Metal for anything that carries load or keeps someone on the boat. Plastic for anything light, below decks, or still being figured out. That is the short answer, and the rest of this guide is the four questions that get you there when a part sits somewhere between those two ends. A printed winch handle holder can look perfect on the bench, survive a dozen calm sails, and let go the one time someone leans on it hard while tacking in a blow. I would rather you make that call on the drawing than on the water.
1. What happens if this part fails?
A cable organizer that lets go drops some wires in the bilge. A cleat, a padeye, a winch handle holder, or anything else carrying load under sail sits in a different category, because a failure there arrives at the worst moment, under load, with the boat heeled. Anything load-bearing or safety-related belongs in metal, machined to a real tolerance, not printed. This question outranks the other three, which is why I ask it first.
2. What load is it carrying?
| Load | Better fit | Why |
|---|---|---|
| Structural or safety-critical (deck hardware, rigging attachment points) | CNC, metal | Needs predictable strength and fatigue resistance under repeated load |
| Moderate, steady load (a mount, a bracket bolted in place) | Either, depending on exposure | A well-designed printed part in the right material can hold up; check exposure below |
| Light or organizational (cable management, small covers, trim) | 3D printed | Low consequence of failure, and printing lets you iterate the fit fast |
One honest note on that middle row. I do not publish a load rating or a strength figure for a custom part unless I have tested that exact part, and a one-person shop cannot test a one-off the way a catalogue maker tests a production run. If your part needs a rated number attached to it, a catalogue part with a published rating is the right buy, and I will tell you so.
3. What is it exposed to, and for how long?
Below decks, out of the sun, damp now and then, is one environment. A cockpit in July sun and salt spray is another, and the material sheet decides which prints survive it. The figures below are the filament maker's own, from the Prusa Knowledge Base, and they are why I treat 60 °C as the line for anything that lives in a sunlit locker.
| Material | Heat | Sun and weather | Where I would use it on a boat |
|---|---|---|---|
| PLA | Softens and deforms above 60 °C | Degrades under UV; listed as not suitable for outdoor use | Fit-check prototypes and below-decks organizers only |
| PETG | Prusa lists it for most exterior use below 80 °C | UV resistance is not addressed on the PETG page | Brackets and mounts that carry little load; plan to replace it |
| ASA | Temperature resistance up to 93 °C | Prusa's outdoor pick for UV resistance | The printed choice for on-deck parts; it warps while printing and wants an enclosure |
| Nylon (PA) | High temperature resistance | UV resistance is not addressed on the nylon page; the filament can absorb water up to 10% of its weight in storage | Tough wear parts; dry it before printing (see the wet filament guide) |
Metal has its own sheet. Seawater carries 18,000 to 30,000 ppm (parts per million) of chloride, and at the temperatures a boat sees, pitting and crevice corrosion are the two main risks the stainless mill names. 316 is the boat-hardware default for good reason, but the same mill rates its 316L for less demanding seawater work and expects it to corrode early in continuously wetted systems such as ballast-water filters. On deck that means 316 for hardware that gets rinsed and dries, with the crevices designed out, because a stagnant pocket under a washer is exactly the condition the mill warns about.
Aluminum is lighter and cheaper to machine. Bare, its natural oxide layer degrades rapidly in marine conditions, so it needs anodizing, and the anodizer's guide names the 5xxx aluminum-magnesium series as the corrosion-resistant choice in saltwater. General practice, no number attached: isolate a stainless fastener from an aluminum part with a washer or sleeve, because the two metals in salt water make a battery and the aluminum loses.
4. Does it need to fit exactly, or does it need to exist quickly?
Printing wins on iteration. If you will not know a bracket's exact dimensions until it is on the boat, print a rough one in PLA and take it aboard with the real fasteners. Mark it up, adjust the model, and only then send the final version to be machined. Two habits make that prototype worth more. Print the mounting holes at the real fastener size, so you are testing the bolt pattern and not just the outline. Then write the tolerance the part genuinely needs on the drawing before the metal version is quoted, which is what the tolerance guide is for.
When neither is the right answer
A catalogue part with a published load rating, made by a company that tests production runs, beats a custom part from me for anything that needs a rating. I will happily build the bracket that adapts a catalogue cleat to your odd deck camber; I will not try to out-engineer the cleat. The same rule runs the other way: if a part is going to be redesigned three times, machining any of them is money spent on a shape you have not settled.
Before you order
- Written down what happens if it fails, and who is holding on to it when it does.
- Named the load: structural, moderate and bolted, or organizational.
- Named the exposure: sun, salt spray, heat in a closed locker, or dry and shaded.
- Test-fitted a printed version on the boat with the real fasteners.
- Put a tolerance only on the features that mate with something else.
- Checked whether a catalogue part with a published rating already exists.
Weigh the consequence of failure first, then pick the material, then prove the fit in plastic before you pay for metal. That order is cheaper than any other order, and it is the one I follow at the bench.
Checked against Prusa Knowledge Base, Outokumpu and Colorex pages on 2026-09-21: PLA, PETG, ASA, Polyamide (Nylon), Stainless steel for ballast water treatment systems, Selecting subsea stainless steel, Anodizing aluminum in the marine sector
