Orca3D News

Tuesday, December 8, 2009

Multihull resistance prediction

We are sometimes asked about resistance calculations for multihulls in Orca3D. Orca3D doesn't have a resistance method that is specifically for multihulls; however, the planing hull and displacement hull analysis can be used to predict the resistance of a single hull, assuming it fits within the valid parameters of the method (which is shown on the report along with the resistance data). Obviously you need to enter the parameters for a single hull (e.g. half of the total displacement), and then double the predicted resistance to get the total resistance (for a catamaran). Depending on the type of multihull, you may want to add some margin for interference drag between the hulls.

If you try to run a prediction for your multihull and get an error, you may want to consider these common sources of error:
  • not selecting only half of the hull (and only up to the chine for planing hulls)
  • incorrect model orientation (positive X should be in the aft direction, positive Z in the up direction)
  • incorrect outward normal direction on the surface(s). When you select Direction in the Analyze menu, the arrows should point into the water.
  • not positioning the centerline of the hull surface (even if it's a multihull) on the Y=0 plane
Note that the current release (1.0.14) has an issue where once you get an error while in the dialog (for example, you enter an invalid speed value), you need to save your file, exit Rhino, and re-start it. This has been fixed in the next release. The next release will also allow you to orient the model in any direction in the coordinate system (i.e.,doesn't require that positive X be in the aft direction).

Also, we sometimes see that people have not set the units as they expect; you may want to go into Orca3D roperties, and in the Units section, be sure that the speed is set to knots and the power unit to HP.

If you feel that you need a prediction method that is specifically designed for multihulls, you may want to consider the NavCad software from HydroComp, Inc. (HydroComp supplies the resistance library that is used in Orca3D). You can read about NavCad at www.hydrocompinc.com. We'd also recommend reading the technical paper "Predicting Catamaran Resistance with NavCad" from HydroComp on multihull resistance prediction, at http://www.hydrocompinc.com/knowledge/whitepapers/HC105-NavCadCatResistance.pdf.