A-frame Rig on 63' Cat
After I posted this reference to this vessel I made a hard copy of his 'Explaination of the A-frame Rig', and reread it several times. It is quite interesting. He makes a good argument for his configuration:
1) that allows for variable tack (sail) locations across the beam of the vessel
2) that might well be called a cutter arrangement
3) all roller-furling sails with no mast interference with leading edge of sails
I thought it deserved being printed out here,...or you could go to his website and see it along with the photos.
http://www.damsl.com/
Explanation of the A-frame Rig
The A-frame mast or bipod rig is my idea (tested on a smaller catamaran) with engineering and construction specifications by Malcolm Tennant and Anthony Stanton of New Zealand design team Malcolm Tennant Multihull Design (tennantdesign.co.nz), who designed the boat. Dave Pope, who was a member of the Tennant team, did the original construction drawings of this boat and has contributed a lot to development of the new rig. Dave now has his own design firm and is based in Whangarei (dpd.co.nz).
The rig was completed in December, 2006. It works very well, is quite efficient upwind, is especially good reaching and running, and is easy to handle because all sails furl and there are other advantages as well.
The boat was built here in Whangarei and originally had a wind turbine rig, a 50ft diameter, 3-bladed windmill, which drove two props. With that rig, the boat could sail straight into the eye of the wind. But there were some engineering problems and construction faults in the turbine, and then I came to realize that I missed the aesthetics and silence of cloth sails. The turbine rig was too mechanical, much like just driving a motorboat, always with some machinery noise and vibration, and I got tired of that.
The new rig, with the A-frame mast, makes perfect use of the boat structure and indeed enhances the strength and integrity.
Catbird Suite has 4 sails, so far, including 2 genoas (one off each bow), a staysail and a mainsail. The staysail and mainsail have 3 positions each, i.e., the tack of the sail can be attached on either side deck or on the centerline of the boat.
For short tacking upwind, staysail and mainsail are both used in their center positions, and are thus vertical, as in a conventional rig. But, unlike a conventional rig, the main, like the staysail, doesn't have a mast in front of it disturbing its wind and therefore is more aerodynamically efficient than on a conventional rig that has the sail running up the backside of a mast, which spoils the airflow onto the sail.
PHOTO
(Mainsail & staysail on centerline of boat, port tack)
PHOTO
(Mainsail & staysail on centerline of boat, starboard tack)
For a long, passage-making tack, I attach staysail and main to fittings on the windward side deck and use one genoa, the genoa attached to the windward bow, so I then have the entire width of the boat for sheeting positions, so no booms or whisker poles are needed as I go from close-hauled to reach. In other words, the sails are sheeted down to the boat itself, and thus the boat is the “boom”.
PHOTO
(Closehauled, starboard tack, using starboard, i.e., windward genoa and staysail and main in starboard sidedeck positions. Port genoa, furled, can be seen at upper left in this picture.)
In that arrangement, I can, with my innovative sheeting system, go from close-hauled right through beam reach and broad reach, with ideal sail shape for every position in between, but no booms, or poles, because the entire width of the boat is available for sheeting positions. The sail plan thus, with sails attached to windward side of boat, looks like a cutter rig that is heeling, even though the boat is not heeling. And she goes to windward very well, like 30 degrees off the apparent wind, and there aren't many cruising boats that can do better than that.
PHOTO
(Reaching, port tack, all sails on port side. Note that, even though the boat is not heeling, the luffs of the sails are “heeling”, i.e., are angled to the right of the vertical, from the port side deck to the masthead which is above the centerline of the boat.)
PHOTO
(Reaching, port tack, sails in same positions as in previous picture taken from different angle.)
The leeward genoa (in the case above, the starboard genoa, which can be seen, furled, at the upper right of the picture) can be added to this sail plan, and, when close-hauled, a whisker pole is not needed, with the clew of the leeward genoa sheeted just outboard of, that is, to leeward of the leeward mast. But easing the sheet to reach, in order to maintain ideal sail shape, requires a whisker pole, because then the clew of the leeward genoa would be out beyond the leeward side of the boat.
