Hi All!!
I am building the hull of a sailboat. I created through the command loft, an open polysurface from several contour lines.
I’d like now to rebuild the open polysurface to reduce the amount of control points but command won’t allow me to select the hull, like if ti was not accepting the open polysurface as input to rebuild.
I’m new to Rhino so apologize if it is a stupid question, but how can I solve this? 
Polysurfaces cannot be rebuilt, only single surfaces. However the main problem is why Loft produced a polysurface and not a single surface. The reason is because your input curves are probably bad - one of them at least may have a kink or other problem that is causing Rhino to make a polysurface. When a Loft makes a poor surface or polysurface, the first thing to do is check and fix the input curves. You should most likely rebuild those, preferably with the same number of points for each, and/or do some point editing. Hard to say really without seeing the curves.
oh I see thanks a lot!!
So if I loft input curves I indeed get a surface. But then I mirror it and join the 2 surfaces (the two halves of the hull) and I think it is there where I get the polysurface.
Anything I can do to have 1 surface from the two halves (instead of a polysurface)?
You don’t want to do that - trying to merge two hull sides into one surface will just create a lot of trouble. Leave it as a polysurface. If you want to rebuild the Lot, do it before the Mirror + Join. Better yet, rebuild your input curves so the loft is perfect and has few control points and does not need to be rebuilt. It’s also possible to rebuild stuff inside the Loft command, but i don’t recommend it. Start with good, clean curves and you will get a good, clean loft.
Boat hulls are usually symmetric from side to side. Because of that it is usually it is best to create and adjust one side of the boat hull, and then use Mirror to create the other side if it is needed for calculations, rendering, etc.
great!! thanks a lot guys!
The bes way I know of (and I’ve created scores of boats using Rhino) is to use the least amount of input curves as possible as with any NURBS surface, the simpler the better to create only one side of the hull. Any refines required you should always make to the single surface you started with and a good idea is to have history on when doing so such that down the road, you can pinpoint a stage of development of that single surface that you wish to change without undoing everything. Here is an example of an 80’ catamaran I designed 12 years ago for a restaurateur in Baltimore for the charter trade. I include this as it has two mirrored surfaces that were then joined, moved away from the centerplane and mirrored yet again to create the twin hulled vessel as part of the process of creating a catamaran.
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