Hello Doug… I am actually thinking of a bermuda rig, not a una rig. Taking a rather truncated broad topped mainsail, not unlike a windsurfer, the mast proper (4mm) would extend to a little above the top of the sail luff. The ‘twitcher’ would extend beyond that and be tied to the rear top corner of the sail/carbon top boom with a line and probably a bowsie. The twitcher could probably sit in a tight fitting silicone tube in the head of the mast so that it’s length is easily adjustable.
Kicking strap tension (not main sheet) will be limited by the tension imparted on the twitcher above of course. Shortenning the twitcher would stiffen it. I think this would give the kind of effect some people are looking for… I would have to build it to find out though. I hope this clarifies my own thoughts to you.
As to carbon forestays… as Angus knows I have been sailing one for awhile now and it works very well. The forestay head is loose in an open bearing tube and having found a way to hold the bottom of the forestay without a jib swivel I now have zero forestay tension as far as the mast is concerned. In a strong wind, hand held, the 1.5mm rod carbon forestay I am using stays arrow straight. This will become a part of the new ‘Siren’ design so I can’t tell it all just yet!
With the twitcher idea though the main mast need not bend to dump mainsail head power.
I think that you’ll find that the ‘inefficiency’ of gaff rigs is based largely on historic strength of materials problems. Tall narrow sails are more eficient to windward than short fat ones. Trying to control twist of wide-topped high aspect sails requires huge leach tensions, even if we are not trying to resrrain the cavortings of half a pine tree in the form of a gaff. Add the wight of said pine tree may up the mast and the case for a Marconi/Bermuda rig is pretty compelling. Result is that gaff rigs are traditionally of pretty low aspect - and hence fairly inefficient to windwad. There is a device known in England as a ‘vang’. It is quite different from what you in the US call a vang. Essentially it is a control line from the end of the main gaff used to control how far off it swings - essentally a kind of upper main sheet. The practical implications of making this work full size were such tat it never really caught on.
Ian,
I assume you are referring to fitting a sleaved luff around a curved mast?
Your unarig certainly has a very appealing look, but I decided to built mine with a simpler straight mast (1/4" wood dowel) and no tensioning line. Thus,
the sleaved luff becomes an easier proposition. As Brett previously pointed out, his design allows lateral bend via torsional twisting of the boom, hence no actual bend of the spar itself. While this type of bend offers the opportunity to simply spill air as a means of reducing heeling moment, it doesn’t directly affect camber or twist in order to depower the rig.
To demonstrate this type of bend, I took a few pictures of my Footy balanced roughly at the CB with a 115g weight at the masthead, just enough mass to lift the 175g bulb off the table surface:
1)“Bow on” shot, showing lateral bend of the rig. Note that the mast itself remains straight.
2)from above, showing the boom configuration which is allowing the bend. The strength of this could be fine tuned to obtain the desired amount of deflection.
3)overall image of the test layout
This rig type is much better on these small boats than one might expect.
when hit by a gust the whole rig deflects as per Bills photos. It either stays like this or whips back to straight when the gust eases.
So in effect it is a shock absorber which helps you keep the little boat on its feet. It can take some time to perfect the ammount of stiffness in a rig but it is very magic when you get it right.
Stiffness in my case is varied by either adiing more glass or sanding some away.
all very #8 wire as we would say here.
The outhaul in my case is also the downhaul and is in the form of a single line.
I also use a wire mast tip crane which is free to rotate,hence allow the sail to twist off…a fixed thinner wire could be used as a automatic depowering device.
I recognize that this is a very old thread than hasn’t been posted to in almost a year, but reading about shrouds triggered a memory.
The shrouds on ice boats are used to rotate the mast, so the mast and sail form a continuous airfoil. The masts are foil shaped and long in the fore and aft direction. The shrouds and forestay are attached say about 50, 60 percent of the way up, either to the front of the mast or a fitting that projects from the front. The shrouds are a bit slack and the mast can rotate freely. With this setup the mast usually manages to rotate into a line with the sail and create a fairly good continuous airfoil.
Am I going to use this method? No, I really like the Mac/Dunedin rig, but it’s nice to think about and realize that it works on “boats” that occasionally have topped 100 mph and normally sail at four or five times wind speed, tacking downwind with the sails close hauled.
I’d love to try it, (ice boating) but I think I will have to wait for global warming to subside a bit.