Patching a steel hull

Discussion in 'Metal Boat Building' started by Wittholz 35, Feb 24, 2008.

  1. Wittholz 35
    Joined: Feb 2008
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    Wittholz 35 Junior Member

    Hi all,

    I have a 35' steel cutter with a glassed over plywood deck. Overall she is in good shape but some time ago water made its way between the plywood deck and the steel hull. She is rusted through in three places along the plywood in 1 foot X 2" long sections. I am planning on replacing the entire deck and restoring the metal underneath. I heard if you are welding in a new plate on a steel hull it should be at least 8" square to avoid distortion. This doesn't make sense to me. It seems like the the smaller area you cut out the less chance you will have of distorting the metal. My plan is to remove the deck, cut out an area of the hull just larger than the rust with a plasma cutter, sandblast and weld in a new piece. Any advice would be greatly appreciated as I have no choice but to do the work myself and honestly am far from qualified to do it. The workmanship on the boat is top notch and I don't want to ruin it.

    Sean
     
  2. M&M Ovenden
    Joined: Jan 2006
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    M&M Ovenden Senior Member

    I wouldn't be to stressed about replacing a small piece of plating, but each situation can be diferent. Is it in a rather "flat" area of the hull or somewhere the steel has more curvature and maybe some inherent stresses, does the section cross a frame or not? It's not all the same. You have to make the call on yourself as you have the boat on front of your eyes.
    No matter the size of the patch I'd cut out I would make sure the steel all around the whole is well supported. If there is any curvature, some stress release can make some buckles appear on the edges. It's easier to avoid them then fix them. If you make a cylinder out of paper and cut out a hole out, it you'll see the buckles I'm talking about. So if the whole is not over frames or not over enough frames to hold everything in place add some temporary structures (flat bar), it only has to stay there until your new plate is resonably tacked in place.
    If there is any shape in the section cut that the new patch will need to "mold" to, make sure again that you have some structure (temporary or not) to pull it against.
    Then take your time to weld it together,use a good weld sequence.

    If the patch is small, it's harder to have the edges fit the shape (if it's not flat). You can get a plate a bit bigger than your patch rolled (slitghly) and cut your patch out of it. Other point about the smaller patchs: Steel is not vrey expensive, a bit more or a bit less welding does'nt make much difference, so if the situation makes it more convinient to cut more...cut more.

    Do some experiments with the plsama cutter, you should be able to get a very neat cut. No matter how neat the cut is, give a grind to it to make sur it's clean and have no slag or your welds will suffer from it. Some smaller cuts can be neater and affect less the area if you cut them with a zip cut wheel on thinner of steel, but by all means plasma cutter is great and use it on long cuts.

    Last point,idea, sometimes in very small pieces, making a circular patch can make a better fix, corners tend to stick out.

    Those a just a few ideas on my approach to patches, probably are many people with good patching experience and good tricks. Just remember to use your imagination and approach each situation in it's context.

    Cheers

    Murielle
     
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  3. Jratte
    Joined: Jul 2007
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    Jratte Junior Member

    I can believe wanting to use a patch bigger than 8" square. It will spread out the heat from the weld. At 8" or under your heat affected zone from welding each side will likely converge in the middle of the patch which will likely distort it. I work on aluminum ships and anytime we patch anything the seams have to be a minimum 100mil (~4") away from each other, usually closer to 200 if we can for this very reason. If the patch is square-ish in shape I would radius the corners of the patch. Best of luck.
     
  4. Dutch Peter
    Joined: Aug 2004
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    Dutch Peter Senior Member

    according to IACS recommendations for steel repairs an insert should be atleast 300/300 mm and cornesr to be rounded with a radii of min. 50 mm
     
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  5. Wittholz 35
    Joined: Feb 2008
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    Wittholz 35 Junior Member

    Thanks for the informative responses. I was about to go about this all wrong. I would proceed to cut out a 16"X16" chunk of my hull and try to weld in a new plate but that would mean going through a 1-1/4" piece of pipe that serves as a rub rail and must also be a structural stringer. What would happen if I cut out everything above the pipe (5") and 16" long?

    Thanks for your help,

    Sean
     

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  6. M&M Ovenden
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    M&M Ovenden Senior Member

    How affected is the pipe itself or the plate on the inside of it? If it's healthy enough and nothing needs to be replaced behind the pipe, the solution is simple to me. I would cut off the whole toe rail and replace it all, or at least some generous length of it. It's not a complicated job, toe rails are usually fully developable and very little stressed, it's a much easier job than patching.The repair would be much more elegant and complete.

    Murielle
     
  7. charmc
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    charmc Senior Member

    Although it may sound daunting, sometimes replacing a larger area is actually easier than smaller patches. If Murielle is right that this is all toerail (it looks that way from your photos), then replacing a longer section would be best. Be careful to grind and/or cut back to clean, sound metal. Welding carefully should make it as good as new.
     
