Composite Forestay Tang

thanks all for your input. I just love it when a question yields answers ranging from "nice concept" to "terrible design". this was just an idea and id like to make it work, but I do have the ability to fit a traditional gammon iron as a plan-B. I do take exception to the 'standard engineering practice' of 3X the diameter over the pin... a short walk down the dock here reveals many many boats with SS chains where the material over the top of the hole is equal to or less than the pin diameter. ive always understood it was the minimum area adjacent to the hole that determined the breaking strength of a part, then again engineering school was many moons ago. also not sure how chain sawing on this part should be a consideration as the chain would always be captive in the SS roller assembly, and if that broke I suspect the rig coming down would be inevitable anyway.

 

I have hopefully located the link to carbon chainplates somewhat earlier in the year. Worth a good perousal, especially how the loads are distributed and note the stainless internal to allow for chafing.

http://www.boatdesign.net/forums/fi...-building/carbon-fiber-chainplates-49724.html

The 3:1 ratio is 'only' for thin flat plate and does not allow for wider sheet and custom applications which require their own stressing. When in doubt, calculate it yourself, even if to double check. Articulation and stress spreading seems to be key for rigging loads.

On occassion I have substituted Al for S/S for shroud plates but by working to similar tensile total load to failure, they have worked fine even if smaller than your project. I'm sure you can make it work with a bit of thought.

 

thanks, I have seen that thread and a few others. one of the reasons ive gone down this path is ive got about a mile and a half of 13oz carbon uni 1" tape that id like to put to good use, but also I really like the idea of a dry bilge and no corrosion. but I don't want to be second-guessing the decision when the fur starts to fly...

 

A few peices of scrap ply or similar, a short length of stainless tube, and your carbon uni tape and epoxy resin is all that's needed for a super duper chainplate, total time to make it would be a couple of hours. It's pretty easy.

Cut the scraps and glue together. Shape it so it sits on top of the square sprit ( if your sure this is well anchored), with the sides vertically flush, and gradually tapering off to an oval shaped top. Cut off the top of this scrap block, approx 1.5 inches of it, and replace the shape with solid epoxy bog and the stainless tube on top of it running fore and aft, aligned at 90 deg to the stay angle. The bog is important so that when you cut the slots for the toggle later, you don't expose the timber underneath to water ingress.

Wrap your uni over this smooth shape from as far down the sides of the square sprit you can reach, preferably going underneath it if possible. 15layers thickness of 13oz uni should be enough to suit the righting moment and max strain of an average 40 fter forestay, assuming approx 15m tall rig? Interleave every 5 layers with 1x 13oz Dbias +-45 eglass or carbon.

Fair it , paint it, your done.

 

yep, that's the pic I was looking for, thanks. ill use the G10 in place if the plywood.

so the key here is to get the uni as straight as possible and as parallel to the load as possible and the G10 is simply core? my hope was to use most if the inherent strength of the G10 and only use the uni for attachment. my only reason for transverse pins is I don't know my rig geometry yet, but I do want to get the deck buttoned-up. probably have to hold off till the forestay angle is known to do it the same as the shroud chains.

just curious why there isn't an equivalent dimension for G10 vs 316? why, say 4X the G10 dimensions will equate to the same piece in SS? or 8X? is it due to the different elasticity mod?

 

For clarity, you want the fibers running in the same direction as the load, in this case tension. The carbon is not doing any work if loaded at 90 deg. The tube is at 90 deg, not the carbon. This puts the rigging screw in pure shear.

The reason for running the tube fore and aft, is for ease of wrapping the carbon so it's alignment is in the same direction as the stay tension, or very close to the same angle.

If you glass it as per your original layout, with the rigging screws running transverse through the g10, you are relying on a very small area of the g10 alone, to stop the rigging screw breaking out the top. This is because the loads are not transmitted from the rigging screw through to all those wrapped fibers. The only part of the fibers that could do any work, are destroyed the moment you drill your hole for the rigging screw.

It might sound the same as when you cut the slots for the toggle in the photo above, but it's in fact totally different. Sure the fibers are destroyed in the cut part of the laminate, but the loads are still transmitted via the long rigging screw, to 90% of the cross sectional area of entire wrapped laminate. Can you see this key difference?

