Dyneema Chainplate Location

[yakimoto]

Hello everyone. This is my first time posting and I hope it draws a lot of interesting discussion.


I have a 1981 Southern Cross 31' with a rig that is as old as the boat, as far as I know. I therefore am in the process of drawing up the plans to replace it in the next few months. I have decided to use STS-HSR 75 from NE ropes as the wire replacement and using lashings for the tensioning system. The mast attachments for the shrounds will be Colligo Cheeky Tangs and the forestays and backstay will be attached via a Colligo thimble and clevis pin.



I currently have 316 SS chainplates (as old as the rigging), bolted each to a knee to connect the shrouds to the hull, all of which I have decided need to be replaced.



I like the idea of attaching "dyneema chainplates" to the bulwark. Allow me to explain.


The dyneema chainplates will be a 10 mm dyneema line spliced to either a sailmakers thimble or a frictionless ring. I will size these accordingly, of course. It will attach to the bulwark via a hole and stopper knot (Stevedore---ref. Colligo). The holes will have HDPE tubing epoxied into them so that they are watertight. For structural integrity, I will use a 1/4" thick by 4" wide and 3' long rectangular sheet aluminum bronze on the hull side and a 2" wide similarly sized piece on the deck side aligned with the respective shroud holes. The bulwark is approximately 3/4" thick FRP.



What are your thoughts? Is this idea structurally sound? Where might a problem lie?



I reference the newer Cape George 31' sailboats with SS chainplates bolted through a 1.25" thick FRP bulwark. Check their website under the "Construction" tab and click the CGC PDF Brochure, scroll to the bottom of page 14.



Please let me know.



I have attached a drawing of the bulwarks.

[GRIT]

Your hdpe tubing might be water tight, but the dyneema within will wick water into the boat.

Why not composite chainplates? Just run a bunch of glass uni up and down the bulwark and through the deck in a loop. Up one side, through the deck, and down the other side. Then it's really waterproof. Run it around that same thimble you had planned for the dyneema, and you're good to go. Be sure the bulwark meets the deck, without a gap.

I don't follow the sheet bronze description... What's that supposed to do?

I understand the drawing, but without knowing where their stations are, it's hard to picture what your looking at.

Cheers.
Paul.

[yakimoto]

The HDPE tubing will run through the hole made in the bulwark, so there should be no way for any water, including that from the dyneema to enter the boat. Essentially I am making the "chainplates" external to the boat.
The bronze is used as reinforcement. There will be a point load from the dyneema stopper knot at the bulwark hole so the bronze sheet would help to distribute that load across the bulwark and down.

I do not understand your idea with looping "glass uni" around the bulwarks.

I have attached a picture showing the stations, if that helps. In this drawing it appears there is a gap between the deck and hull FRP at Station 5. I know from replacing a piece of the cored deck near Station 8 that that gap is not along the entire bulwark.

[GRIT]

Ah, yes. I thought by "Bulwark", you actually meant knee. So many people are careless with nomenclature, I made an erroneous assumption. Appologies.

Cheers.
Paul.

[Breaking Waves]

The Stevedore can slip under sustained load - yes I have tested it. The estar stopper is a better stopper knot for dyneema - it does not slip and is stronger.

However, the stopper creates a weak point in the system (dyneema does not like knots, and it will cut strength by very ballpark 50% - probably more in this specific case) and is not the best/strongest way to do this. Rather it would be better to put a multistrand loop (eg a line looped several times thru/around the hole) with an end to end splice. If the bend radius at the hole is a decent size (bigger than around 1.5:1) then you have a 'full strength' stystem (it would be more than twice as strong as the stopper approach)

as someone has mentionned above - it is decently common to do this sort of 'chainplate' using carbon unifibers - and that is better than with dyneema for several reasons. Stronger, and cut/chafe/uv proof.

