Try to avoid using trimming for a transom that has to blend in with the hull:

What happens when you do try to this is that you get a “shallow intersection” between the hull and transom surface which usually means that the intersection line changes shape rapidly or is even broken when a small change to any of the surfaces is made. It may also cause problems when you switch to a higher precision and the intersection line is calculated differently:

 
No trimming region at lowest precision

Trimming region at medium precision

Use bonding for these cases instead.

The only case where you cannot avoid using a shallow intersection trimming surface is when you want to model a propeller tunnel:

You need the shallow intersection for minimum disturbance of the water flow towards the propellers.

This is very difficult to model properly because it is difficult to get the shape of intersection line right and often the trimming fails when switching between different precisions. How to do this is not covered in the LearningMaxsurf training manual.

You have learnt all the basics about trimming and can now continue to the section on Chapter 9 Bonded Surfaces.