Cracks ion aluminum hull, reinforce or not?

Discussion in 'Boat Design' started by Magnus W, Nov 17, 2018.

  1. Ad Hoc
    Joined: Oct 2008
    Posts: 5,752
    Likes: 265, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    A 90 degree cutout (of a circle) in the frame web below, circled in red, is a mouse hole:

    upload_2018-11-24_9-27-18.png

    This prevents a triaxial weld. That is 3 welds (longt, Trans, Vert.) all meeting in one location. These must be avoided at all times.
     
    Last edited: Nov 23, 2018
    fallguy likes this.
  2. Ad Hoc
    Joined: Oct 2008
    Posts: 5,752
    Likes: 265, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Barry's given you some good pointers.
    Bottom line is that is has become a management exercise.
    Simply mintor the boat each time you use it. Then it'll become a case of what cracks show up and where and then assessing the cost of the repair v the extended longevity of the vessel. And hence is the continual maintenance worth the cost.
    Just like an old car...there comes a point where the replacements/repairs outweigh the cost of the car's value.
     
    fallguy likes this.
  3. fallguy
    Joined: Dec 2016
    Posts: 1,328
    Likes: 53, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Is it possible the mouse holes were there and 'repaired'? Looks a bit that way.
     
  4. Ad Hoc
    Joined: Oct 2008
    Posts: 5,752
    Likes: 265, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    There is zero evidence of that:

    upload_2018-11-24_12-51-0.png
     
    fallguy likes this.
  5. fallguy
    Joined: Dec 2016
    Posts: 1,328
    Likes: 53, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Just don't know how big a mouse hole is....kindest regards
     
  6. Magnus W
    Joined: Nov 2017
    Posts: 75
    Likes: 3, Points: 8
    Location: Sweden

    Magnus W Junior Member

    Thank you Barry for taking the time to detail how a repair/reinforcement should be done. I very much appreciate it :)

    I'll read through it again (and again) and see what questions I might have.

    But about adding another chine. Yes I realise that corrosion might be a problem. But corrosion aside, is it an at least acceptable way if reinforcement? I'm asking because there's only limited access to the problem area so the structural reinforcements you so kindly detailed will only work in some parts of the boat. And I can only assume that the situation is just as bad in the areas we can't see as in the ones I photographed.
     
  7. Barry
    Joined: Mar 2002
    Posts: 1,003
    Likes: 52, Points: 48, Legacy Rep: 158

    Barry Senior Member

    I would not assume that the other lower frame members have cracked. You can probably rent a camera on the end of a cable to look at the other frames.(the sewer pipe inspection type would do it) To me, the incredibly poor butt welds on the lower frames MAY have caused all your issues. As stated before, the butt welds cracked, the plates flexed, poor welding, voids, crater cracks revealed, May have caused the weld cracks that you said are in the inside corner between the chine flat and the hull. My focus is on the butt weld failure as the cause of the other issues. It may not be. The other root cause could be the lack of strong enough stringers close to the bottom hull plate to chine flat weld to keep flex to a minimum.

    If other frame members have the same issue, they should be repaired. To leave them cracked due to access issues, you are leaving this a potential problem even if you add a heavier chine flat

    I guess if you absolutely cannot access these frames or do the repair, then you could add onto the chine flat with an included exterior stringer, sketch attached. But I am not an advocate of it
    and would only consider this as a last ditch attempt.
    The problem with this is that you will need to cap the transom end, creating a submerged chine sponson. You will need, after welding to confirm weld integrity, don't hire the same welder
    who built this boat. The way we have done this, a slightly different reason, is to drill and tap 2 - 1/4 inch pipe threads into the face of the transom cap. Attached a pressure gauge to one of them,
    pressure the sponson up to say 5 psig with a Schrader type automotive style valve see if you have leak down over 24 hours. IF so you will have to soap the joints and repair any poor welds

    Over time with flex in the hull, you may have some ingress of water through a pinhole. You can check for water inside the sponson through one of the 1/4 inch pipe tapped holes. One should be as low as you can make it to the hull.
    A sketch is attached.
    There is an obligation of naval architects and other engineers, to use best practices, I am not sure if this offered option would meet those obligations. I would try to move heaven and earth
    or at least fuel tanks and installed mechanical systems to repair the frames as noted. And if you have access, run a significant stringer from frame to frame to stiffen the inside plate near
    the butt weld.

