If you’ve read my previous posts, you’ll know that I’ve already built the full size boat, so why am I 3D printing a smaller one? Well my initial test boat was a very simple wooden build but it wasn’t very robust. I’m sure you can look at the picture and guess why (the propeller didn’t sit in the water and the water almost came over the sides)!
I needed a way to test the electronics, motors and algorithms and with the full size boat being so large it makes testing very difficult. Therefore I decided to 3D print a new test boat but instead used a new design so that I could make some improvements to the stability. Considering it’s a test boat, it’s going to need to be as stable as possible.
Whilst doing all of this there’s a crucial lesson that I learnt, it’s that I should have built a small prototype, tested that and got it working fully, and then built the full sized version. But hey that’s hindsight for you, and at least now I’ll have a smaller robust boat I can use for testing.
So let’s talk about the design. The full size boat has a narrow hull that was designed for efficiency but it doesn’t make it very stable and the bow sits in the water a bit too low. Therefore this boat has a much flatter hull to provide stability for testing. It’s a fairly large model, and at a meter long this allows me to add extra components such as cameras and mounts so that I can test new hardware and develop new features.
Alongside the inner hull there’s a lip with screw holes so I can slot and add extra components. The back of the boat has a raised platform which will allow for mounting of the servos and the motor pod steering mechanism. This platform also has many holes which can be used to secure different components. The idea is the design should be as flexible as possible to account for future alterations without having to do large re-prints.
The motor pods and their mounts have been designed in the same way as they will be installed on the full size boat. This is so we can test the mechanisms in the smaller boat before drilling holes and installing them on the full size one.
The motor pod consists of a pod that will be submersed under water with a propeller and motor. It attaches to the boat by a stainless steel pipe that will rotate in order to change the direction of propulsion. More about the motor pod design will be discussed in a later article. To allow the steel pipe to rotate and no water to pass through the mechanism contains a bearing and a seal.
The lip on the top of the boat has screw holes in it that allows us to attach a lid (or in boat terms, a deck)! The deck was quickly designed so it won’t create a waterproof barrier but it does give us a decent level of protection from splashing.
The electronics used in the first test boat has been mounted on a removable board. Therefore the 3D printed boat has been designed to be wide enough for this to be installed. This means I can easily move the electronics between both boats for testing.
Due to 3D printing size limitations the boat has to be printed out in 10 parts. We’ve designed these sections to be bolted together and o-rings will be used to create a seal between the parts. Bolts will be used to hold them together instead of glue as this means we can always reprint or modify certain sections of the boat.
As the parts were 3D printed in large sections you have to design them to be printed in this way. For example, you can only print overhangs of about 55 degrees reliably. Otherwise you’re printing the plastic in to thin air! If there’s only one overhang we try to design the object so it’s printed with something underneath to support it (but without using supports as that wastes a lot of material). If we can’t design something to support it we just design the overhang at no more than 55 degrees or rethink the design.
As 3D prints aren’t naturally waterproof we will need to waterproof them. You can print with more walls of plastic, use higher layers or extrude slightly more plastic, and those do help to reduce the permeability of the print. However that would use a lot more plastic, especially for a very large print like our boat. Instead we sealed the outside of the print using some clear coat spray. I did attempt this with lacquer but I prefer the clear coat spray as it contains acetone, so it actually melts the plastic together to create a barrier.
Tip: don’t spray too much otherwise you will get drips!
3D Printing in Practice
The parts took over a month to print. As they were pretty large each one took over 24 hours. However the most challenging aspect was the parts warping and all the attempts trying to fix this is what made it take so long.
ASA/ABS are known for shrinking a lot more than PLA, and large prints are often prone to warping. As the print cools over large periods of time this causes the print to warp and pull away from the bed. This also causes cracking, which is obviously pretty bad for our prints.
Due to warping some of the parts aren’t perfect which has meant that there are either some cracks in the parts or gaps between them when bolted together. O-rings have been used to create a waterproof seal but they weren’t designed to bridge large gaps. So to fix this I’ve put silicone sealant along all the edges and then bolted the parts together.
To prevent this you first need to ensure that you have good adhesion between the bed and the print. To do this I:
- Clean the bed with acetone between prints, and don’t accidentally touch it with any fingers. I’m using the steel sheet from Prusa, so I don’t use anything else such as tape or an acetone blob on the surface, as the prints are generally rock solid to remove, even if they’ve warped.
- I extrude wider lines for the first layer, so the plastic has more contact with the surface.
- Then as warping is caused by the cooling of the print, I keep the enclosure as hot as possible. I found that when I sealed up the gaps in my enclosure this made a notable differences in the warping.
Printing out good parts became one of the hardest challenges, but as it’s only a test boat it doesn’t have to perfect. Stay tuned for more articles on how the motor pod was designed and tested and hopefully some demos of it in the water!