Through-Hull Fittings: Sizes, Threads and Safety for 3D Printing
Through-hulls are among the most safety-critical fittings on any boat: they are the only thing between the ocean and your bilge. That makes them a topic where 3D printing needs a clear head. Some through-hull-related parts are perfectly sensible to print; others should never leave the chandlery as anything but certified bronze or Marelon. This guide walks through the sizes and thread standards you'll meet, how to measure them, and where the safety line sits.
What counts as a through-hull
A through-hull (or skin fitting) is a fitting that passes through the hull to let water in or out — for engine raw-water intake, sink and shower drains, bilge pump outlets, deck scuppers, transducers and more. A typical assembly has three parts: the mushroom-head fitting that passes through the hull, a backing nut on the inside, and often a seacock or valve threaded onto it. Above the waterline the stakes are lower; below it, a failure floods the boat.
Common through-hull sizes
Through-hulls are usually specified by their nominal bore — the internal diameter of the pipe thread — not the hole in the hull. The hole is larger to clear the thread. These are the sizes you'll meet most often:
| Nominal bore | Typical use | Approx. hull hole |
|---|---|---|
| 3/8" – 1/2" | Sink drains, small instrument feeds | ~22–28 mm |
| 3/4" | Engine raw-water intake, bilge outlet | ~34 mm |
| 1" | Larger engines, shower/galley drains | ~42 mm |
| 1-1/4" – 1-1/2" | Heads, cockpit scuppers, deck drains | ~48–60 mm |
Always measure your existing fitting rather than trusting a label — decades of mixed standards mean the marking and the reality don't always agree.
Threads: it's usually BSP, sometimes NPT
Marine plumbing outside North America is overwhelmingly BSP (British Standard Pipe), while US-built boats and much American gear use NPT (National Pipe Taper). They look similar and will start to thread together, but the thread angle and pitch differ, so a forced BSP-into-NPT joint leaks or cracks. Through-hull threads are also frequently straight (BSP parallel / NPS) where a fitting seals on a gasket or backing nut, rather than tapered where the thread itself seals. If you're not sure which you have, our full breakdown in BSP vs NPT Threads for Boat Plumbing shows how to tell them apart with a caliper and a thread gauge.
How to measure for a perfect fit
- Measure the outside diameter of a male thread (or inside diameter of a female) with calipers. Remember a "1/2 inch" BSP thread actually measures about 20.9 mm across — the size names the bore, not the thread.
- Count the pitch — threads per inch — with a thread gauge or by measuring across ten threads and dividing. BSP and NPT use different pitches at the same nominal size.
- Check taper vs parallel by seeing whether the thread diameter changes along its length.
- Note the flange and grip length — hull thickness plus backing hardware determines how much thread you need.
Because these dimensions vary so much boat to boat, parametric, sized-to-fit files are far more useful here than a fixed model. You enter the thread size, bore and length you measured and get a part that matches. Browse the Plumbing & Pumps collection for parametric fittings, adapters and blanking caps.
What is safe to 3D print — and what isn't
This is the part that matters most. A printed thermoplastic part does not have the certified strength, fatigue life or long-term seawater resistance of bronze or Marelon, and no home-printed part should be trusted as a primary below-waterline fitting.
Reasonable to print: above-waterline adapters and reducers, hose-barb connectors on the outlet side, blanking caps and winterizing plugs for seasonal layup, dust and splash covers, and prototypes you use to check fit before ordering the real bronze part.
Do not print as the load-bearing part: the actual skin fitting or seacock below the waterline, anything holding back a standing column of seawater, or any fitting where failure means uncontrolled flooding. For layup and winter sealing, printed caps and plugs are genuinely handy — see our free files collection for simple plugs to start with.
If you do print a wetted fitting
For above-waterline plumbing where a printed part is acceptable, material and print quality decide whether it holds. Choose a tough, water-stable polymer and print for watertightness rather than looks. Our guide to the best materials for 3D printing boat parts compares PETG, ASA and nylon, and how to 3D print watertight fittings that hold pressure covers wall thickness, layer adhesion and the Z-seam issues that cause weeping joints.
Safety note
3D-printed parts are not certified marine equipment. Below the waterline, in any pressurised line, or anywhere a failure could flood or sink the boat, use certified bronze or Marelon fittings and have critical work checked by a qualified marine professional. Always verify material suitability, thread compatibility and sealing before relying on any part — printed or bought. Questions about a specific fitting? Reach us at info@marinelab3d.com.