WishBone Sailing Rig

High Tacker

REPLY TO TOx1c Re SAFETY FACTOR ON CATBIRD SUITE'S A-FRAME RIG

I checked with Tony Stanton who is still running Malcolm Tennant's website (tony@tennantdesign.co.nz), and who did the original calculations for the A-frame rig on Catbird Suite. He had a quick look at the files and says that we had a margin over buckling of 2.5 on 100% of the righting moment. So that would be 250% as compared to your mention of 3 times 80% of the RM, or 240% (assuming that that is in reference to buckling as well). Tony also remarked that this is much easier to achieve on the A-frame than on a conventional rig. There is no question that this A-frame has superior strength to weight, strength to cost, and strength to drag ratios, as compared to a cat of the same size carrying the same sail area on a single mast stepped on the centerline. Aside from the issue of strength, as has been mentioned above, you keep an eye on the weather and reef appropriately, and it helps to have all sails on furlers.

 

Thanks High Tacker. I've just received the same answer from Tony.

It is good to know that one can design aluminum A-frame rigs with a conservative 250%RM, at a reasonable cost.

For me the main argument is the ease of reefing. I don't enjoy anymore the need to spend time putting wet suits and harnessing, to go dancing in front of the mast in strong seas. Especially at night...

 

Wishbone i.e. A-frame rig

Hi

I am the designer of the Voyager with the A-frame mast. The statement "of the self" glass fiber flag pole as masts is, as almost always, incorrect. It is a pity that articles etc are not really read anymore.
First this arrangement is no gimmick but there is a reason why I designed this A-frame mast.
First to unload the bridge deck from the high compression load imposed by a center mast rig (could be as high as 13 metric tons for this boat and sail area). This means the whole bridge deck can be designed much lighter. Result a faster boat and less material thus costs. See sketch.
Second, to set and lower a mast is not easy. Or you have to spend a lot of money for a crane or you need a device to do it by your self. In the Netherlands we are used to lower and set a mast by our self. Common is an a-frame which can be used as railing as well. My A-frame mast is going a step further. The whole mast is the A-frame. So you can set and lower the mast by yourself.
We use flag poles from Sweden. The have made extensive tests. With the results that I have a base for a lay up from UD glass and other material for the necessary strength. By the way the deliver the poles also without gelcoat, so nothing to sand for the extra lay up. The compression tubes are AL tubes.
To EEC regulations. I can only say that the spoiled all there credits and trust as the invented the regulation how much a bend a banana has to have to be suitable for consummation in the EEC. So forget it. What would the reach anyway? A more heavy rig could mean, that the weight when up side down, could be just to much, with the beamy boats of today, that the boat can not right it self anymore. Our Pelican is stable till 89°. With a more heavy mast he would go at about 70°. Attention this is a theoretical stability calculation. Don't try it in waves !!
Next point. I am not an American designer, but a Dutch designer.
A forum can be a good thing, but before you offer a meaning, research your answers for correctness otherwise you do more harm as good.
In any case have fun and remember the best captains sitting always on the wall .

Cheers

Bernd

The photo shows an A-frame mast on a tri. I made the photo last Sunday in Sete/France

 

Bernd, I can appreciate that you went through the engineering for your rig and that you can stand by your work. And if it works, that's fine, and I do not mean to denigrate it. I was very clear that my statements were "in my opinion", and I offered my reasons why, based on my experience working for over 30 years engineering, designing, and testing fiberglass tubes and poles. So I will let my comments stand.

Eric

 

High Tacker

UTILIZING RIG GEOMETRY AND CATAMARAN BEAM TO GAIN STABILITY

Since we're discussing capsizing, rig strength safety factors, etc., it seems appropriate to note that most rigging schemes seem to be still hung up on the conventional fore and aft vertical arrangement, i.e., the conventional monohull rig, whereas, on a catamaran, not only does it make no sense to put a huge mast and its compression load in the center of the boat (requiring a heavy wingdeck structure such that you might as well go ahead and build a trimaran), it also makes no sense to put a huge tension load on the center of a beam between the bows. After all, a catamaran is two boats in one, so why not treat it as such, and in the process take advantage of the width of the boat and utilize a geometry that allows angles for lift and/or anti-capsizing loads.

See www.damsl.com and on the welcome page look to your left at table of contents and click on "A-frame Rig".

I think of Catbird Suite as two boats connected by a wingdeck, each boat having a masthead cutter rig, and the two rigs are leaning together, their masts meeting at the top, over the center of the wingdeck. Or perhaps more aptly, she is two proas leaning together. The head of each sail, genoa, staysail, and main, is connected to the masthead. The tacks of the genoas are attached to the bows of the two hulls. The tacks of the staysail and the main can be attached on the centerline of the boat, i.e., the centerline of the wingdeck, or on either side deck.

For sheeting, there are tracks and various other sheeting points scattered around the decks, and the clew of each sail has two blocks for sheets so that, for each sail, there are four ends of rope that can be moved around to sheeting points and winches so that, when the clew of a sail is inboard, ideal sail shape can be maintained on all points of sail without booms or poles. That is possible when the tacks of the sails are on the windward side deck. With the tacks on the leeward side, then poles are needed.

On a passage-making tack, especially if there is substantial wind, it's good to use the windward genoa and the staysail and main with tacks on the windward side deck. All the gear is then inboard and one can go from close hauled right through all the points of sail and maintain ideal sail shape without booms or poles. With the heads of the sails attached to the masthead over the center of the catamaran and the tacks attached to the windward side, the sails are effectively "heeled" and thus spill some wind.
AND a substantial portion of the force on the sails is pushing the windward hull down, i.e., is an anti-capsizing force.

