As donradcliffe suggests, I’d not attempt a repair/replacement afloat.
Originally Posted by donradcliffe
If its half open, then the head works. I had exactly the same problem with a ball valve less than 2 years old. Kept a tapered plug next to the thru hull and assigned that head to light work only till the next haul out...
... you can make do with a head and a half till the next haulout, where its a half hour job to grind the flange off the thru hull, knock the remains inside, and install a new thru hull and valve.
The consequences of failure could be disastrous.
While an in-line Ball-Valve might
be removed from the separate through-hull (plugged), there’s significant potential for breaking the thru-hull loose
(you’ll want a retaining wrench) from the hull connection.
An integral Sea-Cock cannot be safely removed afloat!
A true sea-cock features a flange that is bolted (or screwed) to a fairing block afixed to the hull. Sea-cocks are manufactured by Groco, Buck Algonquin, Perko, and others in marine
Forespar manufactures sea-cocks in Marelon, that is a formulation of glass-reinforced Dupont Zytel.
Most seacocks feature NPS (straight) threads on the flange (hull) side with NPT (tapered) pipe threads on the top side. Marelon seacocks feature NPS (straight) threads on both sides. Forespar offers Marelon hose fittings with NPS (straight) threads that are designed to work with their sea-cocks.
Ball Valves operate with a quarter turn similar to a sea-cock, but are not designed to be attached directly to a thru-hull fitting. Most ball valves feature NPT (tapered) threads on both ends and are compatible only with tapered pipe fittings.
See the Groco Service Manuals
➥ GROCO MARINE PRODUCTS
Industrial Ball-Valve Dis-Assembly:
1. Remove the lever nut (figure 6, item 1) and the handle (figure 6, item 2) from the valve body (item 3).
2. Remove the packing gland
(item 4) and the valve stem packing (item 5) from the valve body (item 3). Discard the valve stem packing.
3. Remove the valve stem (item 6) and the valve stem bearing (item 7) from the valve body (item 3).
4. Remove the retainer (item 8) from the valve body (item 3)