31' aluminum cruiser with crack in hull

Discussion in 'Boat Design' started by jgdyer, Nov 16, 2010.

  1. jgdyer
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    jgdyer Junior Member

    In March of this year, we ran our 31' aluminum cruiser aground and sustained a crack in the hull. We were able to make it to a marina, where a Yard Bull was used to perform an emergency haul-out.

    When I set about to repair the breach, I noticed broken welds in the framing. I then removed cabin appointments and took up the decking. Inspection revealed broken welds on longitudinal framing throughout the midships area, with the hull skin pulled away from the framing in some locations by as much as 1/2". The main ribs at hull plate panel seams are welded solid and did not separate.

    Inspection under the boat reveals that the hull skin has "ballooned" between main lateral ribs. I also discovered that the keel is warped to one side. I surmise this is an artifact of force against the keel during the grounding incident.

    I believe the remaining damage occurred when our boat was lifted at the marina: Weld separation and ballooning of the hull skin are consistent with stress that would be induced by lifting the boat with several thousand pounds of water in the bilge.

    Obviously, the stretched hull plating cannot be salvaged: It will have to be cut away and replaced. I am soliciting proposals for repair with regional boat builders and a local company that specializes in high-tech sheet metal work.

    Due to the extent of petroleum contamination in the bilge, my inclination is to propose that we scrap everything in the bottom except the main lateral ribs. I would then suggest that the repair shop re-plate with 1/4" material ( original was 5/32" ) and new T-bar for stringers and incidental bracing.

    My reasoning is that the heavier plate will cause less trouble with metal distortion, particularly since the assembly will be solid-welded, and that what we spend on replacing the stringers and other bracing we will make back by avoiding the labor premiums associated with re-using contaminated metal and welding out of position.

    Using the main lateral ribs as a guide, I would expect layup to be straightforward. I would also expect the rigid T-bar now used in aluminum hulls ( this was not available when my boat was built ) to provide a better foundation for the hull form than the original components.

    I know I'll be adding some weight, but I am certain I have enough displacement that it will not be an issue.

    Any thoughts, cautions, or critiques will be welcomed.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    With out really seeing the boat and obtaining more 'fact's...i does appear that either the orginal design strength is weak, or you're taking the boat beyond its design limits. Since any boat should have ample margins for lift, as an example.

    Thus before you embark on any modifications, ascertain what the orginal design limits of your hull is, then you can gauge what to do about repairs and which way is best. Since you may need to do more than you are suggesting, for the role that you wish the boat to perform.
     
  3. jgdyer
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    jgdyer Junior Member

    I agree that lifting the boat out with perhaps 15,000 pounds of water INSIDE the hull would place stress on frame bonds to the plating opposite to the direction of force applied during operation and in excess of design limits ... This was the marina's mistake, although I was standing right there and I didn't guess what would happen.

    I am not sure that this event calls out a need to re-examine the operational mission of the vessel, however.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    What difference does the direction of the applied load make to the plating, whether inside or outside? Also the water inside is a static laod. When the boat is underway, it is a dynamic load, ie greater! (The hydrostatic head of water inside a boat of your size should be minor compared to what the main structure can withstand in a seaway at full speed).

    Thus, without knowing more, i would advise you seek a NA to review the whole structure of the boat and thus provide you with a "safe operating" limits. Then and only then can you decide what to do with regards to adding plating etc and understand what the boat will do post modification, safely.
     
  5. jgdyer
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    jgdyer Junior Member

    I should add that most of what I propose is based on ABS minimum standards for a vessel of this class and size, standards that were not in place when the boat was built in 1960. The vessel has operated safely for 50 years, with the last 21 under my ownership. Under the circumstances, replacing the hull plating and longitudinals with more robust materials would seem to be a plan with little or no potential for calamity.
     
  6. ExileMoon
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    ExileMoon Junior Member

    Hope to see some photos.

    I would like to place easily damaged layer of polyurethane foam, so that in case of a hull cracking, foam layer can prevent the water.

    Fuel, if the fuel tank mounted on separate, not the hold, so the possibility of damage caused fuel leakage greatly reduced. In particular, if any, between the tank and the hull foam layer isolated words.
     
