Discussion in 'Boat Design' started by disneytime, May 8, 2012.

  1. disneytime
    Joined: May 2012
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    disneytime New Member

    Hi all,

    This is a question I've had for ages with regards to chainplates, and I can never figure out what the reason for it is - I have just been taking the literature for granted, because I know it is based on practical experience and must be right.

    The question is this:

    If I want to design a 316 s/s chainplate with a breaking load of 69100 N, Larrsson and Eliassons 'Principles of Yacht design' says I should use a chainplate lug which is 45mm wide, 14mm thick with a 18mm hole centered 27mm down from the chainplate top. "Sailing rigs and spars" by Matthew Sheahan gives a similar answer, so these are the chainplate dimensions I usually use.

    However, if I use basic engineering theory to work out the dimensions, I get the following...

    1. Shear area

    Using the Yield stress of 316 s/s=195MPa I get a shear strength of 0.58 x195 = 113.1 MPa. Note that I have used the yield stress, because I dont want the chainplate to yield at the breaking load of the rig.

    Therefore I need a shear area of (3/2)*69100/113.1 = 916.45 mm^2
    (the 3/2 being a shape factor for a rectangular area)

    Which if I used a 14mm chainplate with an 18mm hole, would give me a dimension down from the top of the chainplate of 74.46mm!

    2. Tensile stress - this works in general, i get a smaller answer than the books - which once I include a Stress conc. factor seems about right.

    3. Bearing stress
    I get a bearing strength of 1.8 x195 = 351 MPa, giving me a required bearing area of 196.87 mm^2, with a 14mm thick chainplate yields a 14mm diameter hole, so the books are slightly conservative which is OK.

    So, basically, what is going on with the shear??? Why do the other calcs make sense and it doesn't? The only thing I can think of is that maybe I should be using the ultimate shear strength instead of the yield? - that would give me a number which was much closer to the book values but why?
  2. masalai
    Joined: Oct 2007
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    masalai masalai

    What are you attaching the chainplates to? grp, steel, wood? - My boat is composite (endgrain balse with GRP on both sides in varying thicknesses - chainplates are taped in place and neatly integrated into the build each to hold a static load of 4.5 tonnes....
  3. tunnels

    tunnels Previous Member

    Can you lift the whole boat complete on just the chain plates ?? :eek:
  4. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    On a smaller boat you should almost be able to .
  5. tunnels

    tunnels Previous Member

    How small ?? a 10 foot dinghy ?? jesus we used to lift our 36 foot racing boat on the 2 chain plates and the back stay anchor !!and never thought twice about it !!:eek:
  6. mydauphin
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    mydauphin Senior Member

    I call anything smaller than 40 feet small... lol. After that hull is usually too big to lift on one point.

    BUT I doubt most boats can take it.
  7. tunnels

    tunnels Previous Member

    One central point of balance of a fully completed like as in total with a 1inch steel bolt through the glass grid ,its all glass construction all interlocking and glassed in the right assembly sequance, impossible for any part to come out or even come loose !! The first of 8 !:p

    Attached Files:

  8. masalai
    Joined: Oct 2007
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    masalai masalai

    On mine before overloading with engines, galley, water and HUGE fuel, THEORETICALLY on CNO YES but the tipped angle would probably place the loads in the wrong direction :eek: :eek: - there are 4 inbuilt 'chainplates' angled to lead to the pasthead 12m above the mast-step... - the stainless steel bush is in place ready for the fitting , pin and the rest of the stuff...
  9. Frosty

    Frosty Previous Member

    The french boat Miramu's can be lifted from the chain plates. They have eyes in the plate for doing so.

    I don't think the chain plate is your weakest point --it would be the swaging or sta lock connections or mast lugs.
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    FAST FRED Senior Member

    Why SS chain plates?

    Burying them causes them to dissolve and sticking them ON the hull shows bolt heads , and they may still be dissolving under any sealants.

    There are many bronzes about as strong with none of the bad news from SS.

    If you must have SS on the boat , a beer mug holder is probably best.

  11. disneytime
    Joined: May 2012
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    disneytime New Member

    I'm not really designing chainplates for a specific boat, just wondering why the Ultimate shear seems to be being used in the literature rather than the shear yield... noone seems to know the answer to that question though :D

    S/S is commonly used, more because it is fairly cheap and easy to weld than anything - yes Bronze is probably an option though.

    Well if these chainplates lift 7044 kg each, yes they would be able to lift the whole boat - well so long as the boat weighed less than 14088 kg that is... The same principles are applied to lifting lugs basically.
  12. philSweet
    Joined: May 2008
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    philSweet Senior Member

    One thing you might be missing is if it is a bearing area failure and a plug of metal gets ripped out of the top of the chainplate, there are two shear surfaces to the failure. Basically, I think you need to go dredge the web for engineering primmers on bolted connections (same as pinned ones, pretty much) and get a handle on the failure modes and what material properties are in play for each failure mode. It is not a simple matter to cookbook an optimal design. But in order to not have to backstop the machinists and fitters every time they put a hole in something, certain proportions have become familiar.
    There are literally hundreds of pages of info on bolted timber connections, and reading some of it would probably be time well spent.
  13. groper
    Joined: Jun 2011
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    groper Senior Member

    When i first read the starting post in this thread, "... 18mm hole centered 27mm down from the top" i almost had a heart attack...

    In civil engineering, when engineering steel plates like this application, there is also general rule of maintaining a minimum of 2.5*Diameter of the hole from the end of the steel plate. This allows enough material to prevent the bolt from pulling out the end of the plate in tension like this... dont know how they figured it, its just a general rule for the engineering standards and varies with material type, for instance its 4*hole diameter when using hardwood...

    So in this case it would be 2.5*18=45mm to center of hole.

    if you used the 27mm to center, you would have 27-(18/2)=18mm of meat left at the end of the plate after you drill the hole... pretty scary if you ask me...

    The 75mm center sounds a bit excessive... dont know whats going on there...
  14. CatBuilder

    CatBuilder Previous Member

    Many catamarans can be lifted by the chainplates, including this 15 meter (45 ft) by the same designer as Masalai's boat.


    This cat was moved into the water on launch day by a crane attached to the chainplates.

    If I remember correctly, these are composite chainplates and not stainless.

  15. Frosty

    Frosty Previous Member

    It makes it even moer impressive when you look at the angle they are pulling them at.

    Thats what spreader bars or frames are for.

    Looking at the crane I assume this was in Afganistan.
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