The original rigging that came with the boat was stainless wire rope with with pressed-on fittings. There was a lot of rust on the fittings, but otherwise it seemed ok. However, I have an innate distrust of wire rope fittings - you can’t see the condition of the wire rope inside the fitting. I had also decided to make the mast 6” longer than the original so I probably needed new rigging instead of just extending all the lanyards.
I had noticed several Wharram people replacing wire-rope rigging with Dyneema and saw that the new Mana 24 was using Dyneema as well. So I did a lot of internet research on Dyneema. In brief, what I found was:
- Dyneema of the same size is far stronger than wire rope.
- Stretch characteristics are different. So Dyneema should be sized according to stretch not strength when replacing wire rope.
- Minimal “creep” will occur over the life of the line.
- “Construction stretch” will occur after splicing and should be removed by loading up the lines after splicing. You also need to account for the stretch and make the line shorter than the size required when making the splices.
- Dyneema can crush a thimble under load so use closed thimbles or sailmakers thimbles.
- Dyneema needs needs curves with a radius 5 times the line diameter to maintain strength.
- Predicted life span is 5-10 years.
I priced the materials out (~$150) and decided to try Dyneema for my rigging. I liked the idea of having something that would be easier to coil and store than wire rope when I was transporting the boat. I also liked being able to do the splices myself instead of needing a rigging shop - good for repairs!
The original wire specs for the Tiki 21 (breaking load) were:
- 4mm for shrouds and forestay (2000-2833 lbs)
- 5mm for forestay bridges and dolphin stay (3100-4400 lbs)
I ordered 1/4” (6.5mm) SK-78. It has more than enough strength and has similar stretch characteristics to the specified wire rope.
I used a Brummel splice to make loops and capture thimbles. I used two variations on the Brummel splice to account for times when I had a free end and for times when the other end had already been spliced (McDonaly method). When burying the tail, I buried 72 times the line diameter. A great source of info for splicing Dyneema is L-36.com. I made my own fids by flaring one end on pieces of 1/8” copper tubing.
I built the dolphin stay, forestay bridge (2 pieces), and put the mast loops in the forestay and shrouds. I did not put the thimbles in the bottom of the forestay and shrouds because I wanted to size the rigging with the mast in place first. That will have to wait until the boat is assembled. I stretched the completed pieces and was indeed able to get out the construction stretch and get the pieces to their intended size.
While constructing I was thinking of how to attach the headsail to a Dyneema forestay. The current bronze hanks on the jib just barely fit over the 1/4” Dyneema. And I am worried about them abrading the line. Options are: removing the hanks and using soft shackles, or put stainless rings on the forestay and clip the hanks to those. Let me know if you have any ideas!
I’m calling this post Part 1 because I will follow up when I finish the rigging and actaully raise the mast.