  7. jgdyer
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    jgdyer Junior Member

    Good points ... My analysis was that the welds bonding the longitudinals to the plating experience little or no stress during operation, since the load inwards is borne by the plating bearing against the longitudinals themselves, not by the welds. Lifting the boat with water inside, however, pulls the plating away from the stringers. In this setting, the welds take as much load as the longitudinals ... At least, until the welds part, which is what they did. Certainly, we could argue that the builder should have done a better job fastening the stringers to the plating.

    And, I agree that I should consult an NA ...
     
  8. Ad Hoc
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    Ad Hoc Naval Architect


    Er, sorry?!!...what supports the plating....the structure (longitudinals/frames)...how are these connected to the plating....welds!!:eek:

    Without welding the structure together, the plate is supported by...er.....dunno...scotch mist???

    Thus, the welds are crucial, it binds all the metal together to work as a 3D structure and shirk loads as required/designed.

    As for ABS....these are almost always either, woefully weak or massively over engineered. ABS are not renowned for their aluminium work!! But this is your prerogative if you considered the vessel suitable for your current needs.
     
  9. jgdyer
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    jgdyer Junior Member

    I still think you're missing my point ... Unless the longitudinals are joined to the plating by continuous welds, the load bearing capacity of the hull is always greater inwards than outwards. I have not seen any authority who recommends all-over continuous welding of framing to the plating. We can argue all day about whether the original designer should have specified enough welds to support lifting the boat with 2,000 gallons of water inside .. Regardless, my boat couldn't take it, and now I have to do something about it.
     
  10. Petros
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    Petros Senior Member

    You should also track down a set of plans for the hull, you will need to re-loft the hull and perhaps straighten it out in a large heavy jig before you weld on the new skin. If you can not locate the original plans you will have to hire a local NA to create some lines off the distorted hull, to rebuild the original lines so you have something to work off of. He/she can also tell you if going to the expense and weight of the heavier skin is worth it, or if it just a waste of money.

    On a sailboat absolute symmetry is not too critical (rarely does the water "see" the hull symmetrically in the water anyway), but you want it close enough to be not noticeable. There will be a lot of people objecting to this statement about symmetry, but the fact is most older sailboats are not symmetrical anyway and rarely does anyone even notice.

    all that water in the hull would definitely damage the skin and perhaps the primary structure as well, it was not designed for outward pressure. this is true for all structures, many years ago a Boeing 747 lost it's vertical tail in flight because the tail got pressurized from the inside and failed the skin outward. The rear pressure bulkhead (the aft end of the passenger compartment) had a relatively minor failure, which should not have caused loss of control or major failure. All the air in the fuselage pressurized the tail cone on its way out of the rear pressure bulkhead, and failed the skin outward on the vertical stabilizer, severing the whole tail off at the base. The aircraft amazingly managed to stay in the air for over an hour, but there was no way to control its flight and it crashed attempting to land, killing everyone on board (one young girl miraculously survived). I was a retrofit engineer in the structures department at the time, our department did an extensive investigation of the cause of the failure.

    I can tell you as a engineer, without hesitation, that the design of the structure has to assume loads in a given direction. If a complex structure like a hull was not designed for loads in the opposite direction, failure will almost certainly occur. We are not talking about homogeneous material but rather a complex assembly of discrete structural elements designed for a purpose. welds do not have the same strength is all directions of loading, that is a fact.

    It seems to me the boat yard should have known better, they are the experts on repairing boats, not you. It should not be your job as the customer to object to the yards procedures. If a tow truck operator damaged your car because they did not tow it properly, they are liable for the damage. I think there is at least some partial liability here for the boat yard. Presuming you have insurance, as well as the yard, there should be some shared responsibility for the repairs. The repairs would not have been as extensive had they done their job properly. I would at least talk to a lawyer about it (or your insurance company's lawyer); he/she might asst you in negotiating reasonable and fair settlement for the repair costs.

    Good luck.
     
  11. jgdyer
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    jgdyer Junior Member

    Thanks, Petros

    Thanks for confirmation regarding my assessment of the failure .. I was starting to wonder if I was too ignorant to be dealing with this.

    Original plans are out of the question .. Marinette has been out of business for years ... However, I really think we're OK on the main structure ... This is a pretty stout boat and there are no signs of failure or distortion except for the ballooning between transverse ribs ... The perimeter of the envelope within which the failure event occurred is very stiff ... I don't think a frame straightening would be called for, nor can I imagine what we would use for a benchmark.

