Hi everyone,

Thank you in advance for the guidance. I am trying to model a polyester rope in MoorDyn v1, and I am very confused about how to populate the Line Types section. The rope I need to model comes from the DeepRope-brand catalog, which provides the following properties:

- Specific gravity = 1.38
- uses rhoSeawater = 1050 kg/m^3

- Diameter = 185 mm
- Total weight = 21.8 kg/m
- in air
- this is the true mass-per-length of the line

- Submerged weight = 5.22 kg/m
- in seawater
- these units are confusing to me, but I assume this is the submerged weight of the line divided by gravity. Therefore, this value takes into account buoyancy, but the line still has positive weight underwater, meaning it does not float.

- Stiffness = 2.16e5 kN

I am most confused about the relationship between the volume-equivalent diameter (*Diam*) and the mass-per-length (*MassDen*) in the Line Types properties. I assume *MassDen* should be the dry weight of the line, 21.8 kg/m, based on this post. But what should *Diam* be?

If I use 185 mm from the catalog, then calculating the specific gravity of the line material yields 0.77 (this comes from *s.g. = MassDen/CrossArea/rhoSeawater*), meaning the line would float, which is not correct. Therefore, water must be infiltrating the line when it is submerged, and its actual cross-sectional area must be less than *pi/4×(185mm)^2*. To account for this, I could take the given *s.g.* and then back-calculate the area, yielding a *Diam* of 138 mm. Is that the correct method for deriving *Diam* and *MassDen* from these given properties of the polyester rope? Is it correct to use specific gravity as the driving property here and keep *MassDen* equal to the total weight value? Should I be using the catalog’s submerged weight value at all?

For *EA*, I can use the catalog value, and for *BA/-zeta*, I will assume -1, critical damping. For the last four properties, I am not sure what to do. How should I calculate the transverse and tangential added mass and drag coefficients? The MoorDyn v1 manual does not explain what these values are or how to find them from the information I have.

Thank you again,

Doron T Rose