Aluminum catamaran crossbeams

At this stage its just a thought process but it is something I am seriously thinking about. I read of a professionally built older design Wharram from South Africa that was built in aluminum and uses aluminum cross beams. I know of at least one other Wharram that has used foam/ glass beams. (Boatsmith in Florida), and have read about a smaller Tiki with CF beams.

I have a Wharram Tiki 38 which has 4 wood I beams for the structural connection. The beams are a 36mm ply web with top and bottom flange 130mm wide and ~ 90mm deep. They rest in troughs with bearing pads and internal structural reinforcement on the hulls inner and outer edges.

The thing about them is that they are seriously heavy. That creates a number of issues for separating the boat for storage or transport or moving them for repair. After building them I remember how hard it was for 2 fit and strong guys to move them around. It was right at my limit.

I know an aluminum beam would be significantly lighter and knocking a few 100 pounds off the boat would be a good thing, though not the motivation for the proposed change.

What sort of specialist should I consult to figure out what I would need to use to provide the same capability as the existing beams? Naval architect, structural engineer? And is there anyone on this forum that would be able to do those calculations?

Thanks

 

To replace these wooden I-beams on the face of it, is simple.
However, it is more about the behaviour of the vessel - by that I mean the response of the vessel to an input, a wave, and how the structure responds to such.

Since designing/building an aluminium equivalents, is straight forward naval architecture/structures. But one cannot design these main structural members in isolation.
Those wooden I-beams, would need to allow for a degree of displacement, or movement. Is that critical, likely yes, since that seems to be the MO of that design type...the wrapping/lashing of these beams (is a partial fixity).
Then what or rather how, is the beam attached to the main structure of the hull, this is the load path. What local structure is there at said fixing locations and can this structure shirk the load to the surrounding structure, and do so without deformation or increasing the stress, compared to the wooden beams.

So, it is simple enough, but doing so, will change the behaviour of the vessel, by that I mean the structural response and vessel motions (owing to a change in displacement between the members).
How much...ahh..that's where the investigation and circulations will dictate what you do.

But I would recommend that you select a section that has torsional stiffness, like a box section, rather than an I-beam.

 

Are your beams a constant depth along their full length? I have seen some Tikis where the beams were tapered (re height) at the ends.
Here is a neat little video explaining how to calculate the second moment of area 'I' of an I beam -


Put your dimensions into the formula (making sure that your units are consistent - eg don't combine mm and metres) and see what 'I' you get for your plywood beams.

Now then, AI tells me that the Modulus of Elasticity for a plywood I beam can vary - they said :
" For a structural plywood I-beam (or I-joist), the Modulus of Elasticity (\(E\)) is typically between 1,500,000 psi to 2,100,000 psi (10.3 GPa to 14.5 GPa), depending on the wood species and the direction of the grain. In I-beams, this stiffness is calculated using composite action between the flanges (laminated veneer lumber) and the web (plywood or oriented strand board)."

In contrast, AI also tells me this regarding the Modulus of Elasticity for aluminium -
"The modulus of elasticity (Young's modulus) of aluminum is approximately 69 GPa (gigapascals) or 10,000 ksi (kilopounds per square inch).
I dunno about ksi; gigpascals are easier to understand.

If we use a Modulus of Elasticity (E) of say 11 or 12 for your plywood beams, then the Modulus for the aluminium is approx 6 times that of the plywood.
Or E(a) = 6E(p)

This link helps to explain the bending stress formula : M/I = Sigma/y = E/R
Bending Stress: Definition, How it Works, Calculation, Types, and Examples https://www.xometry.com/resources/materials/bending-stress/

Or M = Sigma x I/y (the small x is a multiplication sign)

Where Sigma (the little 'o' with a squiggle on top) is the bending stress.

This link tells me that the allowable stress for plywood is between 2,800 and 3,300 psi -
https://files.engineering.com/files...sign_and_Fabrication_of_All_Plywood_Beams.pdf

Scroll down to Section 1.5 'Allowable Stresses' on page 6.

And you can calculate the I and the y for your plywood I beam, so knowing Sigma for plywood, we can then calculate the bending moment M.

I think that we can safely assume the same M for the new aluminium beams (@Ad Hoc is this correct?)

Google tells me that the maximum yield stress for 6061 aluminium is 35 ksi (241 MPa) and the allowable bending stress might be 70% of this.

We now have M and Sigma; if you now look up suitable aluminium box beam sections (as suggested by Ad Hoc) they might give you values for I, to save you calculating them.
If they don't, then you can calculate them using the formula
I = (B x D x D x D)/12
where B is the width of the box, and D is the depth.
The second moment of area (or area moment of inertia) of a box beam is calculated by subtracting the properties of the hollow inner void from the properties of the solid outer rectangle - ie calculate 'B D cubed over 12' for both the inner and the outer surfaces.

You could try using a maximum depth that you want your square hollow box beam to be - in this case the 'y' value will be half of the depth.

And I = M x y / Sigma so you can then calculate what the I for your ally box beam will have to be.
If you reduce the wall thickness of the beam, then you have to increase the depth (and / or the width) to compensate, and vice versa.

