Just so we’re clear, 2 mil = 0.002 mm? In England that means 2/1000 of an inch". Or am I completely up the spout?
Angus
In the USA, we refer to a “mil” as .001 inch, not mm, so it’s the same as in GB…001 mm would be very thin indeed! I agree that 2 mil is a nice thickness to work with, at least with Mylar.
Cheers,
Bill
ps; below are some pix of my recently completed Kittiwake, using a “McRig”
unarig. Preliminary sailing results are positive- it seemed to balance very well.
pps;I was very impressed with the KW kit, both in term of design as well as execution. Nice job Graham!!!
My understanding is that mil = thousands of an inch. I’ve used 4 mil mylar, way too heavy. I’ve seen 2 mil on a Footy and it looks like a good weight. I’m guessing on the wrapping paper thickness, it’s very thin though.
Thank God that misunderstanding’s disappeard. Blame Graham (I think:sly: ).
I know a mil is 1000th of an inch… 
but not until after I told a guy planning on using 2 mil copper foil on a wood routed slot car track that it was WAY too thick!
Looks good Bill, a rather attractive combination. Don’t forget there is a Siren hull here for you which will be even nicer I think.
Graham
Today I tried a McCormack rig for the first time, on the Pepsi Torpedo. For convenience, I sheeted the boom about 3" aft of the pivot. I kept getting a weather helm, even after bending the mast more forward. I finally realized that sheeting in the boom was pulling the mast aft, creating the weather helm. In the end, I bent it forward again and limited how far the sheet could come in. This seemed to work. I was using a big sail (18" luff, 16" foot) with a thin (1/16") Z-rod. It appears that the forward sheeting will solve this problem, although it may create a different problem. Another solution is a stiffer Z-rod (3/32"), which is probably the right thing to do with this big sail. We will find out the answer next week.
Walt, you shouldn’t be tightening the mainsheet down so tight to the centerline that it pulls the top of your mast back. You want the boom cracked off from the center somewhere between 5 and 7 degrees for upwind work. This position will also help alleviate the weather helm you where experiencing and make your boat go faster too.
Please read my earlier comments about forward sheeting and resist changing over until you’ve tried the above and swapped out the Z wire for a stiffer gauge.
Walt,
Niel is right about overtightening the sheet (free advice from a multiple national champion is like gold).
My first Footy had forward sheeting on its swing rig just because the Pitou design i scaled it down from had it that way. It worked fine, but the reason i abandoned it was that these boats often stick their noses under water and I wanted to have the sheet exit as far aft as possible.
I use 3/32 wire on my McRigs and it works fine.
I’ve been thinking about ways to control the bending of the boom on my McRig. I have a rear sheet attachment mostly because I didn’t want to drill more holes and it’s working well for now. I was looking at how much the 1.3mm carbon boom was bending and thought “I wonder if I can make the stiffness adjustable without breaking Brett’s simplicity concept?” Sometimes I want it to depower by twisting and bending. Other times I want it twisting only.
Here’s my idea (see drawing). Move the sheet attachment from a ring I have on the boom to the aft of the boom. Still running through the ring so my sheet helps resist boom bend while keeping the same servo movement/boom angle ratio (solid orange line). A grommet or other slide could move the attachment forward and allow the boom the bend more easily (dashed orange line).
Does this make any sense? Or should I just thicken up the boom to make upwind sail trim controllable and let the twist do all my downwind depower duty.
Walt,
Another way to minimize the problem is to elevate the sheeting fairlead location closer to the height of the boom connection point. This will limit the amount of “down” force, as well as creating a more efficient angle. The force vectors will be oriented mainly for lateral, not vertical motion. The attached image is a bit oversheeted in order to demonstrate the point.
Tallastro,
Interesting concept, but I wonder if the very narrow angle will have much effect in controlling the amount of boom flex. I like the idea proposed by edb in post #113. Personally, I’m inclined to let the aft boom stay fairly stiff so that leach tension is maintained for upwind work, while relying on twist of the Z wire to spill air as needed.
