Boat Heads and Holding Tanks: What You Can 3D Print

The head is the least glamorous system on any boat and the one most likely to ruin a weekend. It is also full of small plastic parts that break, disappear or were never designed sensibly in the first place: vent filter brackets, pump-out cap keys, seat hinge bushings, tank sender covers, hose spacers. Chandlery prices for these are absurd, and half of them are discontinued.

So the question is not whether you can 3D print parts for a marine sanitation system. You can. The question is which ones are a good idea, and where printing a part quietly creates a problem you will only discover offshore.

The one rule that decides everything

Split every part in the system into two groups: wetted and pressurised, versus everything else.

Wetted and pressurised means the part is in the flow path and sees pump pressure, vacuum, or standing effluent — discharge elbows, pump housings, joker valve seats, hose adapters between the bowl and the tank. A failure there is not a leak, it is a biohazard in your bilge, and in some jurisdictions a discharge offence. These parts are also exactly where FDM printing is weakest: layer adhesion under sustained pressure, permeability through the layer lines, and chemical attack from urine salts and tank treatment chemicals.

Everything else — brackets, covers, spacers, organisers, tools, keys, labels, vent grilles — is fair game and is where 3D printing genuinely earns its place on board.

If you do want to understand what an FDM part can and cannot hold, our guide on printing watertight fittings that hold pressure covers wall counts, perimeters and the pressure testing method. Read it before you print anything that carries liquid.

Parts worth printing today

Deck and pump-out cap keys

The classic. Every boat has a waste, water and fuel deck plate, and every boat loses the key. A printed key is under an hour of print time and works on the first try because the geometry is just two lugs at a set spacing. Our parametric deck and tank cap key is a free file — set the lug spacing and thickness to your fitting and print it in PETG. Print two and leave one in the cockpit locker.

Vent filter and hose brackets

Charcoal vent filters are usually sold with a flimsy strap or no mount at all. A printed cradle that clamps the filter body and bolts to a bulkhead takes the strain off the hose barbs, which is the actual failure mode. Same logic for long unsupported hose runs — a printed saddle every 300 mm stops the sag that traps liquid and creates smell.

Seat hinges, bumpers and bushings

Nylon hinge bushings and seat bumpers wear out and are almost never available separately. These are dry, low-load parts. Print the bumpers in TPU so they do not crack, print the bushings in nylon or PETG.

Tools and service jigs

A spanner for the pump base nuts, a guide that holds a joker valve square while you seat it, a funnel that fits your tank fill without spilling. Printed tools cost nothing to iterate and you throw them away afterwards without regret.

Labels and switch legends

Printing which seacock is which, and which way is closed, in raised letters that survive being wiped down. Trivial to print, disproportionately useful at 3 am.

Vents and grilles

Heads compartments are the dampest space on the boat. A printed louvre or grille for a locker door improves airflow enormously — the same approach we cover for beating condensation below deck.

Parts to leave to the manufacturer

Part	Why not
Joker valve / duckbill	Needs a specific elastomer with a calibrated cracking pressure. TPU will not reproduce it and a failed valve back-flows.
Pump housing and piston	Cyclic pressure plus abrasive solids. Layer lines become leak paths within a season.
Discharge elbow to the tank	Pressurised, wetted, and structurally loaded by the hose. High consequence, low cost to buy properly.
Skin fitting / seacock	Below the waterline. Never. See our through-hull guide for the full argument.
Tank itself or tank welds	Volume, permeability and odour control all fail on a printed vessel.

Materials: PETG, ASA and nylon in a tank environment

Urine forms struvite and calcium scale, tank treatments are often enzyme or surfactant based, and some skippers still use bleach or vinegar flushes. That is a wider chemical range than most boat systems.

• PETG — the default. Good chemistry against dilute acids and salts, easy to print thick and dense, cheap. Weak point is UV and creep under sustained load, neither of which matters inside a heads locker.
• ASA — use it where the part sees daylight, such as a deck plate key holder or a cockpit locker fitting. Not chemically better than PETG for tank duty, just far better in sun.
• Nylon (PA6-CF, PA12) — best for bushings and anything that slides. It absorbs moisture and grows slightly, so print bushings a little undersize and expect them to swell.
• TPU — bumpers, chafe protection, soft washers. Not a substitute for a moulded valve.
• PLA — no. It hydrolyses, it softens in a hot locker, and it will let you down.

Our full comparison lives in PETG vs ASA vs Nylon for boat parts.

Threads and hose sizes: measure before you print

Sanitation hose is typically 38 mm (1½ in) for discharge and 19 or 25 mm (¾ or 1 in) for flush inlet and vent. Fittings on European boats are usually BSP; American systems are usually NPT. They are not interchangeable and a mismatched pair will seem to tighten and then weep.

Sanitation hose is also stiff and thick-walled, so a barb that works fine on clear water hose may be impossible to push on cold. Warm the hose, use a barb with a lead-in taper, and double-clamp. Parametric files help here because you set the barb diameter and the hose ID rather than hoping a fixed model fits — see plumbing and pump fittings for sized-to-fit parts, and note that we recommend printed barbs only on vented, low-pressure runs.

Print settings that matter

• Walls: four perimeters minimum on anything that touches liquid, six on anything threaded.
• Infill: 40–60 % gyroid for brackets; solid for threads and bushings.
• Orientation: print threads and barbs vertically so the load runs across layers, not along them.
• Temperature: at the top of the material range for layer adhesion. A cool PETG part is a leaky PETG part.
• Finish: smooth surfaces resist scale buildup. Consider a brush of food-grade silicone grease on sliding faces.

A weekend plan

Print the deck cap key first — it takes forty minutes and solves the most common problem. Then measure your vent filter and print a bracket. Then print the two seacock labels for the heads intake and discharge. That is one evening of printing and three jobs off the list, none of which needed a chandlery visit.

Start with the free files in our free collection, and browse interior and cabin parts for brackets and organisers that suit the compartment.

Safety note

3D-printed parts are not certified marine equipment. Nothing in this article should be used on a pressurised sanitation line, a through-hull, or any component below the waterline. Verify material compatibility and structural suitability for your own application before use, inspect printed parts regularly, and carry a manufactured spare for anything whose failure would flood or contaminate the boat. Discharge of untreated waste is regulated in most waters — check local rules before modifying your system.

Questions about a specific fitting or a size we do not list? Write to info@marinelab3d.com.