Equilibrium rig,how its done at the bottom of the earth.

Footy Class General Discussion (Footy)
112

Brett,
After my attempts to play around with the layout, I’ve come to totally agree with the above & appreciate the design you have evolved. As others have said, the rig has to be closely matched with the hull’s characteristics & you have done a great job.

Neil,
Good points about pivot rake issues. Actually I discovered that some of my downwind control issues were because I had a slight forward rake in the mast tube which made matters worse. So the rake I’ve introduced will be pretty minimal, but still enough to cause a 1" shift in CE towards hull C/L when running downwind (compared to what it had been previously.)

Gary,
I also look forward to hearing more about mast rake trials. I’m a bit surprised that raking it forward caused increased weather helm, I would have expected the opposite.

Getting ready for the trip to Sheboygan. Hopefully I won’t be totally embarassed by Graham, Angus & others. Looking forward to a fun day of racing, as up until now there has been no basis of comparison.

113

Bill, Gary & all - Bill, with my swing rigs on both M class and 36/600’s I angle the mast pivot forward about 3 degrees as standard practice. The reason for this is two-fold.
First, my masts are curved under compression from a stay running from the masthead crane to the aft part of the main boom. This compression can be adjusted to alter the camber of the sail, full for light winds or flattened out when the wind picks up. The curve of the mast shifts the Ce of the main sail aft in comparison to a sail on a straight mast. Angling the pivot forward (for rigs 75 to 85 inches tall) allows the pivot point to be father aft in relation to the Ce of the whole rig than if the mast (at the deck) were perpendicular to the waterline. The effect of this forward angled pivot is when the boat is becalmed with the rig cracked out from close hauled the mast weight (being forward of the pivot axis) helps to pull the sails out. Because the rig is canted slightly forward the weight of the sails insure that the camber is filled out and ready to react to the next puff.
Second, on broad reaches and runs having the rig angled forward enhances acceleration and to a certain degree mitigates diving (on 36’s its most noticeable). The forward angle encourages flow over the sails earlier because the wind is deflected more easily over the top of the leaches. With most conventionally rigged boats downwind sailing involves trapping the wind in your sails. The swing rig is essentially a uni-rig with two sails. Single sailed dingys rarely sail dead down wind. Reaching is the fastest point of sail for a swing rig as it is for a windsurfer. I often sail with my swing rig trimmed beyond 90 degrees to the centerline, particularly if I’m trying to preserve starboard tack advantage as I approach a mark. The curved mast is is under compression, which makes it stronger fore and aft. Therefore I use a lighter carbon mast than I would with a straight mast. The mast is more springy side to side though which deflects some of the energy from the type of gust that would send a boat diving, giving me a second more to react.
The rig under discussion here has a similar spring effect which maybe why reports indicate that the boats are more easily handled downwind, and why angling the pivot forward gave Bill control problems. Since the rig deflects under load, and Footies accelerate really well anyway, angling the pivot axis forward might not help achieve the positive effects mentioned above.

Gary, it sounds to me like the pivot point is in the wrong position. You shouldn’t get weather helm by altering the rake angle of the mast forward in relation to the boom. My guess is that the pivot point (and hence the whole rig) has to be moved forward. If you located your mast pivot position using the location provided for the mast position of a conventional jib/main type rig then you probably need to recalculate your equilibrium rig’s pivot point. The Razor design has a tendency to round up in gusts anyway. That is probably due to its sharp bow and hard chine (and easy to build) construction method. Every boat has its quirks, thats something everyone has to adjust to with their particular hull. But finding the “sweet spot” of rig location may require more holes in your deck.

114

Niel,
Thank you very much for sharing your swing rig experiences in this thread.
It is very much appreciated by me and I am sure the others.
Seems to me that there is a a definite relationship between these 2 rig types.

My rig is really just a simplified swing rig for use on smaller yachts.

115

Thanks for the comment Bill. Made me doubt myself, so went back to the water & guess what, You were right. Sorry for the duff gen guys.

