Mismatching valves or other fittings on thru-hulls is not good practice and when done below the maximum heeled waterline, it increases the odds for failure and water intrusion. It is this risk to boats that provided the impetus for the American Boat and Yacht Council (ABYC) to develop standards for the installation of thru-hulls. They state, “When installed below the maximum heeled waterline, a thru-hull must have a valve mounted so that the system will withstand a 500-pound static force applied for 30 seconds to the inboard end of its connecting fitting.”
But where exactly is the “maximum heeled waterline?” The ABYC places it “for powerboats, at the level of water on the hull when the hull is inclined to seven degrees; or for sailboats, it is the level of the sheer amidships.” Another ABYC requirement is that the valve must be operated by a lever-type handle, usually operable through a 90-degree arc, giving clear indication of whether the valve is open or shut.
Then there is the concern about threads: For threaded joints to have full strength, a minimum of five full threads must be engaged. To accomplish this goal, straight threads need to be screwed into straight threads and tapered threads need to be screwed into tapered threads. Mixing the two types of threads usually results in fewer than five threads engaged, often only one or two, and a weak, thus vulnerable joint.
Bronze thru-hulls, as well as the threads in the bottom of bronze seacocks, have National Pipe Straight (NPS) threads. Straight threads allow a seacock to be turned in any direction that is best for the installation, and let a thru-hull be cut down if needed, and assuming five full threads will engage, still form a strong joint.
The inboard ends of bronze seacocks, though, have National Pipe Thread (NPT) threads, as do bronze in-line valves, nipples, raw water strainers, bushings, reducers, hose-to-pipe adapters, raw water manifolds and safety seacock conversion adapters, so at this end of the seacock, it is usually easy to match threads.
But thru-hulls and the threads on the bottom of seacocks made from Marelon, which are ABYC-compliant Forespar components, will have one of three different types of threads – NPS, BSS (Buttress-Style Straight) or BSP (British Standard Pipe) threads. Since these threads are not interchangeable, a Marelon seacock must be matched to a thru-hull that has matching threads; if in doubt, call Forespar to resolve this issue.
The inboard end of Marelon seacocks all have straight threads, not tapered. To match these straight threads, Marelon tailpieces also have straight threads, as do Marelon in-line valves, adapters, reducers, connectors and strainers. But you should be aware there is an outlier here – Marelon pipe-to-hose adapters. These have NPT threads, and are not compatible with fittings that have NPS threads.
But times change and improvements are made. Previously, flanged seacocks were the only valves that could be screwed onto a thru-hull and pass the ABYC 500-pound test. But nowadays, there is another alternative.
Groco manufactures bronze NPS to NPT flange adapters, also called in-line ball valve flange (IBVF) adapters. In addition to their flanges, they have straight threads in their bottoms and tapered threads on their inboard ends. With these features, the installation of an IBVF on a thru-hull allows the use of in-line valves on thru-hulls located below the maximum heeled waterline, and as long as five full threads are engaged, this combination will pass the ABYC’s 500-pound test.
An additional plus with installing an IBVF adapter is that an in-line valve can be replaced without having to violate the seal of the thru-hull and flange on the boat’s hull, though it is best done with the boat out of the water.
However, keep in mind that since Marelon in-line ball valves have straight threads, they will be problematic if screwed onto bronze IBVF tapered threads; therefore, they should not be used on these adapters.
There is also a relatively new style of thread being used by Groco and Perko on their bronze thru-hulls, which they call a “combination thread.” These thru-hulls are made with straight threads for most of their length, while the last few threads convert to tapered threads.
As long as the thru-hull has not been cut down, the tapered threads at the end of these thru-hulls can use components that also have tapered threads. They also can be used on fittings with straight threads since the first few tapered threads are bypassed.
Since thru-hulls with combination threads have no load-dispensing flanges other than their retaining nuts, their use with non-flanged adapters, valves or other components below the maximum heeled waterline should be questioned because of their ability to meet the ABYC’s 500-pound requirement.
Per ABYC standards, there are three exceptions where a seacock is not necessary on a thru-hull that is located below the maximum heeled waterline. One is on engine or genset exhausts that exit above the static waterline. The second is on cockpit drains that discharge above the static waterline. Third is on any discharge line above the static waterline, but it must have reinforced hose or piping attached that can withstand kinking and collapsing as well as meet ABYC’s H-8 buoyancy standards.
Even though a flanged seacock is not mandatory in these installations, in most cases, a pipe-to-hose adapter should be installed to eliminate the possibility of leaks where the hose would otherwise slide over the threads of the thru-hull.
For those who in the past may have been frustrated when installing thru-hulls, maybe now it will be easier. Use materials that are compatible with one another; use materials that are suited for the medium in which they will be used; match like sizes, fit straight threads to straight threads and fit tapered threads to tapered threads.
Field testing for strength
Dragging a 500-pound test weight into a boat to test a thru-hull’s strength is rather impractical. Instead, a field test can be improvised by having someone stand on the end of the tailpiece, maybe even bounce lightly, or yank the heck out of it; this is best done while hauled out.
Tapered plugs
Every thru-hull should have a tapered plug tied nearby, or better yet, tied to it. One advantage softwood plugs have over other materials is once wet, they swell relatively quickly. This swelling allows the plug to seal tightly and makes it less likely to be dislodged.
Use same type of materials
Since Marelon and bronze expand and contract at different rates, combining fittings of these different materials, even if the threads match and a sealant is used, can result in leaks. So it is generally recommended not to mix the two. Instead, it is best to fit bronze with bronze and Marelon with Marelon.
Tailpieces
When a pipe-to-hose adapter is installed on a seacock, it is then referred to as a “tailpiece,” but traditional tailpieces are not interchangeable with their modern counterparts. Traditional tailpieces have nuts with straight female threads that mate with straight male threads on those older seacocks. To obtain a seal, a gasket is employed. Modern tailpieces have male threads that screw into female threads in contemporary seacocks and rely on sealants.
Sealants
Threads that tighten against themselves — like tapered threads — are self-sealing and generally do not re-
quire a sealant to prevent leaks. If a
sealant is needed, then regular pipe dope or Teflon tape is usually sufficient.
However, all straight threads, plus tapered threads that stop short of self-sealing, do require an aftermarket sealant. If the joint is under pressure, as it would be if it were below the waterline, a harder-setting sealant, such as Life Caulk, 3M-101, 3M-5200 or 3M-4200, is best. In addition, the entire assembly also needs to be well bedded and sealed against the hull to prevent leaks.
Backing blocks
The area where the seacock mates with the hull needs to be flat, perpendicular to the thru-hull with a footprint that is at least 10% larger than the flange on the seacock and with beveled edges. This is usually accomplished with a backing block. Though wood is often used, a moisture-resistant material or a build-up of reinforced resin may be a better choice. If necessary, extra-long thru-hulls are available. n