X-Beam and the Giant

[brian eiland]

This was a note I posted to another multihull forum several years ago when I had made the suggestion to possibly utilize an ‘X- beam’ structural arrangement on a race bred catamaran


X-Beam Cat and the Giant
Lets see if we can look at this situation in a ‘less technical' way.

Forget the rig for now and lets look just at the structure of the hull (actually hulls and including crossbeams for a multihull vessel). Suppose we were a GIANT of a person who could pick this 110' + vessel up as though it was a model boat. Now lets chop off one hull right at its connection to the two major crossbeams and discard it:

1) Then we take the two free ends of the beams in each hand and push one down while we pull the other up. If there is any lack of rigidity in that remaining hull (torsional and/or bending), we will flex that hull to a point were it will build up a resistance and then suddenly transfer a great load acting to break the connections of our beams to the hull structure. Naturally the longer lever arm we have (full length of the crossbeam), the greater the forces we can impose on these connections of beam to hull. And if we have significant flex and /or play in the system, the dynamic loadings are even greater

An X-beam configuration possesses far less cantilevered length than the conventional full-length crossbeam, and thus imposes lighter loads to the beam/hull connections. I think the Giant would have a tougher time twisting the X-beamed boat apart.

Does this happen in real life? As we discovered early on in Formula 40 cats, ANY opening (hatches, cockpits, etc) cut into the hulls between the two major beam connections significantly compromised the stiffness of the boat, and allows the hulls to 'work' independently. And watch out cutting access hatches into your hulls in front of the front main beam or you just might crack off the bow(s).

2) Still holding the two free ends of the beams, our Giant now bangs the opposite side hull on the floor. Both hull/beam connections are highly stressed to a breaking point that is more easily reached the longer our crossbeams are. Again the Giant would have a tougher time with the shorter X-beam vessel.

3) Now lets suppose our model boat has both hulls attached as it was before we cut off the one hull. Then our Giant takes one hull in each hand and pushes one hull forward while pulling the other back, ‘racking’ the vessel. I think it’s pretty obvious that the X- beam configuration would win this battle also.

4) Finally our Giant, with one hull in each hand, tries to bend the boat structure in half, either up or down. Here I believe incorporating an arch in the X-beam should help resist the athwartship’s bending imposed by the shrouds and the buoyancy acting upwards at the width of the vessel and the downward thrust of the mast at the center. And as an added bonus we get greater water clearance under the arch at the center of the vessel.

I've said previously that it should prove interesting to fully inspect those boats involved in the RACE upon their return. I just read today of a problem that occurred with Club Med, "the most alarming problem was a primary structural failure....around one of the main crossbeam roots....exactly where the front beam joins the starboard hull." This type of failure can be directly related to the leveraging that occurs at the beam to hull connection. Team Adventure's beam problem (in the middle) is more likely related to the huge dynamic loads of that compression mast trying to crash thru the beam when you bang thru a seaway.

Those new, really large cats, with their expansive width, might be really stretching the upper limits of connectivity, so maybe we should be looking at alternative configurations.

[brian eiland]

I remember hearing a few years ago of two fellows, Robin Chamberlin and Terry Travers, who made a trip down to Antartica in an Xbeamed catamaran. Recently I became aware of Terry posting to a multihull forum, so I wrote him asking of their vessel. Here is the reply I received;

Hi Brian
The designer Robin Chamberlin has used the X-beam concept for years. In the Southern Ocean we had one incident where the whole boat disappeared under a huge wave impact. That was the one time when we thought we may be in serious trouble. But it reappeared intact. I thought that the other hull would be gone.

I am firmly convinced that the X-beam adds significant strength to the whole arrangement.

The wires under the X-beam are important. We had one break half way across the Tasman and the whole structure became a bit floppy. A temporary repair with spectra line and a Spanish windlass got us back to Australia OK.

I’m including a photo that shows two Chamberlin boats – Slingshot a 30’ more modern design with a composite X-beam, and Excess our 38’ cat that we took to the Antarctic behind it. Excess uses the prototype aluminium X-beam. Robin incorporates the concept into all his boats even if they are not open cats as these two are.
Regards,Terry Travers

[brian eiland]

Regrettably this vessel was a failure as it was a bit overweight compared to her competition.