The incident sitting head to wind, I remember now, was with a high aspect ratio sail I tried where the mast was 700 mm long, foot 270mm, pivot 100 aft of the mast. (The hull carried this sail amazingly well, in light airs of course). The top of the mast was behind the pivot point, which put the C of E too far back & the boat weather-cocked, stalled, & wouldn’t steer into wind. Tipped it forward, problem solved.

The rig I used to retest was mast 500mm long, boom 300, pivot 100 aft of mast with a sail very similar in shape to Ian H-B’s Tane. I did as I suggested previously bending the wire @ the top of the pivot, reducing mast rake angle & moving the C of E forward.

Initially the boat pointed higher & weather helm was reduced but never completely went away. The point, to which I incorrectly referred to previously, happened quite suddenly, the result being the best & virtually only point of sail was a broad reach. It was impossible to turn through the wind as on a normal tack & even needed considerable motion to accomplish the turn down wind where control was poor. I only did this manoeuvre once before tipping the rig back to a better position.

To further try to reduce the weather helm to the hands off stage that others have achieved I began to trim the leech of the sail gradually. After three trims I ended up with a triangular sail, no reduction in weather helm, & diminished performance, I presume due to reduced sail area & loss of efficient sail shape.

I aborted any further experimentation that morning as I was now quite wet from the rain & had very little sail left to play with.

I hope this has gone part way to restoring any credibility I may have once had. Keep at it men.

I look forward to hearing how things went at Sheboygan?

116

Thanks for the advice Neil. I had already resigned myself to the fact that there will be a few more holes in the deck.

117

Hi has anyone used 2 booms-a stiff one attached to the wire with a flexible one (1mm CF) attached above it with sliding adjusters such as cable ties which could be moved along to adjust its stiffness according to the wind strength?? Brian

118

nearly 7000 hits to this thread…amazing.

119

Brett
Interesting that you bring that up. I was just about to post a couple short videos showing the flexing of unarigs using the same sail rig & a floor fan, but with different wire thicknesses. The 1st video is using 2mm “music wire” from the hobby store, the 2nd is 1.6mm (1/16"). This is not intended to be rigorously scientific, but gives a very clear visual comparison with the different wires. The floor fan created a rather turbulent air flow, but you can really see the working flex:
http://www.youtube.com/watch?v=3rexyNQ8DTM

http://www.youtube.com/watch?v=tbdnUSWmdF0

Brian,
Good idea, have you tried it yet? It would make a big difference where you decide to connect your sheeting line to the boom.

Bill

120

I did a little experimenting of my own this weekend. I wanted to see if a really tall rig would work in light winds. I also wanted to beat a friend’s very well executed Razor with a standard Bermuda rig. In the past I could catch him downwind but got killed upwind.

My first McRig followed Brett’s suggested size. This one is a bit taller.

Initial specs
Luff 710mm
Boom/foot 310mm
Pivot 100mm from luff

It worked ok in very light winds but developed a crease whenever the wind picked up a bit. It pointed but not as well as my competition. After a little experimenting I decided the top of the mast was too far back and the clew pulled too much. I bent the wire to bring the mast directly over the pivot. Now the sail needed reshaping, break out the scissors and trim the foot to a better shape.

Now the luff is only 680mm. Much better, no crease and pointing at least as well as the Razor. The winds had picked up a bit by now (we sailed bigger boats for a while too) and the tall McRig still performs great. I can beat the other guy upwind and down. It handles 8mph reasonably well, the bow digs in some but not too bad. It’s great at finding wind when the other boat is becalmed.

I thought this experiment would have me trimming the sail down to the 550mm range but it surprised me. 8mph is probably the max for this size but now I have a light air rig. The next improvement will be better mylar. I need to find 2 mil. This wrapping paper is just too thin, maybe 0.3-0.5 mil (Angus thinks I stripped a NASA probe). The boom could be a bit stiffer too.

121

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?

122

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!!!

123

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.

124

Thank God that misunderstanding’s disappeard. Blame Graham (I think:sly: ).

125

I know a mil is 1000th of an inch… :confused: 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

126

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.

127

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.

128

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.

129

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.

130

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.

131

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.