    Yes, I am insured, and I fully expect my insurance company to try for a recovery from the marina ...
     
  12. jgdyer
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    jgdyer Junior Member

    About re-lofting

    Petros ... Do you really think I need to contemplate re-lofting the hull? I confess I am a hobbyist-engineer, not a degreed engineer .. Still, I've had this boat stripped to the bones 3 times over the last 21 years ... Today, everything I look at around the damage site looks exactly like it's supposed to look ...

    I was just figuring on cutting off the bad stuff and using the transverse ribs as the foundation upon which to mount new longitudinals and skin and just going on with it ... I see nothing to suggest that the transverse ribs are not exactly in the same place they were before the damage event ...

    If they were not, I think we'd see evidence on the cabin roof, the hull sides, the sliding windows ... somewhere ... and we don't ...
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    With respect, you don’t understand basic structural analysis/design.

    I’ll state it again, what supports the plating??.....the frames and longitudinals. This creates a “panel” size which has an aspect ratio, of a/b, where a and b are the small and large length of the panel.

    If, lets assume the frames and longitudinals are infinitely stiff. Thus the panel of plating remains a/b. So apply a load, a pressure load, from inside the hull. What happens, the plating bends. Apply a load from outside the hull, what happens, the plating bends.

    The amount of bending/deflection is governed by the plate thickness and the panel aspect ratio size, with respect to the ‘nominal load’, not the direction of the applied laod.

    So, if now the frames/longitudinals are not infinitely stiff, ie real structure what happens.

    The load is shirked from the plating to the structure that supports it. The weaker of the two are the longitudinals. So, over the given span, between frames, and over the panel size (ie breadth of plate it is supporting) this longitudinal must be able to support the load applied to the plating.

    The direction of the load being applied does not affect the span of the longitudinal, it does not affect the breadth of panel size it is supporting, ergo the bending/deflection of the plate is independent of the direction of load applied, positively inwards or negatively outwards (reversed). The section modulus of the longitudinal remains the same which ever direction the load is applied, inwards or outwards.

    Providing there is sufficient weld length (area) to transfer the shear load from the plate to the longitudinal, and that the longitudinal is stiff enough the carry that load from the plate over the given span, the structure is satisfactory. So continuous welds or not, enough weld area is all that is required.

    However, unless you start looking at cylindrical prismatic shapes as pressure vessels and you start looking at volumetric strain etc.
     
  14. Stumble
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    Stumble Senior Member

    Ad Hoc,

    You are right about the way structures take on load, but in this case when a load is placed on the hull from the water pressing against the hull, that force is directed from the hull plates directly to the stringers/ribs. When the force is directed from the inside out the stiffness of the plate is not at issue but the integrity of the weld is. Since the force directed outboard is point loaded on the welds.

    As for the legal issues...

    At least in New Orleans there would definatly be some liability on the yards part for lifting a boat improperly if that action led to damage of the boat. As a factual problem that might be hard to prove since the boat was grounded just before the accident. However it is possible, and their insurance might be willing to settle for some amount of money to prevent a total loss. I would also point out that in Louisiana they would also be on the hook for the loss of time using the boat, any damage to equipment, ect...
     

  15. mark775

    mark775 Guest

    Without comprehensive pictures we are all guessing here but my guess:
    The boat IS misshapen because of the impact and and that is what ballooned out the structure. Come on - a few thousand pounds of water broke how much weld spread over how much area? Ridiculous. You are either trying to get opinions on how to proceed with the lawsuit against the lift operator or exceedingly naive.
    Also, the lift pulled you out in an emergency! He saved your boat! Not only is it repugnant to suggest he is at all responsible, it is highly unlikely that any court would rule that he should know the structure of your boat and that it might not take water being in the bilge. You said you had an emergency and asked to be pulled. He pulled you - no time to analyze what condition internal structure was in and you were standing there to correct him -he had your implicit okay on everything he did. I have seen aluminum boats full of water picked with no damage - completely full! The keel is warped to one side? Another clue that you dang near folded the boat in half when you piled it on the rocks. This boat is probably not worth fixing and what fixing does happen should be on your dime.
    Stop making MY insurance rates go up with YOUR milking of the system. (and learn to drive)
     
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