You can lock the length of a curve so that it maintains a constant hull length, even when you edit the curve (for example, by moving a CV).
To lock/unlock the length of a curve
You can also lock/unlock the length of a curve by selecting the curve (or CVs), and selecting Curves > Lock Length or Curves > Unlock Length, but be careful you don’t forget to unlock the length. Using the hotkey is better, because you’re not likely to forget to unlock the length.