Composite forestay attachment point.

Discussion in 'Multihulls' started by Corley, Oct 9, 2011.

  1. Corley
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    Corley epoxy coated

    I've been looking lately at the combination stainless forestay tow ring assembly on my kraken 25 its ridiculously heavy like most components of the boat. It consists of a stainless strap about 400mm long with a shackle for the forestay fitting on the top. The load path would seem to be nearly all in one direction (towards the mast) I'm wondering if I could make a simple lamination out of unidirectional eglass and extend it down the forefoot of the boat obviously I could do the same in carbon but for such a low loaded component it doesnt really seem worthwhile. The boat has a nine metre mast 300sqft of sail about half in the 3/4 overlap genoa and carries a twin trapeze. Should I just look at stainless used for the fitting or for the stainless wire used on the forestay as a guide to how strong the lamination should be?
     
  2. Gary Baigent
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    Gary Baigent Senior Member

    You can use glass but you need to overbuild a little, requires more laminates than carbon.
     
  3. cavalier mk2
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    cavalier mk2 Senior Member

    There is an element of side force too if you think about headstay sag. Add in waves and no heel in gusts and the size of the fitting starts making sense. Sending some layers to the sides seems sensible.
     
  4. michael pierzga
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    michael pierzga Senior Member

    Google carbon headstay " Tack Fitting". A home builder building a small performance boat documented its construction and gave valuable engineering insights. Ill look to see if I can find it again and post if I do.
     
  5. Petros
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    Petros Senior Member

    Anything made in steel could be designed in graphite or glass composite, but that assumes you know and can predict all the forces and all the directions the loads will be applied to the connection. Unlike most metals, composites have to be designed specifically for the direction of loading because of the nature of the fiber construction.

    Many traditional approaches to marine design are often simply historical rather than by analysis. That is the designs is simply based on the size of the hull and type of use (off shore vs. coastal vs. inland waters). This is a recognition of the difficulties of accurately predicting loads in complex assemblies in heavy weather conditions. IT is like "we can not predict loads accurately, but 1000 years of history has shown if you make it out of material X it must be y thick to hold up reliably in most conditions".

    So you can design it yourself and take your chances, if it is for racing and day sailing only, it is not a bad calculated risk to stay competitive (and to save weight). I would likely try it. But I am not sure I would do that on a recreational or cruising boat.

    Good luck.
     
  6. Corley
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    Corley epoxy coated

    I was thinking of wrapping a thimble in glass as I've read elsewhere and taking the unidirectional down the front of the curved overlapping bow and leading the other side back down the inside of the hull onto the poured double bias reinforced keelson. The alternative is I ignore the bow all together and install an angled bulkhead inside the hull skin to spread the load and bring the forestay load inline with the bulkhead. Its all very easy to integrate as I have the cold molded hull still unfinished upside down on the mold (one veneer of the triple diagonal to go!).

    There are no sails set on the forestay the Genoa begins about 1/3rd of the way back from the bow to the maststep.

    more details on the boat here:
    http://sailboatdata.com/viewrecord.asp?class_id=6864
     

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  7. Corley
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    Corley epoxy coated

    Fair point Cav there will be some sideways loading at times I'm starting to lean towards the bulkhead solution for the forestay and genoa mount. The boat has to have stiffeners for the deck anyway so its just a matter of a little repositioning (the deck is 4mm ply) I'm tempted to use 3mm as theres no reason to stand on the deck forward except to unjam the roller reefing weight is everything in this boat I'm even going on a diet myself :D

    Looking at the plans it says 3/8" galv for the forestay and 5/8" galv for the shrouds.
     
  8. sabahcat
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    sabahcat Senior Member

  9. Corley
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    Corley epoxy coated

    Thanks for the reply sabahcat but thats not correct. The forestay is not depicted in that sailplan picture but its seperate. It runs to the front of the bow and is lashed on a combination fitting (not shown) with a tow ring. Lots of old dinghies in Australia had a similar system I'm not sure its universal though. I guess Lock left the forestay out of the sailplan for clarity you can see it in the 3/4 view and photo though.
     

