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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?"


See added notes below on Sail Battens,  Feb 2022

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 4.2% and 16 gives 1.6%.   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,



ADDED Feb 2020:    Role of SAIL BATTENS

Also concerning this subject, its important to underline the role of sail battens in achieving the desired sail shape and camber.    When you stop and realise that the main loads from your mainsail are actually taken on only the corner eyelets, the only way there is of controlling the shape of the large central 'bag' of the sail is with either a very flexible mast or with full-length battens.  As most masts are pretty rigid and wingmasts totally so, we need to look hard at the battens.

Learning Test #1:   Just try putting up your sail once without the battens and you will probably be shocked at what it looks like !  This should impress on you their high value.   

These battens need to be strong enough to apply resistance to compression that would otherwise allow the sail to become far too full.  While a thin-walled tube is theoretically the most effective for this, its generally too stiff at the forward end, so a flatter section is typically used.    After making sure the forward end is in its pocket at the luff, this is achieved by pushing on the end of each batten at the leech and firmly tying it off there to match the camber you want, so that it cannot loosen.   For windless days, the lower ones especially will need to be adjusted to hold a fairly high camber (a guidance table to camber values is given at the bottom of the page on Sailing Tips #1).

But the location of the camber fore and aft is very important and just pushing on the end of a batten with equal section over its whole length (as typically supplied with sails), will typically give a camber that is just part of a circle with the fullness about 50% back.   Yes, new sails will fight against this centralizing of the camber, but as sails get softer with use, the material is no match for the batten and will soon become controlled by it (but yes, somewhat less so with expensive hi-tech materials).   

While this sounds bad, it really offers a good side ;)    This means that as the battens collectively control the main bag of the sail, a LOT can be be done to improve your mainsail (old or new), by creating battens that are stiffer in their after part so that when pushed in and compressed, they give more curve to the forward part, to create a much more efficient aerofoil.   (Ideally, a batten should change from a flat section in the forward half to a tube at the rear, but these are expensive).   Here is what I do with ALL my new long, lower fiberglass battens that the sailmaker supplies, and I don't wait for my sail to first go out-of-shape either.    I get the battens giving the shape I want, and then 'dress the sail over them' for the best combined result.   To do this, I sand and then bond on graded lengths of carbon fiber tows (you can buy rolls of CF tows and its really worth having), typically over the rearward 50% of the batten.    The starting point of each tow needs to be staggered say 75mm apart so that the transition from flexible to stiff is over a length of at least 1/4 of the batten length.  If you are not convinced as to whether this is worth the effort ... try this.   

Learning Test #2:   Pull out each stiffened batten and slip them into your sail pockets after flipping them end-for-end.  You will be surprised what happened to your nicely-shaped aerofoil sail and if you sail with this, expect to lose at least 20% of your upwind speed!

So when you get your sails ... put the main up in a gentle to moderate breeze and evaluate what sort of batten profile would help.  If you are not sure, just add 4 or 5 tows on each side of your lower battens at the rear ... and then, you can always sand some off if its too stiff in some area.   It's much easier to fine tune them by sanding off than by adding on though both work.    But ALWAYS sand off your added CF and give at least one coat of epoxy BEFORE pushing them into a sail pocket, as ANY CF fibers sticking out can pierce your sailcloth and do damage, not to mention getting under your skin!     Once you are happy with the shape, give them another coat.   Most sails will perform better if you stiffen up the rear half of your battens.

So in conclusion ... having a fully battened sail is a very good thing.  Not only do the battens increase sail life but you constantly have the means to adjust how a sail sets, even as it gets older and potentially out-of-shape.  Looking at old sails that only have short leech battens is often a very sorry sight and far too many sailors do not realize just how much performance they are then throwing away.  

Avid sailer/racer and Mk ll Waters Wingmast builder in Germany, sent in this link on the controlling battens of the high tech AC75 sails that I think is well worth sharing.  Enjoy .. and thanks Andre.   Relating this to the early inventions by the likes of Herreshoff etc, is also interesting so see this 2012 article on Sail Options for some examples.

Mike 2022

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