Sailing Anarchy reported:
This is a group affair. First, for the failed concepts in Gitana X, the Open 60 tri. In fairness, it is not just Ollier to blame. Gitana X was designed by the Gitana Design Team which included the Gilles Oliler Design Team (France), Duncan Mac Lane (USA), Mario Caponetto (Italy), Sebastien Schmidt (Switzerland) and .

Too many steps were taken away from the design development route pursued by the class, and the boat went the route of complexity (e.g. individually threading xbeam fibers through the main hull!!), and in the end of the second season the boat ended up a confirmed dog, albeit a very expensive and eccentric one.

According to Multiplast The decision to have not a similar boat as the others 60-footer was the one of Benjamin de Rothschild, the armator.
The last nail was hammered in the coffin when the boat was replaced by an older boat in the fleet, Gitana XI (former Belgacom).

[yipster]

good thread brian, i have been looking at those x beam tri's and now see cats also, always good to triangulate in more directions
these are open x beams, a crossbeam house, specialy full beam, would make way for a more box like construction
dont know, there are those in arctic circles who rather heat a small space...

[BOATMIK]

Howdy,

I agree with the general drift of the post above, but there's no such thing as a miracle setup that resolves all problems.

The X set up will be more highly stressed than conventional ones under forces that tend to pitch the outriggers than conventional setups and also more highly stressed when it has to resist any rigging loads where wires go out to the extremes of the outriggers.

In the terms of holding the model in your hands - put the bow of the main hull on the ground - grab the front and back of the ama - pull the back end of the ama up and push the front down. The lever arm of those torsional loads is the length of the ama divided by the much smaller distance between the ends of the X where they meet the hull. Torsional loads of this type will be 3 or more times those of a conventional beam setup.

It also probably necessitates a third crossbeam to deal with these loads as well - which is often not necessary at all in smaller craft with the conventional setup.

Michael Storer

[CT249]

Doesn't an X concentrate pitching loads onto the very short fore-and-aft distance on the centre of the X?

Are the Chamberlain boats, which normally (always, I think) have significant fore-and-aft beams, the same as your example which seems to have no fore and aft beams but only the X?

I'm no engineer, I may well have got it wrong.

[BOATMIK]

Hi CT249,

I'm half an engineer and I agree. Actually the pictured boat in the second top post has three beams.

I wonder what the one in the middle is doing - ie the part of the X that is swept back?

Engineering would tend to indicate that it would be having rather a lazy time as soon as any pitching at all was involved. At times with combinations of torsion and bending loads there would probably be no load at all on it at all compared to the fore and aft beams - at very best it could be loaded equally with the other two beams.

Brian did a good job of reinforcing the problem that openings in the hull cause - I'd never thought about that directly, but of course, he is spot on.

To triangulate the loads maybe a wire is all that is needed? After all the big loads in that plane are going to be from the amas being pushed back by impact with waves and sometimes more massive objects - and you don't need the same degree of triangulation when going in reverse.

The X-beam does deal with the rig loads rather nicely though with little or no induced torsion into the ama/aka structure.

As far as failures of more conventional setups go ... I don't think it is a good guide at all as to the optimum setup.

What you have to do is build (or model?) two boats of more or less the same configuration of hulls and rig. One with an optimised conventional setup, one with an optimised X-beam and then find out which one is lighter for the same loads and acceptable deflections.

Failures just show there needs to be a bit more material in the broken area - rather than it is an unsuitable system. But if the weight needed to make it suitable is excessive - then THAT is what defines it as unsuitable.

My feeling is that the X-beam is less than optimum because it means there have to be three beams rather than two.

Maybe it carries a windage penalty too?

Best Regards to all

Michael Storer

[brian eiland]

I have a concept in mind for the catamaran situation, and it very definitely utilizes a 'full X' rather than the shallow ones on the Gitana tri and the old Elf Aquitaine cat.... just haven't taken the time to sketch it up.

I do not believe the trimaran configuration benefits from the X configuration in the same manner as does the catamaran simply because there is the benefit of the central hull structure already inherit in the tri layout. The cross span distances are greater in the cat than in the tri.

