Interesting idea, using the one
bridle both ways. I think both approaches are reasonable, and both have advantages and disadvantages. Id probably be inclined to go
spectra, as its light weight, high strength and lack of chafe would make deploying the bridle much easier. You can get the stretch from the towrope itself. But heavy nylon with substantial antichafe would
work as well.
Most modern harbour tugs use
dyneema for towlines these days. Sometimes with a short elastic nylon tail to cope with shock.
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