Cracks ion aluminum hull, reinforce or not?

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

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Nope.

    Cracks will always come back if you do not establish the root cause of failure. Establish and address that, then you'll have a proper repair. Not a sticking plaster job destined to repeat the previous repair.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Alloy boats, especially planing boats with their shock loads, that have done a mountain of open water work, without weld repairs, are as scarce as hen's teeth. It depends how much work has been done with this one, but if it has had a hard life, and what is described is the worst it has to show for it, radical solutions seem unnecessary.
     
  3. Owly
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    Owly Junior Member

    As someone else said, the cracks will always come back....... I've done metal working, especially welding, all my life, and if there is one thing that I've learned it is that a repair must be engineered for it to work. Simply welding, will result in cracking alongside the weld. Adding reinforcing must be done with a great deal of thought, as it changes the way things flex, and often just moves the failure point somewhere else. I can't count the number of times I've heard people "brag" that their repair held..... when in reality it caused a failure elsewhere. My experience with aluminum is that once you start getting cracking, it will continue, and more and more cracks will show up.
    Sometimes the best thing to do is what my friend did repeatedly with truck tires on his farm truck. Buy new, use them for a few years, and sell them while they still have lots of tread and look good. That way you get the best use, the least trouble with flats, etc, and the best traction and are never running old tires, and you get enough out of them that buying new is not a huge investment. In the end he calculated that it cost him less, and the neighbors were always eager to buy that nice looking "like new" set of tires, and pay a good price.
    With an aluminum boat, if there is a decent market, you may be money ahead to do the same thing.... Use it and sell it while it's still nice, and buy another.....

    H.W.
     
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  4. Yellowjacket
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    Yellowjacket Senior Member

    Owly is correct in that you've likely used up the useful fatigue life of the weld joints and you will continue to have problems with cracking in other locations along the seams. Remember that the aluminum in the weld joints is not as strong as the sheets that the rest of the hull is made of. The joints are annealed and they don't have the heat treat of areas outside of the heat affected zone. Remember too that aluminum has a fatigue life that is much lower than steel as a percentage of yield strength. That is, typical steels can be loaded up to near their yield strength and have essentially an infinite fatigue life. Aluminum can have a very long fatigue life, but, as a percentage of yield strength, that allowable stress is much lower. Some say that aluminum has no real fatigue limit, that it will eventually fail, but in practical terms it has an infinite life, but the allowable stress is relatively low. Since the boat is cracking in multiple locations it has pretty much used up that fatigue life in the seams and now, if you repair it, it will most likely just crack in some place else along the seams after more use. If you went back and rewelded all of the seams that might prolong the life, but that would be an expensive proposition. If you try to beef up the joint as noted, it won't crack in the joint, but it will likely start cracking somewhere else. Probably time to retire the hull or plan on more weld repairing of cracks in the future.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    There is a significant amount of misconception with aluminium. Steel has a fatigue limit and so does aluminium. Steels’ fatigue limit is roughly 80% of the yield and more importantly it is a “cut-off” limit. Stresses below said limit fatigue no longer is the source of failure; it’s related to other issues. Aluminium’s fatigue limit is varied, owing to the way it is fabricated and the joint under investigation. Additionally the ‘limit’ is not a true cut-off limit. The propensity for failure beyond is significantly less, - it is not zero. Thus aluminium is often described as having no fatigue limit – which ostensibly is correct.


    Fatigue failures occur owing to one of three reasons:

    i) design

    ii) fabrication

    iii) operation.


    In general, the failures occur for around 80-85% of the time owing to ii) fabrication.


    So, in phase i) design, the naval architect analysis the vessel, identifies that certain location will experience a higher stress levels than other areas. Solution, increase the load paths and structural redundancy. Simple. Followed by ensure geometric details do not promote localised stress concentration factors.