In moderate to light winds, the sails can be deployed on the leeward side, and then a substantial portion of the force on the sails is upward, is lifting the leeward hull. Picture a windsurfer leaning into the wind so that his sail is tilted, not "heeled", rather the reverse, canted into the wind.

 

Procyon....a bold new experiment. A-frame mast

I'll bring up 2 points here,...
1) make reference to another bold experiment with an A-frame/Wishbone shaped mast,
http://www.boatdesign.net/forums/sailboats/procyon-project-bold-experiment-10771.html

2) to make note that Procyon did experience some small problems with keeping the tips of those mast legs together (and that was on a monohull base structure compared to a multihull 'spread')

 

Catbird Suite's flexible A-frame rig, see www.damsl.com

Hello Brian,

Quite a while back already, on this very thread, you mentioned Procyon's masthead problem, and I addressed the issue then...uh, well, I thought I had...but maybe I wasn't intelligible then...or, maybe your memory is even more like Swiss cheese than mine...but wait, here's the link:

http://www.boatdesign.net/forums/boat-design/wishbone-sailing-rig-1999-16.html#post499688

Anyhow, some details and photos are there at that link, and my comments then seem as intelligible as my usual gibberish and there's little hope for improvement, but I will add that by the time I threw the bright idea, of putting an A-frame on my cat, to Malcolm Tennant, he had long since thought of such an animal, knew all about Procyon's problems, and had also been dealing for years with issues of flexibility on catamarans, and he dealt quite handily with this one.

In short, the A-frame structure must not be rigid; there must be allowance for some movement, not just at the masthead but at all junctions and especially at all three corners of the big triangle formed by the mast legs and the boat.

On Catbird Suite, each of the two mastheads has a plate, (a silhouette of a bird) of about 400mm by 1500mm, welded to the masthead, so that the bolts holding the two mastheads together are spread over a large area, and there is considerable play in the bolt holes drilled through those plates (which play I'm sure would be appreciated by any red-blooded bolt). Also, a thick rubber gasket is sandwiched between the plates (and the holes in that gasket are EXACTLY the diameter of the bolts, so that no bolt, or dolt, could complain about looseness of fit). See photo at the link given above.

The bases of the two mast sections are cut in a slight curve so that they have rocker, i.e., Catbird Suite's masts can ROCK. (Can yours?) Each mast base is stepped within and loosely pinned through a stainless collar which is welded onto a stainless plate beneath which there is a thick rubber gasket.

 

Hi Bernd,

My mistake for thinking your design was US. Apologies. At least it had the resulting effect of having you joining this thread, which can only benefit from your designer experience.

I agree there are very good reasons to the A-frame design. However I am not sure to follow you on the structural savings/lighter bridgedeck.
On a bermudian rig, the compression of the mast would pass through main bulkhead/beam, deck and hull sides, to the shrouds. On the A-frame with central sails, that would be the exact opposite. So unless all sails are fixed to hulls -and not to central deck- as done on Catbird Suite, I am not sure there would be any structural savings?

Maybe I was mistaken by your drawings of the KD122 Voyager, the A-frame glass pole profiles look so thin (and they stand on the interior of the hulls), so they don't seem to reflect the real profiles needed. And they even seem to be tapered?

What %RM have you taken for the rig calculations, and what xy inertia & glass pole diameter/thickness do you obtain? That would be very surprising and interesting that "off the shelf" flag pole tubes (designed for self standing/tapered or little compression/thin) could be adapted for rigging.
.

 

Thank you VERY MUCH for refreshing my memory...it truly is fading a bit. I had forgotten this explaination of yours. It seems a very important consideration.

 

The advantages I see for an athwart ship 'A' frame mast are:
1.) If you set the sail between the legs, you get a clean luff, and
2.) as stated earlier, if you set the sail on the leeward leg, you get and upward lift component. If you set the sail on the windward leg, you can get it to 'sleep' at its proper sheet angle in light winds.

The disadvantages are:
1.) considerably more extra weight up high, and
2.) extra cost, as you have to buy two masts for a rig that could get by with one.

I guess, like any other rig, it has its pluses and minuses and is not necessarily a panacea, but could be very useful in some situations.

 

The furcula addresses some of the weight aloft issue by forming one spire atop the legs, albeit expensive it does reduce the number of "masts" to one at height. I have always thought it strange that cost and ease of availability (extruded aluminum) are such top considerations when talking about masts. It is arguably the most important structure on a sailboat is it not - at least equal to the hull itself - unless you plan on doing a lot of motoring. Not looking for an argument, just musing....

 

My point to. Every rig has his disadvantages and advantages. The point is to design for the optimum. As I outlined in my other response, here was to have a boat which makes the owner independent of marina facilities. The mast can be set by the crew and lowered as well. Besides getting rid of the compression load on the bridge deck, which enables a lighter construction.
Somebody was asking about the stability and the maximum righting moment.
Here a diagram. The wind speed for the different curves are 0, 5, 10, 15, 25 knots with a safety factor of 70 %.
There many possibilities for rigs. We are so used to the sloop rig and forget that there perhaps other and more suitable solutions for a specific goal.

Cheers

Bernd

 

Agreed. Data backs you up. More than one way to solve the problem. I think we sometimes get drawn in by the "standard" which was not the result of intensive, rigorous analysis but was the result of a glut of product and industrial capacity following the last world war. Vive la difference.

 

Bernd, are these the flag poles intended for use? http://www.formenta.se/default.asp?LangID=2.

I still can't see how there can be any saving on the bridgedeck structure. You just reverse compression/traction in the cross beam.

 

Yes, this are the flag poles we use as BASE.
The compression is about double as big as the tension because through the spread of the loads.
Buckling load of the lower wind mast and cross trusses is the bigger problem.