Your halyard is "shorter" now because when you released the tension that had been on it, it returned to it's original length. But once it gets loaded up again for a little while, it's going to stretch again, just as before, & likely about the same amount.
3/8" Dacron, be it XLS, or something else, is really too small on a boat that size. And XLS is going to stretch as much or more than almost anything except perhaps Nylon. Plus, you probably have XLS or it's equivalent up there now.
Look up how much a halyard material stretches at X% of it's breaking strength, via the cordage maker's website. And see how much 50' of line stretches when the load on it doubles, due to a wind increase, or your boat falling off of a wave (every 10sec). It's not insignificant in the poorer materials.
Any more, using Dacron for halyards is rather silly, due to it's relatively poor stretch & wear characteristics vs. other cordage types.
It'd be like buying
instead of a Rocna
, when purchasing
a new anchor
If I were advocating your buying
halyards that cost $5-$10/ft or more, then they'd be unneeded, due to overperformance for the intended application. But Warpspeed or it's equivalent is pretty much the benchmark any more.
You can go one step lower, to a blended Spectra core
, though you'll run into more of the issues that you're having now, with Dacron. So I can't see going such a route
. Especially if you want good performance out of your new sail that cost you 4-figures.
Not to put him on the spot, but Dockhead
has replaced virtually all of his lines in the last year or two, with Dyneema cored ones. Meaning pure Dyneema cores such as in Warpspeed, Maxi-Braid Plus, or in his case, due to location & cordage availability, Marlow racing line cored with SK75 or SK78.
As his Dacron (Polyester ones) weren't up for the task, both in terms of stretch, & longevity. Some of the latter was due to sizing & braiding characteristics. But his new lines should give him much longer service
, assuming they're correctly sized. As in a decade or more. Without any of the stretch issues of the previous lines.
Edit: Less stretch in a line greatly contributes to it's lifespan. As with each stretch cycle, the line is wearing against itself internally, & that friction adds up. It's what destroys moderate to highly loaded mooring
lines, even when they're not touching anything which can cause abrasion on them.