What Type of Seacock do You Have?
These are multi turn to open and close with the mechanism screwing down a â€œgateâ€ to open or close the outlet/inlet.
These have handle with a 90 degree open close mechanism moving a close fitting â€œballâ€ with a hole through it for inlet outlet. The hole is in line with the outlet for valve open and at 90 degs for closed.
Hence you can tell at a glance when ball types open but generally need to rotate the handle on gate types to find end stops. It is recommended that you don't leave the the handle rotated hard against the open stop in case it jams. Find the end stop and back off a little.
It is also recommended that you execrcise seacocks (especially gate type) every time yo go on board.
Cone type Seacocks - Blakes Style.
There also have handles that only move 90 degrees. Westerly fitted Blakes seacocks to Centaur heads inlet and outlet. They are not ball valves but a form of cone valve and they differ from ball valves in that the handle is on the top of the valve not on the side.
Blakes seacocks need greasing every year with a special seacock grease to keep them from leaking so it is important that you identify them. Later examples of Blakes have a grease nipple to enable them to be greased whilst afloat.Early Westerly fit Blakes Seacocks did not have a grease niple and must be dismantled whilst ashore for greasing. If the cone becomes pitted the seal can be restored by lapping with a valve grinding paste. A coarse paste is recommended as the resulting finish aids grease retention.
Ball valveinfo here: 
Gate valve info here: 
Blakes web site here: 
What Type of Material are your Seacock Made of?
As used for domestic central heating fittings. Not suitable for marine use because the sea water leaches out the zinc leaving a weak copper rich material that is susceptible to impact damage. Affected fittings can be identified by a pinkish colour resulting form the copper richness.
Standing for Dezincification Resistant material. Suitable for marine use.
Excellent for for marine use but expensive.
A plastic material that is not susceptible to corrosion. Information here: It may however be damaged by heat.