"Z40 Roadster" - A Sailing Catamaran

Inquisitor

This is my first hobby project using Mecway. It’s a 40 foot sailing catamaran. I don’t know if it will ever get wet, but the journey in design is half the fun. This is my course model of the full catamaran. Basically, I’ve modeled all the main structural members. I expect this model to be good enough for giving me global deflections and behavior under static, modal and possibly dynamic response. I don’t expect it to give accurate stresses since the element size with respect to stress concentration size is too large. But I am hoping it will help point me to areas where detailed analyses will be required.

Some basics of the model used in Mecway.
• 63,300 Nodes
• 379,600 DOF
• 23,400 Quad8, Tri6, Line2 elements
• Orientation adjusted on all shell elements so that accurate local coordinates systems can be established for the Laminate properties
• Laminate properties on much of the structure
• Offset shell properties on much of the structure
• Gravity, distributed masses, point forces and hydrostatic pressure

I can’t say enough about the new hydrostatic pressure introduced in Mecway version 0.4. Compared to my earlier work with earlier versions, I was really dreading the calculations required to model accurate submerged hull pressures. With this feature in Mecway, it was a five minute task of which most was just reading the documentation.
This model runs in 37 seconds on my three year old i5 computer. It’s almost anticlimactic… I don’t even have enough time to get up out of my office at home and go into the kitchen to get a cup of coffee. This program rocks!

Images below. The first two show the Google SketchUp model. This is what I use to do the geometry and designing the basics like making sure there is proper distribution of weight, adequate headroom and to rationalize how pieces fit together. This was then exported using stl format for import into Mecway. The tedious part then begins. There is probably close to a hundred hours of clean-up and element generation in what you see here to give the near 1:1 element aspect ratios. At least I didn’t have to pinpoint nodal coordinates. That part translates over very well.

Inquisitor

By far and away the most highly stressed components on the Z40 are the Swing Keels. For those not familiar with sail boat jargon, this is a wing that runs vertically in the water so that the lateral loads on the sails are resisted. Instead of crabbing sideways, the boat tends to go forward more than laterally. In this design, they are intended to swing back parallel to the water for several reasons:

(1) For traveling / anchoring in shallow water
(2) In the rare case that they strike some underwater obstacle at speed. The intent is they swing up instead of shearing off as can happen with dagger board designs.
(3) In light winds or racing conditions, one can be raised to reduce drag.

The down side… they are basically being held on by one very large bolt.

Here is the detailed Mecway analysis.
• 64,000 Nodes
• 344,000 DOF
• 24,480 Quad8, Tri6, Hex20, Wedge15 elements
• Orientation adjusted on all shell elements so that accurate local coordinates systems can be established for the Laminate properties
• Laminate properties on much of the structure
• Offset shell properties on much of the structure
• Gravity, applied pressure regions, compression only surfaces.

I’ve finally found a way to get a coffee break…

This model, when using a totally linear static analysis can run in about a minute. However, a portion of the bolt bearing surface would be in tension which isn’t possible in reality. This being the highest stressed region, it would be poor engineering to trust these non-conservative results. Also the bearing surface of the upper portion of the swing keel (the portion pressing against the boat) is only in partial compression. Not all parts are in contact and thus, again, the model would be inaccurate. Fortunately, Mecway, as of 0.4, has a great new compression only boundary condition. This was used to great advantage in this analysis.

This being a non-linear affect, Mecway has to analyze the model and determine which nodes are in tension. In subsequent analyses, it releases those DOF. It then has to make sure that those nodes do not pass through the plane of the element that once had fixed DOF. So… as you can imagine it has to toggle iteratively those DOF until on all are “right” with the world.

I set the convergence tolerance to 0.0001 and it took over six hours. Good coffee break.

VMH

Inquisitor, did you use linear (without midside nodes) or quadratic (with midside nodes) elements? I just curious if there were any differences for your analysis. It seemed that you manually meshed your model that was imported from Google Shetchup exported STL file? Thank you

Inquisitor

I recall importing it in and leaving it with linear elements while I mirrored and stitched the sections together. Then, I would use custom refinement to bump up the refinement in areas of concern. Also, since I'm using composite properties I had to be mindful of local material coordinate systems by re-orienting or re-versing element connectivity and adding material properties. THEN at the very end, I'd change element shape to using quadratic elements.

VMH

Thank you!