foldable cross beams...

[jpquattro]

Ok I am building one 8 m (26') trailerable catamaran.
In my project beams are 3, composite, demountable, bolted to hulls.
But now and then I can't resist and try to redesign the cross beams. The idea is to discuss the possible solutions, and eventually evaluate effects (modification on existing parts, etc.) on my cat…

I try to redesign it foldable.

The idea is that foldable beams would make assembly easier and faster.
I have discarded telescopic beams, both because I don’t have room inside hulls and I don’t thrust on telescopic solution.

Mi beams would be V foldable, that is, each beam would have 3 hinges, one in the middle one at each hull.

The beams would folds in different directions, the front fold in front-back direction, the rear beam fold in up-down direction.

The main beam is more complex, composed by two elements, one which folds front-rear direction, the other in up-down direction. Once opened, the two elements are bolted together, creating a T beam.

The mast is in the middle, exactly over the hinge…

The reason for that complex mechanism, is that if beams folds in different direction, the vertical paralelism between hulls is maintained during folding/unfolding. This means that folding/unfolding can be performed also on water…
I think that the beams can be built from uni glass (more elastic than carbon), with metal hinge included in laminations…
The question is: Would it work? Weak points?

Paolo

[ancient kayaker]

http://www.youtube.com/watch?v=C8kUq4BouS0

[upchurchmr]

Test your metal in glass lamination very carefully. Don't just do a single load test, apply and release the sailing load a lots of times.
Such metal to composite joints are really difficult to keep working for the life of the boat. So much so that the aircraft company where I work will not even try. Some aircraft have had such joints, it usually takes years before the design and manufacturing process makes something that lasts.

Better to just bolt the joints on. Those have enough problems.

Three joints per beam will mean the boat will not be rigid and the continuing motion at the joints will leave a large possibility of them wearing out. Make the joints heavy and as tight as you can possibly make them.

[jpquattro]

Originally Posted by upchurchmr

Test your metal in glass lamination very carefully. Don't just do a single load test, apply and release the sailing load a lots of times.
Such metal to composite joints are really difficult to keep working for the life of the boat. So much so that the aircraft company where I work will not even try. Some aircraft have had such joints, it usually takes years before the design and manufacturing process makes something that lasts.

The problem is that I need one hard surface for hinges. I have some ideas about how I must do it, but I am interested to alternatives...
I think that the metal part is substantially a tube and glass is wound around it, not only laminated over...
But I don't know if ti will last...
One alternative would be aluminium with welded or riveted hinges... the joint between tube and hinge is another weak point...

Three joints per beam will mean the boat will not be rigid and the continuing motion at the joints will leave a large possibility of them wearing out.
Make the joints heavy and as tight as you can possibly make them.

Unfortunately three joints hare the minimum for a foldable cross beam... or not?
One of the purposes of multi direction folding is to reduce the movements...
The hinges would be large, with the pin substantially off axis with respect to the beam, and another wing of the hinge on the opposite side of the hinge.
The wings line up when the beam is alligned and anoter pin (or bolt) can be inserted and lock the hinge.
This should reduce the movements and wear...

Paolo

[gonzo]

The Gougeon Brothers, of West System, have succesfully built high performance multihulls with epoxied on metal brackets. You should check out their website.

[ancient kayaker]

A few comments:

Simplicity works best and lasts longest
The more joints, the more there is to go wrong
Sliding joints are the strongest but also most likely to jam, pivoting joints are weaker
The joint has to support the load folded or extended but the highest dynamic loads will be applied when sailing with the beams extended; it would make sense to build it so the joint is not the only supporting element in that condition . . .

[upchurchmr]

Check out this folding mechanism http://www.cat2fold.com/

"I think that the metal part is substantially a tube and glass is wound around it, not only laminated over..."

With such a simplistic description there is no way to know, show a drawing, get the comments, then get a naval architect or engineer to design it, then test.

"One alternative would be aluminium with welded or riveted hinges... the joint between tube and hinge is another weak point..."

The best thing about this is that it is made of known mechanical elements, the ability to design and analyse such a joint has existed for a long time. See the comment about some kind of an engineer helping.

"The wings line up when the beam is alligned and anoter pin (or bolt) can be inserted and lock the hinge.
This should reduce the movements and wear..."

The clearance of the pin to the lugs of the hinge is where you need to have very close tolerance (minimum clearance from the pin to the plate). A tapered pin will take up the tolerances if both the upper and lower lugs are reamed to fit at the same time. Unfortunately, taper pins are sometimes hard to remove.

You might also look at this site: http://slidercat.com/blog/wordpress/?p=595 I don't think the actual construction details are worked out.