The last part of your question relates directly to material properties. I'm sure g10 is a good material for this application. The problem was more about holding the g10 down, and I still beleive your holes are too close to the edges, the rule was 2x the hole diameter, not 3x as someone else misquoted me.

 

thanks groper for all the input, and yes I do see the difference in the orientations. on the second pic you posted, am I correct in assuming the carbon tows wrap the pin, penetrate the deck, and wrap the sprit tube, or just the underside of the deck?

any benefit to wrapping this longitudinally with 10mm of CF uni? this seems to contain the pins somewhat, although 45 deg from parallel. the gussets/biax hold the tang itself down to the tube and resists side loads. the CF tapes could even lace through to the inside of the tube but I guess the 90deg turns are no good...

thinking out loud, probably rambling.

apologies

 

The problem with that is, it's just getting too messy to engineer it. I wouldn't have confidence in figuring out how much carbon needs to go around the top to give adequate break out strength for the rigging screw. Sure it might work, but unless you have 100% faith in your calculations, how can you sleep easy at night?

The method I showed you earlier depicted in the photo, is a very simple structure to engineer. The cross sectional area of the laminate is known, the carbon tensile strength is known, the fibers are aligned perfectly with the stress, and you should know your forestay tension. If you don't, then you need find out.

I doubt any professional could reverse engineer your design, and tell you exactly how much stress it will handle before failure, and how long it's fatigue life will be, unless a full finite analysis was done on it. Therefore, I don't think anyone could tell you if your design is good enough or not. This is why I offered another suggestion, which I know works, as it's being used everywhere these days with no problems.

 

One other dodge is to pre wrap the S/S tube in carbon/epoxy. Yes it takes away from the load bearing over wraps but ensures a better local bond to the tube. Groper has it right, but a local couple of mm wrap around tube first allows a bit more real world tolerance when you cut the slots as per Groper's pic/instructions. Allow for this primary thickness when you calculate the overwrap layers as the internal wrap is structurally redundant but gives a good bond all round (the tube) and a water proof layer when you cut through - a bit stronger than just bog if you get it right.

I've used this technique on a rather more mundane, never the less still shock loaded application with success. In that case a heavyish 45Kg Daggerboard that gets 'accidentally' dropped from about 600mm. Oooops! funny how the reataining tube has bent and now the glass has disintegrated.... Can you repair it? kind of way. Hence new tube, reglass and re-engineer attachment similar to your forestay even if less total load. A few calcs on tensile, allow safety factor and less than perfect layup and you get there.

 

The stainless tube doesn't do anything structurally, it only serves as a Busch for the rigging screw so it doesn't wear out the laminate via friction.

 

Exactly, but to get a true bond to the bush a wrap helps. It lowers the possibilty of delamination of the glass/carbon and water ingress, plus gives a better angle (by slightly bigger diameter) for the structural layers. If you went nuts you could even create a D of it prior to laying it down. The possibility of water creep up the structural fibres is almost completely eliminated.

 

Hey ther rob

First off, that there G10 is definitely up to the task, crazy strong for sure. I would not trust a epoxy bond only especially when the going gets rough! Can you make those G10 fillets taller and wider at the base? This will get rid of any sharp angles so the uni's will be at their max strength. Can you get some carbon fiber uni's?? What I have done in a similar project is to cut a shallow notch in the top of the plate staying back from the holes for the uni's to go over the top ending up flush with the rest. Remember, no sharp angles in the uni's, they like rounded corners. Is there a way to run some of the uni's down onto the hull then putty over them to fair? Another good trick is to wet out uni's on a table first, you want just enough resin to wet them not dripping wet! Of course pre coat the chain plate. I think this will do the trick for you. Then the weak link will be how the glass tube is attached to the hull.
It looks like it is tabbed in on the inside only. Can you add some more glass to strengthen that area up a bit? The ply over all of this probably won't add much to it, the plate has to be anchored to the hull via the tube. We added some chain plates (called them this} to a Gun boat cat, those uni's went down thru the deck and about 24" onto the inside of the hull fanning them out a little as they went down. Yes sir, carbon fiber rocks! The bushings were made of G10 as well.Uni's up and over then back down is the way. Hope this stuff helps..........peace out.....g