Now what if one could eliminate the need for the usual front bow crossbeam on the cat configuration...this beam that finds its way underwater on many occassions. And the center portion of the X crossbeam is even further aft still, than the crossbeams on the RACE cats that so often interferred with the waves. Might need a different rig as well

This 'racing design' might make use of an 'enlarged volume composit X beam' as both the structural member to connect the hulls, act as the mast base, and be the abreviated crews quarters in its central volume??

[brian eiland]

Originally Posted by brian eiland

I have a concept in mind for the catamaran situation, and it very definitely utilizes a 'full X' rather than the shallow ones on the Gitana tri and the old Elf Aquitaine cat.... just haven't taken the time to sketch it up.

Speaking of the old Elf Aquitaine cat, here is the only picture I had. It doesn't show the X very well. Again a very 'shallow X'. There was a balstrom (Aerorig) type rig mounted at the X juncture.

Anybody have any better photos to add??

[BOATMIK]

Hi Brian,

Unless I'm misunderstanding you I think that if you do some calculations you will find that to resist the torsional loads on the athwartships axis (ie pitch of the hulls relative to each other) that a pure X crossbeam setup will have to be excessively heavy to sustain those loads.

Boatmik.

[brian eiland]

Originally Posted by BOATMIK

Unless I'm misunderstanding you I think that if you do some calculations you will find that to resist the torsional loads on the athwartships axis (ie pitch of the hulls relative to each other) that a pure X crossbeam setup will have to be excessively heavy to sustain those loads.

I'm not sure we have our terminolgy cleared up between us here. If we are talking of "pitch of the hulls relative to each other" aren't we talking of torsional loads in the fore-aft axis rather than athwarships?

Wouldn't torsional loads in the athwarships axis refer to bending the boat in half about the fore-aft axis?


______________________________________________________
You had written:

Originally Posted by BOATMIK

In the terms of holding the model in your hands - put the bow of the main hull on the ground - grab the front and back of the ama - pull the back end of the ama up and push the front down. The lever arm of those torsional loads is the length of the ama divided by the much smaller distance between the ends of the X where they meet the hull. Torsional loads of this type will be 3 or more times those of a conventional beam setup.

Brian had written:

Originally Posted by Brian

Then we take the two free ends of the beams in each hand and push one down while we pull the other up. If there is any lack of rigidity in that remaining hull (torsional and/or bending), we will flex that hull to a point were it will build up a resistance and then suddenly transfer a great load acting to break the connections of our beams to the hull structure. Naturally the longer lever arm we have (full length of the crossbeam), the greater the forces we can impose on these connections of beam to hull. And if we have significant flex and /or play in the system, the dynamic loadings are even greater

An X-beam configuration possesses far less cantilevered length than the conventional full-length crossbeam, and thus imposes lighter loads to the beam/hull connections.

Aren't we describing the same operation here? My giant just cut the opposite catamaran hull off before lifting one crossbeam end, while depressing the other. He could have left the hull attached and just 'pitched the hulls in opposing vertical directions to accomplish the same act.

The point I was trying to make is, that in the conventional design, he has the full leverage length of the crossbeam to excert higher loads to the opposite side's beam-to-hull connection point. In the X configuration this leverage length is likely half that distance.

I'm actually more concerned with the vulnerability of these connection points than I am with the strength of the beams in torsion or bending. It's easy enough to design a proper beam for anticipated loads, but the connection points are much more troublesome.

Back to my Giant. If he has the two catamaran hulls in each hand and 'pitches' them in opposing directions he can exert higher loads at the the beam-to-hull connection points with the conventional designs, than if the X configuration enters the picture to cut down on the crossbeam length. Granted the 'box' at the X's center must be engineered to contend with torsional loads, but this becomes a matter similar to standard practices of how much and what strength material in the 'skins' of the beam's structure, and the overall dimensions (how far skins are from neutral axis) of this 'boxed beam'.....I-beam-ish stuff

[yipster]

roger all that but miss getting the last I beam stuff above

[brian eiland]

http://en.wikipedia.org/wiki/Beam_(structure)

http://www.engineersedge.com/beam_calc_menu.shtml

[yipster]

more at efunda and mathweb but still aint gettin why you mention "I-beam-ish stuff" when talking tubes

[yipster]

than again, i aint no engineer