This adjusting car replaces separate blocks and a connecting member. Tandem sheaves eliminate severe flexing of the backstay, providing a neat, thoroughly functional fitting. The 38 Series with Delrin sheaves is intended for boats under 30ft., while the husky 39 Series with aluminum sheaves is intended for boats up to 40 feet. Note: For maximum car release, use a 1.5 to 1 ratio between backstay base width and car height. Example: If backstay width is 5', car height should not exceed 7-1/2' to 8' in height off deck. Higher ratios may require manual release with aid of boat hook, etc.