Hull repairs.

Discussion in 'Metal Boat Building' started by Nick.K, Feb 25, 2012.

  1. Nick.K
    Joined: May 2011
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    Nick.K Senior Member

    I am starting the phase of hull repairs on my project and need some advice:

    It is a steel sail boat with four chines, length on deck 11m, beam 3.5m, weight of hull approx 10 ton. Bottom chine 5mm, all others 4mm. It is framed with 40/8 mm bar on approx 800 centres.
    The hull was blasted and painted inside but then abandoned for many years during which it corroded in some places. I have to decide how much of the corroded plate to change.

    I have been told that up to 20% plate loss is acceptable (3.2 remaining), is this correct...it seems thin?

    The framing seems light compared to other steel boats I have been in. I am considering doubling the number of frames so that it will be framed on 400 centres. It is a heavy hull anyhow and the additional frames may compensate for plate I don't change. Opinions?
     
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  2. Nick.K
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    Nick.K Senior Member

    Interior

    interior photo
     

    Attached Files:

  3. gonzo
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    gonzo Senior Member

    The acceptable plate loss is determined by the designer, or someone who can recalculate the structure. There is no standard percentage. Some boat are heavily built and can have more loss than others. Also, how did you measure the percentage of plate loss?
     
  4. Nick.K
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    Nick.K Senior Member

    Thanks for your reply Gonzo.
    I measured the plate in the corroded areas with an ultrasound meter, cleaning the surface back to bright steel, calibrating the meter then using with the gel provided. I tested the readings against some new 4mm plate.
    Most of the corroded areas test around 3.7mm (of 4mm) I was thinking of replating areas that are thinner than 3.7, but a well respected boat welder locally told me I could go down to 3.2 (in his opinion)
    The hull was blasted and painted before it was abandoned, the corrosion is mainly limited to areas around a tide mark where it had water sitting in it.
    What do you think of the idea to add extra framing, does it look lightly framed to you in the photo? I do not think the increase in weight will be significant.
    Nick.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    If you have more than 0.5mm or apprx 10% thinner plate, which ever is least, then you should replate.

    The pic you show, the cabin roof structure does not appear to be continuous; looks like they just butt into the side. The bottom structure with the frames and longt.s looks a tad light. But this is steel and only a 10 tonne boat. The plate thickness is fine, you could elect to add frames or long.ts, or both, that's your choice. It wont do any harm, other than add more weight. But you need to ensure that the existing, as well as any potentially new, structure has suitable load paths and not terminate abruptly. And all transitions to be smooth.
     
  6. michael pierzga
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    michael pierzga Senior Member

    How did you measure plate thickness ? I never trust those " gizmos " or their operators. Your eyes and heavy internal scale are the best indicators.

    Logic says that since your hull is open and easy to repair, replace any steel that may be problematic in future.

    Failing survey then Repairing a fully fit out hull is a nightmare and the repairs are never "correct"

    Do your duty

    And prevent futher corrosion. properly prepared and Painted steel is very durable. Either your paint system was poor or poor craftsmanship, design has created areas that trap water. The front end of the bulkhead shown is a potential corrosion swamp...detail it correctly
     
  7. Nick.K
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    Nick.K Senior Member

    Thankyou Ad Hoc

    That gives me a clear guide for which areas to replace.

    Should I continue the cabin roof framing down the cabin sides and under the side decks?
    Originally the boat was built without, the hull has transverse frames with some stringers, but the side decks were longitudinally framed with two T bar frames and there was nothing on the cabin sides. Actually the cabin top is quite small, about 280mm high at side deck and it stops about 300mm aft of the mast step, there is a flush deck forward of that.
    Nick.
     
  8. Nick.K
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    Nick.K Senior Member

    Thanks Michael too
    ...Yes, I have seen a few cases of surveyors getting it very wrong with ultrasound. Last year 8mm plate from a seventy ft yacht was replaced in the yard, (and with no exageration) in one place you could see the epoxy fairing from the inside...and that was under the waterline! ...and the surveyor had reassured the owner that all was fine, some minor issues only!!
    It is easier for me because I can clearly see the wasted areas inside.

