Estimating man-hours for hull welding

Discussion in 'Metal Boat Building' started by makobuilders, Jan 17, 2015.

  1. TANSL
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    TANSL Senior Member

    makobuilders, I'm glad you enjoy our posts.
    If you give me some detail of your boat I'm sure I can give you information as you need.
     
  2. makobuilders
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    makobuilders Member

    Not sure why everyone comments about oil drip from ye ole Gray Marines. I never had a drop of oil in my bilge… of course I had tin cans hanging under the breather tubes and I wiped the block down after every day of use.

    Not a bad compromise considering the engines always started within a 1/3 second of hitting the button. Show me a 4-stroker that can do that :D
     
  3. TANSL
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    TANSL Senior Member

    12 minutes / meter for basic electrode and throat of 3.5 mm, manual welding.
     
  4. Barry
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    Barry Senior Member

    I have to agree with Gonzo. There are two many variables to come up with a number for welding hours per ton and expect to get to a reasonable answer.

    If you are getting the parts done on a CNC machine, the cut file will give you a total length of cut for each sheet that you cut. You could estimate then the time based on applying some percentage to overhead, downhand, horizontal weld and the various plate thicknesses
    The easier way is to search the net for boat plans of the type that you are thinking of building and usually they will have an estimated hull build time which will include fitting etc.

    Also your comment, easy to get primed cnc cut material. Do not rely on the preprimed steel that is available as often it is not sandblasted The primer is normally called a shop prime and does not have enough physical roughness for the primer to stick to it and the thickness of the primer is often very thin, and will not be adequate for finish coating. They often put a light coat on so you can even weld through it with a stick
    You can look up the specification on the internet for the level of sandblasting AND the thickness of easily procured marine primers to ensure that you get a good finished product
     
  5. makobuilders
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    makobuilders Member

    That's actually one of the things that I'm checking into. It is a weld-through primer. I'm suspicious about the treatment of the steel before priming and need to find out if the sheet are wheel-abraded at the mill properly. The heavy sections (I, H, etc.) obviously wouldn't be, by I won't be using those in a small boat. Come to think about it… a 1000lb section of I-beam would make a great anchor :D
     
  6. Barry
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    Barry Senior Member

    There are several sites that will discuss the main levels of sand blasting.
    Generally the three most important are Commercial, Near White, and White with White being at the high end. No staining while near white can have a percentage of stained surface.
    Most preprimed steel providers blast to Near White specifications. I am not sure if this meets any marine building specification.

    The primer must go on thick enough to encapsulate the microscopic peaks of the surface and leave enough thickness after drying to inhibit corrosion.
    What ever paint supplier that you are considering using should be able to provide this information.

    There are quick dry and slow dry primers and I believe that the slow dry primers are better as the primer will sit on the surface for a long time an perhaps give the primer more time to "wet out" and provide a better bond.

    To get better details and recommendations, find a thread on this forum where Adhoc has supplied input and ask him as it appears, from the high quality of information that he provides in his comments, that he would know what the specifications for a finished surface should be.
     
  7. TANSL
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    TANSL Senior Member

    Barry, all that, imo, has little to do with boats. I have not touched this subject for many years but I seem to remember blasting quality should be "mirror" according to a Swedish tables. Something similar to the tables to define colors (RAL).
    On the other hand, in the shipyards plates are blasted and painted in a continuous sequence, the process is very fast (seconds), and are stacked by thickness (or any other system). Therefore, the drying of shop primer must be very fast.
    The thickness applied to the plates is several microns (can not remember how many) but, to my knowledge, in practice no one takes into account the "microscopic peaks of the surface".
     
  8. Barry
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    Barry Senior Member

    The Swedish standards apply as follows, White= Sa3, Near White=Sa2.5,and Commerical=Sa2, I have not heard of a mirror finish

    In an industrial highly corrosive environment, the paint manufacturers will establish a set of application specifications for their product.
    Say that the blast spec is White (Swedish spec 3) the peak to valley difference can be
    anywhere between 3 to 7 microns. The manufacturer will provide a gallons per square foot application rate to ensure that the finished coat will cover the peaks by enough of a buffer to ensure that the primer completely covers the structure.
    Perhaps sometimes the shop preprimed specs may not be thick enough to ensure coverage.
    Depending on the abrasive media, ie the quality of the sizing of the media and the hardness, larger random particles can cause a higher peak to valley variance.

    My comment to Mako was to suggest that perhaps the shop primed- weld through primer may not meet the paint requirements of what ever manufacturer that he might choose for a finished coating.

    Re the fast or slow dry of the primer.
    I have seen primers that have such a fast dry rate that with an AIR DRIVEN nozzle the
    material is almost dry when it hits the surface of the metal. This dust will hit the metal,
    and be covered by wetter material behind. This will leave an area then, without a properly wetted adhesive interface. The technical term is bridging and can be a place for corrosion to start. I am sure that anyone who has worked around a red oxide primer application, fast drying, will have seen instances where the overspray can be rubbed off by hand, dusted off so to speak.

    your comment" in practice, no one takes into account the microscopic peaks of the surface" The paint manufacturers do when they specify application rates.

    The faster the drying times, the more chance of bridging and subsequent unwetted surfaces

    The confirmation of application thickness is a simple guage, a little bigger than the size of a credit card with extremely thin recesses toward the middle the card. When the card is touched to the WET surface you merely read the depth of the paint where the last recess touches the painted.

    If it is a fast drying primer, then the manufacturers coverage specs must rule.

    Again, my suggestion to Mako was to merely confirm that the primer is thick enough to meet standards and not to rely upon the fact that the plate is primed. Primed yes, adequately, maybe not
     
  9. TANSL
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    TANSL Senior Member

    Well Barry, I guess you're right in what you say, which does not contradict my comments, imo. Thanks for your explanations.
    On the other hand, what has to do all that about shop primers with welding meters / hour Makobuilders was asking for?
    Cheers.
     
  10. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Mako said that it was easy to get cnc PREPRIMED material for the build
    My concern was that perhaps he would assume that because it is primed, it is primed adequately to just over coat this shop primer with possible catastrophic consequences when the hull begins to rust, inside and out. Which could cost tons or kilograms of money to fix after all the finishing work was done. Simple as that
    Mako said that he was aware of this issue.
    I always thought that the purpose of this forum was to offer some information compiled with the many different experience levels of the forum responders. If it takes me 20 minutes to write a couple of responses and perhaps save a fellow boat fabricator some
    costly errors, I am happy to do so.
    In a non combative atmosphere of course
    Cheers to you as well, might be holidaying in Spain in a few months.
     

  11. TANSL
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    TANSL Senior Member

    You're right, this forum some claim to help.
    From what I remember from when I worked in shipyards, the life of the shop primer is less than six months. Therefore, it is a temporary and very incomplete protection and is applied just for construction period. After completion of each weld is very convenient to paint it and its area of influence until finally paint the entire surface of the hull.
    I think all this to the OP does not care much, I'm not sure why he was talking about this, but I agree that we can talk about it.
    By the way, another issue is what you say about to calculate the length of weld : I see not convenient just to measure the length of cnc. In some cases an area of the perimeter of the piece is welded on both sides and in other cases it is not welded by any of the sides. Furthermore, this simple method we forget all the parts which are not cut by CNC. Therefore, although it may be helpful, we must treat it with great care and knowing how to be joined each piece with contiguous. I hope all this really helps Makro to avoid erroneous conclusions from what we've told. Many times, with the intention to help, we can get the other making lots of mistakes.
     
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