WishBone Sailing Rig

[brian eiland]

The mainsail tack is fixed on the SMG 50 - the track you see on the photo is for the selftacking staysail. Found that out from a larger resolution photo on their webpage.

You are correct, I misinterpeded that photo when I looked at it too quickly.

 

[brian eiland]

While still keeping with a fixed conventional rollerfurling / forestay setup midships on the longeron, one could easily move the tack of a light air upwind sail, on a flying rollerfurler, to the bow. Like taking the tack of an asymmetric spi out to the bow when going downwind. No need for twin forestays, to both bows, with the associated weight and resistance.

His justification for those twin forestays is twofold;
1) to rig to the stronger bow locations and not have a front crossbeam
2) to have 'heeling sails' on his upright very stable multihull.

For Heavy Air

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.

For Very Light Airs

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.

 

[brian eiland]

Technically not much of an issue, but I'm not sure if that would significantly improve upwing performance in light air.

I'm hoping to ask the owner about his upwind performance in light airs while using that 'windward genoa'. I believe this would only work if he repositions the tacks of his other sails (staysail and/or mainsail) also to windward. Otherwise I believe his pointing capabilities would deminish.

Or perhaps he could make use of his leeward genoa (if there is enough air to keep it full, and he could sheet it properly) to point higher. In other words the vessel so configured with the mainsail tacked down to the centerline, and the geona tacked down to leeward would effectively have the vessel headed up higher than a vessel with the conventional arrangement of the genoa and mainsail tacked down to the centerline of the vessel.

 

[philSweet]

The usual explanation for this geometry (inward leaning biplane geometry) is to reduce the heeling moment experienced. The downforce on the windward side and the upforce on the leeward side counters the heeling force to a substantial degree. In this case it would also tend to decrease the racking moment as well.

 

[philSweet]

In this case it would also tend to decrease the racking moment as well.

Only if sails were set on both stays. If only set to windward, racking would increase.

 

[pool]

His justification for those twin forestays is twofold;
1) to rig to the stronger bow locations and not have a front crossbeam
2) to have 'heeling sails' on his upright very stable multihull.

For Heavy Air

For Very Light Airs

All I see is a lot of complication, weight and windage in that rig, for doubtful improvements in performance.

You can easily stay with the simple, strong and proven longeron / fwd crossbeam setup and a selftacking rollerfurling jib on centerline - then add any type of heavy or light air sail(s) you feel the need for, set flying on a furler and attached to the bow(s).

 

[Kojii]

Bendy mast

Got a question for you Kojii. Is that bow of the wishbone section of the mast distorted asymmetric, or is that the camera angle that makes it appear so??

In other words it appears as though the port leg is more exaggerated in bow (more compression), and the starboard one is straighter (tension).

Both are true, as usual. The bow legs of the arch are the same material as helicopter rotor wings. They flex. Tied together as they are with the inner struts, one pulls the other, and gravity, sail force, heeling angles, all play their part. It gets really interesting when you change the wires.
Move the bow string (shrouds) to the deck tabernacles (they were absent on this day of sailing) and tension them up carefully maintaining balance then tension the fore and aft stays again incrementally to keep the arch symetrical or close and voila she sails in same wind with same heel and the rig remains symetrical.
More interesting (to me anyway) dress the archmast to port or to starboard and then tension up to keep that bend. The mast top is fully a foot over to port or to starboard (keeping in mind the boat is 9 feet wide). Why you ask? The wing (sail) is now in a different orientation to the centerline of the vessel, is it not? This changes the angle of attack to the wind without changing the vessel heading. We saw a good deal of wind on our last outing to Juan de Fuca and much of it on the nose. Results of the experiment are not conclusive to me as yet, but it was very interesting to be sailing as close as we were to the wind. Very strange indeed. I am sure a lot of engineers can fully explain the experience to me.
All this experimentation is only possible because the old mystery man Fred built a carbon/kevlar catenary wing archmast with an integrated spire that has (so far) withstood all the bending and "distorting" that we have thrown at it. Remarkable.
Experimentum periculosum...

 

[brian eiland]

Both are true, as usual. The bow legs of the arch are the same material as helicopter rotor wings. They flex. Tied together as they are with the inner struts, one pulls the other, and gravity, sail force, heeling angles, all play their part. It gets really interesting when you change the wires.

Move the bow string (shrouds) to the deck tabernacles (they were absent on this day of sailing) and tension them up carefully maintaining balance then tension the fore and aft stays again incrementally to keep the arch symetrical or close and voila she sails in same wind with same heel and the rig remains symetrical.