    Before I took on the project I chose two of the places with the heaviest scale and cut out a small circle with a hole saw, served well to drain her too.
    Nick.
     
  9. michael pierzga
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    michael pierzga Senior Member

    The ultra sound survey is a scam. Its needed for double skins, with single skins physical inspection is better. If corrosion is found..ultra sound the area. A few years ago a nice 70 ft steel motorsailer was surveyed for sale. It passed ultra sound plate inspection and I was contacted to control ships inventory then fire up the yacht and introduce the new owner to the systems. With the new owner at my side I took a tank sounding rod to show him how to measure water tank volume. I unscrewed the standpipe cap , inserted the sounding rod, clumsily lost grip of the rod, it crashed into the tank and went right thru the bottom of the boat !!!!!!!!! Opps.................

    And for sounding rods..a soft rubber or wood foot on the rod tip will keep you from chipping the paint when sounding.
     
  10. charmc
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    charmc Senior Member

    Michael,

    "Sounds" like you are contradicting yourself. ;). Seriously, ultrasonic thickness measurement is an extremely accurate and reliable tool, proven for more than 50 years. Like every tool, however, its accuracy is dependent on the skill level of the operator. Lack of proper training in its use is the most likely source of the many anecdotal horror stories similar to the one you and Nick cited. On the other hand, I was involved some years back in the conversion of a deep sea trawler to ROV support ship. The project was delayed when the ship had to remain in dry dock for re-plating after the insurer's surveyor, using ultra sound, detected severe corrosion in plates that had been cleared by the yard as being within thickness tolerances.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    It kind of depends. It is all load and load path driven. If you apply "a load" and the structure is stiff enough and has adequate load paths to shirk the load to its surrounding structure, then it is satisfactory. The arrangement you show has the small cabin side (280mm) as its own long.t girder. So this can be considered suitable. BUT, the deck transverses they just butt into to this with no transition.

    So, you can do either:

    1) Snipe the ends of the FBs just short of the side. This makes the deck and FB stiffener self supporting, the ends are what is called "simply supported". But this does require a knowledge of the loads you expect to experience and hence can the structural arrangement take that load. If not, it may require stiffer FBs. Also, can the 280mm vertical side, is this adequate for the load transfer to the side decks, once the load has passed from the deck plate?..or will it rotate too much owing to no transverse stiffness!
    or
    2) Add small bkts from the FB and down the cabin side. The shear load from the FB would then be taken into the small 280mm sides and one assumes taken out via the welds to the side deck plate.

    If you really wanted "belt and braces", i.e. if you go offshore a lot, then yes, it is better to have a continuous frame, from the bottom up the side and around the cabin. This provides a decent load path that any part of the structure can shirk its load to another "bit of" structure elsewhere. So the deck FB, its web is continuous all around the hull.
     
  12. Nick.K
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    Nick.K Senior Member

    Thankyou again Ad Hoc

    My intention was to put support struts under the side deck down to doubler plates on the hull. The struts would eventually be incorporated in to partial bulkheads or become handholds. I would have a strut each side, two meters and four meters aft of the bulkhead. The side deck would then be supported (from the bow) at 3m, 5m and 7m. LOD 11M

    If this was done and brackets added to the butt joints as you suggest, would it be "belt and braces enough" ?

    (I kept the original longitudinal side deck framing and the new side decks went in reasonably fair, I am concerned that If I cut out the longitudinals and replace with transverse I will ripple the decks.)

    Nick.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Nick,

    Plenty :D

    Just make sure the struts are not too thin walled. Since a very thin wall on a long member shall be prone to buckling under heavy loads. Also line up the location of the strut, on the bottom frame, with a FB long.t Since eccentric loads can cause the frame to trip.

    It is common practice to have bkts, or similar structure either side of such joints, ie in 2 planes (long.t & trans). Thus lining up with a long.t can do the job of both, or if the frame depth is much deeper than the long.t web, you'll still need bkts.
     
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  14. Nick.K
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    Nick.K Senior Member

    Thanks Ad Hoc

    I'll post a photo when its done (will be about mid April)

    Nick.
     
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