A good place to start on sail aerodynamics is with Arvel Gentry:
and then chase the links.
Cheers,
Earl
I’m not sure if this is what you’re driving at, & I certainly have no objection to a paneled sail with broadseams or luff curve. But since I’m kinda lazy, I’ve found that a decent camber can be obtained with a “flat” panel if it is used in conjunction with a pocket luff sleave of sufficient width that will induce draft. Refer to the attached photo of an early version of Brett’s unarig concept. The amount of camber is easily adjusted by outhaul tension. A similar effect can be seen in the photo of a flat panel sail used with a composite wingmast.
As far as demanding scientific proof, I would suggest that the long successful history of paneled sails on the racecourse offers substantial empirical evidence that goes beyond simple anecdote.
For panelled sails in the model context and for some fine craftsmanlike construction, have a look at Making Model Yacht Sails by Larry Robinson <larry35@foxinternet.net>. Published in Seattle by Ragged Symmetry, which is Bob Wells. The email address for Larry is old and may not be current. I don’t hve one for Bob.
Russell
I would like to point out that at least from the photo posted that Bill K.'s sail has a fairly straight roach which is controlled at both the head and the foot. This type of sail mounting allows the twist in the sail to be controlled at the head of the sail. Triangular sails don’t have this type of control at the top, so some amount of induced camber is needed to maintain the airfoil shape throughout the sail for efficient upwind sailing.
The camber in Bill’s sail is a bit far forward to my eye, which is typical of sails that have luff curves that don’t match the spar on which they are mounted (paneled or flat). Mounting flat sails with a pronounced luff curve may seem like an easy solution to create camber, but usually the maximum camber ends up too close to the mast. To the untrained eye this might appear acceptable but if the sail in question is a mainsail it will suffer from backwinding sooner. If the sail is a jib or a unarig like the McCormack rig then the sail will not point high because it will start to luff before it reaches the optimal angle to the wind. Neither of these traits are desirable in a racing boat, particularly since most of the distance sailed in a triangular course is upwind.
Inducing camber by using panels is a much more effective way to create an airfoil that is not dependent on guessing how much luff curve to cut into the sail. Thats why the people that race in other classes (both big and small) use paneled sails.
I would like to point out that at least from the photo posted that Bill K.'s sail has a fairly straight roach which is controlled at both the head and the foot. This type of sail mounting allows the twist in the sail to be controlled at the head of the sail. Triangular sails don’t have this type of control at the top, so some amount of induced camber is needed to maintain the airfoil shape throughout the sail for efficient upwind sailing.
The camber in Bill’s sail is a bit far forward to my eye, which is typical of sails that have luff curves that don’t match the spar on which they are mounted (paneled or flat). Mounting flat sails with a pronounced luff curve may seem like an easy solution to create camber, but usually the maximum camber ends up too close to the mast. To the untrained eye this might appear acceptable but if the sail in question is a mainsail it will suffer from backwinding sooner. If the sail is a jib or a unarig like the McCormack rig then the sail will not point high because it will start to luff before it reaches the optimal angle to the wind. Neither of these traits are desirable in a racing boat, particularly since most of the distance sailed in a triangular course is upwind.
Inducing camber by using panels is a much more effective way to create an airfoil that is not dependent on guessing how much luff curve to cut into the sail. Thats why the people that race in other classes (both big and small) use paneled sails.
I am preparing to make my first McRig and have been following a lot of posts. The post with a lot of interesting comments is 4 months old with Neil posting last on 2/19/08. Have any of you old timers come up with anything new since then? I hope to try the rig on a Razor and B-2 from Brett which I have just started building.
One of the previous posts had the following data:
Luff Boom Pivot from Luff Wind Author
710mm 310mm 100mm 0 to< 8mph tallastro
It would be helpful to see this kind of data extended.
Right now I am considering 1/16" stainless Z wire and 1/8" carbon booms. I have some Icarex PC-31 from a kite shop here in Michigan and will be using this for sails.
Any suggestions or recommendations greatly appreciated.
Best Regards, FastFrank
Brett,
I want to thank you very much for what you have done by positng your sail design on this thread. I also want to thank you for all of your work on this project and on others.
Me being a first timer, your sail is something that I feel even I can do. That's very important for the sport all round, to have something a first timer can feel confident that they can actually make their first try work.
Footy's should be used to promote RC sailing. There are simple designs for hulls, like the Razor (thank you Bill for that design)...... and now there's a simple sail to go with it...... I think the AMYA could make good use of growing the sport by using Footy's simple design to promote first time RC builders (especially young people). I really hope the simplicity will remain and not get swallowed up by all of the carbon fiber, fiberglass and plastic of our societies.
Thank you also to all of you who contribute to threads like this. It forces me to think and to try to learn, which is necessary for the sport and for my old noggin. I may never do more than finish my footy and sail on my pond, but I will learn a new hobby and when someone wants to know about it, I will refer them to this forum first and foremost.....
