[Extruding outside the spline]

hi, pretty new to this software but can anyone tell me why im getting strange results from my extrude.

I have a closed 3d spline, im symmetry over x and z planes, im trying to create a recess where the spline is.

 

but im getting this on the back of the volume

have tried to put the spline in a couple of times and had similar anomalies in different parts of the volume.

what am i doing wrong?

[Programme not responding when carrying out simple extrudes]

Hi,

Not sure if anyone can help, i am a very new user to the 3dcoat printing software and voxel style modeling. I have completed a few tutorials and am going it alone building a spaceship.

Currently i am trying to extrude negatively into a capsule which is cut in half to give an air intake feel, when ever i try this command it takes the computer a good 5 mins to calculate the output. I run 32gb of ram and have a gen 8 i7, yet when i look at the resources being used while it does this command it is only using 45% of the cpu and 4gb of the ram. Is the software limited on how much of the system resources it can use, or am i doing something wrong?

below is the 3d curve i am using and the output, there is also some issus with the back which i cant work out why its happening on the extrude command.

any assitance or advice anyone can give would be amazing

thanks