Ahoy there,
It's more the bobstay and gobline (the boomkin's bobstay) that need attention. You'll see from a simple parallelogram of force that these stays take a great deal more strain than the jib or backstay.
I don't have the Apprentice at hand but I'm sure Brion discusses sizing these stays. So just a couple of thoughts:
Chain looks shippy but is inferior as bobstay or gobline for two reasons: More exchange sites for air-water induced galvanic corrosion; and surprisingly streatch up to catastrophic failure.
Galvanic corrosion? Attach a bit of wire with an ampmeter to each end of a copper bar. Immerse one end of the bar in salt water so it's about half out. Move the bar up and down between almost fully immersed and almost out. You'll see the ampmeter twitch. This is why smart boaters put an extra coat of bottom paint near the waterline. Chain, with all those surfaces bearing link to link, is just that much more vulnerable in an environment of cyclical wetting and drying.
I moved away from bothering with a turnbuckle for either bobstay or gobline. Seemed a needless complexity. Doing without means that hooking up either must be done before attaching the jib or back stay and putting considerable download on the bowsprit or boomkin. That way when you attach and load the stays, you won't just peal the spars up. It also means very accurate measurement but that's what rigging is all about.
A dolphin striker is often useful to open the bobstay angle a bit and reduce the tension required. This should be mounted to a free pivoting socket on the stem at a point behind and above a straight line from one end of the bobstay to the other. Located rightly, the striker will stay in place as it naturally lies along the shortest available path but it can still deflect a litte when an anchor line bears across it, thus preventing fatigue and breakage where it meets the stem.
G'luck
Ian
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