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Sail crafting 101

Making sails for your model is not complicated, or time consuming.

I often experiment with different ways to rig sails, but the way I make them, is almost always the same. Just follow these few simple steps from the start and you will avoid a lot of time consuming mistakes later.

As an example, we will make a common triangular sail with a bit of roach, pretty much the same sail that comes with your kit. Start by taking a sheet of paper that will be used to create the sails pattern. A sheet of standard size paper will do for most Mk I sails, something larger for a Mk II.

"Step" the mast into the hull, (take out the centerboard) we are going to design the sail pattern with the paper rigged onto the mast. This helps you visualize the final product.

Wrap one edge of the paper around the mast with enough left over to allow you to adjust the sail's draft. The paper is then held onto the mast with clothes pegs. Set the paper bottom edge down on the deck. This gives you the space to add the tail at the sails clew, that should come out and away from the sails finished shape. This will reduce the cupping effect caused when the knot is tied that forms the sails clew.

Sketch the sails shape with a marking pen.

Note in the illustration, I added some "belly" to the to the sails foot in this design to increase sail area. Also note the space needed for the "gooseneck" at the bottom of the mast.

After the sketch is completed, cut out the pattern with scissors and re-attach it to the mast for a final check.

Spread out the sail material on a flat cutting surface and check that there are no holes or scratches that will ruin the sails appearance. If you don't have a large breadboard, use cardboard or an open newspaper.

Tack the pattern down onto the plastic with some tape in a few places to hold the pattern in place.

Hold the pattern down to keep it from moving with one hand. With the other hand, cut the sail out by slicing the plastic with a single edge razor, or use a hobby knife, around the edge of the pattern.

Using a straight edge, like a ruler, helps to keep the pattern in place and make straight cuts.

I recommend not cutting through all of the taped parts at first, just to keep the pattern in place.
note:See how to make multi color panel sails.

Tying knots at the corner of the sail is done by holding onto the sail material at the tab and lightly twist the corner of the sail so that it stays together.

Tie the sail with an overhand knot, made very lose and open, roll the open knot carefully down, closing the knot as you go, until you reach the tab, and the knot is closed into the tab. The knot is easy to tie when the end of the sail is looped over a finger and the finger is twirled around in a circular motion.

Do not pull the knot tightly closed, leave it somewhat lose. This leaves you with a larger knot that is easy to tie rigging line around. The sail material will stretch and weaken if you pull the knot too tight.

For a jib, rig the model with a head stay, then with a few quick folds, and scissors "rough in" the shape of the sail pattern. Leave some extra paper to fold one edge over the stay

There are different ways to make jibs for your model. This is the easiest, (not necessarily the best) way, the sail is attached to the stay with a strip of clear cellophane tape (3/4 inch), gripping each side of the sails luff.

The tape should come in contact with the stay, and be firmly squeezed together to keep out any water.

A boom can be made from a length of bamboo skewer, or you can use a strip of recycled plastic (just like making a batten) the boom is also fixed to the sail with tape. The stay can be tied to the fairlead at the stem and then made fast at the mast head.

A small fairlead is then placed on the deck in front of the mast to guide the jib sheet.

Calculating sail area and C.E

To find the area of your models' sail, use the paper pattern that you used to make the sail. Trim off the areas on the pattern that are not put to use by the wind. You will need a ruler and in some cases you will need a 90 degree angle and a compass.

The same methods are used to find the areas of any plane figure. Here are some basic formulas that are used to calculate the areas of different shapes.

For a square: Area equals the length of any side multiplied by itself.
A = s square

For a rectangle: Area equals the base multiplied by its height.
A = bh

For a right angle triangle: Area equals one half of the base multiplied by height.
A = ½ bh

Note that a right angle triangle has a 90 degree angle, and is one half of a rectangle or square.

For a triangle (three different sides) Area equals one half the base multiplied by the height.
A = b x h / 2

Note here that the height is measured at a 90 degree angle from the base, and is not a measurement of one of its sides. For a trapezium: Area equals the average lengths of parallels multiplied by height.
A = ½(a + b)h

For a parallelogram: Area equals base multiplied by height.
A = bh

You may find yourself dividing the sail pattern into two or more of these shapes, calculating the areas for each, and adding the total.

For sails that have curves, such as the roach area of a sail shown in the illustration, you estimate the area. In the example shown, the length from the sails' head to its clew is measured and divided into the units of measurements that you are working with (inches or centimetres).

The next step is to find how many full square units there are using a parallel line to form a grid of square units.

