I am trying to understand the mechanics of this thread.
I believe when someone refers to a compression vang they mean a vang that holds the boom down, supports the boom in light air and also yields to sheet tension when the boom is near centerline. It seems to me that there are two ways to accomplish this. 1. As pictured on page 109 of Optimizing - with a spring - on Bob Wells's boat. 2. Using a thin rod (2-56 if it is long enough) which will support the boom but which can be easily (but not permanently) bent when the sail is sheeted in near the centerline. Also a compression vang has to have zero (or near zero) play or else the support feature is lost.
I believe that there is no support feature on the SailsETC vang.
Near the centerline the sheet is pullng down on the boom. The solid vang is not yielding and is actually putting an upward force on the boom at that point. A compression vang would yield and allow the boom end to drop reducing twist. That is my understanding of the term "sheet vanging."
Speaking of homemade vangs. I admire Earl Boebert's version as pictured on Chad's discussion group - New Owners section - Gooseneck for Victoria.
http://www.rcsailing.net/forum1/topic.asp?TOPIC_ID=425
His design has a great feature that fires the imagination. He uses a fishing swivel as part of the vang. That particular swivel might not be up to the rigors of an EC12, but how about one of those Bass Pro 75 lb swivels with the loops slightly squeezed and inserted into an aluminum rod on one end and a threaded coupler on the other. Probably an aluminum tube over the coupler to increase the dia where the forward loop is inserted. Then a couple of well placed pins to help out the epoxy. Probably have to pull the loop with a string into the aluminum tube to eliminate play. And a long 2-56 rod to make it "compression". Just a thought. Sounds like a lot of work.
PS. Thank you Gregg for the image. I have seen the US1M construction guide. It just never ocurred to me until I saw Earl's vang that that sort of design could be applied to a vang.
Scott