retro fitting composite chainplates

Discussion in 'Multihulls' started by Jcatmad, Apr 24, 2012.

  1. Jcatmad
    Joined: Apr 2012
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    Jcatmad Junior Member

    I'm building a 12.5 Meter Catamaran the design called through hull bolted chainplates right next to the bulkhead. Hull and bulkhead both 14mm cedar core, Hull layer up with 2X600db e glass on outside and 600db on inside. The hull was then reinforced locally 4X more 600 db 400X150 pads layed up on inside plus 2X600 db on outside where ss chainplate to be attached via 4X1/2" 316 ss bolts. All bulkhead Hull joints are 300mm half half bulkhead hull 600DB eglass.This has been done but no holes drilled or chainplate fitted.

    Its a 5000Kg sailing weight, 13.5 :1 length to beam ratio hulls, narrow hulled cruiser racer carrying a 17 meter high rotating wing mast and 88 sQm working sails and a large 150 sQm asymmetric downwind. The Chainplates are at the 8.05 meter point from the bow on a bulkhead which is 3.45 meters from centreline, Hull centrelines are 2.75 from centreline. All close approximates from drawings. The mast is stepped at the 5.42 meter bulkhead from bow both shrouds are attached at the 15 meter mark on the mast that gives full triangulation for loads

    My question is how difficult would it be to now fit composite (Carbon) chainplates. My thoughts were that the area is generally strong enough given it was designed to take load.

    Its just the spec of the layup of the chainplate and execution. Any suggestions of someone who could engineer it for me without going crazy on cost?

    I'm thinking wrap 100 wide 300 gsm carbon UNI 4x either side of bulkhead encapsulating SS tube for shroud retention and rapped through a letter box hole say 600 down in bulkhead. The bulkhead having been patched with say 400 by 800 and 200 by 700 300GSM carbon UNI either side before hand.

    Do i fit then onto the bulkhead through the strip planking or onto the hull sides inside and out either side of the bulkhead. Either way in cutting out some hull which i then re-glass filling hull or bulkhead 14mm cedar core with glass.
     
    Last edited: Apr 25, 2012
  2. Stumble
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    Stumble Senior Member

    They are slowly taking over as the material of choice, in part because you eliminate the need for stainless to be bedded through a deck. Eliminating both corrosion issues, and a ingress point for water.

    Do a google search for "composite chainplates" and a number of design firms will pop up that have done them in the past. For a part this heavily loaded I would defiantly want a professional to spec out what I needed.
     
  3. luff tension
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    luff tension Junior Member

  4. Jcatmad
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    Jcatmad Junior Member

    The email @ clever fox website bounced I use Gmail and it sent Fatal error which means it is really not working
     
  5. luff tension
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    luff tension Junior Member

  6. catsketcher
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    catsketcher Senior Member

    Why not find someone who has built a similar cat and ask them - about the same as your boat and nice composite chainplates. Don't have to go carbon though - the ones I saw being made were glass.

    cheers

    Phil
     
  7. catsketcher
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    catsketcher Senior Member

    Be careful still

    I was kayaking the other day in the Gold Coast and saw a nicely built cat from a reputable designer. The quite thin (about 30mm) composite chainplates were sticking a long way above the deck (about 100mm). Problem was the chainplates were not in line with the cap shrouds. They were almost straight up and the shrouds led forward at about 20 degrees.Really silly engineering from a designer or builder who should have known much better. From my back of envelope calcs (actually in my head) the torque would be close to 500Nm when the boat is loaded up and it is a totally unnecessary load.

    Keep the chainplates as low as possible and wrap the unis right onto a bulkhead. Fan them heaps wet on wet and vac bag as well if possible. Then go over the whole lot with mutliple layers of double bias to tie the unis in shear.

    As to how far to go with the unis - that is a hard one. If you think about it - unless the ends near the fitting creep then the long uni ends will not do much. So it is best to use more strands and fan them rather than increase the length.

    On Kankama I worked out the strength of the wire. Then I looked up some specs on laminates. I then calculated how much area was required ( on each side of the chainplate) to handle 3 times this load. Then wet unis out and started wrapping.

    The great thing about composites is that they will crack before they let go. So do your calcs, add extra safety margins and then add a little more and have fun. Who cares if your chainplates are 5 kilos heavier than if they were engineered lighter. For me the best type of engineering is looking at a similar boat that has done serious miles and checking with the owner about the design and if they are happy.

    cheers

    Phil
     
  8. Jcatmad
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    Jcatmad Junior Member

    My boatbuilder has not done composite chainplates before, I found lots of examples on the WWW. All start with good alignment to load i.e. shroud the old school was fan out from there the newer school is reasonable width i.e. 100 mm carbon unit rapped thro letterbox hole in bulkhead. both covered in DB Glass or Carbon patches to distribute load. All I need really is how many raps of the Carbon uni for say Max load 5000Kg given boat weight fully loaded.
     
