Through hull jammed from holding tank

[arshearer]

Any suggestions on how to free up seized or overly stilff ball valve at through hull from holding tank

[Alan Mighty]

Careful application of measured force.

Groco added a 1/2" square hole to its flanged ball valve sea cock levers so a socket wrench handle could be applied for extra leverage. The company did counsel applying the additional force in a careful way, because it is possible to break the handle before your overcome the stiction and the ball rotates.

I've successfully used an old box wrench (an old spark plug wrench) to be a helper.

Most all ball valve seacocks have drain plugs, so all water can be drained from the seacock (for boats that are overwintering). If you remove the drain plug, you should be able to squirt in a PTFE lubricant or a spray lanolin-based lubricant.

Some ball valves have PTFE or other linings. I'm not clear if what solvents (WD40 etc) could be used.

As you know, working the seacock once a month disrupts calcium scale deposits and marine life.

[a64pilot]

Use of a little heat like from a heat gun, not a torch couldn't hurt.
I freed one by putting cooking oil on the ball portion after removing the hose and letting it sit overnight, sink valve in my case. I think the oil helped, who knows, cooking oil because that was what I had, not because it was the best