Many people would like to change the sail plan of their dinghies. Unfortunately it can be a rather expensive business but second-hand sails are sometimes available. You can use them as they are, or alter them or have them altered to suit your ideas. I have attempted to give some guidance in the diagrams below, to ensure that you do not ruin the balance of the sail/hull combination when trying to change the rig for the ‘better’.

Most boats sail best to windward when carrying a little ‘weather-helm’. That is when in order to be able to keep a straight course the helm has to be pulled a little to the windward side of the boat against some resistance. It not only allows the craft to work to windward better but is also safer in that if you leave go of the helm the boat luffs up into the wind. Many boats also luff up if they are allowed to heel. An excessive weather helm however makes the boat very hard work to steer, particularly in breezy conditions. It also slows the boat as the rudder is acting at an angle to the centreline causing drag.

When doing the original design calculations, the yacht designer will only consider the sails used to windward and he will normally ignore light weather sails. He has to find the Centre of Effort of the sails which is the central point of the combined areas of the sails, dropped vertically down to the LWL. Then he calculates the Centre of Lateral Resistance of the hull, which is the centre of the area of the hull as seen from abeam and which resists the sideways pressure of the sails. He brings this point vertically to the LWL as well. He can then see their relative positions fore and aft. In modern hull types the C of E should be slightly forward of the CLR in order to give the right helm balance. The exact amount of lead is impossible to calculate scientifically but depends upon the experience of the designer and his knowledge of similar boats.

There is not normally much you can do about the CLR so your problem is only in designing the C of E of your new rig so as to have the same relative position fore and aft as the old. A few other points I would mention. The diagrams show a quadrilateral mainsail but any triangular sail is treated the same as a jib. If you have more than two sails, a yawl with a jib for example, then you calculate the C of E of any two sails first and then consider them as one sail when calculating their combination with the third. Notwithstanding your accurate calculations, certain sails have a tendency to give a little more or less weather helm than others. It is as well to have some options open to you after the first trials of the new rig, such as adjusting the rake of the mast or being able to change its position fore and aft if required.

Whilst we have not considered the height of the C of E of various sails, it should be taken into consideration. For example the height of the C of E of a Bermudan sail is higher area for area than the average quadrilateral sail. If you have a tender craft you may be considering changing to a gaff or lugsail from the existing Bermudan. Likewise, if you are considering changing to Bermudan rig you should be aware that the taller rig will have a greater heeling force. Area

Multiply either diagonal Multiply any side by by the half sum of its half its perpendicular perpendicular distance distance to the from the opposite corners opposite corner

Centre of Effort

To find the common centre, draw the line YM through both centres. Divide the product of multiplying the area of the jib by the scaled length of YM, by the sum of the areas of the mainsail and jib. The result is the scaled distance of X from Y.

Example :

mains’l = 350 sq ft, jib = 75 sq ft, YM = 10.0 ft

YX = 75 x 10 = 750 = 1.76 ft 350+75 425

Note - It is usual to ignore the area of the roaches of a sail in computing the Centre of Effort.

Draw the diagonals AB & CD Draw straight lines Note - A Bermudan mainsail intersecting at E. Make BF = from any two would be calculated the same AE and CH = DE. Draw FH. corners to the as a jib (i.e. as a triangular sail) Draw straight lines from any centres of the both for area and C of E. two corners to the centres of opposite sides. the opposite sides. Their Their intersection M. intersection Y is the centre. is the centre.