My sheeting system does require a lot of rope, but rope is much cheaper and easier to handle, and less dangerous and potentially destructive, and less heavy, than booms. The advantages make handling the rope worthwhile.
Why do they call purchase systems "purchase" systems? Because the more mechanical advantage you want, the more rope you have to purchase.
The geometrical advantage for sheeting of the genoa attached to the windward bow is especially great. Even on a beamy cat, with the genoa in the middle, i.e., between the bows, as in conventional rigs, there's only half the boat for sheeting range and, therefore, with a big genoa, you cannot maintain ideal sail shape for beam reach, and certainly not for broad reach, not without a whisker pole, because the clew of the sail is then out beyond the leeward edge of the boat, so the sail develops too much belly. I have the advantage of the entire width of boat for sheeting of the windward genoa, and one genoa is usually enough on any point of sail except dead downwind in very light winds.
PHOTO
(Broad reach, port tack, mainsail & port genoa)
PHOTO
(View of mainsail on broad reach, port tack)
Whisker poles would be of use dead downwind in very light winds for poling out both genoas at the same time, and covering the gap in the middle with the main and staysail. That arrangement would capture a tremendous cross-section of wind.
But my boat is so easily driven and my genoas are so big, that most of the time around here, one genoa is enough, and it's handy to sheet across the entire width of boat and maintain ideal sail shape with no need for a pole, and to be able to alter course drastically without having to worry about a pole, or having to fuss with it in the first place. Indeed, downwind with a good breeze, one genoa, sheeted right across the bows, moves the boat along very well, and it's quite amazing how much you can alter course, almost 180 degrees, that is, almost from beam to beam, without jibing. In other words, you can sail “by the lee” on one tack, with the leach of the sail becoming the luff.
I've provided for addition of whisker poles, with fittings on both masts, topping lifts, etc., but have not added poles yet.
The staysail is my storm sail and has very good sailing balance, being directly above the keels, whether in a side position or on the centerline.
PHOTO
(Staysail in starboard side position)
The A-frame mast structure is incredibly strong and simple in comparison to conventional masts with all their necessary complications. With sails on wires, which are the most efficient kind (no mast in front, spoiling the airflow), and you want to be able to furl ALL sails quickly and easily (so they have to be on wires), the big problem is that sails on wires, which wires must be very taut, create a lot of compression on a mast, so the mast must be very strong and MUST be kept in column, i.e., not allowed to bend, which requires spreaders and more wires, stays, adding more compression, and, on conventional masts in the middle of the boat, the stays are at angles that add even more compression, and thus more stress on the boat, and, on a catamaran, right in the middle of the boat, trying to break the two hulls apart.
The larger the angle between stay and mast, the less compression on the mast and the less stress on the boat. That's why big single masts have lots of spreaders, to increase the size of the angles of wires to mast, but at the cost of lots of wires and fittings and complications, increased windage, wear and tear on sails, etc., etc.
My stays, in this geometry, have very advantageous angles, with therefore less compression on the mast structure, and a lot of the existing tension and compression forces, especially the side loadings, which are the greatest on any boat, are contained within the A-frame structure. There are fewer wires and the wires are smaller, lighter, less expensive than would be required in most conventional rigs. Altogether, there is less material involved in this rig, and thus less weight as well as less expense.
Thus the essence of this rig is in its geometry. Just look at the size of the angles between wires and masts. There are no really acute angles as there are in most rigs. And with respect to strength to weight ratio, and strength to cost ratio, I dare say that few rigs could compete.
The compression load on the boat is not only shared by the two hulls at their strongest points, the load is not trying to break the two hulls apart, as it is in a conventional rig. On big cats, it has never made sense to me to have a huge mast in the middle of the boat, trying to break the boat in half. A cat is two boats in one, so why not have two masts stepped right down to the keels through the parts of the structure best able to take the compression, as well as dividing the load by two, and increasing the side-loading angles to the fullest extent possible.
