts the gap is forced open by the corrosion products and off it goes.

    In a rigid heavily framed keel with the surface cement coved up at the edges and even integral sumps within the cement plug it can work very well even in a wet bilge. The keel is relatively narrow of course and usually with integral floors etc it is very stiff and doesn't move.

    Thick layers of cement in other areas of the hull will often separate as the plating and the cement will move differently, this is common in anchor lockers. Even then if the steel is prepped properly and well painted and you use a flexible sealer Sika - polyurethane around the margins then there will be no problems.

    6mm is fine for the keel sides particularly if the keel is cement filled, the keel base plate should be thicker 10 - 20mm.
     
  3. Carr
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    Carr Junior Member

    Mike, Landlubber and Gonzo

    Many thanks for the information. Im afraid through ignorance I posted bally and included concrete in the posting rather than cement.

    At issue of course is how well the areas were prepped. Its difficult to know that or to know how well they are holding up. One wonders how big a risk this could be. She was launched in 1998 and provided the coatings were well done then she should have a number of years left before hopefully there is an issue .

    Again many thanks to everyone for the interest and helping a neophyte out.

    Regards
    Carr
     
  4. mydauphin
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    mydauphin Senior Member

    A quickie note for Newcomers, things that you can do with steel boat are a definitly a no-no with Aluminum. A lead keel in an Aluminum boat is call a battery and will corrode like one...
     
  5. thudpucker
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    thudpucker Senior Member

    Two bits worth of experience here.
    Whatever acids are in Concrete are pretty strong and eat Bronze and Brass quickly.
    I bought a wooden boat that had Cement for Ballast and all the wood was rotted under the ballast. Chances are your Cement might do your steel in faster than you think.

    If you think Cement might be what you need, try using Shale or larger round rocks instead. That will let you pump out or at least aeriate the bilges.
     
  6. apex1

    apex1 Guest

    Carr, there is (in this case) no difference between cement and concrete except for some sand and gravel in the latter.
    A massive amount of cement in a bilge IS a no no!:!:

    Regards
    Richard
     
  7. MikeJohns
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    MikeJohns Senior Member

    Yes you need to be cautious.

    But concrete is very strongly alkaline (not acid) , this high alkanlinity encourages some metals to corrode such as aluminium, brass, zinc and alloys containind zinc, and also on that list is tin and chromium.

    But steel is very well protected. the steel surface is pasivated and protected very effectively providing it was bare, even slight surface rust (but not scale) gets bound up very effectively. Old steel and iron ships bilges were painted with a thin layer of cement and this protected the metal even after years of immersion as we see when wrecks are lifted.

    Wood also doesn't react well to alkalinity and although wooden boat building is not my area of expertise I would expect good practice would be to paint with a thick layer of some mastic bituminous product prior to any concrete pour.

    I would not recomend loose material in a steel boat, the idea of the cement is to cap and seal all voids which it can do very well provided some common sense is applied.

    cheers
     
  8. thudpucker
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    thudpucker Senior Member

    Well now you exposed my ignorance.
    I dont know which was in that boat. I normally look at it all and call it concrete.
    In AK some folks tried to put hot water through copper pipes in the Cement floors. The Cement ate the Copper pipes and Bronze valves.

    Excuse my ignorance if I led anybody astray.
     
  9. apex1

    apex1 Guest

    Mike I do´nt like to disagree with you, but have to, in this case.
    As I mentioned (one page back) a thin layer can be a good sealant and hold for ages.

    But a massive amount of cement / concrete is not poured in for sealing voids, but for having the cheapest stuff at hand for ballasting. Let´s stop to make a difference between cement and concrete, its the same stuff in this case, the cement is the agressive part. And it definitely is a water trap for which reason so ever.
    So, better leave it out of any boat.
    Regards
    Richard
     
  10. MikeJohns
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    MikeJohns Senior Member

    The reality is that many sailboats will be ballasted with steel cuttoffs mortered into place. Lead is expensive and designers will ften give a cheaper alternative and a slightly deeper or wider keel base. Then the question should be how to do this with confidence.

    This may be a little confusing a rich portland cement mortar mix with some smooth small particle agragate is often poured over lead and or steel and iron pigs, cutoffs, punchings etc which are in the keel as ballast. The cement mortar locks everything in place and seals the voids. As I said before the problem is if the hardened mortar-cement-concrete (call it what you will) seperates from the steel plate and salt water and chloride ions get in the interface.

    If the mortar is trowelled in thick in the centre but 'coved' thinning at the sides so as to end up a thin skin it tends to stay bonded since even the mortar has some elasticity. The alternative is to seal the interface or even pour a layer of mastic material over the top, in the past tar-epoxies were common, now polyurethane sealants seem to be ideal.

    The best course is always to cap the void with plate and keep the water out forever but I was initially trying to allay the fears of people who already have this very common setup of concreted-in ballast.

    When hauled out you can tell pretty well whether the steel remains adhered to the concrete by tapping even with your knuckles or a small plastic ended hammer.
     
  11. apex1

    apex1 Guest

    Mike, I do not contradict, but, that´s it:

    "When hauled out you can tell pretty well whether the steel remains adhered to the concrete by tapping even with your knuckles or a small plastic ended hammer."

    And if not?
     
  12. MikeJohns
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    MikeJohns Senior Member

    Richard please note that I'm not advocating this as ideal practice in a new build. Ideal is to cap with plate over the mortar-concrete and ballast infill then there is never any problem.

    If an existing installation has a void and is not sealed (capped) with plate then you need to thickness check and also make sure that no water can into that void, if it is already full then it should be drained, rinsed and then inject a slurry of cement and lime will bind and re-passivate and fill the void better than injecting paint or epoxy.
     
  13. Gilbert
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    Gilbert Senior Member

    For cementing in ballast in wooden boats the practice is to 'paint' all the wood which will be in contact with the ballast with a mixture of only cement and water. When this cures the slurry or mortar will bind to this cement coating. Wood and cement give off and absorb moisture at about equal rates and at any given time the moisture content of both will be very similar, and so they 'get along' pretty well with each other.
     

  14. Guillermo
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    Guillermo Ingeniero Naval

    Boatyards over here commonly use cement with sand as ballast for wooden boats and also to fill spaces able to retain water in the bilges, as Gilbert says.

    Capping over the cement has to be done only after the cement has completely cured, as otherwise the curing may stop. In my first professional job I had to travel two times to Mozambique from Spain to correct all the ballasting of some 20 commercial fishing boats which had been sold to that country some months before. Hulls were of GRP and ballasted with cement and manganese balls. As the cement was capped with some GRP layers before it was completely cured, the balls remained free to move in a soap of cement. With vibrations and the boats' movements these balls punctured the hulls from the inside, making them to look like baskets. Incredible.

    Not a nice job to dismount all 20 engines, shafts, etc, chip all ballasts off, repair the hulls, put new ballasts and mount all engines again, etc. All this on a muddy beach with daily temperatures over 40ºC. We recorded one day of 52ºC. :(

    Cheers.
     
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