Arched Or Flat Crossbeams ?

[mcm]

Why do some catamarans connect the hulls with arched crossbeams but most use flat crossbeams?

Aren't arched beams inherently stronger ?

 

[Ad Hoc]

It depends upon the width between the hulls. the further apart the hulls, is better to curve, or arch them, to prevent the beams from being submerged by on coming waves. Which would add a sudden and significant drag, rather like a crash stop...not ideal at all!

So the distance apart, and the freeboard tend to dictate how the beams are arranged.

 

[gonzo]

Also, on some beach boats like the HobieCat, it prevents the canvas trampoline from sagging and collecting water.

 

[mcm]

Thanks for the responses.

So, keeping the beams above the waves is the reason for arched beams - not increased structural strength.
Oh, and reducing tramp sag.

 

[SamSam]

So, keeping the beams above the waves is the reason for arched beams - not increased structural strength.

I think you need more input before making that assumption.

 

[mcm]

@SamSam,
i agree, more input would be more useful.
but at least we all know Ad Hoc is very knowledgeable.

 

[SamSam]

http://www.john-shuttleworth.com/Articles/52AeroDesign.html

I don't know anything about it, but looking at the above diagram it intuitively looks to me that an arched beam would better resist the mast loads, especially in a stayed mast, and would better resist the deflection caused by the two moments. It intuitively seems that an arch gives you a pre-stressed beam and it intuitively seems that a pre-stressed beam made flat by forces is stronger than a flat beam given a negative shape by forces. Intuitive meaning wild *** guess.

Actually though, looking through images, I find very few arched cat beams.

The detail below is mainly because I like the looks of it.

http://www.john-shuttleworth.com/Articles/52AeroDesign.html

 

[Ad Hoc]

I don't know anything about it, but looking at the above diagram it intuitively looks to me that an arched beam would better resist the mast loads, especially in a stayed mast, and would better resist the deflection caused by the two moments.

Unfortunately the diagram shown is not correct. The beam shown indicates that there is no moment on the windward hull at the beam hull connection, just a 6.75tonne vertical load. Well, this implies the beam to be simply supported, which it clearly is not. Thus there shall be moments on each beam end.

Additionally looking at just the mast compression load is a "simple" 2D analysis. These cross beams must resist global loads, i.e. global bending, shear and torsional loads. Which, depending upon the arrangement can be significantly greater than the simple mast compression load. Not to mention any torsional load from an eccentric sail force on the mast base itself too.

 

[SamSam]

http://www.john-shuttleworth.com/Articles/52AeroDesign.html

Here's another diagram from the site, showing the beam with a stayed mast. I didn't notice it before.

I don't think he's trying to show every load/force on the beam, just the major ones. One is the mast load and the other is from the dagger board. He only uses one retractable dagger board in his catamarans, so he wouldn't show any dagger board forces exerted on the other end of the beam, as there isn't any db. In this view the beam ends are still shown deflected differently from each other, one being effected by the daggerboard, the other not. The left half is an s curve, the right half a simple arc.

Actually though, I just used that diagram because it's the only one I could find that attempted to show the forces on the beam. Whether it's correct or not is not the main thing. I was questioning the assumption that catamaran beams are arched solely to keep them out of reach of waves and to drain the trampoline.

 

[mcm]

I don't know anything about it, but looking at the above diagram it intuitively looks to me that an arched beam would better resist the mast loads, especially in a stayed mast, and would better resist the deflection caused by the two moments.

intuitively, i would think so as well.
but unfortunately like you, i don't know anything about it either.
maybe someone who does know, will give us a clear example proving or disproving our intuitive assumptions.

 

[Ad Hoc]

maybe someone who does know, will give us a clear example proving or disproving our intuitive assumptions.

Like most things, it depends. It depends upon the radius of curvature to the depth of the beam, as well as where, if any, points of inflection there are along the length of the beam and it depends upon the end fixity and it also depends upon the method of fixing to the hull, i.e. direct or as some are turned near vertical, and the flexibility/stiffness of the hull it is attached to as well.

Since to calculate this we establish the “work done” in bending the beam to find the amount of strain energy and then using Castigliano’s Theorem, which means a displacement in a direction of the applied load is equal to the strain energy in the system, wrt the load applied. We may be able to reduce the bending moment but the overall deflection may increase, thus it’s Structural stiff, EI, is best to calculate, since the “E” part is the material property. And low modulus materials are always deflection driven not stress driven.

But as noted above. The mast compression load is just one load to consider. The cross beams are designed to take the global loads, not just the mast compression..

 

[El_Guero]

Why do some catamarans connect the hulls with arched crossbeams but most use flat crossbeams?

Aren't arched beams inherently stronger ?

From a simple perspective, yes. BUT.

Let's look at a very simple example - Compare 2 rocking chairs.

On 1, you bend the wood to create curve for the rockers.

On 2, you cut the curved rockers out of a much deeper piece of wood, say 2x12". Now the grain does NOT go all the way across.

The second rocker will not support as much load as the first. And you should expect the wood to split much sooner.

So, the arched beam must be built for the arch.

 

[rwatson]

I dont see any reference here to the incorporation of the Dolphin Striker to cat beams.

Very few cats using simple aluminum beams that dont have a support underneath the beams.

The other big concern about bent arms is simply the extra trouble of construction and assembly. Simple beams help in these areas.

 

[Petros]

the deflection of the beam is a function of the load, the cross-sectional properties (i.e. area moment of inertia), and the elastic modulus of the material. The beam being straight or curved is unrelated to total deflection.

IOW, a curved beam and an identical straight beam deflect the same amount under the same load. a curved beam does not reduce deflection, it cost more to make and uses more materials (a curved path is longer than a straight one).

The only two reason for a curved beam, one is as stated above, to keep the beam out of the water and waves. The other is because it looks sexy and more high tech (helps sell the boat design, a very important consideration).

 

[Ad Hoc]

IOW, a curved beam and an identical straight beam deflect the same amount under the same load.

That's not correct. There are many factors that influence this and is not necessarily cart blanche either, see my notes above on the caveats.