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  8. Wittholz 35
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    Wittholz 35 Junior Member

    Thanks for the advice folks! Murielle is right. It is all toe rail. I might try a 16" patch first and if that does't turn out well then I can cut it off and do a larger section. Hopefully this is the first step towards going crusing. Thanks again. I'll post pics of the carnage when I'm done.

    Sean
     
  9. Wittholz 35
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    Wittholz 35 Junior Member

    No matter how old this thread gets please reply if you have any opinions based on experience. I am not going to start this project until the spring and since I am a huge procrastinator maybe not even then!

    Sean
     
  10. pengreg
    Joined: Feb 2005
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    pengreg Junior Member

    Patching

    Hi Sean,

    I have put around thirty patches into my 38' cutter in the last year, and can now do one in around half an hour. It really is quite simple and enjoyable if you are set up well. I cant add much to the good advice already in this thread, except to recommend a book: The Steel Hull - Roger McAfee. In addition I have found that corrosion often indicates areas of poor drainage, were water is ponding. You may consider cutting "scuppers" where your corrosion is now. In any event, I would first go over the whole area with chipping hammers and a wire brush, and then put a thin coat of red oxide etch primer. Only then will you clearly see the extent and be able to plan a patch size and shape. You may find more than you expect.

    Success,
    Greg
     
  11. ETEE
    Joined: Jul 2005
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    ETEE Junior Member

    Our firm does a great deal of Engineering for clients with ABS Classed vessels. Download the U.S. Coast Guard NVIC 7-68 . It is a fairly definitive guide to Steel Hull inspection and repair proceedures.
     
  12. welder/fitter
    Joined: Jun 2008
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    welder/fitter Senior Member

    Sean,

    A few thoughts/questions on your repair:

    My first question would be, is the sheer pipe welded to transverse frames? If so, before cutting out the section, you'd want to temporarily brace the area between the frames & ensure that the boat is well-supported, so that you do not lose shape when cutting out the rusted section. If the sheer joint is good, you won't have this concern, but I'd check very carefully. You may have to remove the plate before you can assess the pipe. So that the pipe is not damaged when removing the toe rail, I'd suggest using a zip-cut where the plate lays over the pipe.

    For any corners that are not running into weld seams, you must radius the corners.

    You may wish to tack supports to each end of the toerail, just beyond your vertical cuts, to keep them from losing shape/alignment.
    If I am understanding your description correctly, I would suggest that going
    2" past the shear pipe and the same distance beyond the transverse frames on either side of the damaged area should give you the best chance of avoiding distortion. you can make up small stiffeners - thicker than the plating - with mouseholes, to tack across the seam at regular intervals, after tacking in the insert(don't make the stiffener tacks too large). The mouseholes need to be big enough for you to weld through them.

    When you weld it up, use back-step technique with small stitches to reduce the possibility of warpage.

    Which welding process are you using? If stick(SMAW), because the welds are well above the waterline, I'd downhand the verticals with 6010/11, as less heat will go into the joint this way. Use small diameter rods, for the same reason. If using GMAW or FCAW, I'd downhand the verticals, for the same reason.

    Take your time when welding & grinding, let the heat dissipate between stitches and grind a bit, stop, grind a bit more.

    If you take your time, the repair job will be as good as anyone else could do.

    I'm assuming that you have steel transverse deck beams that are welded to each hullside, probably to the sheer pipe.
    I'm not keen on a steel hull/wood deck combo, but am guessing that you're not willing to replace the deck with steel?
    Hope this helps.
    Mike
     
  13. welder/fitter
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    welder/fitter Senior Member

    Sean,
    Maybe, I should have posted AFTER blowing up the pictures! I see, now, that the toerail is centred on the sheer pipe and, I expect, welded to it. If this is the case, and if the sheer pipe is good, I'd support the top of the toerail from just beyond each side of where you'll make your cut and, if the toe rail is welded to the pipe, zip-cut it along the seams & it's not necessary to radius the corners.
    Mike
     
  14. Herriott_1999@y
    Joined: Jul 2008
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    Herriott_1999@y New Member

    My 1955 Welin 37 footer has galvanized hull.

    My 1955 Welin 37 footer has galvanized hull.Does this change the nature of how it is repairs versus the advices below

    Richard Herriott
    Huron, OH
     

  15. Wittholz 35
    Joined: Feb 2008
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    Wittholz 35 Junior Member

    Thank you everyone.

    As you can tell I haven't checked this thread since the last time I posted. Ironically I was googling for advice on how to patch a steel boat and my own thread popped up! :rolleyes: I finally took the deck off of my boat (right at the start of the rainy season, but I have to work when I'm motivated) and have started cutting out the rotten areas and cutting new plate to match. I have not noticed any distortion but I have not welded anything back in yet. So far the sheer pipe is not compromised and looks like it will take a weld. I think I will read the book that pengreg suggested before I go any further or I might just go for it using Mike's suggestions. By the way what is a zip cut? I am using a plasma cutter and a miller tig welder. I will post some more pics of the carnage soon.

    Sean
     
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