Seized seacock

[jan watson]

Sailboat in water. Never had this problem or would had checked when in dry dock. Used wd 40 to no avail. Not mechanically inclined and 104 lbs.
Help !

[Scott Berg]

What type (ball valve, tapered cone, buna-plug) and what material (Bronze, Marelon); different actions for each

S

[HopCar]

A photo would help.
If it is a modern bronze ball valve type of seacock, they can usually be freed with just brute force. A piece of pipe to extend the handle works well. Exercise them a couple of times a year to prevent them from seizing up.
You can buy nice ready made pvc extension handles.
Seacock Helper

Older style tapered cone seacocks require a different approach.

[delmarrey]

While putting some good pressure on the handle, tap the side of the valve with a hammer. But don't get carried away, depending on how it's mounted. A true thruhull valve can take a beating. But a screw-on could break off.

Another way would be heat it with the use of a propane torch. Only heat it hot enough to still be able to touch and hold with your hand. Some ball valves have plastic seats that could be damaged with too much heat.

If these don't work your next option, besides a haul out, would be to put a plug into it from the outside, pull the hose and full it with diesel fuel to soak it for a day or two.