bolt
The standard linear measurement of canvas for use at sea: 39 yards.
bolt: a standard roll of sailcloth, 39 yards long
A bolt is a fixed length of canvas wound on a roller, traditionally 39 yards (about 35.7 meters) long, used as the basic unit of measurement and sale for sailcloth in the marine trade. This standardized length dates back centuries when sailmakers needed a consistent way to order, stock, and cut material for sails of various sizes. A single bolt can yield sails ranging from small jibs to parts of larger canvas projects, depending on the width of the cloth and how the sailmaker plans the layout.
The 39-yard standard exists partly for practical reasons: it represents a length that one person could handle and store without excessive bulk, and it divides reasonably into common sail panel widths. Sailcloth itself comes in widths typically between 24 and 60 inches, and the bolt length accommodates economical cutting patterns. When a sailmaker orders canvas, they specify both the bolt length and the cloth width; a bolt of heavy Dacron or traditional linen at full width weighs between 50 and 100 pounds depending on the weave and fiber type.
Not all sailcloth is sold by the bolt today. Modern synthetic sailcloth, particularly high-modulus materials like Spectra or Pentex used in racing sails, may be sold in different roll lengths or by weight. However, traditional square-rigged and working vessels, as well as sailmakers working in heritage traditions, still rely on the bolt as their primary ordering unit. The term persists in maritime supply catalogs and in conversations between sail lofts and canvas merchants.
The bolt also matters practically in the loft. A sailmaker laying out a mainsail or topsail must account for the width and length constraints of the bolt to minimize waste and ensure the strongest possible fiber orientation. Diagonal cuts across the bolt (called bias cutting) can waste material but may be necessary for certain panel configurations. On older vessels with large canvas requirements, a single sail might use multiple bolts, requiring careful seaming and planning to align thread direction and weight across the finished sail.