The rest of the units are estimated using decimals (one half of a square equals .5 ect.). Then add them all together for the total.

Finding a sails' centre of effort

At the geometric centre of a sail is its centre of effort (ce). This is approximately where the over all force (effort) that is produced by the sail has the effect of pulling, or pushing the sail at a right angle from the surface of the sail. (The true centre of effort is usually found slightly above and forward of the geometric centre but this method is close enough.)

The centre of a square and a rectangle is easy to find by drawing a line from each corner to its opposite corner. A big "X" marks the spot.

To find the centre of a triangle draw a line from each corner to the centre of its opposing side. Where the lines all cross is at the centre.

For other shapes the simplest way to find the centre is to copy the sails shape onto a piece of cardboard using the "working" area of your models' sail pattern. The cardboard from a cereal box works fine. Take the pattern and balance it flat on the tip of your finger, where it will balance without tipping off will be its centre.

Multiple Sails and TCE

When a boat is rigged with more than one sail, the boat is driven from each of the sails centre. With sails of different sizes on the sail plan we use a few calculations to find out where the combined or total centre of effort (tce) the sails produce will be located.

We first calculate where each of the sails' centre is and then calculate the area of each sail in square units of measurement (inches or centimetres).

You can use the paper pattern of your sails to do this exercise, try to lay the patterns out in the same way that your sails' would be located when rigged on your boat.

Draw a line connecting each of the centres, and measure the lines length.

The total centre will fall in between the two centres at a point proportionately closer to the larger of the two sails.

If the sails were the same size it would be in the middle.

To find out where along the line the two different areas will balance, first use the area of one sail divided by the total sail area.
This is expressed by a fraction, sail area over total area. Multiply this by the total length of the line between the two centres, expressed as length over one.

The two fractions are then multiplied.
Multiply the top (numerators) and the bottom (denominators) and the resulting fraction will be the proportionate length of the line that the sails' effort will have of the total.

To check your work, repeat the process using the other sail area, the total of the two fractions when added together should equal the total length of the line.
See the example above with a blue jib, and a white main sail.

The jib has an area of 24 square inches and the mainsail an (estimated) area of 36.5 square inches

When you add 24 and 36.5 you get a total area of 60.5 square inches.

The centre of each sail is calculated and marked on each pattern, the distance between the centre of each sail is measured to be 2.5 inches, once the sail patterns are put in place.

36.5 over 60.5 multiplied by 2.5 equals 91.25 over 60.5 or 1.51
(to the nearest one hundredth of an inch)

24 over 60.5 multiplied by 2.5 equals .99

1.51 + .99 = 2.5

You may note that the distance between the two centres would be longer if the jib was not a lapping jib. In theory, the sails could be far apart and the TCE could be located in thin air!)
The overlapping sail areas do not cancel each other out, thus, if you were to try to find the total centre of effort by balancing a cardboard silhouette of the two sails together, the balance point would not be accurate.

For boats with more than two sails, such as a schooner, calculate the total centre of effort on two of the sails, such as the jib and foresail, and then calculate where the balance point is between it and the mainsails centre.

The hull's centre of lateral resistance.

Knowing the sail plans' centre of effort is only part of designing a well balanced rig, the sail plan should also be balanced with the hulls centre of lateral resistance.

One method can be used to find the hulls CLR is to make a cardboard pattern (in profile) of the hull's wetted surface, the part below the waterline. This includes the rudder and the centreboard. Use a needle (or pushpin) to find a spot along the top of the pattern where the pattern will hang level. This will find the vertical location of the CLR. Of course, moving the centerboard changes everything.

There is one method that really works, and that is placing your model in still water where there is no wind and gently push the boat on the side of the hull with a finger or the end of a pencil to find a point on the side of the hull where the boat will move sideways through the water without turning.

This can be done in a bathtub, or a pool of still water, while you are at it, notice how the centre changes as you adjust the centerboard to different positions.

Balancing the design

Although with some experience in experimenting with different sail designs you can judge (more or less) the balance of your design by looking at it, using some practical math can be rewarding when your calculations turn out to be (more or less) correct.

In most cases the centre of effort is designed to be slightly ahead of the centre of lateral resistance, this is because as the boat heels in the water, more of one side of the hull area (lee side) is in the water than the other.

Having more of the curvature of the bow in the water on one side gives the boat a weather helm, meaning it will tend to turn in the direction of the wind. By having the centre of effort slightly ahead of the centre of lateral resistance helps to balance this effect.

How much of a lead depends on the design of the hull, for most boats the lead is 10 up to 20 percent of the hulls length.

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