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  10. waikikin
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    waikikin Senior Member

    Crikey, thats a Kraken shroud!
     
  11. Corley
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    Corley epoxy coated

    you said it, I'm sure the actual mast doesn't have a 5/8" shroud. I dont have the mast at home just now will have to look to see what stainless shroud it actually has.
     
  12. cavalier mk2
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    cavalier mk2 Senior Member

    I went with the angled bulkhead/frame when I redid our forestay attachment and it does a great job of keeping the loads in tension and distributing side loads. The original deck fitting had bolts fail in sheer, aligning everything with the new bulkhead/frame solved the problem. Well bonded to the hull sides and stringers it also eliminates hull panting or flexing/spreading caused by a deck to keel system. On a really light boat the flex can strain the deck attachment when the hull sides bow loosening the to the keel wires or chainplate. We beefed ours up with lots of carbon so it is way overbuilt yet light. Lots of ways to do these things but when a forestay fitting lets go the mast tends to fall aft at you and you do have to be quick.
     
  13. John Perry
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    John Perry Senior Member

    A few years ago we made some test pieces to represent chain plates made in this way. The test pieces were made by wrapping unidirectional carbon pre-preg round and round a former made from MDF covered in parcel tape then heat curing. The ends of the former were full radiused so we ended up with what looked like oval chain links but square section. We trimmed the rough edges then tested them to destruction by pulling them appart in a frame to which was attached a 200 tonne nominal capacity hydraulic jack. The attachment to each end of each link was made through a close fitting pin machined from Nitronic 50. I wish I could remember the exact results of the tests but I cannot and the firm I was working for at the time has now gone bankrupt. However, I can tell you that the breaking strength of the links was less than you would predict from the cross section of carbon fibre in the two straight sides of the oval. It seemed from the way that failure occured, that load was not equally shared between the fibres around the ends of the links, so these started to fail progressively with a series of loud cracks. Failure was always at the rounded ends, not the straight sides of the oval. The breaking load was certainly less than a simple calculation for the straight sides would indicate, unfortunately I cant remember how much less, I am not a lot of help am I!

    I am certainly not saying that you should not make chainplates this way, quite a few builders of composite hulls are now doing it. But I would say that you should allow for the above effect by picking a reasonably generous factor of safety/ stress concentration factor (whatever you want to call it).

    When a typical stainless steel chainplate is loaded close to breaking point there is also uneven stress distribution arround the holes used for fasteners and clevis pin, however SS316 is lovely and soft (relatively!) and yields to mitigate the stress concentration at least a little bit. You need to allow for a stress concentration factor round the holes of a stainless steel chain plate (the ESDU engineering data books are one source of informaton on this) but I suspect that you may need a rather larger factor for a unidirectional carbon construction.
     
  14. catsketcher
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    catsketcher Senior Member

    Carbon aint soft enough

    I was told a story about the first few sails of the AC boats built in 1994 in Australia. When they first went sailing they had all of these bangs occurring. The boats were built lightly (witness One Australia sinking) and then engineers said that the cause was the slightly tighter fibres failing as they did not stretch enough to give their neighbours any load.

    Corley - That sound sounds way overboard - I don't have 5/8 shrouds on my 38ft cat - unless Lock meant circumference which some people used in the 1960s (Eric Hiscock mentions this use in a book of this time). Personally I would look at the wire cross section and increase this by a factor of about 6. According to my composite design book E glass laminates have a tensile strength of 140 MPa and Stainless about 600 so E glass is about 3 times less strong. Double the wire size as you never want to break a chainplate and then make an equivalently strong glass one.

    One thing that works in your favour with glass is that you can spread the ends of the uni out to approximate the load paths - think of the clew of your radial cut main. This also means that you get much lower stress on the boat so it is stronger at the boat/chainplate interface.

    cheers

    Phil
     

  15. cavalier mk2
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    cavalier mk2 Senior Member

    I think your right about the circumference measurement Catsketcher, it is the old English way of measuring ropes etc....I built my frame strong enough to take the load and added the carbon uni for insurance. As soon as those fibers go horizontal they are operating in sheer not tension.
     
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