Keep up the good fight........ :D
Thank you,
Luff Attachment - a new simple method
The basic McCormack Rig uses a sleeve luff, which works very nicely. However, it is difficult to optimize the luff curve, and I have using a straight luff. Some pseudo curve can be generated by putting a spacer inside the sleeve at the tack, and using a hook halyard to pull the head forward.
Others (e.g., Scott Spacie, who always wins) have used a conventional luff attachment, using strings a few inches apart to attach the luff to the mast. This affords the ability for exact luff curve adjustment, but it is tedious. It also takes advantage of the mast bend to flatten the sail in stronger winds.
I built a sail a few weeks ago, intending to use the above method. The string attachments were not perfect, but I sailed it anyway in our Thursday races, despite some wrinkles. It wasn’t terrible, but obviously needed improvement. While taking out the individual attachment strings, I noticed that it took a nice shape without the strings. So I sailed it this week without the strings, and it worked very well (a second and a first). It must be noted that this was not an ordinary simple luff. It was reinforced with a strong continuous tape from top to bottom, which probably acts like a jibstay. A strong downward force was applied at the tack. The sail naturally moves slightly to the leeward side of the mast, so the mast presumably doesn’t cause a separation bubble on the leeward side of the sail. This sail has the advantage of being very easy to build, however it doesn’t flatten automatically with mast bend.
I was wondering why the carbon tows with epoxy is being used to attach the boom. Why use such a stiff mechanical bond?
Wouldn’t 2 or 3 pieces of shrink tube be enough? It would allow easy replacement of the boom or possibly even allow the boom to be moved to adjust the foot.
thx
Common practice in UK is elastic bands.:graduate::devil3::zbeer::zbeer:
i go even easier than that and use a couple of 5mm slices of fuel tube to bind the boom to the z wire
In true kiwi style I use Electricians PVC tape for “lash up” experimental rigs.
This thread has had nearly 21 thousand page views now,I am very flattered that so many of you are using my simple rig design.
I would like to update some of the information here if I may.
I see many of you have the pivot to far aft.It is natrual to want to do this as it is better downwind as the rig is more balanced.This has an adverse effect upwind though and I would recommend no more than 25% of the sail area in front of a vertical line of the pivot.
As for the position of the pivot /mast step…well this depends on many things but what i have noticed is that in most hulls the mast step shoul be approx 40-50mm in front of the leading edge of the fin.
If you have trouble with the boats having weather helm(boat turning into the wind on its own when sailing upwind) then keep moving the mast step fwd until you acheive balance.if you experience lee helm then move the mast step aft.
Z wire construction…I now use stiffer wire,2-2.5mm s/s wire seems about right.
sail shapes.
I belive the sweet spot is in the 15% - 18% range for the camber of the sail at the foot.this is quite a bit fuller than on larger models and will give you the power you need to drive through waves.
sail twist…much more than other models also…not sure why(surface effect of wind shear??)
but we have found here that quite a lot of twist is fast…if only to give a large sweet spot to making sailing easier.
Hope this give those of you messing with these rigs a bit more idea of what I belive to be fast,this rig design works like no other and has some special challenges,the rewards are there though when you get it all working right.
Good luck to all footy sailors for 2009.
Brett
Thank you Brett and many others for a most entertaining thread - it has kept me enthralled for many evenings, also visiting quite a few of the links and studying the photos attached.
I am engaged in a challenging project - design and construction of a model dhow, roughly 100cms long. My plan was to rig it authentically, and to that end I located many photographs and diagrams, concentrating on the settee and lateen sails, how to rig them and how to go-about. This final manouver, essential in a tacking sail boat, is most challenging if one is to avoid having the sail pressed against the mast on the opposite tack. To avoid this, I found it necessary to go about via gybing, pull the yard (that holds the suspended mainsail) foot in to the base of the mast so that it can shift to the opposite side of the mast, then pull in the opposite mainsheet. Translating all of the above to Radio Control was giving me nightmares, then I stumbled upon Brett’s wonderful McRig-Unarig concept. It looks like a lateen sail, and perhaps could be adapted to suit my design.
I will post when I have completed construction, and begin trying out a unarig similar to Bretts.
Keith:cool:
I know it will not help your challenge but here is my solution.
If you wanted to avoid interference between the sail & boom, you could use the offset boom design, like the Hoyt rigs use (www.garryhoyt.com). It will probably look odd, but it would work.
Sorry just found a later version with a better head fitting shot.
Thank you Ian - that is fantastic!
Here is a link to a construction blog I have published for the dhow so far.
I will mention both you and Brett’s Equilibrium Rig there as I think my pathway will go along those lines. My experience with R/C is not strong, so I want to keep the controls as simple as possible, and this seems to be the trick I have been looking for.
Cheers, Keith
With regard to tacking a conventional lateen rig, it is done every day by thousands of Sunfish. the sail sets a little differently on port vs starboard tack, but it still works reasonably well.