    In phase iii) operational, this is where the vessel is used beyond the limits of the design. For example, if the vessel is designed to satisfy 1.0g vertical acceleration, yet experience 1.5g vertical accelerations when in Hs= greater than it is designed for, then it is purely an operation issue. Going beyond what the vessel is designed to cope with – the SOR. Solution, slow down to ensure the vessel does not exceed the vertical accelerations it is designed for, or, ask for the vessel to be designed to a higher vertical acceleration. Simple.


    Thus in i) and iii) the solution is simple.


    This brings us to ii) fabrication. In i) the designer notes that increasing the plate thickness locally and adding an additional long.t stringer will create the load paths and reduce the stress levels. Then ensuring there are improved and smoother radius transitions etc. etc.


    Oh, procurement says, adding long.t stringers will increase production time = money, Increasing plate thickness + adding stringers = increase in weight = money.


    Changing the vertical acceleration limits from iii) into i) increase plate thickness + adding heavier long.ts = weight = money.


    Then in ii) Ahh, the designer wants tight tolerances on plate gaps, so we need more time to fabricate = time = money. We need to ensure the plates are rolled properly = better machines = money. The designer wants to ensure improved smooth radii everywhere and guarantee weld returns around mouse holes/joints = more time welding = money. Fabricators build quickly to save time = saving money. But the boats distorts during fabrication = poor weld sequencing = frames being pulled into place owing to distortion etc. Changing the weld sequence = attention to detail = increase fabrication time = time = money. Etc etc.


    Thus most fatigue failures and failures in aluminium, in general, are owing to poor fabrication. And the root cause is money. Training fabricators not to do XX and YY, using better machines for rolling so no more pulling or forcing plates into place, changing weld sequence, increasing vertical acceleration limits and/or plate thickness etc etc all = money.


    The aircraft industry uses aluminium, they suffered from fatigue failures. Now, it is ‘designed’ out because time = people’s lives, not just money. The fabrication side was addressed to ensure attention to detail in the fatigue design is maintained in the manufacturing process etc.


    Designing vessel to have long working life, is not rocket science. It just = money. Money which clients can choose a cheap boat A or an expensive boat B. All clients only care about their initial capital cost not the through-life longevity of the quality of design and build. You only get what you pay for.


    Thus, if a boat cracks, the first thing that must be done is establish why. Not addressing this, will lead to a repeated event. Apply any load to any structure, it must go “somewhere”. Just rewelding a cracked joint will not alter this simple fact.
     
    Last edited: Nov 18, 2018
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  6. Magnus W
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    Magnus W Junior Member

    IMG_6856.jpeg IMG_6857.jpeg IMG_6858.jpeg IMG_6859.jpeg
    Some pics from the forward hold, about the only place where there's access.

    The first crack, about two years ago, occurred here in close relation to one of the ribs. The ribs was poorly welded so (in the bend where the plating had cracked) so it was fixed. I have put about 1500 hrs on the boat since then woointh the problem coming back so I leaning towards the assumption that the first crack was a result of the badly assembled/welded rib.

    The second crack can't be accessed from the inside without tremendous effort, time and cost and downtime so it has only been welded up from the outside and time will tell wether it will crack again in this place. This crack may be related to a bad weld in the plating or a bad rib. Or fatigue. Or a general poor design. Or overload.

    Considering that the boat "only" is 20 years old and only has 3000 hrs on it in what I would describe as usage within limits I don't think the boat's at the end of its life. This compared to other similar boats in similar conditions. And I don't think I'm pushing it to hard.

    So I'm stuck with bad design, bad welding or more likely a combination of those. And since the boat hasn't developed any other cracks in any other areas I'm thinking this is at least to some degree probable. So the question remains; what would the least bad solution be?
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    MW
    Thanks for the pix.

    Oh dear..what a terrible state.
    Very very poor attention to detail, and the welding, very low quality too. Just ignoring the poor quality welding for now, these regions are classic failure locations and all avoidable:

    upload_2018-11-21_19-40-23.png

    No mouse holes (red circles), and welding the flange to the transverse frame web (yellow box), just for starters.

