While reading Alan’s thread about “rig and sail fine tuning” I was asking myself : how to control or keep the wing shape of a sail. As we can see on many pictures, there are lots of disformation, concerning the mainsail.
So I am thinking about building a semi-stiff sail, made of two layers of 20g/m² FG, epoxy resin stratified on a shaped mold.
This way we could get “one piece” sails, as light as cloth or mylar but more stiff and keeping the wing shape on both sides (hope so…). Reinforcements with tape and eyelet as standard sails.
I’m asking if someone tried this way, or knows about, or just has comments or suggestions before I build a mold to make a sample.
I’m afraid that it will be too rigid even with 1 layer. and the wind force not sufficient to chage the shape from one board to the other.
Instead I found that using heavier milar cloth, 85g/m² generally used for strong winds, the main form is more stable in spite of the aditional weight, an other idea is to make multiples panels , 6, 7, 8 , etc. and relying on the dual-face adhesive for stiffening . Having said that, try and if I’m wrong I will glad to know it.
Are you saying two layers, one from one side of a mold and the other from the other side? So you would have a hollow airfoil shape? You may be able to build lighter with a lightweight structure covered by a film, like the C-cats.
Take a look at how the windsurfer sails are designed. They are out of very stiff mylar and full battens and use a lot of downhaul tension to hold their shape. They do not tack, without a jerk on the booms by the sailor, to pop the battens to the opposite side.
Hew, the idea is not to create a wing like the BMW Oracle multihull. I thought using two layers to cross the fiber: first 90°, then 45° in order to reinforce the sail and keep the shape “memory”. I noticed that fiberglass leafs (3x 80g/m²) was tearing up almost easely, so with hard winds…
Claudio,happy to see you’re back. The biggest fear I have is that you told first : Wind not strong enough to change the shape from one board to the other…
Multiply the panels must be a very good solution, as I’m already using 85 or 90g mylar cloth.
Whatever, I have to make a sample of the semi-stiff FG sail ! but next year… I have to imagine a mold I could use for different sail size and kind.
Any idea is welcome.
I once had an idea to build seamless shaped Mylar/polyester sails by heat shrinking over a foam core. Basically, the sail would be laid over a veneered foam wing core of the desired airfoil section, the core bent spanwise to the right degree of curvature, and the film laid over it and firmly fixed. The film would then be ironed to conform to the shape, as is done for film covering of many model aircraft.
I found out later that some full size sails are made in a similar way.
This is the idea, but the aim is getting a stiffer sail that won’t disform or less than a mylar or polyester one.
With two layers of 20g/m² and the resin I could get a sail of about 80g/m² or less that should be twice stiffer than a “standard” cloth sail.
A seamless mylar or polyester sail would be even less “stiff” than a seamed panels mylar or polyester sail, so…
But I’m curious, so if you tried the seamless shaped sail technic you described, tell me more.
In his shop he had a sail mold he made out of blue foam. he used a hot wire to cut a uniform chord shape block that was 72" long or so. Picture an airfoil shape that was 15" wide, 2" deep and then extruded 72" long. if you laid a piece of mylar on it, it was a ruled surface so it would induce NO sail shape. to put draft in, hal would curve the entire block along the long dimension by weighting down the two ends and proping up the middle. He could control draft along the entire length but constraining the block differently. Ill see if I can dig up a picture.
I had a thought similar to this about 4 years ago . . . I concluded that it would be fast and feasible, as long as i added a micro servo to the boom that would “pop” the sail when there wasn’t enough wind.
I can tell you the outcome of my experiment with molded sails for RC boats. . . I couldn’t get it to work. I tried normal mylar as well as monocoat and other products.
Most of the experiments resulted in uneven shrinking and sail “blanks” that looked like crap. The one sample that looked good on the mold, as soon as I popped it off was completely uneven.
Keep in mind that a lot of foams you may use start to melt at temperatures lower then the films want to shrink.
For my experiments, my “mold” was constructed out of plywood. I simply took a piece of plywood, cut a triangle sail shape that was about the right size for a footy sail. Using a belt sander I free hand shaped it into a 3D sail mold using the plywood contours as a guide. once it looked good, I mounted that down to a piece of plywood that was 1" larger then the sail shape. This allowed me to clamp and or tape my film as tight as I could to the mold before shrinking. If it worked well, I was going to make a 3D mold that would have worked to make 1 meter size sails (US 1M, IOM. 36/600s etc).
I still have a couple plastics that I wanted to try. my last conclusion was that in order to get this to work right, I would need to do it in the oven allowing the plastic to shrink all at once rather then one zone at a time.
If you want to heat the mold in a hoven or with “thermic heater” (“décapeur thermique” in french), you could made a synthetic plaster (“Zellan”) mold. By using fiberglass, there is no need to heat.
Concerning the mold, my problem is that on a 140cm high (55 inch) sail, the twist is not the same from basis to top. So I don’t think we could use a simple NACA profile all along the mold, like an aircraft wing. I’d like to create a standard mold that we could use to make any kind of sail : different mainsail shapes/sizes and jibs…
Using an additional micro servo to pop the sail seems to be the solution.
Waiting for the pictures…
my little contribution in French !! If you need more try to get in touch with René Villeret that wrote some manuals.
I did it one for M class and used with assembled Mylar panels, but was a failure.
I had thought about vac forming sails. essentially make a mold that is your final sail shape, then heat a sheet of mylar, move it over the mold and vacuum it down until cool. Lots of things are made this way so it should be possible. The problem I ran into is if you wanted a different shape then you would need a new mold. Way to expensive vs standard broad seaming. I will stick with broad seaming so I can make a lot of variations to see what works best.