Thru hulls and seacocks

Thru hulls and seacocks -10 things you need to know

1. Your thru-hulls and seacocks should be as strong and failure proof as your hull.

2. The American Boat and Yacht Council (ABYC) regulations require that all thru-hulls that exit below the waterline be fitted with a seacock. For a sailboat the definition of the waterline is the heeled waterline when the rail is in the water – so most of your thru-hulls should be fitted with a seacock.

3. The function of seacock is to protect your boat when the hoses attached to the seacocks fail. You must be able to access your seacocks easily or they are useless in an emergency. Service your seacocks at least once a year and attach a softwood plug to the seacock to plug it if it fails to operate.

4. To comply with ABYC regulations your thru-hull-seacock-tailpiece unit must be able to withstand the weight of 500 pounds on the end of the tailpiece without breaking. That’s about three men standing on the tailpiece.

gate valve5. Gate valves and globe valves are not seacocks and should not be used on a sailboat. They are prone to corrosion and can be broken if excessive force is used on the handle. Also, you cannot tell if they are open or closed by looking at them and they can be jammed open by marine growth.

6. Thru- hulls and seacocks are made of silicon bronze or fiberglass reinforced nylon (Marelon).

7. The thread problem. All thru-hulls have parallel (NPS) threads. Most gate and globe valves have tapered (NPT) threads. A tapered thread gate valve will only engaged three to four threads with a parallel threads thru-hull before it binds. This arrangement is not safe and would not pass the ABYC 500lb requirement.

scockdiaA8. A seacock has:

  • A lever handle that operates through 90°
  • A body and valve made from high-quality cast silicon bronze or fiberglass reinforced nylon (Marelon).
  • An internal tapered bronze plug or a Teflon coated plastic or stainless steel ball to shut off the cock
  • A flange to allow the seacock up to be bolted to the hull..

9. Fiberglass reinforced nylon (Marelon) sea cocks are lighter and sometimes cheaper than bronze seacocks and do not suffer from any corrosion problems. However, they can be over tightened which can cause the valve to bind. Using excessive force to open or close a bound valve can break the handle.

10 Reputable seacock manufacturers include Blakes, Apollo, Groko, Perko, Willcox and Forespar Seacock is produced by these manufacturers are labelled “UL-1121” or “Marine UL Listed” to indicate the fittings have passed rigorous tests and inspections and are approved by marine underwriters for marine use.