Brian replied: I assume your vessel is rigged near the same as the photos of Relentless I posted back in HERE in#98 ?? These are the deck tabernacles and shroud attachments you are speaking of??

More interesting (to me anyway) dress the archmast to port or to starboard and then tension up to keep that bend. The mast top is fully a foot over to port or to starboard (keeping in mind the boat is 9 feet wide). Why you ask? The wing (sail) is now in a different orientation to the centerline of the vessel, is it not? This changes the angle of attack to the wind without changing the vessel heading.

Brian replied: Interesting experiment, but not a practical adjustment underway.

We saw a good deal of wind on our last outing to Juan de Fuca and much of it on the nose. Results of the experiment are not conclusive to me as yet, but it was very interesting to be sailing as close as we were to the wind. Very strange indeed. I am sure a lot of engineers can fully explain the experience to me.

Experimentum periculosum...

Kojii, do you have some photos of the details of your rigging?

 

[brian eiland]

Chief, I suggest you take a look at some of the pictures and specs of my boat - I've posted the link above but here is a link to the pictures: http://www.sail-works.com/KOLIKA/html/picture_gallery.html

I just looked thru your gallery of photos again, and I can not determind how the masts tubes are mounted on deck (how rigid or flexible). Nor can I determine how rigid or flexible that masthead is (joining of the two at the top)??

I'm particularly interested in the top joint as it has been expressed that Procyon experienced some problems here. And of course this could be more problematic with twin tubes mounted on a wide beamed 'individual' hulls of a big catamaran.

 

[Kojii]

Kojii, do you have some photos of the details of your rigging?

I suppose all this is quite troubling to the straight stick "keep it in column" extruded aluminum crowd...fair dinkum.

Orca's rig represents a simplification of the rig from the photos of the old Relent you mention.

Simply put the shrouds go from top of the mast to the deck tabernacles outboard attachments (cast aluminum). These tabernacles are through bolded to the inner frames which conform to the hull and are bolted through the hull down to about the waterline. This inner frame transfers the stresses to the hull/deck near maximum beam.
On their way they pass two critical points. First, the distal spreaders are aluminum rounds (very stout) articulated at just the below the articulation of the legs at pelvic joint and angled upward slightly from the mast. The shrouds then pass close enough to the arch legs at the knees (where the interior spreader is located) that I have placed UHMW guides. The shrouds rest against these pads, which also guide the shrouds. I have yet to determine the extent these points of contact play in the maintenance of the catenary under loads, but it is obvious they are keeping the flex of the bow "limited" shall we say?

The shrouds originally (when we first sailed it) went from this arch knee (point of the lower inner spreader) to the mast top without any wires going to the tabernacles. That also resulted in a lot of flex under heavier loads (15 knots and up). This was a modification by someone after the initial redesign of the rig from the tall to the medium height it is now (39').

Surprisingly, I have no photos of this that would help you much at this point. Will be going up mast soon and get some at that point. The photo attached may help some - it shows the inner spreader (Radar mount) and you may be able to make out the UHMW knee caps. Regards, K

 

[brian eiland]

I'm sure this vessel reference has been posted before, but in looking thur some old postings of mine, I did not see it in these particular discussions

50 SMG Catamaran
http://www.sail-the-difference.com/DESIGN.28.0.html?&L=1

Here is another reference to some good windward performance by the SMG catamaran;
http://www.sail-the-difference.com/fileadmin/users/9/news/2009-05-05-Kornati_cup_results.pdf

 

[brian eiland]

A-frame Mast with Lateen Sail

Just happened across this interesting A-frame rig making use of a lateen sail up front,.....and a mizzen sail as well. Some good photos over HERE:
http://www.boatdesign.net/forums/boat-design/frame-mast-25696.html#post449122

http://www.puremagma.com/page5.html

 

[High Tacker]

High Tacker (www.damsl.com)

Hello to all, especially Brian Eiland and philSweet,

I'm a newcomer to the Boat Design Forum and first of all want to thank Brian for his references here to my A-frame rigged boat Catbird Suite and her website www.damsl.com. And it's good to see that quite a few members recognize some of the benefits of an A-frame rig with all sails on furlers.

In particular, the reference above to the good upwind performance of the SMG-50 catamaran is informative because usually the first question sailors ask is whether she goes well to windward. I can say yes for Catbird Suite and am glad to hear about SMG-50. But there are many other benefits in an A-frame rig, especially with furling sails whose tacks are movable.

With respect to philSweet's comments re the geometry of some of my sail configurations, it is correct that the heeling moment is reduced when the tacks of the sails are attached to the windward side of the boat, effectively "heeling" the sails, re-directing the heeling moment downward and also allowing for some spilling of wind in gusts. And, heeling moment is further reduced if sails are attached to both sides of the boat.

