Chine Lap Joint Removal?

Discussion in 'Metal Boat Building' started by NorCal, Oct 14, 2013.

  1. NorCal
    Joined: Nov 2011
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    NorCal Junior Member

    A primary reason for my recent haulout was to thoroughly address the corrosion in the stern parts of the bilge. For the most part this 40+ year old boat is not showing signs of rust in the bilge but along the last 4-5 feet of the stern there is some pack rust between the lap joints on the chine. The lap joint is about 6 inches wide, fully welded on the outside but only tacked on the inside.

    As I said, there is pack rust in the stern corners of the boat that I am removing. For good measure should I remove the lap joints along the entire length of the chine? OR should I finish the weld on the inside that is only tacked?
     
  2. pdwiley
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    pdwiley Senior Member

    Some photos of the affected area would help you get better answers.

    If there's rust between the lapped plates I, personally, would do my best to cut off the lapped area and properly clean it back to bare metal.

    PDW
     
  3. michael pierzga
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    michael pierzga Senior Member

    Water can penatrate a lap then move along the joint and cause trouble. Do a carefull repair..as pd stated,cut back the lap , grind back to white metal.
     
  4. Nick.K
    Joined: May 2011
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    Nick.K Senior Member

    I have always found on my boat and others I have worked on that where there is pack rust there is significant metal loss. Are you planning to test the remaining thickness where you have chipped the rust?
    What are you using to remove the scale? I don't have a needle gun but use an SDS drill with a rounded off bit, it works fantastically but your neighbors will hate you.
    Nick.
     
  5. NorCal
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    NorCal Junior Member

    Sorry for the delay, been slammed at work and haven't been to the yard lately to snap some pictures.

    Here's a picture of the issue.

    [​IMG]

    Yes, in the original post I linked to my other thread asking about the order I should tackle haul out tasks. I think removing the lap and performing that repair throughout the boat will be the first order, then ultrasonic the entire hull, and the rest of the tasks as listed in that thread.
     
  6. pdwiley
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    pdwiley Senior Member

    I take it that the bottom plate is folded up on the outside of the topside plate? Cutting it all out could be an interesting exercise. That's got to be a structural joint, what's going to hold the boat together while you're rectifying it, and how do you do it?

    So, sorry, I can't offer you any suggestions. This one is beyond even my assumed internet level of knowledge & competence. If it's possible to clean the internal joint *thoroughly* and get all the rust out and there's no sign of the lapped plates being forced apart due to rust in the lap, I'd be tempted to weld it up & forget it. Some things are better put off for another time (owner).

    PDW
     
  7. NorCal
    Joined: Nov 2011
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    NorCal Junior Member

    PDW,

    Not sure I entirely understand but here's a quick MSpaint version of the joint.

    As you can see it's the side of the boat that is bent in and the outside joint is fully welded. The bottom plate is NOT folded up on the outside. The inside edge is tacked every 6" which has allowed moisture to build up and pack rust to form, pushing up the inner part of the lap. It should have been sealed or fully welded to begin with but this boat was built in 1970.

    Do you think gussets are needed if I remove that overlap?

    The boat was free so I'm that "another time" guy. I do not mind some sweat equity and would like to complete this project if it is not a futile effort. The plan is to blast and coat the entire bilge/interior metal which is good other than these seams.
     

    Attached Files:

  8. pdwiley
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    pdwiley Senior Member

    Ah, ok, I know some boats were built with the lap the other way around. That way is a lot easier to cut out.

    I'd cut that one back as close to the chine as possible then weld it completely after removing rust. It's going to take a lot of cutting disks though. I'd do it in smallish sections rather than long runs and wedge each section free so you don't need to cut into the underlying plate by accident. Not the end of the world if you do but why make more work.... Not sure there's any other way to do the cut but someone else might have ideas. Using oxy-acetylene is likely to give you 'interesting' blow-backs every time you generate a steam pocket so I personally wouldn't. Ditto plasma cutting.

    Gussets - don't know. Depending on how close to the chine you cut, welding in some flat bar bisecting the angle (with lots of rat holes for drainage) will strengthen the chine quite a lot, at the cost of 2 extra weld runs. It's easy to make gentle bends in flat bar incidentally, just support it edge up on a piece of heavy channel and tap it with a decent sized hammer.

    As long as you got the boat for the right price, it's all good.

    Oh yes, buy yourself some good ear muffs, breathing masks and a full face protective shield, plus a lot of riggers gloves. Those thin cutting disks can ruin more than your day if you get on the wrong side of them. I have a nice scar right over the main tendon of my right thumb to demonstrate the point. I was lucky, it was only a flesh wound - through my rigger's glove.....

    PDW
     

  9. tazmann
    Joined: Aug 2005
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    tazmann Senior Member

    Without actually seeing it, it is hard to give advice on how to tackle it.
    I would say to start with take the worst looking area and cut the welds loose on the inside seam of the flange, just a foot or two then take a chisel and separate the seam enough to see in there , if the bottom plate is pitted bad with deep pits then you would need to RR the section, if it is just mild surface rust then cutting the flange off like PDW described would be an easier option.
    Tom
     
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