Steel Boat Building Techniques

Discussion in 'Metal Boat Building' started by CB Haws, Sep 17, 2006.

  1. CB Haws
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    CB Haws New Member

    I am looking at a custom (homemade) 40' steel boat. The seams appear to be "welded lap joints". I can see not rust from the inside below the water line. The bilge was completely dry. Are lap joints a big no-no in steel boat hull construction? The bow apperas to be double plated. I guess to break ice? Oh the boat is about 30 years old.
    Thanks,
    Charlie
     
  2. Chapman
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    Chapman Junior Member

    This is an interesting question. I have spent many years around steel vessel construction (none of it on recreational vessels though), and have only seen butt joints used on shell plating. I dug out an old book called "A Guide to Sound Ship Structures" and they talk almost exclusively about butt welding sheel plate with one exception. Apparently many years ago it was thought that lapping joints and using a partially welded/partially riveted join was a good methodology. Does the vessel you are looking at have a fillet weld on either side of the lap? Given the uneven surface that a lap joint produces, how are the longitudinal and transverse structural members connected to the plating? Are the lap joints visible on the exterior of the hull, or is some filler compound used to fair it. If a filler compound was used, I would wonder what the condition of the welds under that was. Sorry to answer your question with more questions, but I've never seen a lap joint used on shell plating with the possible exception of doubler plates being added.
     
  3. timgoz
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    timgoz Senior Member

    Let me give you some more questions.

    Is the boat somewhat conventional other than the lapwelds?

    I cannot remember which book I saw it in, but I once saw a boat built in the lapstrake (clinker) style out of steel. This is a real rarity though.

    As Chapman mentioned, the joints would need to be continously welded inside and out. If not, interior wise, there would be moisture intrusion into the seams, bad news.

    TGoz
     
  4. jehardiman
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    jehardiman Senior Member

    You know...that was my first thought. What you may be seeing are light weight doubler plates welded over an older pitted hull. As Chapman and Timgoz ask....What is the thickness of the exposed lap? Do the laps show up on the inside? Are the plates joggled (i.e. in-out-in-out) or are the lapped (out-out-out)? And do the laps run fore-&-aft, or up-&-down, or both?
     
  5. timgoz
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    timgoz Senior Member

    Have you guys ever seen a lapstrake hulled steel boat?

    Like I said above, I saw one once in a steelboatbuilding book. May have been in "Steel Away" or Bruce Robert's "Metal Boats".

    Although all those welds & resulting heat would be a negative, the longitudinal strength would be impressive. But other than possibly ice-work, I think a proper steel boats much more than adequate strength wise.

    All those laps would also present alot of corrosion possibilities, exterior & especially interior. The same would go for a repair where someone welded plate over a bad section instead of properly cutting the section out and insetting a new piece, a matter of fact, plate over plate would be much worse I would think.

    TGoz

    TGoz
     
  6. MikeJohns
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    MikeJohns Senior Member

    The overlap method was common for a decade or so after the move from riveting joins to welding. The odd hull is still produced that way

    I've surveyed a few welded hulls built to the overlap method. the plates are appied longitudinally similar to trad planking but up to around 1m wide the overlap of around 2 inches is treated in several ways.
    Either alternate under over plating, or lapstrake or jogged seam where the overlap is heated with a torch and hammered over the lower plate till it fits snug .
    In all types both the inner and the outer plate edge are then fully welded to the plate below.

    The boatyard saves a lot of time in fitting since you can have a large tolerance on the edges.

    The problem is the internal void along the overlap and if the welds don't seal completely it corrodes. Generally it seems to be ok. When they do rust the rust puffs the seam up quite visibly. Repair is easy if you can get at the inside at the trouble spots.

    All the best
     
  7. timgoz
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    timgoz Senior Member

    Thanks Mike.
     
  8. jehardiman
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    jehardiman Senior Member

    TGoz, yes I have heard of them, but why on earth anyone would do it is beyond me. The resulting hull is actually weaker (the corner weld is a hard spot that reduces the FOS of the hull), and the layout and cutting is 4 to 10 times longer (depending on the strake to chine ratio) than conventional forming. I could never see anyone skilled in shipfitting doing it for a reason other than "that's what the customer wanted", and only a neophyte doing it because they thought that a corner joint is eaiser to weld than a butt and/or they wanted the traditional look. It was the age of the hull and the doubling of the bow that lead me to the plated over (a common quick fix in the 2nd life commerical world) conclusion.
     
  9. CB Haws
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    CB Haws New Member

    Thanks guys for the input. The boat was built around 1975. It is lap jointed not joggled or over under under over or what he said. Since the interior is pretty much non-existant visual inspection was easy. All seams are welded inside and out. You can see each lap strake from the inside and no rust is visible below the water line. As you get above the water line the inside looks to have been tarred or something. The bow section is doubbled for no apparent reason. The bow section is rust free on the inside and the outside has paint. None of the seams are faired; just very nice welds. The guy was a welder who built this one up. I just figured lap joints were easier to do; but my concern was water in the lap. I could see no evidence of rust. The bilge had no water at all. The boat was in the water and running. The only rust I could find was was in the butt welded deck plate where one side door has been leaking for some time. This boat was never really finnished and I have not found out why. It may be because the original owner builder lost interest. The last two owners were going to finnish her; but got interested in bigger projects. looks like she just needs a little love. The tops sides are aluminum. The inside trim work is all stainless. The guy who did the trim work was a master fabricator. The engine hatches are stainless not welded together; but bent and rivited. A work of art. Other wise the boat looks real crappy, and needs a total redo.
    Thanks again. I will let you know what the surveyor says. Oh by the way what is a 40 foot boat worth in scrap metal? I don't want to get stuck having to be the guy who has to have it cut up!
     
  10. timgoz
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    timgoz Senior Member

    Jehardiman,

    I hear you. It was an interesting odity though. I never gave thought to what might have motivated this boat being built.

    Take care.

    TGoz
     
  11. MarkC
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    MarkC Senior Member

    when in doubt ask the Dutch about steel builds!

    Was it sort of like this (see below) - can be used to good effect.

    Dutch steel built - BLAUWE BOEI SLOEP BB750

    taken from http://www.botenbank.nl - dutch boat sale web-site

    contact for the company can be found on the site - i dont know if they do English.
     

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  12. MarkC
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    MarkC Senior Member

    or like this steel motorsailor 1978 - 10metersx3.1x1.5 - full keel for sale €49K
     

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  13. CB Haws
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    CB Haws New Member

    Thanks for the photos. Yes that is pretty much how she looks. The plates are a little wider; but that is the fabrication method. I am sure it is really strong; but the laps had me worried.
    Thanks
     
  14. CDBarry
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    CDBarry Senior Member

    Actually laps have an advantage for some welding issues - you can use drag rod.
     

  15. MikeJohns
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    MikeJohns Senior Member

    Christopher
    Have you ever supervised a hull built this way? We had a yard bulding a boat here that was offering a discount to plate this way but local DNV were not happy.
    I always think of ways to fill the void.. like grease nipples and pump epoxy in, ...there are some very low viscosity 100% epoxies available now which could work well.

    Cheers
    Mike
     
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