The Onboard Spares Kit: What to 3D Print Before You Cast Off
Every skipper has a spares locker: impeller, fuses, hose clamps, a spare belt. But the parts that actually stop a weekend — a cracked deck-fill cap, a split hose barb, a lost locker clip — are rarely in it, because nobody stocks fifty variants of small plastic hardware. If you own a 3D printer, you can. This guide walks through a compact, sensible kit of printed spares to make before you cast off, roughly in order of how often each one saves the day.
Why print spares in advance?
You could print a part when it breaks. In practice, the printer is at home, the boat is not, and the failure happens on a Saturday twenty miles from the dock. A printed spares kit flips that: each part weighs a few grams, costs cents in filament, and lives in a small box in the nav table. Print them in a marine-suitable material — PETG for most parts, ASA for anything living in the sun — and they'll wait for years. If you're not sure which filament to use, start with our guide to the best materials for 3D-printed boat parts, and see how long printed parts actually last in saltwater and sun.
Tier 1: the parts that fail underway
Caps and plugs
Deck-fill caps, tank caps, drain plugs and inspection covers are the most commonly lost or cracked plastic parts on any boat. They unscrew, they fall overboard, their tabs snap. Print one spare of each size you have aboard. Because most of these are parametric on MarineLab3D, you can size them to your exact thread rather than hunting for a chandlery match — measure once with the method in our hose and thread measuring guide, and the file fits.
A cap key
The key that opens your deck and tank caps is small, loose, and famously overboard-prone. A printed one takes twenty minutes and costs nothing to lose. Our parametric boat cap key is free — print two, tie one to a lanyard, and drop the other in the kit.
Hose barbs and adapters
A split barb on a bilge, galley or washdown line is a genuine problem at sea. A printed PETG barb, printed solid and oriented so the layers run around the hose axis rather than across it, will hold a clamped low-pressure line long enough to get home. Keep one or two in the sizes your boat uses — the plumbing & pumps collection covers straight, reducing and elbow variants. For anything that holds pressure, print a fresh part at home afterwards and treat the spare as the temporary fix it is.
Tier 2: the parts that fail at anchor
Clips, hooks and small deck hardware
Locker clips, hatch tie-downs, fender hooks and line organizers break from UV and fatigue rather than load. They're also the parts guests break. A handful of printed hooks and clips from the deck & rigging collection weighs almost nothing and turns a minor annoyance into a two-minute fix. Print these in ASA if they'll live on deck.
Knobs and handles
Stove knobs, hatch handles and valve handles crack at the splined bore. If any on your boat are original plastic and older than a decade, print the replacement before it breaks — you'll have the geometry handy while the intact original can still be measured. A snapped original is much harder to reverse-engineer.
Tier 3: consumables and conveniences
These aren't emergencies, but they earn their place: winch handle pockets, a caulk nozzle, cable clips for that one wire that's always loose, a soap holder for the cockpit shower. Many are free — browse the free files collection and add a few to the next print batch. The marginal cost of printing one more small part while the printer is warm is essentially zero.
Packing and printing the kit
| Part | Material | Qty | Notes |
|---|---|---|---|
| Deck/tank caps | PETG or ASA | 1 per size | 100% infill on threads |
| Cap key | PETG | 2 | One on a lanyard |
| Hose barbs | PETG | 2 | Solid, layers around the axis |
| Drain plugs | PETG | 1 per size | With a TPU gasket if needed |
| Hooks & clips | ASA | 4–6 | UV exposure on deck |
| Knobs/handles | PETG | as needed | Print before the original breaks |
A few practical points. Print everything at 4+ perimeters — small parts fail at the walls, not the infill, and the extra grams are irrelevant. Label parts with a marker (size and thread) so you're not measuring in a rolling cabin. Store the kit in a zip bag with a silica gel sachet; PETG and ASA don't mind humidity much, but the bag keeps everything together. And once a season, check the kit against the boat: if you've changed a hose size or added hardware, the kit should follow.
A note on safety
3D-printed parts are not certified marine equipment. A printed spare is an excellent get-home fix for caps, clips, organizers and low-pressure plumbing — it is not a substitute for proper hardware in critical applications. Never rely on a printed part below the waterline or anywhere failure could endanger the boat or crew, and verify material suitability before use. When a printed spare gets you home, replace it with the proper part — or a properly printed and tested one — before the next passage.
Questions about sizing a spare for your boat? Write to us at info@marinelab3d.com — most parts in the catalog are parametric, so the spare you print is the size you actually need.