Do You Know How Sails Work? Find Out More Here
August 26, 2009 by Chris Wethered
Filed under Sails
Daniel Bernoulli
The question how sails work was answered by a Swiss scientist in the eighteenth century called Daniel Bernoulli. Before elaborating, it is important to remember that the wind has the same characteristics as a liquid: Wind flows.
When wind reaches an obstruction it is splits and goes around the obstruction, then joins up again. The speed of the wind as it negotiates the obstacle varies.
Concave And Convex
This is what happens with yachts moving through the water. So lets look at the physics in tangible language! As you are aware, the different parts of a sail or foil, play a specific part in the process.
How sails work: As the wind moves around the sail shape, the speed differential moves the boat through the water
What is happening?
When the wind reaches the foil:
- The wind must find a way around the foil
- As the wind flows around the foil, the windward side of the foil makes a concave shape and the leeward a convex one
- The draft of the foil is measured in the middle
- The draft is the point at which the wind on the leeward side is moving fastest
- More draft equals more speed
Push And Pull
The push and pull between the windward and leeward develops due to pressure differential. High pressure on windward repulses low pressure on leeward. As air reaches the foil it splits up. The air that flows to leeward speeds up and that to windward slows.
An area of low pressure develops to leeward and high pressure to windward. The “push” and “pull” combine together to move the boat through the water.
the result - lovely efficient shapes!
It’s all about pressure change:
- To leeward a channel develops between the accelerated air and the unaffected air
- The draft of the foil narrows the channel
- The channel accelerates the wind towards the unaffected air
- The faster air leads to decreased pressure and a chain reaction begins
- As air heads towards the foil, more is drawn to the low pressure area
- This increases the difference in pressure
A different approach is used for flying the spinnaker
Point of sail
The result is that the boat accelerates to it’s maximum speed for that point of sail. The angle of wind to the direction of travel has sparked the whole industry. It’s impossible to sail straight in to the wind. The reason is that air reaches the foil evenly on both sides. There is no pressure differential, the sails just flaps uselessly.
Moving at angles to the wind
Moving at angles
Harnessing wind, therefore requires the yacht to move at angle to it. This is why yachts tack to change direction. Back in 1783 Daniel Bernoulli published the now famous Bernoulli Effect. How wind works, has fascinated sailors ever since!
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Find Out Why Sailmakers Are At the Leading Edge Of Wind Technology
August 25, 2009 by Chris Wethered
Filed under Sails
Along with boating technology as a whole, sailmakers have come along way, through using hi technology fibers and techniques.
Find out where we’ve come from
In the early days they were made of heavy cotton, flax or hemp. The problem with these materials were:
- They needed to dried out before being stowed, or mold would quickly grow
- Folding when wet would lead to creases that took ages to come out
- The material used was heavy and awkward so the wind had to be constant
- As ships grow in size, so did the sail area
- Sun damage was more of a problem
The trouble was ships were growing in size and so the wind harnessing ability also needed to grow. Under these circumstances they did not hold their shape well. The one other problem which faced these early fibers is UV damage.
The sun has a depreciating effect on unprotected materials.
The sailmakers finished products
This is were we are now
Fortunately nowadays things have changed:
- Using top of the range materials, specifically designed for the purpose.
- So different sail cloth serves different purposes.
- What suits spinnaker sailing does not suit the head sail.
- Sail making is a highly coveted skill in the industry. From cruising inshore on weekends to crashing through the Southern Ocean, all yachts need theirs maintained.
Design
Naturally design plays a role too. Requirements change with the length of boat. Getting a better angle to the wind whilst maintaining speed is a key priority. Something that most sailors understand, but novices don’t is how sails work In essence they are the same as airplane wings. click on the links to refresh your memory or advance your learning.
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Learn How Sail Making Improves Boatspeed
August 24, 2009 by Chris Wethered
Filed under Sails
The stages of making this important aspect of being on the water are below:
Modeling
Sail making is geometry. The dimensions of the boat need measuring. This is usually done using a scaled plan. Key measurements include: height of mast, length of fore-stay, distance to shrouds and length of spreaders.
As you can appreciate, this data is paramount to the operation. The calculations are put into a 3D modeling program. The shape described is a reflection of the measurements typed in. The model allows visualization of the shape of the sail. Once this step is complete there four factors which need assessing:
- Chord depth %
- Maximum draft position
- Wind entry & exit angle
- Percentage twist of sail from clew to head
The Sail Making loft
Check Out bateau plans!
As most boats are designed before the sails, this means that an optimum shape is rarely achieved.Here are two examples: The Genoa track is pleasing to the eye, but not the wind. Spreaders can also be too long, interrupting the curve of the sail. The implications of this lack of foresight tends to be a drop in speed as the angle of exit is too broad in the sail. Generally speaking the narrower the angle of exit, the better.
Getting the shape right
Wind Tunnel & Pressure Distribution
The prototype now goes to the next stage. The objectives are to determine the lift and the drag effects of the boat. Again what suits one scenario, may not suit all. So several initiations can follow. The make of the cloth and the load distribution on the sail are looked at next.
Essentially the cloth is chosen for it’s properties including strength in all directions, weight and ability to be pre “moulded”. graphs are generated to demonstrate the stress and strains on the cloth in different directions. The result is a prediction of the optimum shape. The shape is tested through a range of wind speeds and directions.