    And then:

    upload_2018-11-21_19-42-16.png

    Not highlighting the lack of mouse holes, as in the image above. But, the trans frame is a hard corner, no bkt or smooth transition.
    The mousel hole for the butt joint, is terrible and not smooth, it is already a crack.
    The joint of the 2 members, poor weld but coupled with the region of poor mouse hole and insufficient additional of strength to account for the discontinuity etc...very very poor.

    The attention to detail and weld quality, or rather lack of...is what is causing you issues here.
     

    Attached Files:

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  8. JSL
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    JSL Senior Member

    Glad you accept that the construction is not the best.
    Ad Hoc has illustrated lots of details of poor work and classic no-no's, including a good example of tri axial stress (photo 1). My first comment is a caution: don't push the boat too hard. Higher speeds, rough seas, heavy payloads, can be a problem. Even on land, bottom loading points when trailering or shore blocking,etc. can be trouble. I have seen similar boats before...we call them Rice Krispy Boats..... Snap, Crackle, & Pop.
     
  9. Magnus W
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    Magnus W Junior Member

    My other boat as also aluminum and it has been put together with just about the same lack of finesse as this one. It's also made from 5 mm aluminum sheet (but bars and stringers are beefier than the Rice Krispy). It's ten years older and it has thrice the hours on the hull and it has seen some very rough times, especially since it's being run in ice. Save for the fresh coat of paint its a dented old bucket and you can clearly tell it has taken a few punches over the years. While cruising at 5-7 knots less than RK it's still a planing boat and it has a shaft so vibrations are an issue too. And it takes three times the amount of cargo and weighs 6000 kg vs 3500 kg But despite all this it doesn't crack.

    One shouldn't compare apples and oranges but if I do it anyway it would seem to indicate that a bad design will still hold up if it's beefy enough (this makes sense to me anyway). If this is the case with RK, a big IF, then perhaps the design and execution flaws aren't much of a problem they are as far as the whole boat goes but rather being isolated to this specific area which leads me back to what can be done. If anything that is.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Quick recap then:

    So, you're now left wondering:

    If you don't think it is an issue...then happy boating :)
     
  11. Magnus W
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    Magnus W Junior Member

    Oh, I do think it's a problem but I'm (foolishly perhaps) hoping that despite whatever the root cause is it's isolated to this specific part go the boat. The best way of being absolutely sure that it doesn't crack again is to not use it but that's not an option until the boat actually falls apart. And I'm not going to sell it and let some other poor guy deal with it.

    So either I just keep on boating, happily as you put it (but with knowledge of the potential issue in mind) and deal with cracks if and as they appear. Or I try to reduce the stress in the specific area if there's a feasible way of doing so. Like in welding in a strip as I mentioned in the first post. Or removing the weld from the outside and re welding it. Or adding a new weld on top of the old one. Some input here would be greatly appreciated.

    And thanks for taking the time to give me feedback. In this case, bad news are better than no news as far as I'm concerned, and at least I'l be knowing more about what to look for when I buy my next boat.
     
  12. fallguy
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    fallguy Senior Member

    Interesting stuff.

    I am only observing.

    What is a mouse hole?

    And I think Magnus quickly was relieved when you showed him workmanship errors vs design errors.

    His first reply perplexed me a bit until I reread all of it.

    What should he do was my thought after seeing AH critique.

    I will return to my spot on the wall.
     
  13. JSL
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    JSL Senior Member

    The cause could be elusive and the 'cure' even more so.
    Yours is not the first boat that I have seen with cracking problems and, like yours, appeared to be what could be called 'manageable'.
     
  14. Barry
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    Barry Senior Member


    Some disclaimers here
    You have sent some pictures but they do not really show what is into the next frame area as this area can impact the repair that you do in the areas that you have cracking.
    The limited pictures do not really show how much access you have to weld

    I will offer an opinion as how I would fix them to get at least an improvement in what you have keeping in mind that without seeing more of the inside of the boat a complete
    attempt to limit further cracking or increase stress concentrations in other areas of the boat cannot be accommodated

    So taking the 3rd picture, the one with the nylon strap showing on the right hand side
    This appears to have been rewelded and could be the cause of cracking along the inside angle of the strake that you have indicated. Ie this weld cracked, allowed movement and hence
    cracking further up the hull.

