If you're lucky enough to have proper bronze tapered sea cocks fitted to your boat (or to your putative boat. as it may be) don't try to re-fit them with the boat still in the water.
Once out of the water, you can back off the locknut and adjusting nut with the right sized spanner (or a carefully adjusted monkey-wrench (as opposed to a pair of mole-grips)) and tap the end of the thread to loosen the barrel in its sleeve, before tapping the handle to break the torsional seal and withdrawing the barrel.
Once withdrawn, inspect the barrel for "coppering" - see Sailingdog's post above -and if it looks ok, polish it and the inside of the seacock's bore with fine wet-and-dry sandpaper.
refit the barrel in its sleeve with petroleum jelly (vaseline) or a similar high-density grease - anti-corrosive greases are available - and re-tighten the packing flange and locknut. the handle should be stiff to move, but not excessively awkward (I speak as a 200lb male, so for lighter builds, it should be moveable with a box-spanner over the handle and leverage of about 8-10 inches. Protect your knuckles with cotton working gloves (or similar) or you will surely bark them!).
Shut the valve, and upon relaunching, whip around the through-hull fittings and make sure they're not leaking. Any which are, tighten, and for those which aren't and are stiff, loosen the lock and backing nuts 1/8 turn before re-locking and trying again.
Finally, if you're replacing through-hull fittings, don't even dream of usinf plastic fittings, they're far too easy to break accidentally - and don't use ball valve fittings, the spigot that turns them can snap off in the ball meaning you can't shut them off.\
Employ the tried-and-true transverse tapered cone fittings (Groco) or the old-fashioned-but-still-effective in-line tapered cone fittings which can be re-ground and re-fitted in situ. they sometimes leak (a very little, and mine are 30 years old) but they abide by the KISS principle, and can be refitted at sea. By banging a bung in the hole from overboard!
see also: http://www.swainsons.com/gallery/dis...?album=5&pos=7
- for a sorry looking vale with handle pointing directly upwards (ie in the open position)