    Alternatively, you could try the new exterior welded sponson addition as an alternative. There is no guarantee of course that if the frame butt welds are not properly dealt with
    that you will not have issues down the road. But you have issues here, inadequate fabrication/welding/joint design NOW and you need to try something.

    This sponson addition, for lack of better words, will give you a wider chine flat with perhaps a slight increase in stiffness in the ride. Probably not noticeable .

    If you decide to go this way, even if you decide to add your heavier chine flat which I would be concerned about it accentuating the cracking issue you NEED to follow proper welding procedures
    Mig, of course, after tacking, run 12 inch welds, let cool, cut back the termination
    end of the last weld to ensure no crater cracks caused by contraction in this area, then start your next bead.
     

    Attached Files:

    Last edited: Nov 24, 2018
  8. Ad Hoc
    Joined: Oct 2008
    Posts: 5,752
    Likes: 265, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Big enough to prevent all 3 welds from touching, and the ability to perform the return around the end too.
     
    fallguy likes this.
  9. Ad Hoc
    Joined: Oct 2008
    Posts: 5,752
    Likes: 265, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Picking up from Barry's suggestion. If you do elect to go this route, I would suggest the following:

    upload_2018-11-26_10-44-4.png

    Make the chine out of 2 plates - 1.5 x thickness of existing shell.
    The lap on the side kept to a minimum 30-50mm. This small strip and be welded to the side and also on the underside of existing chine to the strip.

    Then add the web insert with the mousel holes. This too fully welded.

    So how to weld the last run, the web to the new chine strip?...well...this is where it is a bit messy, but a more secure joint.

    If you cut slots in the chine strip and the web has a tab (to fit the slot dimensions) that pokes through the slot. Then fully weld with a fillet either side, then trim the excess tab to the chine plate.

    Or, simply remove the existing strip (which is not a pretty weld anyway)...and then you'll have full access from inside the hull.

    Which ever you feel is easier for you.

    The new web to the new chine strip will also now have sufficient support as the welded joint of the chine is supported by the side and the bottom hull along its length. Unlike existing - has none.
     

    Attached Files:

    fallguy likes this.
  10. DCockey
    Joined: Oct 2009
    Posts: 4,036
    Likes: 156, Points: 63, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Ad Hoc, what is the reason for not having 3 welds meet at one location? Is it specific to welded construction; perhaps due to stresses caused by heating during welding? Or is it due to a general structure concern of not having three panels meeting at a corner, which might also be applicable to glued plywood construction?

    Also, what is done at the corner of the transom where the side and bottom also meet?
     
  11. JSL
    Joined: Nov 2012
    Posts: 660
    Likes: 30, Points: 28, Legacy Rep: 41
    Location: Delta BC

    JSL Senior Member

    in some cases you can't really avoid a tri-axial joint* but with proper welding procedure you can lessen or avoid stresses and resulting cracks. (* transom/hull side/hull bottom, hull transverse bhd, long'l bhd).
     
  12. Ad Hoc
    Joined: Oct 2008
    Posts: 5,752
    Likes: 265, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Firstly let's start with an easy one. Typical construction, as also noted in the pictures previously posted.:

    upload_2018-11-27_9-0-18.png

    A long.t member, in this case a Tee, could be an angle bar (as in this thread's boat) is subjected to a load, say from a slam load. The flange of the Tee Long.t experiences tensile (or compressive) load.
    The transverse frame that is supporting the plating and the long.t, its web carries that laid as shear. The shear load, in this case is the vertical shear load, from the vertical applied slam load.

    If there is a weld across the flange of the Tee to join it to the transverse frame web, the weld is subjected to 2 loads, i) long.t and ii) vertical. For the same of simplicity, if the load in each case is 1, the resultant load on the weld is srt of 2 = 1.41. In other words this weld is subjected to a load that is 41% greater than if it were only being subjected to a load in 1 direction alone. Thus most designs do not take into account this higher than "designed" load case. The calculations are always based upon a single load application - the yield stress which is based upon a single load application.

    In addition since the weld is being pulled in 2 direction, this will also initial shearing. The allowable design shear stress is also based upon a single load application. So this experiences a higher than expected shear load.
    Thus the joint shall fail much quicker than if not welded at the flange. This is based upon simple static loading. It is much worse when considering fatigue.

    So, now let's look at a different scenario where there are 3 welds joining in one location:

    upload_2018-11-27_9-9-14.png

    If the smaller long.t stiffener passes through the transverse frame web, as shown 3 welds are meeting in one location. That being the long.t stiffener to plate, the trans web to plate and the 2 webs of the longt. and trans frame respectively.

