Welding in Stern Tube

Discussion in 'Boat Design' started by Salmoneyes, Aug 28, 2019.

  1. Salmoneyes
    Joined: Sep 2018
    Posts: 89
    Likes: 1, Points: 8
    Location: Southern Oregon

    Salmoneyes Junior Member

    Hi All
    Just roughed in my stern tube and it dawned on me I have options..

    Original stern tube was 2.5 inch and was welded in only at its ends around the tube. It was wider than the keel so the keel was cut to allow the tube sides to stick out and it was covered with a modified piece of pipe so as to be water tight.

    We upped to a 3x .5 inch tube so some trimming of the keel was necessary.

    My question is..

    Can I weld the stern tube in the entire length of the keel slot? Eliminate the pipe covering for water tightness.

    I can see heat being an issue distorting the tube possibly. If I weld 3 inches then move 180' and weld 3 let cool then repeat until finished, that might prevent warping the tube?


    Attached Files:

  2. Deering
    Joined: Feb 2005
    Posts: 481
    Likes: 25, Points: 18, Legacy Rep: 44
    Location: Juneau, Alaska

    Deering Senior Member

    I did exactly what you are proposing to do and it worked fine. Warping did not prove to be an issue (mine was aluminum), but your careful approach is never a bad idea. I assume you have enough gap between the shaft and tube that a small amount of warp could be tolerated.
  3. Barry
    Joined: Mar 2002
    Posts: 1,858
    Likes: 510, Points: 113, Legacy Rep: 158

    Barry Senior Member

    To minimize distortion
    With a 1/2 inch wall you should not have too much distortion but you can minimize it with the following procedure
    You will use a progressive tack system, this is where you will make a small tack only to stabilize the tube without any chance for distortion, when the entire shaft is locked in with these small tacks, say 1/4 inch in length, you will then go back over these tacks and make them about 1/2 inch long. These tacks will be cut out before welding in your longer welds, say 3 inch or whatever length that your total weld length suggests

    The upper right hand sketch shows a couple of straps that you will use to stabilize the tube- the first weld will be to the keel, and the second tack to the shaft, do both upper and lower

    The main reason here for the temp straps is that your gap is large and if you try to make the joint up, you will need a lot of heat/filler metal which could cause some distortion.

    At this point you could jump inside the boat and move the tube slightly to get it where you want it.

    Then make 4 small tacks, at say 1 1/2 inches to the right of position "0" using the weld sequence 1 -2 -3-4

    Move to the midpoint, position 6 - ---small tacks same weld sequence----------then the same at position 3 and 9 or midpoint to whatever your total weld length dictates
    Then catch position 11. You will note that other than the straps, there is not a weld at position "0" or "12" more on this later

    Now the tube is stabilized but the tacks may not stand up to a full weld

    Go over the small tacks with a 1/2 inch tack but with the same sequencing. This will give a stable platform for the tacks to withstand any pull from the welds

    Cut off the straps with a thin cut off blade. The reason that I left no welds at position 12 (and 0) is that you should start your weld at 11, moving to the right, across the keel/tube and
    back forward along the back side to the same 11 position. It would then look like a U shape weld when viewed from the top.
    This will minimize a chance of a porous weld as it will be one start and one finish. Then do the lower opposite weld

    Move forward to position "0" and make the same U shaped weld, ie from 1 moving to the left, then up the short vertical weld, and then forward, one continuous weld

    Cut out all 4 tacks at position 6, weld position from 4 to 8, same welding sequence
    Cut out all tacks at position 3, weld, cut out tacks at position 9, weld
    Now you will have a fully stitched tube in place

    Review Crater cracks,
    Using a thin cut off blade, slightly cut into the start and finish of each completed weld to remove any cracks that may be there.

    Then just fill in the rest of the void areas.

    Attached Files:

    Salmoneyes likes this.

  4. Salmoneyes
    Joined: Sep 2018
    Posts: 89
    Likes: 1, Points: 8
    Location: Southern Oregon

    Salmoneyes Junior Member

    Perfect.... We can do this.. Thank You
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