Mismatched Hoses On Board? 3D-Printed Reducers and Adapters That Actually Fit
Every boat older than a season has at least one hose joint that doesn't match. The new bilge pump comes with a 19 mm outlet and the old hose is 25 mm. The freshwater pump is 1/2" and the tank fitting is 3/4". The deck-wash spigot was replaced with a metric part and the hose stayed imperial. Chandleries stock a handful of common reducers; the odd combinations are special orders, and the price of a two-inch piece of plastic is often absurd.
This is exactly where a 3D printer earns its place on board. A barb-to-barb reducer is a simple, low-stress part that prints in under an hour, and a parametric file lets you enter the two hose sizes you actually have rather than hunting for the closest stock item. This guide covers how to size them, what barb geometry works when printed, which materials to use, and where a printed adapter is the wrong answer.
First, measure the hose — not the fitting it came off
Hose barbs are sized by the hose inside diameter (ID). The nominal size printed on the hose ("3/4"" or "19 mm") is the ID in most cases, but old hose stretches and cheap hose is often a millimetre off. Cut a clean end and measure the ID with callipers in two places at 90°. If the reading is 18.6 mm on a "19 mm" hose, use 18.6.
Useful reference for the common sizes you'll meet on a boat:
| Nominal | Hose ID (mm) | Typical use on board |
|---|---|---|
| 3/8" | 10 | Fuel feed, pressure water branches |
| 1/2" | 13 | Freshwater, shower drain |
| 5/8" | 16 | Fuel fill vent, raw-water on small engines |
| 3/4" | 19 | Bilge pump, sink drain, raw-water |
| 1" | 25 | Bilge pump, cockpit drains |
| 1 1/8" | 28.5 | Bilge pump outlets (common on 1100–2000 gph pumps) |
| 1 1/2" | 38 | Head discharge, cockpit drains, sanitation |
If you need the full measuring procedure, including threads, read how to measure a hose and thread for a perfect printed fitting.
Barb geometry that seals when printed
A moulded brass or nylon barb has sharp, thin ridges. Printed at 0.2 mm layers, a sharp barb becomes a staircase and the tips are weak. The rules that work on FDM:
- Barb outer diameter = hose ID + 5–8%. For a 19 mm hose, a barb crest around 20–20.5 mm. Too small and it leaks; too big and you'll split the hose or never get it on.
- Shank (valley) diameter = hose ID − 3–5%. The hose relaxes into the valley and the clamp compresses it there.
- Two or three barbs, 6–8 mm apart. More barbs don't seal better; they just make the part longer and harder to push on.
- Ramp angle 15–25° with a flat back face. The ramp lets the hose slide on; the flat face resists pull-off. A rounded crest (0.3–0.5 mm radius) prints far cleaner than a knife edge.
- Bore ≥ 70% of the smaller hose ID. A reducer inevitably restricts flow; keep the bore as large as the wall allows. Wall thickness of 2.5–3 mm for sizes up to 25 mm, 3.5 mm above.
- Clamp landing zone. Leave a straight, barb-free band of at least the clamp width (usually 9–13 mm) behind the last barb so the clamp sits flat. Position it over a barb valley, not over a crest.
The Hose Barb Connector / Reducer (Barb–Barb) exposes exactly these inputs — the two hose IDs, barb count, wall thickness — so you can dial them for your hose rather than accepting a fixed catalogue size.
Straight, elbow or tee?
A straight reducer is right when the hose run is already gentle. But if you're forcing a hose into a bend to reach a pump that faces the wrong way, a printed elbow with the two sizes built in is a much better solution than a straight reducer plus a kinked hose. Kinked hose is the number-one cause of bilge pumps that "work fine on the bench". The Elbow Hose Barb Connector / Reducer handles 90° with different sizes on each leg; the 3-Way T Hose Barb Connector is for splitting a deck-wash line or joining two drains into one outlet.
When one side of the joint is a threaded port rather than a hose, you're into BSP or NPT territory — that is covered in BSP vs NPT threads for boat plumbing. The rest of the plumbing & pumps collection covers the threaded variants.
Print settings that matter
Orient the part with the bore vertical wherever possible. Barbs then print as concentric rings with no overhang problems, and the layer lines run around the circumference, which is the direction a hose clamp loads them. A straight reducer prints standing on its larger end; an elbow needs supports on one leg or a split at the corner.
- Layer height 0.16–0.2 mm; thinner gives smoother barb ramps.
- 4 perimeters minimum, 5 for sizes above 25 mm. The barb tips must be solid perimeter, not infill.
- Infill irrelevant if walls are thick enough — most reducers are 100% perimeter anyway.
- Slight over-extrusion (flow +2–3%) or a 0.6 mm nozzle helps close micro-gaps between layers.
- No ironing needed; the hose seals on the barb surface, not on layer tops.
For pressure-side lines (freshwater pump, deck wash), the watertight fittings guide explains how to make the walls themselves leak-proof, and sealing 3D prints covers epoxy wipe-coats when you want belt and braces.
Material by application
| Line | Material | Notes |
|---|---|---|
| Bilge discharge | PETG or ASA | Low pressure, intermittent. PETG is fine below deck. |
| Freshwater (pressure) | PETG, PP for drinking side | Typically 2–3 bar; PETG with 4+ walls holds this easily. |
| Deck wash / raw water | ASA | UV and salt on deck; see the UV article. |
| Engine raw-water | Nylon (CF) at most — prefer none | Heat plus vibration; a failure here floods the engine bay. |
| Sanitation | PETG or PP | Odour permeation is a hose problem more than a fitting one. |
| Fuel | Don't | See below. |
Where a printed adapter is the wrong answer
Being honest about limits is what keeps the boat afloat. Do not use a printed reducer:
- Below the waterline on a through-hull or seacock line. Anything between a seacock and the first high point should be certified marine hose and fittings, double-clamped. A printed part here is a sinking risk. The through-hull article explains why in detail.
- In the fuel system. Diesel and especially petrol attack common filaments and the consequence of a leak is fire. Buy the metal fitting.
- On engine cooling lines at temperature. PETG softens at around 80°C; raw-water after the heat exchanger can exceed that.
- As a permanent fix on a hose that itself is too soft or too hard. A reducer doesn't rescue a hose that has gone brittle.
A bilge pump outlet reducer, a shower sump join, a deck-wash tee, a sink drain adapter, a rain-catchment splitter — these are the everyday joints where a printed part is cheap, quick and appropriate.
Fitting it properly
Warm the hose end in hot water for a minute before pushing it on; it slides over the barbs without tearing the ridges. Use a stainless (316) hose clamp — two clamps on anything that pumps water overboard — sitting on the landing zone behind the last barb, and tighten until the hose just bulges either side of the band. Over-tightening a clamp on a printed barb can crack it along a layer line; a second clamp is better than a crushing first one. Check it after the first day's use and again a week later.
If you're building up a full set of joints for a plumbing refit, the weekend plumbing project lays out a print order and a test procedure.
Safety note: 3D-printed parts are not certified marine equipment. Verify material suitability and test every fitting under realistic pressure before relying on it, and never use printed parts below the waterline, in fuel systems or in other safety-critical applications. Questions on a specific joint: info@marinelab3d.com.