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Sail Adjustment to improve an old mainsail

QUESTION: "I have an old mainsail that’s stretched out of shape. As the material still seems solid, is there a way to figure out how much I need to modify the luff and/or seams to do this myself and get some drive back into the sail?"


While this will not be easy with bigger sails because standard sewing machines cannot handle all that material bunched up or even the cloth thickness, something can certainly be done with most materials under 6oz. if the material justifies the effort. Typically, standard beach catamaran sails are in the 4-5oz range and the material is soft and supple to work with.

You first need to locate where the sail has gone out of shape. Typically it’s got very baggy about ½ way up the mast, so we’ll assume that’s the situation for the purpose of this reply.

You need to mark where the issue starts and stops … along the mainsail luff. Go sailing with a helper and with the sail hauled out tight, get a measure of how much ‘bag’ or camber you presently have at various heights. Having a straight plank with you nearly boom length can help. This ‘camber’ is your closest estimate of how much the sail bellies out from a straight, roughly horizontal chord line between the mast and the leech. Could be somewhere between 150mm (6”) and 300mm (12”) … or even more.

By dividing this value into the chord length, you’ll get a ‘Camber Ratio’ at various heights.. and then you need to decide on what Ratio you want to achieve. Some of this ‘bag’ can be pulled out by removing the luff tabling (that holds the bolt rope) and then cutting back on the luff (re-sealing with a hot knife), before restitching the luff tabling back on. Where you cut off the luff will more or less image the location of the bag that was to the rear of it. If it’s a lot, you may well need to also open the sail panel seams in that area and overlap them more to recuperate the original shape vertically as well …. a little on each seam, overlapping more where the most belly was, but tapering off to nothing at the luff and also rearwards towards the leech. The question that comes up most often is this. If I want to change the existing Camber Ratio from say 5, to a flatter 10, how much material do I need to remove at the luff and/or, how much total EXTRA overlap do I need to make on the panel seams? (assuming of course, this is a regular ‘crosscut’ sail with quasi-horizontal panels). So this article will address both those measurement issues.

The following explanation and curves will help you roughly calculate 'how much does what' .

For horizontal stretch, the attached curve shows the % increase in the arc for different camber ratios. It's drawn for a parabolic shape (see green line) but if you calculate the figure and deduct 35% if the sail is also full well aft, or deduct 45% if its flat aft, it gives a reasonably close figure.   Deduct 40% if it's average.


Now, if your present chord-camber ratio is say 8:1 but you want it to be 1/2 that full ..., ie chord-camber of 16, then from the curve,  8 give 4% and 16 gives 1.4%.   The difference is 2.6%. Deducting 40% gives 1.56%. So, if the chord is 100 inches, then the luff needs to advance 1.56" at that point.  Obviously, the amount will vary up and down and reduce to zero at the peak and tack.  In most cases, some distance from both, and in fact, may only need to start about 1/4 the way up as the lower part is often less stressed so seems to stretch less.    OK?

A sailmaker might want to combine both approaches that I've given you, reducing the amount from each, but I think I'd personally start with pulling the luff forward and then seeing how it sets.    You could take a temporary 3/4" tuck just to the rear of the luff rope and stitch it by hand.  3/4" 'pinch' would shorter the arc by 1.5" at the most needed part.

In order to assess the vertical stretch, another approach can be used, but this requires that one works accurately with the calculations, particularly when calculating the angle A shown in the drawings given here. The vertical sail camber is herewith assumed to be a segment of a circle with a certain radius and the formulas show how to find that radius.

The important formulas are those for the Radius ‘r’ and for the Chord ‘c’.

First, we know the rise ‘b’ (camber) and the chord ‘c’, so we can readily solve to find radius ‘r’. Taking a W17 sail as an example, might give ‘b=0.4’ and ‘c=6.5’ in metres. But as the deformation is likely to be more over the centre part of the sail, it would be reasonable to assume that effective ‘c’ could be less .. so let’s assume 5m for ‘c’. (NOTE: As this distance remains a constant once decided, it’s precise value is not very critical in the calculation)

Solving for ‘r’ in this example, we have (4 x 0.4^2 + 5^2)/ 8 x 0.4 = (0.64 + 25)/3.2 = 8.0125m for ‘r1’

The chord formula needs to be re-expressed to find the Angle A

If c = 2r Sin (A/2), then Sine (A/2) = c/2r . In our example, 5/(2 x 8.0125) = 0.3120.

From Sine tables, that gives A/2 as 18’11” deg, or A = 36’22” deg (36.367 degrees).

36.367 deg is 36.367/360 of a circle or 0.10102. [Circ = 3.1416 x 2x 8.0125 = 50.344]

So the Arc length is 0.10102 x 50.344 = 5.0857m, reflecting the present vertical cloth distance after stretch.

Now repeat the calculations with the required ‘b’ camber value … say ½ of 400mm or 0.2m.

Solving again for ‘r’, we have (4 x 0.2^2+ 5^2)/8 x 0.2 = (0.16+25) /1.6 = 15.725 m for the new ‘r2’

We now need the new perimeter over the same segment of the circle. ‘c’ will be fixed, but the included angle will reduce as the radius of now greater.

Sin A/2 = c/2r = 5/(2x 15.725) = 0.159. So A/2 is now 9’09” deg., making A = 18’18” deg. (or 18.300 degrees)

18.3 deg is 18.3/360 of the circle, or 0.050833. So the required corrected arc length will now need to be: 0.050833 x 3.1416 x 2 x 15.725 = 5.0225m [ π = 3.1416]

As the stretched arc was 5.0857, this means that the sum of all the seam adjustments needs to be 5.0857 - 5.0225 or 63.2 mm, to reduce the sail bag by half, or take in about 12 mm per seam at the worse part, tapering to zero at the luff and towards the leech.

(Note that after the ‘b’ and ‘c’ values have been fixed, it’s important to work to a high degree of accuracy with the calculations (as shown), as the difference over the arc length is very small compared to the input values). This is especially true when determining the angle A …. as even ‘1 second’ can have a significant effect on the arc length.

If the required camber reduction is as much as 50% of the measured stretched camber, then one should plan to use both the vertical and horizontal adjustments, but if the required adjustment is 30% or less, then starting with only the luff modification (see first method given above) would seem to be the preferred choice. And as indicated earlier, taking in a temporary pleat 50-80mm aft of the luff and then rehoisting and remeasuring the sail camber, would seem a wise approach. Use polyester thread for stitching and zig-zag is preferred for horizontal overlapping panel seams. Vertical stitching at the luff can be a straight stitch but not too small as this can perforate the material.

Hope this helps,


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