[Benz]

Your idea is sound enough, if you engineer the parts correctly. One question is whether the bulwark is strong enough to take that sort of pulling load without reinforcing the fiberglass. You don't want the bulwark just tearing upward. Also you need a tube thick enough that you can put a generous radius on the edge where the dyneema goes around it.
Rather than a knot of any sort, consider a spliced loop, or even a covered loop (like the Harken Loups). Push the end of the loop through the tube, slip a pin though it (like what's commonly called a 'dogbone,' though its really a toggle), and you're all set. Devising a way to keep the pin from sliding out isn't difficult.
I'm going to do this on the next boat I build, but I'll purposely reinforce the bulwark during construction to ensure proper strength.

[yakimoto]

I like the idea of not using a stopper, because of the idea that it can and perhaps will slip. The reason I even think about it is from the success of "Witch Sailor" on YouTube using it in the same manner in which I proposed.
Breaking Waves---I want to keep from stressing the bulwarks more than I need to, so making a loop as you suggest may be out of the question. The reason being is that I am already stressing the bulwarks in ways unintended perhaps, so the squeezing of a loop would only add to that. The carbon unifibers would be great for this, I will give it some thought.
Benz---Do you have a CG 31? Does it have the chainplates as described in their "Construction PDF"? I like the idea you have presented. I am planning on drilling a hole 1" down from the top of the bulwark making a steep angle (10deg). This will pass through the bulwark and down and out the hull about 4" from the rail. I will reinforce with G10 epoxied to the outside of the hull and perhaps inside? I will also epoxy G10 to the deck-side bulwark. The knees that the chainplates are currently bolted to should give a good deal of strength as well. The continuous loop of dyneema passed through the single hole with a dogbone is a great idea and one that I am leaning towards now. The dogbone worries me only in that it is one more link in the chain.
Thank you both and any other great ideas are greatly appreciated.

[Benz]

Originally Posted by yakimoto

I like the idea of not using a stopper, because of the idea that it can and perhaps will slip. The reason I even think about it is from the success of "Witch Sailor" on YouTube using it in the same manner in which I proposed.
Breaking Waves---I want to keep from stressing the bulwarks more than I need to, so making a loop as you suggest may be out of the question. The reason being is that I am already stressing the bulwarks in ways unintended perhaps, so the squeezing of a loop would only add to that. The carbon unifibers would be great for this, I will give it some thought.
Benz---Do you have a CG 31? Does it have the chainplates as described in their "Construction PDF"? I like the idea you have presented. I am planning on drilling a hole 1" down from the top of the bulwark making a steep angle (10deg). This will pass through the bulwark and down and out the hull about 4" from the rail. I will reinforce with G10 epoxied to the outside of the hull and perhaps inside? I will also epoxy G10 to the deck-side bulwark. The knees that the chainplates are currently bolted to should give a good deal of strength as well. The continuous loop of dyneema passed through the single hole with a dogbone is a great idea and one that I am leaning towards now. The dogbone worries me only in that it is one more link in the chain.
Thank you both and any other great ideas are greatly appreciated.

My CG 31 is home-finished with a gaff rig I designed myself, so it does not have the chainplates the factory finished ones do. I have external bronze strap chainplates, kicked out from the hull with chainwales for a better staying angle. However, your idea is one I have long entertained for other projects, and similar stuff is done on some high-end racers around here. It's pretty common to have a loop-and pin setup on carbon masts, and there's several different versions of heavy-duty aluminum bushings designed for that purpose with a slot to hold a pin secure. Others keep the pin in place with a machine screw.
Conceptually it's a good method, as long as you engineer the parts correctly.
To use carbon uni to build chainplates would require lots of grinding and resin work, knowing exactly what you were doing, and being confident in your skills. It's not exactly a trivial thing to build.

[Breaking Waves]

Originally Posted by yakimoto

The carbon unifibers would be great for this, I will give it some thought..

there are a number of different ways to make them from carbon . . . . https://www.boatdesign.net/threads/c...nplates.49724/ is just one pretty simple example approach