While checking the 6″ x 12″ stern zinc before launching, I found one of the 1/2″ thru-hull studs to be not bedded in tight and with the potential of being turned by a diver when replacing the zinc. Inside, they look clocked around more than a quarter turn. Fortunately, both studs are sound, with no leaks.
On my boat, the thru-hull studs are a 6″ long section of 1/2″-13 UNC threaded stainless steel rod bedded with a sealant thru the hull and fastened with a washer and nut on both sides. An inconsiderate diver could easily twist one around as may have happened to you.
To prevent turning in the future, I measured the distance between the studs and made a stainless steel strap of 1/8″ x 1″ stock with 2 ea. 5/8″ holes to fit over the projecting studs on the inside (lazarette on my boat). I then placed nuts, strap, lock washers and outer nuts on the studs and tightened the nuts hard against the strap.
By the way, my reasonably competent diver explained his zinc fastening procedure: Use two wrenches always. Apply nuts to the studs, place the zinc in position over the studs, add lock washers, then tighten nuts against the zinc, only moderately tight. Add a second, locking nuts on top and tighten these outer nuts against each other firmly.
Sea Racer – NT 42-041