MeGrimm,
It looks to me that you should set the pivot point further forward.
I also think you will need something about 3mm dia for your torsion boom.
You have quite a bit of sail area to handle there.
That should make the Stowe crowd sit up and look, Martin. and you get rid of the shrouds as well.
That’s now 2 of us (and probably Angus as well;) ) thinking of scaling up to MM/505 size.
Thanks Brett. Unfortunately 3mm stainless is a bit harder to come by and the pivot point is pretty much set unless I do horrible surgery :scared: But the potential benefits are great and I havn’t given up yet…
McGrimm,
Yes, it would change the location of the sailplan’s Center of Effort (CE) in relation to the hull & keel, depending on whether or not you angled the wire to oppose the tilt of the pivot in the hull. I’m not sure what would be gained by doing this, but perhaps it could be an advantage. The concern about the boom scraping on the deck could be easily avoided by having it angled up to compensate. Also, the angle on the forward end of the wire where the CF mast rotates will affect all of this. In the interest of keeping things simple, I think I’ll continue to use a vertical pivot
Shown below are pix of these orientations when the hull pivot (drilled dowel held in vise) is slanted back
about 20 degrees:
1)parallel with hull centerline (C/L), wire angled 20 deg to oppose pivot
2)perpendicular to hull C/L, wire angled 20 deg to oppose pivot
3)parallel with hull C/L, wire square to boom
4)perpendicular to hull C/L, wire square to boom
One item of interest is #2, which shows that the CE will be closer to the hull C/L when running downwind. This has the advantage of having to use less rudder to resist yaw moment. Of course, CE will still need to be somewhat offset from the axis of the pivot rotation or the sail won’t swing out properly
Hi Bill. Yes that’s my idea. Get the sail CE closer to the CL of the hull to improve running efficiency. Also, if (due to unusual design) you have a pivot position in a place other than ideal for the sail CE, you may be able to fiddle it by adjusting pivot rake fore or aft. I don’t know, it’s just an idea. I’m sure there could be other advantages too but without much experience in sailing I don’t know how realistic those ideas are!
I’ve been thinking:nuts:
A way to use a single adjustable torsion “Z” piece.
a) swap the deck leg of the “Z” piece for a separate “L” piece
b) extend the boom forward to about half way to the mast
c) Make the bottom leg of the “Z” about 10 mm and fix into hole drilled in boom.
d) fit a slider to boom and 10 cm length of “Z” piece with set screws to clamp both.
[
](http://img215.imageshack.us/my.php?image=macriggk9.jpg)5.imageshack.us/img215/8738/macriggk9.th.jpg]](http://img215.imageshack.us/my.php?image=macriggk9.jpg)
Thanks to ImageShack for Free Image Hosting
Then by moving the slider forward or aft and locking, the torsion can be adjusted by about a factor of 2.
The photo ain’t a working version;)
One ahead of me and one behind. Using more or less the same trick you can adjust the bendiness of the boom.
Hm. Interesting how two of the benefits of Brett’s rig design, it’s simplicity of construction and lightness, are already being eroded by design ‘improvements’.
Whoops, I’ll keep my trap shut in future:tapedshut
Martin,
I don’t think you need to feel that way about it. Who knows, perhaps it’s this sort of innovation that could open up new possibilities?? Personally, I prefer to keep things simple (KISS concept) & play around with only 1 or 2 variables at a time until they’re understood, but I have no problem with someone else trying something new as long as we all can learn from it. It could very well be that my thermoformed luff sleave idea could be seen as adding unnecessary complexity.
Speaking of torsional twist, here’s a video of my most bendy rig, the one with 1.6 mm wire. The wind was puffy & shifty, but one can clearly see several instances of rig flexure, including on downwind runs. The rig felt fairly balanced most of the time, but would show weather helm in a gust so I may try moving CE forward on my next sailplan. This was taken at the pond across the street from my house & I was trying to aim the camera at the same time, so the camerawork & sailing were a bit erratic at times.
http://www.livevideo.com/video/B24FCF82E25043369852E479783E5EEE/footy-rc-sailboat-at-pond.aspx
Must be quite amusing for Brett to sit back and watch us all in a flurry of developing ideas and experiments reinventing the wheel he’s just spent so long getting to where it is now!
There is an intrinsic problem in the Brett design so far. Both the adjustment of the Z-bar torsion and of the boom bend by using a wooden boom and whitling (as I suggested somewhere) is a one-way process - one wind too much, one stroke of the sand-paper too much and you go back to the beginning. I think it is wortgh investigating whether this short-coing can be overcome without excessive weight or complexity.
Martin. I wasn’t going after you, merely musing. As soon as I have time to drive the 40 miles to my hobby shop to get carbon tube and rod I’ll build a couple of ‘Dunedins’ and with permission wil try your slip joint on one of them. New light, skinny hull is almost ready to receive them.
