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)!  

The wooden test boat
The first boat with extra weight (a water bottle and battery) tapped down to push the propeller in the water.

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. 

The 3D printed test boat
The 3D printed boat!

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.

Design

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. 

A profile of the fibreglass hull design.
The underside profile of the full sized fibreglass boat.
A profile of the 3D printed hull design
The underside profile of the 3D printed test boat.

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.

Motor Pod

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.

An example of the motor pod submerged in water for testing.
The motor pod being tested for leaks.

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.

An example of the servo mount and the back of the boat.
The left side of the boat shows the holes on the deck for mounting and the right side shows an example of the servo and motor pod rod mount attached.

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.

An example of the finished boat.
Example of the boat with most of the lid on.

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.

3D Printing

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.

Waterproofing

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.

Part warping.
You can see the warping on the left-hand side part of the print where the corner is pulling away from the bed.
3D printing gone wrong
An example of when 3D printing has a mind of its own!

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.

Warping

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!


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