Savitsky method result understanding confusion

So i am using the code from openplaning python: https://github.com/elcf/python-openplaning

I have an understanding that for hulls with a large deadrise the stability of the hull is supposed to increase but when i am using the program based on savitsky formulation i got a different result, an opposite one.

basically for stability i am just plotting the eigen value of the system from this code and i see that for increase in deadrise i see the eigen values shifting to RHS of the complex plane i dont understant why because i was under impression that the increase in deadrise should make the hull more stable and have the eigen values more towards LHS plane i am uploading the result as well.

 

Welcome to the forums
Have you run Lyapunov's second method on your algorythm equations to verify the form of solution you will get? Or did you just "plug and chug" and are now confused by the results? And what "stability" are you expecting? roll? trim?
Remember that the Savitsky method is about planning resistance related to trim, not about transverse stability (indeed, the models used were fixed in roll). Much like the classic calculation of GM, the vessel is initially unstable but oscillates between two final stability positions where dynamic lift is sufficient to push the vessel back to the other side (i.e. chine walking). All v-type hulls chine walk to some degree, and depending on the parameters of the hull, chine walking can become excessive

 

As far as I can see, there is nothing in that calculation procedure that refers to transverse stability, but there is a calc module dealing with porpoising, ie longitudinal trim instability. This is a dynamic phenomenon that depends on a number of factors, including deadrise, and there are specific limiting boundaries for all configurations. I strongly suggest that you read Savitsky's original papers before "pressing computer buttons".

 

It's not as bad as you think. The eigen values near (-6, +/-5i) are the dominant response. The 30 deg deadrise has more damping and less vibration than the 10 deg deadrise; however, the 20 deg deadrise is the least stable. Apparently the study indicates the design is near a maximum instability point. Maybe re-run the test using deadrise from 5 deg to 45 deg, on 1 degree increments to narrow down the deadrise with the least stability.

 

From what I know, the Savitsky method has some assumptions and simplifications that might cause results that don’t always match intuition, especially with big deadrise angles. Also, eigenvalues moving right doesn’t always mean less stability in all contexts—it depends on how the system is modeled. Maybe double-check the code details or try some simpler test cases to see if the trend holds. Sometimes, numerical methods can behave unexpectedly for certain hull shapes.