    The weld as before, if we assume a simple unit of 1 for each alod, the resultant vector is srt of 3 = 1.73 = 73%. So this weld is now subjected to loads that are 73% higher than if a single weld - the 'as designed' allowable stress.

    Additionally, each weld that is deposited will cool. So when the welder finishes the trans frame to plate weld, they will then go back do the long.t to plate run. And the the vertical joint run. In each case the weld will have cooled. So, the welder will not be able to grind back the end of the weld. Thus the 2nd and 3rd weld deposit will be running over a cold weld. This is bad practice because you can never guarantee the heat from the weld will melt the weld pool totally - this means a void is left upon cooling. And again the 3rd time. So you also get a void and in addition, as the wled cools each time, owing to the void created, it begins to get pulled upon contraction. Since the weld from the Trans frame is much stiffer and will thus pull the long.t weld. And again when the last vertical run is done.

    So the weld has 73% greater load applied to it (than allowed for in design calcs), and upon cooling is subjected to internal cooling stresses which initiate fracture, microscopically. In other words, it wont last long!
    And is above this is simple static loading before taking into account fatigue...another whole new ballgame.


    There are some areas where it is impossible to avoid, as noted.
    Thus one must mitagte these locations by several means.
    i) Increase plate thickness to lower the general nominal stress levels
    ii) use collars where possible (this is where a mouse hole exists and is then cover afterwards with a small plate - this helps to make any loading out of plane).
    iii) use the best welders and welding sets you have to ensure these regions are welded to the best of your ability to avoid any stress raisers and inclusions/foreign debris and voids.
     
    fallguy likes this.
  13. DCockey
    Joined: Oct 2009
    Posts: 4,036
    Likes: 156, Points: 63, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Ad Hoc, thanks for the explanations. I have not seen similar considerations in glued wood construction but I also was not looking for them.
     
  14. Magnus W
    Joined: Nov 2017
    Posts: 75
    Likes: 3, Points: 8
    Location: Sweden

    Magnus W Junior Member

    Thank you for the detailed explanation.

    Q: If I take the "Barry suggestion" route – which currently looks like the best one given the circumstances – should the webs that are detailed in Ad Hocs drawing be positioned so that they meet the current bars on the inside of the hull exactly or doesn't it really matter? Or is it preferred that they are positioned as far from the current bars as possible? And should I have the same number of webs as there are bars today or should there be more (like evenly spaced along the length of the hull)? And if not the same as the bars, what's an appropriate spacing?
     

  15. Barry
    Joined: Mar 2002
    Posts: 1,003
    Likes: 52, Points: 48, Legacy Rep: 158

    Barry Senior Member

    My underlying premise in my suggestions are two fold

    1) The proposed external chine work is to build, in essence, a beam made up of the existing bottom of the hull, the new internal stringer for the right side, then the addition of the
    chine flat and somewhat vertical hull side plate.
    2) The location of the internal stringer, under the existing chine flat, would be just inside (toward the keel) to limit flex of the existing weld that you are having cracking problems with.

    If you can support the flat plate in the area of this butt weld, ie existing chine flat to the 18 degree deadrise flat, this weld should not crack again.

    Adhoc has suggested webs as well, which will also limit flex in the existing butt weld. He has also suggested, I believe, that the new chine flat have slots to weld the new stringer onto instead of the two piece chine flat that I have indicated. This is a better idea as there would be less heat distortion in the stringer as the welds would not have to be continuous. It might be a little more difficult to fit, but at the benefit of less heat/distortion.

    Do not ignore the importance of properly repairing the interior frame cracks. There were two items here, the 1 1/4 inch (not 60 series material) flat bar formed to the shape on top of the
    original frame and then the side plate that would tie the frame components ( short 1 1/2 x 4 short piece, the main 1 1/2 x 4 main frame piece, and the 2 x 2/1/2 flat bar) together

    Of these, the upper flat bar component is the most important fix. As this should prevent this frame weld from cracking in the future.

    Regarding getting a shape of the side plates to transfer to a piece of material to cut

    Take a light cardboard file folder, the type you put documents into before putting them in a filing cabinet and a pair of scissors. You can then just lay the folder against your
    the frame, and use the scissors to cut the paper to suit the frame that you are trying to repair. Ensure though that you mark this side "bow" or "stern" as the case may be.
    This is not high tech but, with various angles and/or curves, is a very easy way to produce patterns in minutes which can then be traced onto the material that you intend to use
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.