Sail Design Process
By Tom
Let's look at the design process for the Speed Demon as an example.
The first thing to do is to establish the conditions, equipment and level of sailor that the sail is for and what they want from it. In this case we are making an all-round sail for generally flat water with some chop and swells thrown in. It will probably be used on production boards 90-140 litres and 65-80cm wide with a fin 35-45cm in length. The sailor will probably be from intermediate to advanced level but probably not expert. The sailor wants to blast around as fast as the can get a few chop hops in and overtake their friends. They might do the occasional local race but are really just into zooming around for fun.
Now we can identify the qualities the sail needs to have. The end goal is to have a fast sail, but it must be powerful enough to drive a big board and accelerate quickly from those muffed gybes. It needs to be easy to rig and to handle. The owner will probably fall a lot so it needs to be easy to waterstart and be strong enough to handle the crashes. It will need to have smooth rotation and transition nicely and have good manouverability.
So now we can start making choices. Obviously a wide sleeve and cambers are not for this customer. A moderately narrow sleeve that will accomodate both RDM and SDM masts (so the customer does not need to buy a new mast). A narrow sleeve will not fill up with water and makes waterstarting easy.
Seven battens will make the sail nice and stable and resist compression at the loads that we be imposed on it with tubes in four of the upper battens. Eight battens would be overkill and add extra weigh and cost to the sail.
The outline is determined by many factors but simply put it needs to provide the right platform to deliver the power and twist charicteristics required. In this case we want a fairly powerful sail with good acceleration, wide wind range and a high top speed. A relatively low aspect ratio outline with a wide head with lots of roach, moderate boom length and a medium-low foot will deliver. This outline helps to make the leech more reactive for better power and acceleration. I like to use a little trick I call "Reduced Boom Length" to aid the twist and stability of the sail. The idea is that instead of the clew being at the corner of a triangle between the leech and the foot it is moved in effectively shortening the boom but maintaining the chord length (batten length) above the clew. This moves the tension line from the clew to the head forward aiding in stability and and allowing a more responsive twist in the lower leech. This also makes the sail feel smaller than it is when it comes to handling as the foot is the size of a smaller sail. Mini-battens and scaloping help eliminate leech flutter. The head batten is also slightly shortened for better aerodynamics and to prevent backwinding.
The luff curve is chosen as the best compromise between power, handling and ease of rigging. Generally more luff curve provides more power and less gives a softer more forgiving feel.
The shaping must be matched to the luff curve and outline of the sail. The 3 dimensional sail shape when sailing is created by controling the tension in the body of the sail and is a dynamic system. This is the most technical, complex and closely guarded part of sail design. The goal is to create a complete system that works together in unison. The more shape a sail has the deeper the camber and the more lift it produces. The problem is that with more lift comes more drag so again a balance has to be found. Fast sails tend to have a more draught forward foil as they are going faster and have more apparent wind. This makes the apparent wind move forward so the sail is always going up-wind even when sailing off the wind, and a forward draught is more efficient for going upwind. The Speed Demon needs a moderately deep camber which will provide good power but still be very easy to handle.
The materials are selected next according to their properties and asthetic appeal. Stretch, weight and cost are the primary considerations. We want a sail that is light, does not stretch much and is not too expensive. Lightweight monofilm is the material of choice in these curcumstances. X-ply is used in the luff area where the sail is wrinkled during rigging and extra durability is required.
Reinforcement is added as needed for the expected loads and materials used. The batten tapers are specified to match the sail shaping and Voila we have a complete sail design!
Let's look at the design process for the Speed Demon as an example.
The first thing to do is to establish the conditions, equipment and level of sailor that the sail is for and what they want from it. In this case we are making an all-round sail for generally flat water with some chop and swells thrown in. It will probably be used on production boards 90-140 litres and 65-80cm wide with a fin 35-45cm in length. The sailor will probably be from intermediate to advanced level but probably not expert. The sailor wants to blast around as fast as the can get a few chop hops in and overtake their friends. They might do the occasional local race but are really just into zooming around for fun.
