If the handle refuses to move don’t force it. Moving parts can break or, worst of all, the whole unit could part company with the hull. Tapered softwood plugs should be kept at hand (one hung from each fitting?) to seal off a hole or pipe if the worst comes to the worst. Wherever possible hoses should be double clipped at each end using stainless steel hose clips.
Rotten wood around any skin fitting should be treated as a matter of
urgency and, likewise, any signs of compression on FRP sandwich laminates
Most surveyors would be happy never to see another gate valve on any vessel ever again. The handles tend to corrode and fall off or shear off if put under any pressure, there is no indication to show whether it is open or closed, the whole unit is prone to corrosion and failure and they tend to be bulky. Failed gate valves are not worth servicing and should be replaced either by a marine ball valve or a cone valve. The only thing that can be said in favour of a gate valve is that it is cheap but the advice is don't be tempted!
Ball valves do not suffer from these deficiencies and have the advantages that they give a quarter turn between open and closed, the handle position indicates this, and, because they are used domestically and mass produced, they are comparatively inexpensive. However, it is important to be sure that marine versions rather than domestic are used for boats.
The best valves are traditional cone valves. They are simple to operate and to maintain. They are very durable in marine conditions and the cone can be re-ground if required. The handle indicates when they are open or closed. The one disadvantage of these valves is that they are expensive (£80 - £150+ depending on size).
It is important that all skin fittings below the water line are regularly maintained and are sound because insurance companies will not cover a failure resulting from neglected seacocks or hose clips. Most underwriters will not cover a boat unless surveyor’s recommendations relating to seacocks and skin fittings have been carried out before the vessel goes afloat. Electrolysis can also lead to weakening of skin fittings if anodes are not properly maintained and connected.
Finally it is unacceptable to have redundant skin fittings below the waterline and, should they fail, many underwriters will not cover the resulting losses. Redundant valves should be removed and the remaining hole permanently blanked off.
With the amount of domestic kit carried on many of today's yachts the number of holes drilled through a hull below the waterline seems to be on the increase. Even so it is well to remember the traditional boatbuilder's principle that the fewer the number of holes the greater is the chance of keeping the wet stuff on the outside.