    As AH has stated, the visible welding is extremely poor. Not just the repairs but the original welds. Not surprising that you have problems. Of extremely important note is that
    when welding say a butt weld, you should weld both sides (barring backing bars, though there some other procedures that can work) in order to get a sound weld. When ever we welded
    a butt weld, we welded one side, back cut the weld from the opposite weld down into the first weld to ensure that there were no voids etc and then weld that side.

    So going back to the 3rd picture.
    The bottom of the frame is formed and hence I suspect it is a 5000 series alloy. The forming presents itself as there is a large radius and you can see the bottom die marks still
    embedded in the material. Magnus, I will use imperial measurements as I am most familiar with them.
    So you have a major poorly repaired and probably poorly originally welded butt weld here. I doubt you will be able to weld the side facing away from you due to accessibility issues so
    I will offer a one side only repair.

    Taking a 4 inch grinder 1/8th inch zip cut, ie cut off blade for aluminum, cut into the 4 inch vertical side of the weld all the way through. By looking at this picture I will assume that this angle is say 1 1/2 inches by 4 inches. If this has cracked before you need to remove all the weld and enough to allow a tig bead. Find a qualified tig welder and weld up this removed material. After this is done, do the same for the top flange. There should not be enough heat from the tig to distort surrounding members and with tig you can control the bead a little better
    than mig, ie make it more homogenous as you probably do not have the access to back cut and weld from the other side.

    When this is done, grind the weld smooth as we will be adding other plates which will need to make contact over the welds.
    The top flange is say 1 1/2 inches and you are going to bridge this weld with a piece that will carry the tensile loads in the top of the flange, Ie span this weld.
    Taking a strip of sheared 5052, 5086 , not any 60 series flat bar, that will cover the upper flange from the right hand side of the picture going to the left side and up say 6 inches on the
    2 1/2 by 1/4 inch flat bar frame, ie the vertical flat bar against the vertical side of the hull. This flat bar will sit on the top of the 1 1/2 x 4 inch piece and be bent in a brake to the angle
    up the side of the flat bar. The side of this piece closest to you will be flush with the web of the angle, not projecting past the edge of the web.
    So it will look like an L shape laying on its back. If the flange is 1 1/2 inches, my guess, this piece will be 1 1/4 inches. So this flat will be short on the back side of the flange as you will
    be using this created area to lay in fillet welds.

    NOTE NOTE, no welds will ever cross the flange at 90 degrees, instead use a 3 inch stitch weld along the long sides of the flat bar, but ensure that one of the beads cross over the
    weld that you did in the previous step. These welds will be mig welds. You might need a flexible nozzle on a spool gun to reach the areas.
    As prep for welding, clean with a cleaner that you can use in an enclosed area, and then use a stainless wire brush to brush the area to be welded. Provide a fan to bring in some fresh air
    to the welder.

    Then I would add in another piece of 5052/83/86 to re face the web of the angle that you have. I will have to do this with a sketch as it might be difficult to explain the shape properly
    in just text. When you have this face done and the opposing frame done similarly, you might consider running another inside stringer between the two repairs. Personally I would get
    a small bottle jack, install by clamping a bar that crosses the two frames, put a bit of pressure on the flat area where you said that you had cracking of the butt welds of the hull,
    to see how easily this area deforms. If it appears to move quite easy, then you might consider this other stringer to stiffen this chine flat area.

    Regarding your suggestion of adding another chine flat/strake. For corrosion issues it is not recommended to weld a plate on top of a plate when IMMERSED in salt water.

    I will send a sketch in another post also note that I will include a few plug welds

    And I will reiterate that you should not run a weld in the existing angle repairs length wise to the keel of the boat.









    I will attach a sketch later
     
    Last edited: Nov 23, 2018
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  15. Barry
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    Barry Senior Member

    upload_2018-11-23_11-5-20.png upload_2018-11-23_11-5-20.png

    Magnus, not pretty but hope this gets an option across
     
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