Massive Stem bars ..........Why

Discussion in 'Metal Boat Building' started by LyndonJ, Mar 18, 2010.

  1. LyndonJ
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    LyndonJ Senior Member

    Looking at class rules for a 60 foot motorsailer

    The bow is a raked relatively sharp with the plates coming together onto a solid round bar.

    The rule asks for a stem bar of something like 10mm by 100mm but the well supported and restrained bow plate is already so strong that the stem bar adds nothing structurally just keeps the shape during construction. There's no analysis I can do which shows even the remotest amount of significant stress in that member.

    The class construction surveyor isn't happy leaving out the stem bar
    but it seems superfluous as I already have a massively greater (100 X) structural result with the acute plate angle.

    Is the stem bar aimed more at rounded bows which are relatively flat?

    I was wondering what experience other designers had here ?
     
  2. tazmann
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    tazmann Senior Member

    I am not an expert on the subject but my take on it, without a stem bar would be strong enough if you never hit anything but if you figure your displcement at full speed hitting somthing solid, you would be glad you had a stem bar
    tom
     
  3. marshmat
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    marshmat Senior Member

    My (admittedly limited) understanding is that such "overbuilt" stem bars are there mainly for impact resistance, as "normal" operating loads ought not to put such huge stresses on this part of the boat.

    Consider the case where, at 10 knots, you hit a 40' container sitting an inch above water (as they occasionally do). If you just have the bow plating up there, with no stem bar, the bow plates are likely to buckle outward- the stem bar takes up some of this load and serves to keep everything roughly where it ought to be during the collision. So instead of a cascading structural failure that tears plating from the forward bulkhead, you end up with a nasty but more manageable hull breach.
     
  4. LyndonJ
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    LyndonJ Senior Member

    Thanks
    The plates are 6mm and the round is 25mm solid bar, I think on the event of a collision it will ride up regardless. Anyway if you have to have a collision bukhead it's all very redundant.

    I think it's more a case of ship structure ending up being in small vessel rules. I know ABS ORY is now obsolete but they didn't even have a stem bar requirement.
    Then if you want to consider collision even without the stem bar its about 5 times as strong for collision and considerably harder than layered wood or foam core.
    So I really think its a case of apply directly to the rule society and show them te calcs and ask for an omission. I was really wondering if anyone else had been there.
     
  5. gonzo
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    gonzo Senior Member

    Why do you want to eliminate the bar, is it the wieght? If you are using it to keep the shape during construction, it can't be any extra labor to keep it in place.
     
  6. waikikin
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    waikikin Senior Member

    LyndonJ, the only advise I can give is to leave it there- it may be stuctrually redundant but if it aids set up in holding the round & bobstay attachments or forestay attahment are incorperated the surveyor has a valid point(his job is to verify the boat is built as drawn), also importantly(very) building boats is a business & getting on with it gets the job done & paid for. All the best from Jeff.
     
  7. Wynand N
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    Wynand N Retired Steelboatbuilder

    why not get the designer's view on this?
     
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  8. LyndonJ
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    LyndonJ Senior Member

    Yep the designer thinks it's total overkill too! but if you want class approval then you do it.

    Just seems some things are so heavy that it's dumb, I'm a recent grad engineer and I've just spent too much time analysing this every which way and I just cannot see that it's required, I can see it's needed on a flattish stem but an acute angled bow is so strong anyway.

    Never mind I can take all my calcs to one of the senior engineers and get him to sign it off then they will be happy. It's all about having someone who can carry the blame if it goes pear shape :)

    It saves on weight and allows the welders to get at the inside of the stem, you can offest the stem bar too, then it wouldn't help with collision or attachments.

    Makes me wonder just how much framing is actually superfluous on heavily skinned steel boats.

    Thanks for the replies
     
  9. jehardiman
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    jehardiman Senior Member

    There is actualy very little superfluous structure rerquired by the rules. Almost every thing required is there because it provides maximum reasonable assurance that in a damaged condition the loss of structural intigerity is limited to the initial damage site. As you are a recent grad, and say thay you have analyzed the structure, I have to ask if you analyzied the structure in ALL the damaged conditions?

    As you gain work experience, it will quickly become apparent that the limiting factor in steel design is not strength, but stiffness and keeping structural geometry. The reduncency of ship structure is not for strength, but to prevent loss of sectional properties which leads to cascade failures. As tazmann and Marshmat pointed out, damage forward can be quick and large. If your vessel is designed for 10 knots, did you design the forward compartment for an internal pressure of a minimum of 406 psf (3 psi) with the stem removed? I've seen photos of bows opened up like a flower with the shell plating ripped away from the collision bulkhead, or the collision bulkhead stove in due to ram pressure. Will your bow support all the loads with all structure from the LWL down back to the colision bulkhead gone? Damage can cripple major framing structure and force the loads in the secondary and tertiary structure. There must be sufficient structural material with adequate geometry available to take up the primary loads.

