Rope/wire: Outer, inner and contact diameters

The line type wizard sets up outer and inner diameters for a rope/wire as follows.

The inner diameter is set to zero for all rope construction types.

The contact diameters are set to ~ for all rope construction types.

The line type outer diameter, $O\!D$, is given in terms of the specified nominal diameter $d$ as \begin{equation} O\!D = \begin{cases} 0.85 d \text{ m} & \text{for nylon ropes}\\ 0.86 d \text{ m} & \text{for polyester ropes}\\ 0.80 d \text{ m} & \text{for polypropylene ropes}\\ 0.82 d \text{ m} & \text{for wire ropes with fibre core}\\ 0.80 d \text{ m} & \text{for wire ropes with wire core} \end{cases} \end{equation} This outer diameter is an effective diameter that gives the line type a displaced volume per unit length corresponding to the estimated displaced volume per unit length of the actual rope/wire. The line type thus has the appropriate buoyancy. Note that this effective diameter is less than the given nominal rope diameter, because there are gaps between the fibres and so not all of the specified nominal diameter contributes to buoyancy.

The above formulae for the line type OD were obtained by equating the line type displaced volume per unit length, $\pi O\!D^2/4$, to the displaced volume per metre, $m/\rho$, where $m$ is the rope/wire mass per unit length and $\rho$ the average density of the material.

The following average material densities $\rho$ (in te/m³) were assumed: nylon 1.14; polyester 1.38; polypropylene 0.91; wire with fibre core 6.87; wire with wire core 7.85. The average material density for the wire with fibre core was estimated by assuming a ratio of 6:1 between the wire and fibre volume, with the fibre taken to have the same density as (fresh) water.