Designing the Structure of 38' Catamaran

[TYD]

I'm designing the structure of a 38' aluminium one-off catamaran. I would like to know from you some tips and advice regarding this. Please write and post your comments.

The boat is a cruising one like a Lagoon for example.

Thanks and Regards,

Enrique Mich
Tridente Yacht Design
www.tridenteyachtdesign.com

[Manie B]

TYD your website is

horrible

get rid of your webdesigner and start over

it will loose you business

it is slow and hopelessly over complicated

[TYD]

Thank you very much for your comment about the webpage. I'll take it into consideration.

But for now I appreciate more some comments about aluminium multihulls.

Regards,

Originally Posted by TYD

Thank you very much for your comment about the webpage. I'll take it into consideration.

But for now I appreciate more some comments about aluminium multihulls.

Regards,

Wait till the aussie´s have finished breakfast, their business.
And your website is awful!

Regards
Richard

[Ad Hoc]

what type of catamaran, is it to class, flag etc...displacement etc etc??

[Ad Hoc]

just looked at your website

says you work with naval architects, engineers etc. So you're just a stylist as such?...in which case you'll need significantly more help than tips & advice in designing the catamaran! Understanding of structural principals, fabrication and materials for starters...

[TYD]

The catamaran is for Category 0 of ISO or RINA. Have a displacement of 7300 kgs.

I worked with others Naval Architects and Engineers of course. I know about structure, and I thought that could be a good idea talk on this forum about the work and maybe share thougths and images, but for now I only can see ''bad'' comments.

So, thanks again.
I'm wondering if was a good idea start this......

[TYD]

I'm going to put more information, so maybe somebody wants to write something clever to share with us.

Principal Dimensions
LOA: 11,48 m
Max Beam: 6,32 m
Displacement: 7300 kgs

On the FEA drawing you can see the Stress in Pascals.

Upwind sailing condition, principal forces.
D1: 5498 kgf
Forestay: 4099 kgf
Mast Compression: 14120 kgf

Regards,

Enrique Mich.

[Ad Hoc]

TYD
Interesting FEA plots.

You quote the stress in pascals. Is the plot, membrane stress, and which direction is the stress plot? What is the mesh size, and how did you apply the loads and also constrain the model etc etc? Have you modelled just the plating or have you included stiffening, if so, is it just orthotropic? So you see a simple "tip" is not so straight forward.

You misinterpret "bad comments". I have friends who are yacht designers. One just simply styles and shapes things, very successfully, but wouldn't know a stringer from a butt joint. Whereas another is a classically trained naval architect who is also a very successful yacht designer. The foundations of a discussion and assistance need to be understood before proving advice. Terminology and basic principals of engineering are not understood by everyone, no matter how often one gives "hints and tips".
I've been training up draughtsman and graduate naval architects for years, if it takes them years to understand the basics, how is a simple tip going to help someone without the prerequisite knowledge?
Said person will take "tip" and apply it everywhere without knowing if it is right or wrong to do so.

[TYD]

The stress plot that you see is the Von Mises stress. I modelled the plate and the stiffener members. You can see these as a white line on the FEA plot. Yes the plate and the sections and longitudinal stiffener, all are made of aluminium, so all is orthrtropic.

Ad Hoc, if you are Billy Doc, I' would like to have a nice conversation with you about your aluminium 44 footer and I can tell about this catamaran. I'm sending the next week the sections to cut.

Regards,

[Ad Hoc]

TYD

No, I'm not Billy Doc or anyone else for that matter.
You need to plot the stress in all 3 planes to ascertain which is the dominate axis and hence the structure is then designed accordingly.

How did you apply the loads to the model and how did you restrain it? What type of load case is this, plot suggets a pitch connecting moment, but could be anything... and how many load cases have you investigated?
What type of elements have you used and how is the thickness of the plate 'modelled'. Are the elements all plane, and what about their aspect ratio?

[simon]

the aluminium catamaran, I looked at, had bulkheads for mainbeam structures and for watertight bulkheads. When is a bulkhead stronger than a frame structure? Is there a weight-saving in using frames over bulkheads? Could bulkheads be replaces by frames and filled with lighter material inserts(aluminium or composite) to make them watertight?

Is it possible to reinforce the critical areas, like the transition between hull and bridgedeck with composite? I heard that on the big alu catamarans ferry steel is used to reinforce areas.

Simon

[Ad Hoc]

Simon
Woahhh...many Qs all leading in different directions!

To address this one "..Is it possible to reinforce the critical areas, like the transition between hull and bridgedeck with composite?.."
Structurally anything is possible, just requires creativity.

The main obstacle to main novel solutions is the practical application and where the solution sits in terms of regulations.

To reinforce "critical areas", how do you know this is a critical area?..if it is critical why use a material that has a low modulus? So, what do you mean by critical areas, can you clarify please?

[simon]

Ad Hoc,

aluminium seems to be more prone to stress cracking, if not properly built or designed. Apparentely, where the bridgedeck meets the hulls, higher loads exists, because of the different pitching movements of the 2 hulls.
looking at the aluminium catamaran designs, most have square transitions bridgedeck/hull. I guess that introducing the chamfer panels, like used in other design, could reduce the stress-risers in this area.

But for sure, you can tell me more about this. These are just my interpretations.

What if the aluminium mainbeams would have composite strengtheners to reinforce locally the structure? Either bolted or/and bonded on.
Of course this could be done by beefing up the aluminium, but has a weight penalty

[Ad Hoc]

Simon
In a pitch connecting moment only the last few frames (fwd and aft) are affected, not all.
Yes a transition of sorts is best to reduce the stress concentration.
The stress you calculate will determine what scantlings to use and how to load shed and transfer the shear loads.

"...aluminium seems to be more prone to stress cracking.." more than than what??...please explain.

Why would you reinforce with composite??..what is the stress and more importantly, the deflection in said region?

The deflection is the main driver, and you wish to use a low modulus material?

You need to establish what your stress is in all 3 axis and how to reduce any stress concentration by simple geometry and localised scantlings before you even consider any other alternatives. You need to prove to yourself, mathematically, that another option or solution is warranted. Not just say...i'll add composite to reinforce....this is just a throw away statement that has no meaning without some form of validation.