With respect to the racking moment, I want to point out that with main, staysail and genoa all tacked down to the windward side, every sail (and on all points of sail) in its entirety is over the boat and within the cage of the rig, i.e., everything is inboard, as opposed to the conventional configuration in which, during reaching and running, substantial portions of a mainsail and a big jib are outboard, exerting leverage on the boat. In my sails-tacked-down-to-windward configuration, on a beam reach or broad reach, the luffs and leeches of the sails effectively amount to a rig within a rig, almost doubling the strength of the standing rigging, and without a boom putting a bending load on the mast in addition to racking the rig.

Most of the time, there's no need for sails on both sides of the boat. Mainsail, staysail and one genoa total some 2100 sq. ft. of sail, and having those three sails attached to the windward side of the boat allows the entire 30-ft. width of the boat for sheeting, eliminating the need for booms, and thus that old bugbear the accidental jibe becomes a non-event. And having the total sail area directly above the boat greatly reduces the tendency to broach.

See www.damsl.com for more details on the advantages of unusual sail configurations accommodated on a catamaran with A-frame rig and furling sails whose tacks are movable. The rig of course also allows for the straight up and down, fore and aft conventional configuration. Uh, perhaps we should call that the square configuration.

I would also like to answer some comments about weight and windage. It is a fallacy to assume that two legs of an A amount to two times everything, far from it. If designed properly, an A-frame rig, in comparison to a single-masted rig that would support the same total area of furling sails, has superior ratios of strength to weight, strength to complexity, strength to cost, and strength to windage.

Again, see www.damsl.com for detailed explanations. See the photos and notice how slender the mast sections are on Catbird Suite.

Cheers, Tom Bradshaw

 

[downunder]

Tom,

I was wondering who designed and built your A-frame rig and the cost versus a standard rig.

I would have thought it would have been more expensive.

I am looking at having 46 ft cat built in 2013 and if not more expensive would be a consideration.

i yust noticed some of the info on your website. very interesting.

best regards

John

 

[High Tacker]

High Tacker (www.damsl.com)

Hello John

To answer your questions about design and cost of the A-frame rig:

Malcolm Tennant, world-renowned New Zealand multihull designer, designed the rig. It was our Plan B in the original design of the boat, in case the wind turbine rig turned out not to be completely satisfactory, which is indeed what happened. In the meantime, I did very extensive testing of the A-frame concept with a much smaller version (on a 14-ft. cat) and my boating partner did some testing on one of Malcolm's Great Barrier Express cats of about 30-ft. But what the hell, what's to test? The furling sail is one of the simplest and is indeed the greatest invention in sailing in the last couple of centuries. And A-frames have been known as among the strongest of load-bearing structures for thousands of years.

The final version of the rig on Catbird Suite, including hard furlers for the genoas and solf furlers for the main and staysail, plus the sails, but not including winches, cost about NZ$144,000. That's from memory and from files I have on my laptop with me (am traveling now). Malcolm had told me that a conventional rig and sails would have cost at least NZ$200,000. My mainsail is not very big, but then I have two genoas, so my sail area would total quite a bit more than on a conventional rig. Of course, a big genoa is much cheaper than a big and complicated, fully-battened main. (And there's less likely to go wrong with it, a lot less to maintain!)

Have a close look at the pics on www.damsl.com and you will see that the masts are very slender and of the simplest kind of aluminium section, no tracks, no open slots to spoil the integrity of the tube, and only fittings for stays and sheaves for halyards and topping lifts for whisker poles.

You will see that the design is very simple and elegant, so the result is lower cost, less maintenance, and replacements over time will be cheaper and easier. Everything is very standard, off-the-shelf available worldwide. Furling sails are very simple and very light in comparison to a complicated monster main, and of course far easier to handle.

The builder of the rig, Silhouette Spars, sold the firm, now Whangarei Spars & Rigging, and the main guy who built my rig is no longer there, but he might be available. However, you might not be able to get quite as good a price, because he and my former boat partner were good friends and had done a lot of boatbuilding together.

My best bit of advice is DON'T BUILD A BOAT, AND DON'T HAVE A BOAT BUILT FOR YOU. You would be much better off, and get much, much more for your money, and a tried and true boat, and rig, if you simply bought my boat! She is like new and in today's market I am forced to offer her at less than one-quarter of her replacement cost. You will never find a better deal. She was built by some of New Zealand's finest boatbuilders, closely supervised by my partner and me. She is THE brick shithouse of catamarans.

You're in Australia, right?

Cheers, Tom