You must dig deeper before you start with calculations; fatigue is more complicated than the white paper suggests. The figures (I notice they couldn't keep to one comma convention, even within a single
table) did not identify any specific rope
construction. For example, it is well known that nylon double braid tries to saw itself to bits, while laid rope
is less prone to this. Without stating the construction and wet/dry, the data is not ready to use.
In actual practice, I believe the load factors are even more in favor of high mod ropes than 3:1 because of internal abrasion numbers. This tends to be true with any low stretch fiber, because of reduction in rubbing. The specific reaction to the rubbing is also a factor.
However--there is always an however--we also need to consider whether the nylon will experience a lower load because of the energy absorption, and that is very difficult, perhaps impossible, to simulate with any accuracy. We've been discussing that.
As for the fatigue life of the rope, since modeling the stain is so difficult the only practical answer is to design for a fatigue life so long that the product would be replaced first for other reasons, perhaps 50-60% in the case of Dyneema
What strain do we use? If we use the high factor for calculating fatigue, we insure failure unless we know the maximum strain (if the average is running at 65% we are going to break the line with one bad break, the only time, perhaps when we really needed it), so the reality is that we will design for 4-8 times the average storm strain. And we have circled back to the question of whether nylon will see a lower load. We know that chain vs. nylon are very different with a ground anchor
, but the answer at sea will vary with the waves and the drogue
. Smart money
is guessing that with Dyneema
the line and the attachment point will need to be stronger than for nylon... but how much?
Some vendors (Seabrake) recommend polyester as a good solid compromise; much better fatigue life than nylon, more immediate loading of the drag devise with the passage
of a wave, and some give. Unfortunately, there is little space and weight savings.
Handbook of Fibre Rope Technology - Henry A. McKenna, J. W. S. Hearle, N. O'Hear - Google Books
I don't think the group is unaware of fatigue. To me, that is what the thread is about. Lamentably we have no data on brait and fatigue life. I inquired through manufacturers some time ago--folks I've worked with before--but I guess a phone
call is needed.
Very complicated with many variables. The best answer may prove to be a hybrid rode
, using several materials. Or perhaps as staying with nylon and going up a size.