I have found a simple way to attach the boom to the Z-rod. The boom is a 3/16" diameter wood dowel. A vertical hole is drilled at the forward end to fit the Z-rod. The forward end of the rod is put through the hole and then the mast is slipped onto the rod, with the boom staying above the horizontal section of the rod. The aft end of the Z-rod is taped to the boom. This works because the forward end of the boom wants to press down into the rod and slip off to the side, but the hole keeps it aligned. The part of the boom near the pivot wants to go up, and the tape keeps it down. The Z-rod is relatively free to twist as needed. I have also used tape at the forward end, just to keep things in place, but there is little force on it.
The dowel can be tapered if desired to get the proper spring effect. Other diameters can be used, but it has to be wide enough to drill the hole without breaking.
This scheme has been tried out, and appears to work OK.
I have read every response to the Equilibrium rig and I am going to use the design on my Razor that I am building. I understand the concepts and this is so much easier than anything I have seen to date. This is my first sailboat and I would like to know how to rig the line to control the sail. A photo or diagram would be great. I know this is basic stuff but for a newbie it is essential.
Thanks
Larry I
Larry,
The AMYA website has an excellent construction guide for the US 1 Meter class. Go to
http://www.modelyacht.org/us1m/pdf/us1mpt4.pdf and scroll down to page 57 or 58 & there are diagrams for single arm sail servos that could be adapted to work with the single sheet line you’ll need to use on the McRig. There is more than one way to do this, but the important thing is to get full range of motion of the rig, using good geometry for mechanical advantage.
Good Luck,
Bill
Thanks Bill,
This is gold, and I need it also.
Pete
Thanks Bill,
I appreciate the help. I bought the wire, carbon rod and tube today at Hobby Town so I am ready to give it a shot. I have some .75oz sailcloth coming as well with which I will be making the sail. I will let you know how it turns out once I finish.
Larry I
Just a thought.
Rod
Interesting, let us go away and do some geometry.
:zbeer::graduate:
I have long wondered just what benefit might be gained with panelled (and expensive) sails in model yachting. I can visualize no benefit when using a loose-footed sail, as we do in models, while the only benefit in full sized yachts with sails attached along the length of the boom, is to compensate for the needed transition between the fully curved belly of the sail and the straight section along the boom. Some yachts have a zippered extension to the foot of the sail; does anyone know of specific effects and/or improvements due to this arrangement?
Are there any published wind tunnel data dealing with this aspect of sail technology? I wonder if this is all proprietary information, and therefore secret. Perhaps I’m looking in the wrong places but all the wind tunnel photos that I have seen are made with metal sail replicas.
Rod
Paneled sails help maintain camber from top to foot on model yacht sails. Generally, the sail twist on models is much greater than is used on larger craft, the reason being that we must set our sails for the average because we are not on board to finely adjust the trim. Forcing some camber into the sail helps control the twist off of the leach of the sail. Without it a sail looks more like a funnel than an airfoil.
The strange wrinkles that I’ve seen many posts complain about form when a sail doesn’t fit a rig well. When the boom vang is tightened to control the leach on non-paneled sails these wrinkles typically emanate from the base of the leach. That is because the sail is a flat panel. The line of stress will extend along the leach and any roach (the curved edge on the trailing edge of the sail) will tend to fall off to leeward. There may be other lines of stress that point toward the midpoint of the sail along the mast. This is caused by the stress line trying to adjust to fit a flexible spar.
A cambered sail doesn’t require as much tension to control twist in the leach because its shape already incorporates some amount of tension along the leach. In most cases cambered sails will adapt to spar flex to some degree. Tension along the luff may have to be adjusted as well.
Footies, like other model boats, would benefit from paneled sails. Three panels (or four for very tall sails) would be more than enough for our purposes. Making paneled Footy sails is a sight less difficult than hull construction.
I have seen all the explanations, and you present them well in your reply, but I still would like to see some photographic or tabular proof. It does still seem to me that a loose footed sail, properly tensioned both vertically and horizontally at the end of the boom, should have a shape that is more consistently aerodynamic than one that is distorted in its lower portion by panelling, such that the sail is “baggier” in its middle than it is at the top and the bottom.
As I said, photographic evidence? Not wanting to be obstreparous about it, but I really haven’t seen “proof”!
Can you refer me to any evidence? I was trained as a scientist, and find it very hard to accept “anecdotal” evidence.
I am very interested in any answer that anyone cares to give.
Rod