Some thoughts on the McCormack / Dunedin / MacRig, rigs. :graduate:
There are at least three main elements of the concept that directly relate to the tuning of each set up.
-
The thickness and tensile strength of the wire Z bend main element with it`s associated wrapping system.
-
The thickness/stiffness of the boom material.
-
The stiffness of the mast material.
Some of the other variable factors that will effect performance amongst constructors are;
weight of sailcloth material, area/cut of sail, leech tension, rig mounting and pivot system, overall weight of rig, ballast ratio, hull form and others which you will think of.
Suffice to say that there may never be two rigs the same.
So, how can one ensure that the design chosen for yourself is the most suitable one? You can`t. :devil3:
“Suck it and see” is proberly the best method of development for everyone.
We are fortunate that with this forum those who choose to do so, share their success and failure so we may all learn what works, or not, for them at least and learn from that.
The nature of modellers is that we are all fiddlers, that is why we model and is IMHO to be encouraged.
Brett was generous enough to release his baby to the world (for better or for worse) and subsequently has now lost any control of it.
He most proberly could not care less as he has perfected the rig for his own use as witnessed by the posting of a new "Intergalactic Record " for the internet course.
Anyway back to my treatise.
If you use a solid wooden boom instead of the wire element as per my photo elsewhere then it is the mast flex that would be the main factor in the tuning.
If you would use a solid mast fixing and a flexible boom you will obtain a different result again.
What I am trying to say is that the basic concept of an “Equilibrium Balanced Rig” can be approached in many ways.
There are three million stories in the naked city, this has been one of them…:lol: (that shows my age)
I agree with Ian - an unusual event since we often approach things wih rather different philosophies!
The more I think about it, thge more I realise that the |McCormack rig is a very subtle and very tuneable device. I am sure that tuning in reality will be by suck–it-and-see, but anyone who is prepated to point the way by theoretical analyses should be encouraged.
well said gentlemen!
Ian,
You’ve covered all the bases with this & your other comments, thanks.
And I think the order you’ve given properly reflects the relative importance of these variables. I’ve had very positive experiences with the twisty Z section’s ability to spill air & keep the boat manageable. Item #2 will also be very interesting to play around with. A bendy mast (#3) may have difficulty accomodating the luff sleave without causing big puckers & wrinkles, and may be simply duplicating effects that can more easily be had with #1.
Martin & Angus have raised issues with trying to create a “tuned” amount of flex in a rig. One of the beauties of Brett’s Z wire is that it’s so easily made, & therefore multiple units can be produced for comparitive testing. Also, the wire flexure can be easily measured & duplicated. This will come in handy during testing, as well as when the “sweet spot(s)” is(are) determined.
In an earlier post I had noted some weather helm, but am now thinking that was a result of the shifty wind that day. I have since sailed in steadier air (about 10-12 mph) & found that I could turn off the transmitter & the boat would track upwind as if it were “on rails”. Maybe it was a fluke that day, but it sure felt nice.
Bill
Footy US 17
How about putting a ‘moratorium’ or sorts on the mods & discussion of the equlibrium rig? Or just not posting about what you are doing to it? It’s a good design as a start, and we should just sail them for a while, taking note of the thing you see, and then discussing them.
Just likethe Footy in general, there a number of designs, and people are sailing all of them, and eventually, people will get together to compare notes on design, and come up with something really good. Just remember that there are the moulded boats, and scratch-built ones of panels and strips.
thx
I will choose to ignore the comments made by our mate above.:mes::rasta::dunce:
During one of my early trials with the first version of my “Dunedin Rig” …(so called because it is based on the McCormack Rig but sufficiently different so that any failures will not be attributed to you Brett)… I had trouble with too much flexability of the wire Z bend and consequentially lack of drive.
Mark II version 3 is made using thicker wire and a 3mm tube boom.
Versions 4, 5, & 6 with various wire thickness and boom material.
More trials tommorrow and reports to follow.
The ballast ratio of your boat has a huge influence on the amount of power which is desirable to built into your rig and will vary with the wind strength as well.
Running downwind with the rig over the bow was an interesting look but it worked a treat. Hmmmmm?:snorkel:
…Ian’s comment (above) was made in response to one made by someone other than myself, which has since been deleted:
"How about putting a ‘moratorium’ or sorts on the mods & discussion of the equlibrium rig? Or just not posting about what you are doing to it?.. "
Ian,
I agree that the ballast ratio is a very important in how these rigs will work. It is literally “the other side of the equation”. It would be very interesting if you would weigh how much it took to deflect the rig 2" as I had done previously, just to have a basis of comparison.
Bill