  9. catsketcher
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    catsketcher Senior Member

    Why the letterbox?

    I really don't get the letterbox idea. I saw it on a building blog and thought it was plucking defeat from victory. I'll explain

    All materials deform - the idea with a chainplate is to get the load from a concentrated single point and distribute it radially. As unis can really only take load along the axis the unis will need to be laid up in the direction of the load shedding. Unis go in the load path direction.

    Sails have been built this way for the last 20 years. Gone are the days when you would put a cringle in some cloth and sheet on. Now we use radial cut sails to have lighter and better shaped sails. See the lack of radial creases in a nice sail. Look at a cheap crosscut sail and there is a heap of difference.

    The letterbox idea is daft. Why keep on concentrating load in one part of the bulkhead when the bulkheads job is to spread the load out to the hull skin, floors and interior furniture? It will increase shear stress on the bulkhead requiring it to be heavier. Uni is like rope that we can get to stick where we like. To reproduce all the failings of a load concentrating piece of immovable stainless is counter intuitive. On top of that you have the top laminates of the stack greatly increasing the shear stress on the lower laminates as they have to feed their load through the laminates below to the bulkhead. The end result is that this design could break away from the bulkhead at its upper end. Feeding the unis through the letterbox about a metre down is daft too as the highly shear stressed lower unis will delaminate before the ends do any work.

    Just dumb

    Phil
     
  10. Jcatmad
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    Jcatmad Junior Member

    Fan out Layup seems to be better option

    I have looked at Hulls but under paint and fairing cannot see how chainplates are executed.

    The Bulkhead is 600db either side of 15 mm Herex Foam. The bulkhead is bonded to the hull sides with a 100mm overlap of both hull side and bulkhead of 600db. Hulls are 2X 600 db outside onto Strip planked 14 mm Cedar with single 600DB inside.

    The hulls are curved very Shuttleworth Style to give illusion of wide hulls with narrow Hulls at Waterline.

    A few questions . should I attach to bulkhead or to hull sides or both. Either way I understand align attachment point to shroud load

    To what degree do I Fan them out to and to what length given restraints of floors hull sides etc

    the glassing over to spread loads, how far beyond fan? is it better to to these before or after uni's?

    Note .. its a reasonable low freeboard hull side but I do have 1.95Meter clearance at this point in Hull.

    Im happy to pay for this information if someone out there is an engineer and can be definitive.
     
    Last edited: Apr 30, 2012
  11. Stumble
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    Stumble Senior Member

    You might want to check with http://www.graingerdesigns.com.au/ . I don't know them personally, but they have experience designing composit chainplates, and have a pretty good reputation from what I hear.
     
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  12. catsketcher
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    catsketcher Senior Member

    Go the bulkheads

    I prefer to bond chainplates to bulkheads. If you bond to the hull side you can't really align the hull side to the load path. You can angle to bulkhead (not really necessary). Probably the main reasons for the bulkhead are that you can ensure the bulkhead is straight and will not deflect under tension. The shaped hull sides are not and will want to straighten under load. The bulkhead nearby will stop this so use the bulkhead first up.

    Making the chainplate is a heap easier on a bulkhead. Put the walkway slightly to the inside side so you get a heap of meat under the chainplate. (The chainplate should be way off to the side anyway to get it on to the side of the deck) Then you can fan out the unis out from the chainplate onto the bulkhead for a great distance both sides.

    If you do the hull sides it is going to really muck up your fairing. You can try it (I used this on one of my boats) but I didn't do it again. The next cat had a small bulkhead which led the unis to the hull inside eventually.

    Look around for load paths. Ensure you do not have any hatches near the chainplates on deck so that you don't get any stress concentrations. The compression side is probably more problematic so no hatches in front of the bulkhead (if it is vertical there will be a slight forward moment) for a fair distance. I don't like hatches in the deck anyway so it was easy for my big cat. (MY hatches are not in narrow areas like the side decks - narrow ? - mono sailors could have a picnic on them still)

    I did not paint my chainplates so I could check how they were going. I check them still and will get round to it one day (outside is painted though).

    cheers

    Phil
     
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  13. Sand crab
    Joined: Feb 2011
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    Sand crab Junior Member

    Chinplates

    Do you like these?
     

    Attached Files:


  14. Jcatmad
    Joined: Apr 2012
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    Jcatmad Junior Member

    love them

    thinking further outboard but they look great
     
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