Now we can identify the qualities the sail needs to have. The end goal is to have a fast sail, but it must be powerful enough to drive a big board and accelerate quickly from those muffed gybes. It needs to be easy to rig and to handle. The owner will probably fall a lot so it needs to be easy to waterstart and be strong enough to handle the crashes. It will need to have smooth rotation and transition nicely and have good manouverability.
So now we can start making choices. Obviously a wide sleeve and cambers are not for this customer. A moderately narrow sleeve that will accomodate both RDM and SDM masts (so the customer does not need to buy a new mast). A narrow sleeve will not fill up with water and makes waterstarting easy.
Seven battens will make the sail nice and stable and resist compression at the loads that we be imposed on it with tubes in four of the upper battens. Eight battens would be overkill and add extra weigh and cost to the sail.
The outline is determined by many factors but simply put it needs to provide the right platform to deliver the power and twist charicteristics required. In this case we want a fairly powerful sail with good acceleration, wide wind range and a high top speed. A relatively low aspect ratio outline with a wide head with lots of roach, moderate boom length and a medium-low foot will deliver. This outline helps to make the leech more reactive for better power and acceleration. I like to use a little trick I call "Reduced Boom Length" to aid the twist and stability of the sail. The idea is that instead of the clew being at the corner of a triangle between the leech and the foot it is moved in effectively shortening the boom but maintaining the chord length (batten length) above the clew. This moves the tension line from the clew to the head forward aiding in stability and and allowing a more responsive twist in the lower leech. This also makes the sail feel smaller than it is when it comes to handling as the foot is the size of a smaller sail. Mini-battens and scaloping help eliminate leech flutter. The head batten is also slightly shortened for better aerodynamics and to prevent backwinding.
The luff curve is chosen as the best compromise between power, handling and ease of rigging. Generally more luff curve provides more power and less gives a softer more forgiving feel.
The shaping must be matched to the luff curve and outline of the sail. The 3 dimensional sail shape when sailing is created by controling the tension in the body of the sail and is a dynamic system. This is the most technical, complex and closely guarded part of sail design. The goal is to create a complete system that works together in unison. The more shape a sail has the deeper the camber and the more lift it produces. The problem is that with more lift comes more drag so again a balance has to be found. Fast sails tend to have a more draught forward foil as they are going faster and have more apparent wind. This makes the apparent wind move forward so the sail is always going up-wind even when sailing off the wind, and a forward draught is more efficient for going upwind. The Speed Demon needs a moderately deep camber which will provide good power but still be very easy to handle.
The materials are selected next according to their properties and asthetic appeal. Stretch, weight and cost are the primary considerations. We want a sail that is light, does not stretch much and is not too expensive. Lightweight monofilm is the material of choice in these curcumstances. X-ply is used in the luff area where the sail is wrinkled during rigging and extra durability is required.
Reinforcement is added as needed for the expected loads and materials used. The batten tapers are specified to match the sail shaping and Voila we have a complete sail design!
posted by Jeffrey, Glenn and Tom at
2/20/2008 01:07:00 PM
5 Comments:
Hi Tom,
I have read that Neil Pryde claims to be the first one to have introduced the "Reduced Boom Length" concept. Is that true?
Check Levebvre sails, I think it was 1986 when they first did that.
The concept has probably been around for at least 20 years! Neil Pryde usually adapts other peoples ideas. We had boom cut-outs, a wide sleeve and zipper boom opening on the SR1 back in 1995 for example.
Ok, it seems that NP inclines to plagiarism. Anyhow, I thought that the most innovative concepts become "essential features" of all subsequent sail designs (think of the shear-tip sail). If it has been around for 20 years why the RBL concept "comes and goes"? I suspect it is more of a marketing expedient than a conceptual breakthrough.....
U am no sail desinger but a sail with a floppy leech designed into it seems perfectly complimented by the RBL concept. With RBL the clew to head "strainline" is now distal from the actual clew attachment, as such more" flop" can happen right down to the clew.
hmmmm Or maybe i dont have a clew.
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