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Thread: How strong?

  1. #11
    Join Date
    Oct 2013
    Location
    Shropshire - Sundance, Bristol Channel
    Posts
    1,088

    Default Re: How strong?

    Man maths, I imagine you need to question what and how it's bolted to more than the pad eye.

    Man maths tells me to have the shackle closer to the base then further away so the load to the right of the pic but it also tells me to have a substantial backing (and probably top plate with only a 95mm diameter) as well as transom reinforcement to stop the quarters pinching inwards under load dependent on design.

    As much as I would love to computer model my boat I go on the if it looks right philosophy.

    Oh I've found those Jester decals out, just need to find a scanner big enough.
    Tom, Sundance
    2wheels1keel.com

  2. #12
    Join Date
    Nov 2013
    Location
    Perth WA
    Posts
    2,318

    Default Re: How strong?

    Harken padeye.jpg

    http://www.ybw.com/forums/showthread...Harken-padeyes

    EDIT
    What is going on here? I've just read Sarabande's post and now I am utterley confused
    (Better do something on the yacht. YBW website is driving me mad too).
    Last edited by coopec; 09-11-19 at 01:14.

  3. #13
    Join Date
    Jun 2019
    Location
    Lowestoft
    Posts
    760

    Default Re: How strong?

    Why not use a much larger padeye, which can take unambiguously beefy bolts? One which looks capable of handling drogue sized loads without any nagging doubts or frenzied computer calculations?
    The thing in the picture is more suitable for a bathplug chain or a gerbil dungeon.
    It will also have weird mixed metal corrosion issues.
    "Now shall the gentleman haul and draw with the mariner"
    John Hawkins

  4. #14
    Join Date
    Jun 2013
    Location
    Oxfordshire
    Posts
    1,806

    Default Re: How strong?

    Quote Originally Posted by Ohlin Karcher View Post
    Why not use a much larger padeye, which can take unambiguously beefy bolts? One which looks capable of handling drogue sized loads without any nagging doubts or frenzied computer calculations?
    The thing in the picture is more suitable for a bathplug chain or a gerbil dungeon.
    It will also have weird mixed metal corrosion issues.
    I would imagine that 4 12mm bolts will outperform the structure itís being bolted to and the strop being attached to the item, but I donít know the boat or the application.

    Calculating a safe working load for that item is an engineers nightmare, there are far too many unknowns.

  5. #15
    Join Date
    Feb 2014
    Location
    The Northern Powerhouse
    Posts
    3,062

    Default Re: How strong?

    More relevant I suspect is deck strength. Don’t forget a hefty backing plate to spread the load. Good quality bolts too.

  6. #16
    Join Date
    Nov 2016
    Posts
    2,722

    Default Re: How strong?

    According to Jordan ( http://www.jordanseriesdrogue.com/D_5.htm ) the overall Design Load should be around 6500lbs. and that for a single leg therefore a single 'chainplate' around 4550lbs.

    The 'standard' way of attaching a drogue-bridle is like this...





    I'm looking to achieve much the same facility using the Harken 'deckeyes', with very substantial backing plates inside. The hull skin there is solid GRP about 3/4" thick.
    I reckon that I should be able to achieve an SWL in the titanium deckeyes/bolts/backing plate/hull combo much in excess of the Single Leg Design Load of around 4550lbs.

    Sewn lifting strops rated at 10,000lbs, with antichafe, will likely be used for the bridle legs. These I expect to attach to the Harken goodies using Soft Shackles, again with antichafe sleeves, rated to 'about' 8000lbs. Should inspection make it necessary in use, I could doubleup with additional - but slightly longer - Soft Shackles to take over should the primary ones chafe through.

    As to 'why use the Harken thingies?' I've got 'em. What else could I use them for?

    But I'm still wondering about the UTS of the Harken jobs. Specifically, the web-loop which is about 9mm x 10mm section.

  7. #17
    Join Date
    Dec 2013
    Location
    Deale, MD, USA
    Posts
    1,823

    Default Re: How strong?

    The best answer. Take courses (or read books in engineering statics and deformable bodies. This is just like asking for free legal advise.

    a. The strength of the screws is often a reasonable starting point. It was designed by an engineer and there is a good chance he matched them to the pad eye. This is nearly always the case.

    b. Saying the "screws with break first" of the "hull will fail first" is a cop-out, since properly designed, they can all be strong enough. It is also incorrect to say only two bolt are holding it. Two are resisting primarily in shear and two primarily in tension, but even that loading is mixed and requires evaluation of bolt stretch and how the surface and backing plate will deform under load (everything deforms, although often not enough to be seen).

    c. The primary reason DJ suggested chain plates was almost certainly to spread the load. That is why it is done that way on chain plates. You could analyze the structure, but if you would not use pad eyes for chain plates, you have your answer.

    Without knowing the boat, I'm going to guess that this is not just about backing plates. Even with a thick backing plate, the bolts will have considerable shear load and could shear through a cored deck sideways. I would be laying up more glass on the inside and then adding a backing plate. You probably need 7-8 mm solid glass (not counting the core) to resist the shear load safely, and that load needs to be transferred to a larger area.

    BTW, it is difficult or impossible to engineer the straps to be a safe break away. The problem is that the fabric part has a different ratio of SWL to BS and chafe. If you make the straps robust enough to last (remember the boat lost in the Golden Globe when the JSD failed?) they will not be weaker than the attachment point. Also, a JSD that is undersized to reduce stretch (just in case someone suggests this is NOT stable. A JSD becomes unstable if the tail weight is not heavy enough or if it is towed at normal drogue speeds. Jordan knew this and reflected it in his specs. I have tested shortened JSDs.
    Last edited by thinwater; 09-11-19 at 15:43.

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