Rudder stops with hydraulic cylinders

Discussion in 'Boat Design' started by Esprit Marine, Jul 19, 2011.

  1. Esprit Marine
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    Esprit Marine Junior Member

    I need the advice of the boat design forum community on the installation of rudder arm stops on a hydraulic steering system.

    We've sold our first EC certified Spirited 380 catamaran, completed in 2009, to a Norwegian customer who is a naval architect/engineer. On her trip to Oslo, the top of the rudder housing reinforcement teared. The suspended rudder is very small (0.24 m2), and the housing reinforcement is made of five epoxy resin / 400 gsm double bias fiber taped over a PVC tube. So the strength is largely above ISO 12215 part 6 and 8 minimal requirement, and to our opinion the housing could not tear without a collision or rope entanglement. But the owner is adamant there has been no such event.

    After querying the opinion of other surveyors on the www.marinesurveyor.com network, the owner's independent marine surveyor issued a report stating that the housing broke because of defectuous installation of the arm stops. He says that the hydraulic cylinders should go to maximal/minimal extension before meeting the rudder stops, and that the housing broke because the cylinders were pushing hard on these stops. Now the cat is under repair in Gibraltar, including modification of the stops.

    ISO EN 10592 states that the rudder stops should prevent maximal extension of the cylinders for outboard engine installations, but does not cover standard steering rudder stops. However, the steering cylinders and pumps manufacturer, Lecomble et Schmitt, has confirmed to us that the stops should stop the rudder before the cylinders are in minimal/maximal extension.

    To my opinion, it seems logical that the mechanical stops prevent the cylinders from damage by limiting their course. On the other side, a naval architect/engineer, and a reputed surveyor who is also the Spanish European director for AMS, are of the opposite opinion. Before carrying out any incorrect repair, could I please have a lead on what is the correct answer, and if we missed something there ?
     
    Last edited: Jul 19, 2011
  2. Ad Hoc
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    Ad Hoc Naval Architect

    ISO like Class rules, are just guidelines. They are minimum values, not maximum.

    You really need to work out from first principals, what is the max laod, and hence torque that the rudder could ever possibly experience. Then apply that load to the structure as designed.

    Does it fail...?

    That is your course...
     
  3. Frosty

    Frosty Previous Member

    I can see both points of view. An ideal situation would be both the ram and the rudder coming to its stop at the same time by an adjustment bolt and lock nut.


    It might be a good idea to fit a relief valve on the ram supply set so as not to fail under heavy seas but to relief when the ram or rudder has hit its final stop.

    There already may be a relief valve in the system that needs setting.
     
  4. tunnels

    tunnels Previous Member

    You really need to post pictures or its pointless speculating what should have been done and what could have been done . :confused:
     
  5. CDK
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    CDK retired engineer

    From an engineering viewpoint using the travel of a cylinder is acceptable. The part is very strong and designed to withstand the forces it exerts. By doing it that way excessive loads on swivel points and external stops is avoided.

    A reason to use external stops can be the stroke of a standard cylinder being longer than desired or not precise enough if the cylinder contains spring washers to soften the end of travel. That is a consideration in precise hydraulic installations like presses to prevent damage to a die, for the simple movement of a rudder it is of no concern.

    Because the rudder housing broke, I assume that the end stops were part of the housing, so very near to the center of the shaft. That is not the way things should be done: external stops should be as far from the center as possible.
     
  6. tunnels

    tunnels Previous Member

    Post some pictures we are just speculating !!:D
     
  7. Esprit Marine
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    Esprit Marine Junior Member

    Here is the part of the Spirited Designs drawing showing cylinder and stops.
    The rudder arm is 160mm long, the stops are as far as possible from the housing and not linked to it.
     

    Attached Files:

  8. waikikin
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    waikikin Senior Member

    Esprit Marine, it would be good to see what else comprises the steering system. The most reliable system for catamaran steering is to use long tiller arms(sometimes with a toe in for some ackerman effect to rudder angle) with a link bar, if using a pull pull cable system the steering can actuate to the bar system, if using hydraulic ram this actuates to the tiller arm at apropriate distance from rudder stock center for steering angle required, if the tiller arms are longer & the stops further from the rudder stock center they incure less load. All parts of the steering system must be very robust, groundings impart plenty of load, the shelf for mounting the ram must be very strong considering the short link point to the tiller arm, this sometimes incorperates also the rudder tube/top bearing support in the same shelf.
    Regards from Jeff.
     
  9. Esprit Marine
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    Esprit Marine Junior Member

    Hi Jeff, thanks for the answer. The system is fully hydraulic, see attached drawing from Lecomble & Schmitt.
    The Spirited 380 has not been designed for link bars or cable steering. In Australia, some 380 use push-pull cables, but from discussions with Lewmar who tried to develop such a system compliant with ISO EN 10295, this seems impossible in Europe.
     

    Attached Files:

  10. outside the box

    outside the box Previous Member

    Esprit marine
    Wouldn't it be wise to take the problem back to the designer instead of this forum as all Forum members can do is speculate given the very limited information provided? After all The designer Craig Schionning has enginerred and specified the system, if the builder has followed the plans and specification for the system in build to the letter then isn't it the best solution to ask the designer why??
     
  11. CDK
    Joined: Aug 2007
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    CDK retired engineer

    I marked a better location for the stops in your drawing.
     

    Attached Files:

  12. Esprit Marine
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    Esprit Marine Junior Member

    Outside the Box, indeed we have followed Spirited design, and queried Craig about the issue. Craig's point of view is same as ours, ie housing damage can only result from abnormal use. The arms on Australian Spirited are strictly the same 160mm long, with stops limiting angle to +-30° angle.

    From the posts above, the answer does not seem to be straightforward. The ISO 10295 specifies that manufacturer's recommendations should be followed, and as stated previously, the cylinders manufacturer Lecomble & Schmitt confirmed that the stops should limit the course of the cylinders. Not following their recommendations might void their warranty, and pose non conformity issues with a standard mandated by the EC/94/25 directive ?
     
  13. outside the box

    outside the box Previous Member

    Agreed very interesting another solution we will all learn from no doubt when one is found, please post the outcome.
     
  14. Lurvio
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    Lurvio Mad scientist

    Has anyone tested the pressure in the hydraulic lines? Even a small cylinder with a 40mm diameter piston pushes ~12,5 kg per 1 bar of pressure.

    Just a thought
    Lurvio
     

  15. Easy Rider
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    Easy Rider Senior Member

    I recently installed a hydraulic steering system in my Willard 30. I wondered about the stops and called the manufacturer and they gave me the green light to use the fully extended cylinder as a rudder stop. That was five years ago and 600hrs. It still works perfectly well w absolutely no leaks. I should add though that my rudder cylinder is at least twice as large as specified. I also use 45 degrees of rudder deflection and love the extra steering ability in tight quarters. Another Willard owner was aboard with my in the Ketchikan harbor and thought I was going to hit pilings and other boats. He grabbed my helm once thinking he was going to avoid a collision but we cleared w lots of room. The notion that rudder deflection past 30 degrees does no good seems to be false.
     
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