Forestay chainplate & load

Discussion in 'Sailboats' started by vinceUK, Apr 8, 2010.

  1. vinceUK
    Joined: Feb 2006
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    vinceUK Junior Member

    Dear All,

    I have found the following statement in one of my lectures (dating from some years now so getting a bit rusted) concerning the load transmitted by a genoa:

    "The genoa can only impart its lateral load to the mast at the attachment of the masthead stay. The total sail load is given by the area x wind pressure, and we must assume that something between ½ and 1/3 of that load is transmitted at the mast head, it is safer to assume ½ the load."

    Is this true to state that the load transmitted to the forestay chainplate is the remaining and then can be assumed to be:

    (genoa area x wind pressure) / 2

    Then including a sag varying between 2 to 4% depending on the type of boat, can we assume that the load in the chainplate is:

    (genoa area x wind pressure) / (2 x pi x sag)

    Can someone confirm me this or maybe give some advice about what should be used?

    Thank you very much. Vince
  2. alan white
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    alan white Senior Member

    Wouldn't the remaining load also be shared by the jib sheet?
  3. vinceUK
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    vinceUK Junior Member

    I don't seems more like a very safe rule-of-thumb than an accurate formula. And this is my question. What is the best and the most used? Skene's formula based on the righting moment? The one I have quoted above and that is very conservative? Somerhing else?

    Thanks by advance for any advice you will share. Vincent
  4. Paul Kotzebue

    Paul Kotzebue Previous Member

    It certainly would be. The hard part is figuring the distribution of the load between masthead, stemhead, and jib sheet. The even harder part is estimating the lateral (transverse) components of those loads.

    There is very little transverse load on the jib sheet, so it may not be unreasonable to divide the transverse load between masthead and stemhead.
  5. vinceUK
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    vinceUK Junior Member


    Yes it is true that it makes sense. But again, is this the formula you are used to use for determining the forestay tension?

    I will compare next week the results given by this formula based on wind pressure & sail area vs. the one that is given in "Principles of Yacht design" and that is based on righting moment. But I would like to know if there is another way to estimate the load and which one is used by NA's to do the scantlings of the chainplates and cable.

    I am actually working on a case of a chainplate failure and would like to get a good estimation of the load that has come into it.

    Please, I know this is always the problem with empirical formulae & estimate but what do you usually use as a guide for determining the loads in the forestay due to a jib?

    Thanks by advance, Vincent
  6. Paul Kotzebue

    Paul Kotzebue Previous Member

    My comments were based on the statement "the genoa can only impart its lateral load to the mast at the attachment of the masthead stay" from your first post. The lateral load on the mast due to wind pressure on the jib is not the same as forestay tension. The formula you are considering, (genoa area x wind pressure) / 2, will not give you forestay tension.

    Wind pressure on the jib will contribute to forestay tension, but not as much as backstay tension, running backstay tension, and shroud tension if the spreaders are swept back.
  7. vinceUK
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    vinceUK Junior Member

    Ok I see Paul. You would then advise to use Skene's formula which is more "global", would'nt you?

    It may be worth looking at Iso 12215-9 but I don't know if the draft available on the Internet has been validated.

    Again, thanks by advance if someone agrees to share the formula he would use, being Skene's, Iso, mine, etc.

    Thanks to help me sorting this. Vincent
  8. Paul Kotzebue

    Paul Kotzebue Previous Member

    I won't go so far as to offer any specific advice for your situation. You need to do some real engineering from first principles to analyze a chainplate failure. For instance, the design tension on the forestay is probably not the same load that caused the failure. You need to have a firm grasp of the fundamentals of engineering (i.e. formal training plus experience) to do the calculations required for even a simple failure analysis.
  9. vinceUK
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    vinceUK Junior Member

    Hi Paul,

    Sorry for not replying earlier I was having a long (and appreciated) week-end!

    I am trained and experienced to analyse the chainplate failure from a purely mechanical point of view but also by making fractography, etc. This is really in my skills.

    But what I would have liked, even if the failure indicates some fatigue, is the maximum intensity of the load that has gone through the chainplate.

    I do not necessary need it, as it is obvious now that the dimensioning & scantling of the part was wrong but it is more to have a global picture of the scene.

    I will compare the formula I have in my lecture, Skene's one and the ISO 12215-9 but I would have enjoyed comparing with others and knowing what is used by some designers.

    Thanks anyway, Vincent
  10. Landlubber
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    Landlubber Senior Member


    Rigging (standing) is pretensioned conventionally (1x 19 wire) to about 15% of the breaking strain of the wire. This is easily measured by using a 2 metre rule, and 5% is 1mm, 10% is 2 mm, 15% is 3mm of stretch from new untensioned wire.

    The chainplate obviously has to way exceed these tensions till wire breaking has been exceeded, safe reasoning is usually 3x the maximum wire breaking strain, so if a chainplate failed under standard sailing conditions (this includes knock downs of course as they are in fact standard sailing conditions), without the rigging having failed, then one would be certain the scantling of the chainplate was insufficient.......
  11. Paul Kotzebue

    Paul Kotzebue Previous Member

    I use the method from Principles of Yacht Design for estimating forestay and backstay loads at the design stage.

  12. BATAAN
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    BATAAN Senior Member


    Skene's is good. L.F. Herreshoff's "Common sense of yacht design" is a constant revelation. One often sees toggle setups with no allowance for side to side movement, or a poorly thought out chainplate that was easy to build but wrong for the job.
    Doesn't hurt to look very carefully at successful prior vessels for the small details, chainplate size for comparable stability and sail area etc. and compare that to anything you come up with from a formula.
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