Jeff,

Maybe you've been following my thread on that other beachcat site. I had the same idea, and ordered the parts. The official Nacra and C2 double blocks used in the adjustable setup are ~$55 EACH. The ronstan double block is a ~$25 part. There is a knock off of the Ronstan/Harken part but I don't trust knock off's in a safety critical application, when the name brand guys see failure.

So I went with the shock blocks at $16 a pair from a local supplier (sorry Rick I might return these don't want to double pay shipping). My intention is to splice them together to form a double block using dyneema. To be fair to Ronstan I don't think this is the intended purpose but others have done it with Antal rings. My concerns are the following:

1) MWL; in a 2:1 setup I'm 185lbs wet so you're looking at 370#'s of load on the block. Other have pointed out you may see 2-3g's of acceleration occasionally so you may apply up to 3x that number, 1110 lbs. That exceeds the breaking load of the shock block. However, it's an aluminum block so it won't just snap. It'll likely deform and since you are only applying high loads for temporary periods of time. Finally, one must remember Ronstan designed these for similar applications/load cases on kite boards.

2) Friction. I went with 3/16" (5mm) diameter line which is commonly used in adjustable trap setups. Somewhere I saw (Murrays?) the shock block ideal line diameter tops out at 1/8". Now that I've seen that I'm going to go down to 4mm (another reason to shop locally) line, which actually increases my line cost. Previously I spec'd Excel Pro which has a 1511$# tensile strength in 3/16" diameter, but a mere 695# tensile strength in the 5/32" diameter (4mm). Hence I'll switch to a dyneema cored line, FSE Dinghy Control Line which is a nice running line and has decent price/performance ratio. In 4mm this has a tensile strength of 1574#'s. I want the blocks to explode before the line does.

I'll post photos and reviews of the system when I'm done. I'm doing this right but trying to save money where it makes sense. Also want a reliable system for ocean racing so if the shock blocks don't work they'll get returned.