ArcSail
I have finally launched the ArcSail Proa test model (see article : http://proafile.com/magazine/article/the-arcsail-proa ). There was a bit too much wind for the first sail, so after a brief run and a capsize I gave it away for the day rather than risk loosing the model altogether. The video is up at:
http://www.youtube.com/watch?v=-_Ymp7LQnWg&feature=plcp
The rig may need some fine tuning, but I’ll wait and see how it goes in lighter winds before making any mods.
Cheers,
Mal Smith.
Im very interested in this kind of sail. I think it sails very good in the video. It does not stop the boat from turning over, but i believe the problem would have been much worse with a normal tall mast and lots of sail-area up high.
Please write more about them. I want to test it myself on my advanced sharpie proa.
Cheers,
Johannes.
I would be happy to answer any specific questions you have. For now I have attached a vector diagram showing the basic principle. On the digram note that I have neglected the effect of the hulls. The short model test seemed to indicate that the lateral resistance and drag of the hulls is significant enough to cause the heeling moment which probably resulted in the capsize. I can counter this by altering the shape of the sail, that is, by placing the centre of the arc below the waterline. The downside of this is that it will reduce the efficiency of the sail further.
Cheers,
Mal.
To me it looks like there is a feedback loop where the further the “now bow” digs in while going down wind, the more force the wind causes pushing the arc sail over the top of the bow, hence causing the bow to dig in more. (or in this case the ama appears to have dug in)
Another way to remedy that would be to change the planform of the sail. I wonder if that sort of change would also improve the L/D of the sail without compromising its other attractive qualities?
Adam,
I think you are right about the feedback loop. This seems to be a problem with any force balanced system. It’s difficult to devise one with negative feedback to make it self stabilising, without making it very inefficient. In this case, the arrangement you propose in your sketch would be more stable, but you loose the ‘ring wing’ effect provided by the constant chord, thus the effective aspect ratio of the rig is considerably reduced. Also, the effective sail area is reduced by about 1/3, but the weight of the rig would not be significantly reduced. I think I would prefer to to try and prevent the nosedive from happening in the first place so that the system remains more or less in equilibrium.
One thing I did with this model is to use a low volume wave piercing ama, to reduce weight and windage. Perhaps it would be better to have larger and longer high bouyancy ama which would better resist nose diving? Of course it must be remembered that the wind was quite stong on the day. Realistically, there will always be a point where any design will fail due to the conditions, so the question should really be what conditions am I trying to design for? The ultimate goal is to design a boat that can cope with any conditions, but It’s probably a holy grail.
Cheers,
Mal.
That is a wondrous sail idea. I hope you can get it to stay upright, but while it was sailing it looks as good if not better that an ordinary rig.
Is there anything happening with this sail?
As i stumbled on Roy Stuarts tunnel-foils i got interested in all things circular…
I have been drawing a lot of variations on your ArcSail, but I have not built anything yet.
I have to complete my reefable Gibbons first. I like the ArcSail concept, and believe there is a lot of potential in it.
Cheers,
Johannes
Is there anything happening with this sail?
Not much happening with the ArcSail at the moment. I still haven’t gotten around to doing any more test sails with the model. A few months ago I did a CFD analysis on the ArcSail comparing it to a conventional sail and to a canted biplane arrangement.I only had access to the CFD software for a short time so it was a bit rushed and I found out that I hadn’t used a fine enough mesh to get accurate results, but as a comparative analysis it does have some value. The results do confirm that the ArcSail can be more efficient than a comparable biplane rig or a conventional rig of the same height. However I also learnt that the un-cambered sail I used on the model produces significantly less thrust than a cambered sail would. It would be better to use a cambered canted biplane rather than an uncambered ArcSail, both being of about the same structural complexity. The chart below shows the results of the CFD analysis. The horizontal axis shows drag coefficient and the vertical axis shows lift coefficient.
The camberd ArcSail seems EXTREMELY (im sorry about the screaming there) interesting.
That chart is very very encouraging for future experiments! Its seems to have very high lift at small angles where it is needed the most for fast sailing. (apperent wind)
Thank you for posting that information here.
An cambered ArcSail could be build with a central arc-shaped lightweight boxbeam or carbon-tube in the middle where the foil has its maximum thickness. That should be an easy way of creating camber and stiffness without too much extra weight.
Cheers,
Johannes
I don’t mean to highjack your thread, but it just occured to me that it is possible to build a reefable ArcSail.
The 180 degree arc can be made out of 15 12-degrees segments of reinforced foam that are all linked together like a bicycle-chain. The parts should only be allowed to fold inwards, not out of the arc. This way one could realese whatever way one tensions the arc, pull down one end of the arc as many segments as needed, then just lock it and tension it again with a strap of some kind. I hope my explaination makes sense.
Cheers,
Johannes
hey, have you got anywhere with your sail?
hey, have you got anywhere with your sail?
I haven’t done any more work on it, nor even sailed the model again. Among other excuses for not progressing it, such as lack of time, not the right conditions, more important things to do etc, the idea has morphed into something else which is not quite so proa related…...
hey, have you got anywhere with your sail?
I haven’t done any more work on it, nor even sailed the model again. Among other excuses for not progressing it, such as lack of time, not the right conditions, more important things to do etc, the idea has morphed into something else which is not quite so proa related…...
Where is the front?... 😉
That is a good question Laurent. The boat travels crabwise on a diagonal, say 20 to 30 degrees off the longitudinal axis on either tack. Hence the three small triangular planing hulls of which one side will be roughly aligned to the direction of travel on either tack. Each hull has a rudder, the rudders on the two aft hulls are set up to align with the direction of travel and swing though about 60 degrees. The forward rudder rudder has a bit more travel and is used for steering. The rudders are the only major moving parts, apart from the forward sail assembly, which is used to assist steering through tacks and may not be necessary.