rudder failure

Discussion in 'Boat Design' started by french44, Aug 27, 2012.

  1. french44
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    french44 Junior Member

    Hi gentlement,

    I need help on rudder failure. Look the pictures and tell me if you have an idea for that. I'd never see that before and the safety coef is 2 . the calculation with 3 differents methode arrived at the same results the scantling is good so ? and the for 4 at the same time ? the crew is mute obviously

    Waiting your point of view
     

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  2. daiquiri
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    daiquiri Engineering and Design

    Did I understand it correctly - four rudder stocks broken at at the same time? That sounds almost incredible! :confused:

    To me it looks like a torsional failure, perhaps due to fatigue. But how could it happen at the same time, that's beyond me...
    Did the crew feel any unusual vibration or flutter before it happened? It could be interesting to see the way they were mechanically interconnected.
     
  3. JosephT
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    JosephT Senior Member

    Definitely looks like fatigue failure to me. Corrosion is minimal on the shafts. No doubt the loads on it were either too excessive or they just exceeded their fatigue life. Then again, if the shafts were produced using a faulty method it could be a manufacuring defect. No doubt the boat manufacturer has some questions to answer.

    Would help to get some specs on the boat/rudder in question as well as the use cases (e.g. How was the boat sailed/operated? etc.). A beefed up rudder design would definitely be a consideration. I suspect the root cause is either unforseen loads on the rudder shaft or perhaps fatigue from continuous turbulance coming off the keel onto the rudder...which could lead to eventual structural failure. If all the boats in question had an equal amount of operation time on them that explains the similar failure rates.

    More info on the boat in question as noted above would help.
     
  4. SamSam
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    SamSam Senior Member

    This is great, 3 replies with 3 different answers. 1 is torsional overload, 2 is fatigue and I'm going to go with door #3 Bob, straight on bending overload.

    With "the crew" and "the for 4 at the same time" it sure sounds like all 4 rudders were on the same boat and all broke at the same time. The two center ones are bent a little, which suggests they ran aground or something and the too brittle steel snapped. The bends compared to the turning yokes looks like a straight on collision at speed going straight forward. I can't imagine all 4 snapped at once from torsional twisting and fatigue breaks would not break all at once and would also show older crack corrosion on the edges somewhere. Torsional breaks also usually show a twisting pattern unless the steel is very brittle, like a drill bit.

    I bet if you could find the rudders they might tell you something. They're probably not too deep. "The crew is mute" is funny.

    What kind of boat has 4 rudders and how big is the crew? None of them have the slightest idea what went wrong?
     
  5. waikikin
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    waikikin Senior Member

    Maybe a tug? Maybe that if they backed up on something or some how connected with an object I'd rather see the stocks bent. Maybe fatigue & contact. The structural integrity of those rudders has definately been compromised............ that I can say for sure;) Jeff.
     
  6. daiquiri
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    daiquiri Engineering and Design

    The three replies do not exclude each other. There exists a torsional fatigue too, which I was referring to. A bending could be a result of a collision, but it can, imo, also be a result of a too-thin stock with pivot axis placed too aft along the foil chord.

    But it is imo all a useless guesswork until we get to know a bit more about the design of this boat and it's rudder(s).

    Cheers
     
  7. BMcF
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    BMcF Senior Member

    Another vote here for a torsion+bending failure with fatigue a contributing factor.

    The OP referred to a factor of safety of 2. Based on what? I've designed many an appendage that has a factor of 6 or 8 on normal operating loads after fully-reversing cyclic loads and more extreme "rare" loads were accounted for and used in the design process.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    And another :)

    Ditto.

    a FoS of 2 is woefully low for such an appendage in a high corrosion and fatigue environment!
     
  9. SamSam
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    SamSam Senior Member

    Yes, who knows? "The crew is mute" says an awful lot though. Time for some flogging and yardarm hanging to get to the bottom of this mystery. Argh.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Hard to see the true surface texture, but zooming in on the grainy pic is sure does appear "classic" fatigue:

    SAM_1531-1.jpg
     
  11. SamSam
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    SamSam Senior Member

    There might be marks on the hull that could help indicate what happened. If the rudders were turned and they hit something going forward or backward, that would put a torsional stress, a bending stress, and if the corners of the rudders jammed into the hull, a tensional stress on the shaft all at the same time. I'm assuming there was no support at the bottom of the rudders and they were just dagger type.
     
  12. daiquiri
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    daiquiri Engineering and Design

    You are also assuming that all 4 rudders were installed on the same boat at the same time, and that all 4 of them hit the ground. But it's imo a risky assumption, unless French44 confirms that it was really a boat with 4 rudders. Also, if the hull was damaged due to grounding, that info would probably be given to the designer/manufacturer together with broken rudder stocks. But now it's me assuming things... ;)
     
  13. tunnels

    tunnels Previous Member

    you need to post pictures of the whole system not just 4 broken shafts . wheres the blade in relation to the shaft how is it mounter , the size etc etc
    to even brake one shaft is poor design that dia shaft if it has a rudder mounted and attached is way to small . how big is the boat ?? whats the dia of the shaft ???:confused:
     
  14. philSweet
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    philSweet Senior Member

    So tell us a story. What happened to the people on the boat. What happened to the boat. What was it doing at the time of the incident? Boat have a name and an owner?
    How about some better photos, with the parts identified. Where is the cross connect link. did the vessel have dual hydraulic steering with a cylinder on each shaft?
     

  15. SamSam
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    SamSam Senior Member

    Yes, I'm severely assumptionating.
    'The crew' seeming to be singular and along with '4 at the same time', I find it hard not to assume a boat with 4 rudders.

    This is interesting

    [​IMG]

    and along with the analysis http://www.resnapshot.com/MP1100.htm leads me to believe it was mainly torsional, some bending and maybe some fatigue.

    This part makes me purely guess that they backed into something (or something ran into them) either with the wheel fully turned or the collision forced the rudders over to the stop position where the torsion + some bending and maybe fatigue wrecked the rudders. ( a fos of 2 seems low, but a fos of 5 might not have saved them. A low fos of 2 might actually have prevented even more damage)

    I'm thinking the force came from the back because otherwise the propeller(s) would have got whacked first. There would be more leverage on the rudder shaft coming from the back, and if torsional overload creates a rough surface, those things suffered a catastrophic overload, as they look like broken cast iron.

    But, this is just pure speculation on my part, and if my stock market speculating is any indicator, it's probably wrong.

    [​IMG]
     
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