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Hi I’m struggling to get a successful run of the Holtrop analysis Speed /power. I’m using Rhino 7 and Orca 2.0.21 in Win 10 64bit.
Is there a basics guide somewhere? What are the requirements of the input surface? So far I have a half of the hull surface that has had the appendages trimmed and has been globally smoothed. The surface is made up of 2 open curves. Is this a problem?
I have tried with .IGES and .Steps files and I get an error to check my surfaces. I have also crashed Rhino on some attempts. The best result I had was a series of outputs but all the power requirements reading ‘NaN’.
The job is a 10M motor cat with a 9598.5kgf displacement.
Looking forward to your kind advice.
Br
Ben
Hi Ben,
Yes, the Orca3D Help file has a section on the Holtrop routine, including a section on Model Requirements.
Via your support ticket I have requested your Rhino file, if that’s possible. We treat all customer data as proprietary.
Thank you,
Bruce
Dear Ben,
Thanks for sending your model. It looks like there are a few things going on, mostly straightforward to address.
- First Orca allows the user to choose any coordinate system orientation for their model. With that flexibility comes the requirement that the user inform Orca which way is considered forward and which way is up. You need to set that in the OrcaProperties command as shown below:
- Orca uses the Design Condition to define the flotation plane at which the Holtrop analysis is performed. It is important to remember to check the mirror checkbox since you are only using half of a hull and you must specify a reasonable LCG location for the model
- The last item is a little more philosophical in nature. As you know the Holtrop method was not designed for multihull resistance prediction. We do have users sometimes use it for a catamaran like you are doing, but you must remember that there were no catamarans in the database of experimental results used to develop the Holtrop regression. Therefore I would view any results with a somewhat “suspicious eye” since things like hull wave interference effects and other physical effects are not captured. Having said that, if you are going to try to use the Holtrop for a catamaran the idea is that you are treating one hull of the catamaran as a vessel by itself, and then double the results to account for the other hull. Therefore, you need to take what appears to be the starboard hull in your model, and split it down the centerline and then only use the starboard half of that hull. Again, remember you will need to double the resulting resistance prediction to account for the other hull.
I hope this answers your question but let me know if you run into any issues.
Best regards,
Larry Leibman
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