Ok so the first challenge is to see if I can actually make a 3D print waterproof.  if not this project is sunk in the water. 

For this to work I'm not just talking about whether the print can hold water. But need something that can stand up to some serious water pressure without springing a leak....

I need a pressure test rig for that.

Ok so to handle this kind of pressures I’ve gone for a design that consists of a polycarbonate cylinder with 3D printed endcaps  also out of polycarbonate. These will form a tight seal with the cylinder using a large silicone O-ring. And either end secured in place with 6 pieces of M10 threaded rod. It will be filled with water and a ¼ inch  air hose fitting taped into the top so that I can connect it to an air compressor to pressurize the test rig. 

The samples will be attached to this load spring and submerged in the tank to measure how buoyant it is, or how strongly it wants to escape the water over time. The idea being that either water is going to ingress and compress the air in the cavity – reducing the buoyancy, or water will ingress and air will leave as a stream of bubbles – reducing the buoyancy. As  a back up I’ll also weight the samples before and after to measure if they have taken on water.

I’m going to be testing scaled down, simplified version of a sea scooter hull to make the testing as representative as possible. (white object above) 

If you do a quick search you’d find quite a bit of material on ways to water proof a 3d print. It’s not all entirely consistent. but after a bit of thought I think I found a good set of parameters to test. (details in the above video).

I'll try ASA as the main contender and PLA as a reference. I'll try these at a few different pressures up to a few Atmospheres (Atm) water pressure  and try a few post processing techniques like acetone smoothing and epoxy coating. So I will need quite a few test hulls......!

There where some surprising results. 

But post processing was definitely the way to go.  The Epoxy coating in particular was able to stand up to more pressure than you might expect.