Certainly, there are multiple ways to do this in Rhino, but here is one way we have found that seems to give good results:
1) Start with a half-hull of your sailboat model. Here we have used the Orca3D Hull Assistant to create the hull geometry. Then create one side of the keel geometry. In this case we used the Orca3D foil assistant and made sure to uncheck the boxes “Create Both Sides” and (Cap the) “Root” so that we would only get one side of the keel and the top would not be capped since it will be trimmed off anyway. We’ve also placed the keel in the proper longitudinal and vertical positions, ensuring that the keel protrudes through the hull bottom. The details of the hull and keel design are of course up to you! They don’t have to come from the Orca3D assistants. The result is shown below.
2) If you have a connected bulb or a capped tip on the keel, then you will need to explode the keel polysurface into individual surfaces.
3) The keel generated from the Orca3D foil assistant has a flat trailing edge. It is necessary to separate the trailing edge from the rest of the keel foil shape in order to be able to fillet the surfaces. Split the keel surface along an isocurve at the trailing edge crease as shown below (Surface->Surface Edit Tools->Split at Isocurve). NOTE: If you find that you are unable to split the surface, then you will need to decrease your model absolute tolerance. (Tools->Options->Units)
4) Now, you can use the FilletSrf command to first create a fillet between the main part of the keel and the hull surface and then between the keel trailing edge and the hull surface. You need to set the Extend command option to “No” for the FilletSrf command and of course choose a reasonable radius for the fillet. When you fillet the surfaces, make sure to select the surfaces on the sides that you want to save. The result should look something like the figure below where the keel is shown in red, the hull in blue, and the keel trailing edge in green, and the two fillets in gray.
5) Take a look at the two fillets and notice that at the keel end they do not line up. If this is the case for you it is a simple matter of moving the control points at the keel end of the trailing edge fillet along the keel until they line up with the other fillet.6) Trim off any remaining length of the trailing edge.
7) Use the Rhino BlendSrf command on the open edges of the two fillets to connect them with a fair curvature continuous surface. You should end up with something like the figure below.
8) Join all three fillet surfaces together and join the trailing edge of the keel back to the keel.
9) Use the fillets to trim the hull surface. You should now see something like the figures below where we have also mirrored the surfaces.
10) Check the surface normals with the Dir command and correct as necessary so that all normals point outboard. If you wish you can join all surfaces together into a single closed polysurface with no naked edges.