    The reason for the heavy stem bar is to keep the deck and bottom plating attached and acting together in the event of collision or allision. It's size is not predicated on strength, but on the historical probability of surviving the impact, damaged, but still functioning as a strictural load path. A fair amount of the rules are predicated on "we've never seen a part of X by Y dimensions fail, so the minimum sizes for certification is X by Y".

    And then, for laughs and giggles, are you RTing your stem to ensure you are actually getting the assumed strength? Or have you done an analysis for the case of a weld root flaw if there is no NDT? What is the maximum expected pit size expected in the seam weld between drydockings? There is a plethora of other considerations besides strength for the structural requirement of a 10mm x 100mm stem bar.

    Edit to add, remember, the rules are not about optimum structure. They are about providing reasonable assurance to the insurer or owner that the vessel will perform adaquetely throughout it's useful life.
     
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  10. LyndonJ
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    LyndonJ Senior Member

    JEhardiman
    Thanks, I'm still frustrated though

    There are lloyds approved foam cored FRP boats sitting here right now that have nothing similar re-inforcoing the stem, just thicker laminates on the hull itself. Way weaker than the steel structure.
    Having seen photos of the damage resulting form collision in this sort of construction and it's extensive but even then it all survives.

    A steel boat with bow plating at a relatively acute angle already has a very large structural capacity and in this design the longitudinals run into the equivalent of horizontal floors every 2 feet or so up the stem, this same frrame -floor type construct carries the massive stem bar as well. Those brackets tie the hull plates together and stiffen the stem anyway. If the first frame at the waterline were a full bulkhead appropriately stiffened (with a massive stem bar :)) you could apparently call the bow a 'false bow' and do what you like.

    If it were ice rated the plate would be 8mm thick on the bow .

    As the stem is well raked I think that you could run the vessel onto a low rock all day and not even dent the stem, if it were a sharp rock and ripped into the plate then it's going to do it regardless.

    What is interesting is that the different classes orgs all seem to differ on whether or not it's required and what size it should be.

    I think under ISO and it can be left out completely too.

    It's frustrating when you can't see behind the blind standards and what the intent is. And Istill think a lot of these are generalisations that will catch problems with other bow types. Like a big lazy U shaped bow.

    There's so much framing tieing the hull deck and all together anyway.

    Welds they only check 5% or so the overhead butt welds seem to be their favorite. But on vessels this size weld failure is apparently unknown.

    Now Lloyds will accept specific calcs and so long as the eng is appropriately cross insured. So yep it's about liability

    Still leaves me thinking some overkill is often present in smaller vessels. I can definately see the requirement in a ship, but in a boat?

    Thanks
     
  11. waikikin
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    waikikin Senior Member

    LyndonJ, 100 x 10 mm is hardly massive on a 60 footer, I can understand your frustration but as I see it the main issue raised is ease of welding access(a very important consideration esp' to the guy cramped in the bow doing it)- in that case you could & have suggested terminate the bow in another fashion- maybe another flat set in front at 90 degrees to the 100x10 & attach the pointed part of the bow as a non structural addition to the outside in the possibility of a range of materials(steel, staino, composite, timber?), a vessel my dad built had similar to this.
    All the best from Jeff.
     
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  12. LyndonJ
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    LyndonJ Senior Member

    Thanks Jeff
     
  13. baeckmo
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    baeckmo Hydrodynamics

    The stiffness against local damage in case of hitting a solid object is what counts, like noted by Jeff, Jehardiman, and Matt above. Twin 6 mm plates meeting a 25 dia bar are easily damaged; there will be sharp buckling with plates ripping and/or welds opening!!!! Believe us, you don't want that when you realise that you can't steer away from that floating container you just spotted......! Man, you are talking about a 60-footer, there are forces.
     
  14. LyndonJ
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    LyndonJ Senior Member

    Yes but I just look at the foam FRP composite vessels also built to classification rules and they don't have anything like the strength, so why for one construction already stronger do you make it yet stronger again ?

    The trad stem bar runs right through and has the plate butting at some angle into the side, if the plates meet on a solid round instead then the stem bar has to be added as a separate item . In transverse frames we were always told that floors are better than massive Central keel bars and to put more floors in and reduce the CVK size. Shouldn't this apply to the bow too?
     

  15. LyndonJ
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    LyndonJ Senior Member

    I've been intending to come back to this finally I remembered:

    The stem bar was not required it was a typo and the error grew. There was a bar called a stem bar in the specifications which was not really a stem bar just a shape defining curved bar. The plan approval process just beefed it up to the required dimensions for a stem bar.

    It was actually a plate stem ( no stem bar required ) The plate has to be a minimum thickness and it has to be 'shape supported' with horizontal webs every 1.8m or so max. I thought it was daft to have two 6mm bow plates meeting at a moderately acute angle and to have to add another bar.

    Just thought that might be of interest to designers in metal. Lloyds register were very helpful in the end. It's all in their SSC, plate stems or bar stems and a table for plate thickness for a plating only stem . Funny that it's not in GL <24m, just need to know I guess or stick to LLoyds.
     
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