From the webpage-   Snatch Blocks | West Marine

"Blocks without rotating sheaves or any moving parts offer the advantages of the lightest weight, extreme reliability and, surprisingly, very low friction. They’re made from slippery hardkote-anodized aluminum and work best with low-friction Dyneema line (stuff that’s so slippery that it doesn’t hold knots terribly well, and actually runs more easily over the block as the load increases)."

Will prove cheaper too. See lower half of page-