Strip-theory member joints swapping and hydro coefficients

Dear NREL team,

I have noticed something that doesn’t feel right in the way HydroDyn handles strip-theory members and their coeffcients in particular cases, depending on how the user defines them. Say I take the example of a vertical tapered cylinder, and I define it from the top down : MJointID1 = 7 and MJointID2 = 8, joint n°7 being above joint n°8. HydroDyn will have to swap those joints (as done in Morison.f90, lines 3888 to 3902) : MJointID1 will become 8, and vice versa, and the values of MPropSetID will be swapped as well.
But if the user defines his member that way (as I did), then he will also define his member-based hydrodynamic coefficients in the same order, for example MemberAxCa1 = 0.4 if he wants that value on the upper part and MemberAxCa2 = 0.6 for the lower part. And in that case, if I’m not mistaken, the code doesn’t swap those values and in the end we will have 0.4 on the lower part with joint n°8 (MemberAxCa1 with what became MJointID1 after swapping), and 0.6 on the upper part with joint n°7.

Of course now, I will make sure I always define my members from the bottom up. But still, others might not know. So if you agree with my observations, maybe it would be good to adapt the code so that it swaps the coefficients as well.

Thanks for all the work on FAST,
Best regards,


Hi Romain,

Thank you for your message. Jason has opened an issue about it. I’m having a look at it and reporting the progress here: