STEEL HULLS with Composite Superstructure / Topsides

Discussion in 'Boatbuilding' started by brian eiland, Jun 16, 2013.

  1. MikeJohns
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    MikeJohns Senior Member

    Titirangi
    Good posts thanks. I've seen several failuires of polyurea in marine environments and it's quickly disbonded where sea water got through tears and scrapes in the coating. But as you say it's critical that application is done in a controlled manner. The problem I see is that it may be some years before the quality of the bond becomes apparent so I'd be looking for a 10 year g'tee.
    On a boat such as Brian is proposing I'd limit the polyurea to the rub rails there's no point applying it to any other part of the hull.


    For a
     
  2. brian eiland
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    brian eiland Senior Member

    External Coating, Sharklet

    To tell the truth I had NOT been considering it for any external application....at least not to this point. What caught my attention was the possibility it might be good application in the bilge areas of a steel hull. It does appear as though there are many different formulations depending on each particular applicator, so lots more homework would need to done to convince me of its use.

    For external application, I would be hoping that some sort of adhesive membrane of this Sharklet material makes it to the marine market
    Antifouling Technology, based on Shark's Skin
     
  3. brian eiland
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    brian eiland Senior Member

    Sikaflex Caulking of Steel Frames vs Composite Frames

    So the Sikaflex works here really well,...that likely would be easier that making the interior framing from composite I-beams as I was suggesting here:
    Composite I-beams for Framing Structure

    I was also making a case for this 'composite framing' over here:
    http://www.boatdesign.net/forums/boat-design/bulkhead-tabbing-now-redundant-47578-5.html#post639254
     
  4. brian eiland
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    brian eiland Senior Member

    You are on your way to convincing me of that.

    You have not sold me on fiberglassed plywood for the deck houses, nor the decks themselves. The composite prefabricated panels built like Derek Kelsall make much more sense here for a number of reasons we will likely get into more discussion of.
     
  5. Ace Dragon
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    Ace Dragon Polyurea and Spray Foam

    yep, 85% humidity is the ceiling for the chemicals I have been dealing with.

    I have found companies that will put it their spec's that their products can be applied at -45 deg. F , but when you get their product the paperwork does point out that the finial product will not be as good if it were applied at +50 deg.F
    That's all well and fine but who doesn't want the best finial results possible?
     
  6. Ace Dragon
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    Ace Dragon Polyurea and Spray Foam

    I am hoping to visit a company in Ca. in Aug. that does fenders with polyurea.
     
  7. Titirangi

    Titirangi Previous Member

    There's two companies in UK, Fendercare Marine that offer a really top quality range of PU fender designs, another is Hippo Vessel Fender.

    They both produce cast section modules and coated PU laminates with full range of colours and glossy finish.
     

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  8. brian eiland
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    brian eiland Senior Member

    Just reading back thru this thread, and I wondered if you had retained any 'details' as to how the GRP deck was to fit to the steel hull? I want to keep all options on this idea open.
    Brian
     
  9. MikeJohns
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    MikeJohns Senior Member

    Brian

    Bilges can stay pristine and on a steel craft . Just give them extra epoxy coats if you are worried. But again a hot zinc spray after sandblasting followed by a thick epoxy coating will see your bilges in pristine condition for the life of the vessel providing they are mostly dry, which they should be these days.

    Polyurea is only an advantage where severe abrasion or impact is expected its not a great protective coating otherwise for example it absorbs relatively a lot of water (over 1% ) that means transpiration is also an issue and then you are still relying on the epoxy coating underneath.

    IMO a very good protective coating can be implemented simply with sika polyurethane sealant. We have applied approx 1/4 inch coatings of this to areas such as anchor lockers. Simply troweled on over a polyamide cure epoxy paint system. It protects the paint form chipping and has worked terrifically. But again you don't need to worry about chipping in the bilges I added this as folk should be aware that polyurea is not the only option and there are much cheaper and easier coatings that protect from impact very well.


    Composite decks whether plywood or cored GRP are attached the same way. there is what's called a deck shelf say 400mm wide runs around the deck edge and is welded to the hull plate, it should be hot zinc sprayed epoxied and then bedded in sealant. The composite deck fits over the shelf and is bolted down while the sealant is still uncured, part of the shelf is left exposed as a gutter ( never take it all the way to the bulwark) . Cored GRP decks are reduced to a single thicker skin in the interface.
    The cons of GRP is a completely different thermal expansion coefficient so the attachment has to be considered an "expansion" join and colours should be white, white or white :).

    Steel decks are best IMO. leak proof tough and they add a lot to global hull strength.
     
  10. brian eiland
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    brian eiland Senior Member

    Cabin Sides and Roofs

    ....just a little addendum to these thoughts that I had penciled in my original notes and forgot to include in the posting

    Look at the cabin sides and the superstructure of these two vessels....basically big flat panels. Why build them of steel? Even the cabin roofs are a single curvature camber, not compound,...easily fashioned from PP panels, and self-insulating. One question will be choosing the right core for solar exposure.

    For those that are concerned about the main decks being constructed with PP cored panels, I would offer that a sub-frame or a space-frame of steel beams could be placed across the ship, then the PP panels placed over them or betweem them in lieu of a sheet of steel. Remember the gunnels are all steel, an extension upward of the hull sides. These are available to weld items to.
     
  11. MikeJohns
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    MikeJohns Senior Member

    Yes, with a steel hull and composite deck the deck beams are steel. usually angle or channel.

    Brian, Those sorts of boats make great homes I like the river boat style of the second pic..
     
  12. luckyjr
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    luckyjr Junior Member

    I feel 18 ga 2.5" Structural steel channel studs are the answer for the floors
    Ceiling and floor supports , and the walls . 2 rmax 3/4" instulation
    4 x8 panels in walls and ceiling and exterior and interior of 4x 8 poly aluminum panels.

    This is awesome structure that will last a life time in fresh water.

    Pontoon flotation I have watch two hull house boats sink in the last 6months
    Good luck
     
  13. Ace Dragon
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    Ace Dragon Polyurea and Spray Foam

    I am sure there are several coatings that will do the job, Just do not be quick to think all polyurea's are the same. To say polyurea's will absorb water is not true. I am sure there are some polyurea's I have no use for, but we are building secondary containment's for chemicals and primary tank linings where water absorption is the lease of their concerns. Do you think ConocoPhillips would have me coat a Sea Water Tank with a coating that absorbs 1%?
    Third party lab testing is going on now to measure chemical adsorption with 1000 psig forcing the chemicals into the polyurea. 1% Chemical Absorption is not allowed with these polyurea's.
     
  14. michaeljc
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    michaeljc Senior Member

    Reliable sources have told me that steel is capably of absorbing moisture. This means that anywhere bare steel is exposed it can absorb, after which moisture can migrate laterally beneath a coating. This explains a lot. Clearly bonding and toughness of the first coat is critical. Having a water proof coating is not enough in itself.
     

  15. MikeJohns
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    MikeJohns Senior Member



    Nylon, Polyurea, Polyester and even Epoxy for example all absorb water when immersed, the amounts vary considerably but they absorb water.

    Looking at material properties for Polyurea show 1% water absorption at 50% humidity and 2.5% at 100% humidity. Ask your supplier for a tech data sheet and specifically the immersed long term water immersion absorption figure. If they tell you it's zero they have simply not tested it or they are just being dishonest since this data is widely available.
     
    Last edited: Jul 16, 2013
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