[jpquattro]

Originally Posted by upchurchmr

Check out this folding mechanism http://www.cat2fold.com/

I know it, but i see two weak points:
1) don't fit in my hulls...
2) it relies mainly on dolphin striker to support mast load

"I think that the metal part is substantially a tube and glass is wound around it, not only laminated over..."

With such a simplistic description there is no way to know, show a drawing, get the comments, then get a naval architect or engineer to design it, then test.

You are right, I must prepare at least some sketch...
I have a plan, I am only evaluating one alternative, for now let's neglect the engineering work (I have computer competence and software, engineer friends, I can do the homework...) but numbers sometimes tells something different from the possibilities of homebuilder...
more, there are problems that go beyond the proper mechanical design: epoxy on metal last long enough?
If I increase the glue surface, the metal-epoxy joint last significantly longer?
a tapered pin will seize?

"One alternative would be aluminium with welded or riveted hinges... the joint between tube and hinge is another weak point..."

The best thing about this is that it is made of known mechanical elements, the ability to design and analyse such a joint has existed for a long time. See the comment about some kind of an engineer helping.

disregard alu, I don't trust of my version of metal beam...

"The wings line up when the beam is alligned and anoter pin (or bolt) can be inserted and lock the hinge.
This should reduce the movements and wear..."

The clearance of the pin to the lugs of the hinge is where you need to have very close tolerance (minimum clearance from the pin to the plate). A tapered pin will take up the tolerances if both the upper and lower lugs are reamed to fit at the same time. Unfortunately, taper pins are sometimes hard to remove.

If I replace the pin with a bolt, I rely on friction, and tollerances are less stringent... or not enough?
A main beam is usually a tube or a box... If i make it with a T section it will take the mast load, but I have lost part of torsion stifness... Is this a problem?
I have proposed a lot of question mainly to esplain my doubts... I know that math would answer most of them...

Paolo

[upchurchmr]

Paolo,

Increasing the area of the metal to glass/epoxy joint is the best way to make it last longer.

The tapered pin allows the clearance between the pin and lug to be taken down to zero, but it is easy to push the pin with enough force that the friction is difficult to overcome. Seizing is another possible problem.

If you use a bolt and nut, you may need to have a separate bolt on both upper and lower lugs. If you have only one bolt you will squeeze the lugs together, which results in less friction force and allows the joint to move producing wear.

A T section will work for the bending caused by the mast load, but will be heavier than a tube or square tube. You also need to have an upper and lower plate (the lugs) for the joints which will be easier to attach with a closed section (tube or square tube) than a T. Torsion will be better with some kind of a tube, some people want flexure, but it still needs to be able to survive the twisting.

When you get ready, make up a test sample of the joint you want - connected to 4 foot beams on each side, support it on 2 inch blocks at each end, invite your friends over and stand on the beam to see what happens. If it takes one person add two or as many friends as you have. Save the beer for after the test. You will quickly see just how stiff your design is.

My wife and I did the standing test and were completely satisfied as a result - just a simple hollow rectangular crossbeam.

[Fanie]

Have to agree with Upchurchmr, simplicity is best. Hinges needs to be oversize to wrestle torque's and comes out heavy.

Off late I designed a folding cat using a short round alu tube hrough the hulls to keep them upright, then at the water insert the wide ones and a fold down deck.

It would be nice to see what you have in mind, 8m is a nice size for daysailing and have some space.

[jpquattro]

Originally Posted by Fanie

...
Off late I designed a folding cat using a short round alu tube hrough the hulls to keep them upright, then at the water insert the wide ones and a fold down deck.

It would be nice to see what you have in mind, 8m is a nice size for daysailing and have some space.

I have evaluated exactly the same solution, discarded it because the tube would take too much internal height (I am 6'2")...
Here you can see my "shipyard"...
http://digilander.libero.it/Paolo.Zi...%20Catamarano/

about foldable beams I will post some sketches...

Paolo

[Sand crab]

Or, you could just buy one of the old MacGregor 36 cats. These are made to disassemble and are trailerable and are very affordable and pretty fast.

[HollaStern]

You could attempt to remove the hinges from the loading equation by using mated conical ends to each beam section then running an internal cable to pre-load the beam. - think tent pole meets pre-bent. Spin pole. - you will have to over build the beam due to the extra compressive loads but you'll have to overbuild something and this should give you a stiff beam without extremely precise hinges.

[ThomD]

There was an interesting Australian or NZ design where the deck expanded like a play pen. Seemed really promising, but I have not see anything new on it in a while. Anyone remember that one?

I think the folding cat is a whole lot of pointless pain. Trimarans work in that format, and so do bonded trailer width cats. I have yet to see a folding cat that I would want to build for trailer sailing. And there are virtually no multihulls I would want to own for off the trailer sailing.

It takes X effort to built a multi. Xx2 to make it trailerable, and Xx3 to make it trailer sailable. That could be a slight exageration, but you will go less wrong to believe it than not.