Issues with synthetic rigging?

[echidna]

So let's look at some actual data for coefficients of linear expansion:

Aluminum 6061: 24E-6 per deg C
Stainless steel 316: 16E-6 per deg C
Dyneema: -12E-6 per deg C

I'm not sure how accurately the value I found for Dyneema applies to Dynex Dux, but I'll assume it does.

So, if for the sake of simplicity, we assume a shroud of equal length to an aluminum mast (as would be a good approximation for a deck-stepped, masthead rig), we have the following differential coefficients of linear expansion between mast and (cap) shrouds:

SS 316 rigging: 8E-6 per deg C
Dyneema rigging: 36E-6 per deg C

So the effect of differential expansion is 4.5 times greater for Dyneema in this case.

To get an idea of what this means with Dyneema/Aluminum, consider a variation in temperature from 0 to 30 deg C. This gives a relative length change of 30*36E-6 or 0.0011. Assuming all the elasticity is in the Dyneema (i.e., neglecting elasticity of the hull and mast), this corresponds to a change in load in the shroud of order 6% of breaking strength, based on the Colligo data. This value is probably greater than the pre-tension acceptable in terms of creep.

For comparison, with a SS 316 shroud, the change in load would be of order 2.5% of breaking strength, which should be compared with a typical pretension of say 15% of breaking strength for SS.

I worked these numbers quickly, so I’m open to correction on the math, but assuming it’s correct, the difference between Dyneema and SS is quite significant in this regard. Obviously I’ve made a number of simplifications in my model, but I think the general results will be reasonably valid.