However you will also have to take into account how the ends of the beams are attached to the hulls, as mentioned by Ad Hoc in his post above.

 

@bajansailor, the reasoning, as a whole, seems quite correct. The theory is sound. How to put it into practice is another matter.
It would be necessary to specify the design stresses of the various materials mentioned, along with the allowable stresses, according to the regulations that will be applied to the boat.
It's possible, though it would need to be verified, that the plywood crossbeam is significantly oversized. For an I-beam to have a sufficient moment of inertia about its Z-axis, it needs to be considerably oversized. Therefore, in my opinion, the existing bending moment cannot be deduced based on the geometric properties of that beam. Furthermore, since the aim is to save as much weight as possible, the described method doesn't seem to be the most suitable for obtaining the best possible solution.
On the other hand, and perhaps most importantly, it's necessary to define how the new aluminum crossbeams are joined to the existing structure. Without having designed that connection beforehand, everything else may not be correct, since the greatest stresses will likely occur at those fixed supports.
What is your opinion?

 

Wow, thank you AdHoc, Bajansailor and TANSL. That is all very helpful. I had decided on hollow box section beams, my intuition of what would work best.
I suspect, but really don't know, that the cross beams are massively overbuilt. These things are heavy! Did I mention that? I will look over the calculations and see what I can understand of it all.

The beams are tapered with a curved top and straight bottom. The longest beam for example is roughly center height at 330mm, tapering to 220mm over a half length of ~3m. I imagine a tapered aluminum beam would be a custom item, so can a symmetrical beam be used?

There is one short central beam that holds the main (aft) mast, that is supported only on the insides of the hull. The others sit in rectangular troughs that cross the hull. Each of these have significant support structures beneath them.

Many of the other elements of the boat rest on the beams, including the main mast, the cockpit, or pod as Wharram likes to call it, the box that supports the foremast and anchor rollers etc. So a beam of the same height would work best, avoiding building up the tops to provide the right height.

Each beam is lashed on the inside and outside of the hull through a hull mounted bearing block. There is a significant amount of reinforcement under each trough.

I really don't know how much flexibility is ideal. I don't remember Wharram giving any figures in the plans or building guide. Given how the boat feels when the lashings are tight enough to allow minimal movement, my preference is for less rather than more flexibility. I find the boat feels and sails better that way.

 

For the sake of simplicity, and in the absence of any other data, yes.
But, as noted, is the assumption that the new aluminium beam, is to mimic the same responses as the wooded structure and lashings, but, it is just made out of aluminium?
That's the tricky part...since wood is a lower modulus material, it shall deflect more than aluminium. So, getting the EI, of the wood and the EI of the aluminium to be the same is the key, if the feeling and response of the vessel is to be maintained.

 

Yes, it would certainly be much easier to use a symmetrical beam of uniform cross section area.

It sounds like you would prefer to have a more 'rigid' connection of your new aluminium beams to the hulls, rather than the traditional lashings?
Can you maybe post some photos showing how the beams are currently lashed to the hulls?
What are they lashed to, and what sort of supporting structure is underneath?
Are there bulkheads or maybe deep web plywood frames underneath?

 

Thanks. I can post some pics later, I have a few projects going right now.

The beam troughs sit on top of ply bulkheads. They also have small timber reinforcements from hullside to hullside underneath the trough and ply doublers the width of the trough on the hull. I am no expert but there does seem to be a fair bit of structure.

I have a thread on here about the beam lashings coming loose. If you can find that there is a description of the lashings and a couple of pictures that will give you an idea of how it's done..

 

Here is a link to your long running thread last year that mentions the beam lashings coming loose -
Wharram and webbing beam straps https://www.boatdesign.net/threads/wharram-and-webbing-beam-straps.70334/

There are a couple of photos in post #37 on page 3 of the beam lashings.

The surrounding supp0rt structure (not to mention the end section of the beam) does all look fairly substantial.
If you decide to have fabricated some square or rectangular hollow section aluminum beams, I am wondering if it would be feasible to weld plates on to the ends, and then use multiple bolts through the deck and backing pads to secure the beams?
Or would this then create 'hard spots' in way of the attachment plates, because everything is now too rigid?

 

I am hoping that I can find suitable stock sections. If I have to get custom beams I suspect the cost would be out of my reach and not worth it for a boat with as little value as a Tiki 38. Many of the deck structures are fastened to wooden cleats glued to the beams so there will need to be some welding to be done as it is.

I'm not especially worried about the bit of flexibility in the structure, though I'm also not convinced there's enough to make any serious difference when the lashings are properly tightened.

TANSL and I have been talking about the idea and I am going to provide him with the scantlings and details of the beam supports when I have the time to put it all together.

The loosening of the lashings is part of the thinking that led to alternative beam material. For me to move these to redo support pads or just to check the beam troughs for rot, which I understand is a common problem, will be a serious and expensive undertaking. Lighter beams would allow me to easily do it, perhaps with a bit of help.