Installing Thru-Hull Fittings and Valves
Let It Flow
We recognize that many different combinations of valves and fittings are employed in the marine industry. This bulletin is intended to provide recommendations for the safest installation of thru-hull fittings and related components.
FIGURE-1 shows a sample seacock installation with two possible methods of mechanically fastening shown. On the left side is the method of bolting through the flange, backing block, and hull. On the right side is the method of bolting through the flange and into the backing block, but not through the hull. We consider either method acceptable, as both represent a safe and secure installation.
Cut a hole through the hull and backing block slightly larger than the thru-hull fitting OD, and measure the thickness of the hull and backing lock together (Dimension-A).
Measure the total thread depth of the female NPS threads in the bottom of the seacock (Dimension-B). Add A and B together.
If the thru-hull neck length exceeds this dimension cut the thruhull length 1/4″ shorter than A+B. If the thru-hull length is shorter than A+B by more than 1/4″, use a thinner backing block or use an Extra-long thru-hull fitting (GROCO THXL Series)
To prevent the seacock from turning, bolt through the seacock flange into a backing block, or bolt through the hull from the outside in, and through the seacock flange. Figure-1 shows both methods. Use hex nuts, flat washers and lock washers as shown.
It is acceptable to fasten pipe fittings directly to the seacock, but DO NOT install components such as pumps, valves, or strainers directly on top of the seacock unless these components are independently supported.
IN-LINE VALVE INSTALLATION
FIGURE-2 shows a sample installation of an in-line valve used as a seacock.
Choose a location that is out of the way of foot traffic.
Cut a hole through the hull and backing block slightly larger than the thru-hull fitting OD.
Insert the thru-hull into the hole and tighten the lock nut.
Apply TFE thread tape to the thru-hull fitting and screw on the in-line valve securely. Use a thru-hull installation tool (GROCO THT-530) to hold the thru-hull during valve tightening. If the distance between the lock nut and the inline valve exceeds 1/2″, remove the valve and thru-hull lock nut, and use a thicker backing block.
GROCO does not recommend the use of in-line valves as seacocks for these reasons:
- An in-line valve has no means of attachment to the vessel hull or backing block, so the valve can turn or loosen from the thru-hull fitting with vibration or with normal use.
- If the connected thru-hull fitting becomes damaged or broken, as might occur if the vessel struck a submerged or floating object, or if the fitting was inadvertently damaged or broken inside the hull, there would be no way to shut off the flow of water into the vessel.
- In-line valves have NPT threads, which are not compatible with NPS threaded thru-hull fittings (unless the thru-hull fitting is machined with “Combination Thread”. Installing an in-line valve onto a thru-hull fitting will create a mismatch of threads resulting in minimal thread engagement between valve and fitting, and an unsafe installation. Property damage, personal injury, or both could occur. If you choose to utilize an in-line valve as a seacock, the thru-hull fitting used must have “Combination Thread”.
- Do not use pliers to hold the threaded thru-hull neck while tightening the in-line valve.
- Do not cut the thru-hull length, as this will remove part or all of the “Combination Thread”
- Do not over-tighten the in-line valve on the thruhull fitting. This may weaken or break the fitting.
- Do not install components such as pumps, valves, or strainers directly on top of the inline valve unless the components are independently supported.