Laser Cutting
By now you will appreciate the precision and intricate nature of sail making. The 3D model is split into 2D sections, which are then cut using laser. The process involves a technique called geodesics. Essentially the curved distance between two points is measured and then cut to simulate the 3D model depicted on the computer screen. The finished product is a “pre” curved 2 dimension copy of the computer generated image.
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Flying The Spinnaker – Your Guide
August 20, 2009 by Chris Wethered
Filed under Sails
Kites, chutes or spinnaker sailing. Tend to be asymmetric in shape.
The three points on which are tied the halyard and two guys are designed to bring the tension into the middle of the triangle – filling it.
How materials are used
Yarns are made from:
- Variants of nylon
- Woven between layers of laminate on either side
- Depending on the size of your vessel, your chute maybe made from Twaron, Vectran or Dacron
- The modern day fibers offer the sail better durability and helps maintain the shape
Watch some really smooth spinnaker action
In order to set the kite correctly here are some key points:
- A starboard set means the kite bag will be clipped to the shrouds on port
- Set the pole up prior to launch – after the skipper has determined tack
- Make sure the halyard and guys are outside everything
spinnaker pre launch
Launch
Depending on your level of comfort, you can either drop the headsail before launching the spinnaker sailing or keep it going until the launch has been completed. Which ever method you choose, the launch needs to go smoothly, otherwise you could wrap the fabric around the propeller. The same goes for bringing it down.
Trim
So the chute is up. Now you need to get the pole at the correct 90 degree angle to the wind. The clews need to be level too. The pole can be adjusted to the horizontal. Trimming is key. Play the kite so the clews are level and the belly full of shape, but when the sail has too much shape it will collapse, watch the leach of the sail for flapping. In the same way restricting the sail will also lead to collapse. Having the helm steering a straight course is helpful!
Gybing the spinnaker
Gybe
Gybing pole to pole is easiest. Allow the helm to bring the boat downwind and main position to bring the main to the middle. Then as the direction of the bow changes, unclip the end of the pole attached to the clew. Now the trimmer is single handedly flying the kite. Unclip the pole from the mast, so it is suspended on by the topping lift. Now clip the pole to the other clew, then the mast. Helm can now bring the boat onto course.
Drop
Bring the boat onto a broad reach, so the mainsail acts as a barrier to the chute. Grasp the sheet close to the clew, release the after-guy, which was attached through the pole. then bring the fabric down in big armfuls, with the help of a help of a steady lowering if the halyard. The objective is not to get the fabric wet!
Packing
Just set up the bag in the cabin. Then position the halyard clew forward and hook it to something. Then hook the other clews port and starboard. Now bunch the chute into the bag. eventually you’ll have three clews outside the bag ready to be ties of using the ties attached to the bag. Job done!
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Sail Cloth Is Key To Better Performance – Start Here
August 17, 2009 by Chris Wethered
Filed under Sails
Advances
Today advances in fabrics through research really benefit sail cloth (foils) and laminates. In the past the foil material was subjected to distortion from the wind. This lead to reduced aerodynamics and lower speed.
Another hurdle that has been overcome is developing tri radial foils rather than radial. The difference is that previously similar size pieces of fabric were woven together giving a less efficient area for the entry / exit of the wind.
Nowadays according to the point in the sail where the fabric is, it is cut to specific requirements of enhancing the performance of the wind. They are tri radial as they are attached to the boat at three points.
Computer Aided Design
When the fiber is translated from computers a technique called geodesics has been invented. The optimal 3D curve is replicated to 2D. The result is a pre curved sheet, which is precision cut from sail cloth by laser. The result is a curved, wind efficient sail.
Bringing the sail cloth to life
Here are some common sail types
Spinnaker
Clearly what suits one type of sail does not suit another. It’s useless making a spinnaker out of mainsail fabric and vice versa! Spinnaker fabric is a derivation of nylon, often referred to as ripstop. It is between half an ounce to three ounces per square yard in weight. That depends on what is required. Light wind or heavy, thirty foot weekender or fifty foot racer.
The characteristic of the fabric in sail cloth is that it is not universally strong in all directions. This is why it is interwoven in order for wear and tear to be minimized. The development of tri radial sails means that the stress in the sail is directed to run along the seams on the outside rather than into the center.
down wind with spinnaker flying
Main
In order to maintain the main, battens are incorporated in the design. They aid sail shape and durability by reducing flogging. Another factor to remember is the use of quality hardware at the luff end of the battens. This reduces jamming and so wear.
Different cloth are used on different boat lengths. Mylar tends to be used in boats of fifty foot or less. Its characteristics lend themselves to the foil area necessary. It is light, strong and durable. Most yachts will have Mylar on board. Dacron is used on board forty foot yachts or larger, again for the same reasons as Mylar. Finally Spectra is most often used on yachts over fifty foot in length.
The mainsail set - with spreaders
Genoa / Head
As the similarities of the main and the genoa are closer than those of the spinnaker, the fabrics used are the same. What is different is the shape and size of them. In the same way you can reef a main, there are different sizes of genoa, depending on conditions. Genoas are designed around the lay out of the deck. Contributory factors, include the shrouds, the mast and the spreaders.
Close hauled with genoa
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