Design: The Basic Principles and their Relationship to Sail Trim Upwind Sail Trimming

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ou’ve probably read articles on trimming according to differing wind strengths. Theobjective e is always to make his sails work across the maximum wind range possible. Forthe designer of the sails, the task is to produce sails which will respond to the sailor’s adjust-ments with an overall goal of versatility.Sails are made up of flat panels of sail cloth. Their three-dimensional shape—the airfoil—isproduced by cutting the panels narrower at each end than the middle. (see figure 7) In addition,the curve of the sail’s leading edge (the luff curve) is cut to match the curve of either the forestay sag (for jibs) or mastbend (for mainsails). By carefully controlling these two variables itis possible to produce almost any shape in a sail.Although each sail is a separate entity, when used together they are highly inter-related. Theyshould be thought of this way both on the design computer and when considering sail rim.At Neil Pryde, we make a full range of cruising and race sails. Each is meticulously developedusing the CDT design system program. CDT allows us to describe sail shape in a numerical format so that sails, subject to size and aspect ratio, can be scaled up and down and transferredfrom one boat type to another with consistent results. The computer imagines the designedmould and literally drapes the panels (in the defined layout) over the mould. Those areas where the fabric overlaps are plotted and cut by a computer driven cutter. By literally cutting off theoverlap from the cloth it is possible, after joining the panels, to re-create the defined shape. Thiscutting is known as “broad seaming”, and is applied to both the cross-cut (horizontal panel) and radial-shaped sails.

Basic Principles

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ost people assume that there is an ideal trim setting for every given sailing situation. Thisis theoretically true, but involves balancing many factors according to conditions and perfor-mance goals. The constant changes in wind and sea, the relative disturbance of the air, and even, thetactical position in a race can affect the sail trimmer’s goal at any one time. We’ll see later that idealtrimming can go beyond perfect-looking sails.In order to simplify matters let’s begin by generalizing that the majority of upwind trimming situationsfall into one of two categories: power or pointing.“Power” means trimming for acceleration, and generally involves fuller, more twisted, sails. “Point-ing” is trimming to flatter, less twisted, sails once you have attained higher speed, and want to headcloser to the wind.The only time when these principles do not apply are in very light, almost drifting, conditions whenacceleration is improved not through additional power from the sails but through reduction of drag.This is achieved by flattening the sail: the wind will flow more easily over the flattened surface (less drag) than over a rounded surface.For the cruising sailor following the principles of full or flat sails, excellent performance can beachieved over a wide variety of conditions.The following graph, which shows when to trim for wither power or pointing (and basic characteristicsof each), can be used at any stage of a day’s sailing, You might even find it useful to keep on deck.