There are a few cats around with so-called bipod rigs, a mast stepped in each hull, but vertical, the masts not connected at the top. That's not nearly as strong a rig as an A-frame, which is a triangle, actually, or a system of triangles. A cat with vertical bi-pod rig is simply two monohulls connected at the hulls. My goofy analogy is two one-legged men side-by-side with one water ski each, with the skis connected but no other connections, not as stable as one two-legged man on water skis.
As you may or may not know, a much stronger rig, than on a monohull, is necessary on a big cat, because big catamarans are very unforgiving, in that they don't heel, so you don't get the shock-absorber effect of heeling like a monohull, not only heeling and absorbing some of the force, but also spilling wind, when you get a big gust. That is, a catamaran with vertical sails does not have the shock absorbing heeling effect, but my sails, when the tack of each sail is attached to the windward side deck, the sails themselves are "heeling" from the start, and thus spill wind a bit when you get a gust. That's a significant safety factor.
You don't "fly a hull" in a cat of this size, rather, you'd better reduce sail long before that, and a conventional rig would not be strong enough to lever the boat over. The rig, or the boat, would break first. However, my rig is much stronger than a conventional rig, maybe even strong enough to lever the boat, but if she ever did start to heel a bit, let alone fly a hull, the spilling effect of my already tilted, or "heeling" sails would increase very quickly.
A catamaran with a conventional vertical rig, in a gust, accelerates much more than a monohull, because none of the energy is absorbed in heeling or lost in spilling. So with my "heeled" sails, yeah, I may sacrifice a bit of abrupt speed, but my boat has speed to spare, indeed, goes TOO fast at times, so I can afford to sacrifice a bit of speed in the interests of less shock loading, more safety, and not having to be so constantly on the alert and ready to slack off in the gusts.
On the other end of the wind scale, in very light winds, my sails have an advantage over the sails on conventional, vertically rigged boats, in that their sails require a certain amount of wind just to lift the sails away from the vertical enough to set them into their proper aerodynamic shape for sailing to windward at all, and in winds of less than the required amount for setting, which is considerable on a big boat with heavy sails, the sails are worse than useless, they are all drag. That's why you move yourself over to leeward in a little sailing dinghy in very light winds, to heel the boat a bit and thus get your sails to hang and set properly by gravity. It would take a lot of heavy crew to accomplish the same effect on a big cruising monohull, and would be impossible on a catamaran.
My sails, attached to the windward side, because they are "heeled" to start with, as soon as I unfurl them, they assume their proper shapes by the effect of gravity and I'm away with the gentlest puff of wind, rather than with sails drooping like laundry and the boat going backwards.
There is nothing "radical" about the rig. All the components of the mast structure, and the sails and rigging, are standard off-the-shelf, tried and true and available world-wide; it's only the geometry and the sheeting system that are different and yield several advantages and built-in safety factors, in comparison to most rigs.
The A-frame mast structure is not experimental, except in a relative sense in its rare use on boats. It has been used in various pylon structures for ages now, is tried and true, and indeed less complicated, requiring fewer and smaller wires, and is much stronger than the typical yacht rigs, especially on catamarans, with far less stress on the boat and the mast and fittings. The geometry takes full advantage of the beam of a cat, in the standing rigging as well as in the geometry of the sail plan and the sheeting.
This geometry is 3-dimensional, whereas the standard, vertical, monohull rig is 2-dimensional, up and down and fore and aft, and then you have to complicate things in order to hold the sails out to the side (in the 3rd dimension of the real world) at the proper angles, with booms or poles, and then hold those down with vangs. Why encumber your boat with all that gear when you can move the sails over to one side of the boat, and then, with some extra rope and a few tracks and padeyes strategically placed, sheet right down to the boat which itself becomes the "boom"? And why limit a cat to a monohull rig in the first place?
...just a couple of photos to give an overall picture of the arragement
