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[D B Campbell]

I have absolutely no practical experience of sinew (it seems that few people have!), so all of my remarks are theoretical.

Hmm, strange to use sinew, given the fact that it loses it's strength when damp or wet.

Philon says much the same as you, Robert. He praises the experimental bronze-spring catapult because it remains unaffected by dampness. "When sinew springs are wet or broken, machines cannot avoid detrimental effects. Often even machines kept with every precaution in covered places deteriorate considerably through atmospheric changes" (Philon, Bel. 72, trans. Marsden, adapted).

Cow sinew would be cheap, in fact may be free from my friendly wholesale butcher. I have heard that cow sinew is a bit greasy and not that ideal for backing bows or strings, but for rope, maybe it would be fine.

Heron actually praises bull sinew, Dane (also the tendon in deer's feet). The only sort he deplores is pig, which he says is useless.
I was under the impression that manufacturing sinew rope was laborious, fiddly and messy. Let us know how you get on!