This is a model of a generic torpedo for operating with a 500 size electric motor, although other smaller ones should be easily adapted.I have designed the parts, except the propeller, with Sketchup and then exported to STL. I have designed the propeller with jSDN and then exported to STL.

BOM

4 x Fins (plastic, printable) 2 x Body tubes large (plastic, printable) 1 x Body tubes small (plastic, printable) 1 x Nose (plastic, printable) 1 x Engine mount (plastic, printable) 1 x Tail (plastic, printable) 4 x Coupling rings (plastic, printable)

4 x M3 40 mm screws 4 x M3 nuts 2 x M3 8 mm screws 1 x 3 mm diameter shaft 1 x 3 mm inner diameter brass tube 1 x Electric motor 500 size 1 x 7.2 V NiCad battery 1 x 25 A electric speed controller (ESC) 1 x Arduino-style programmable board

Printing

I have used a XYZPrinting Da Vinci Jr 1.0 printer, with XYZPrinting natural PLA. The general printing parameters are:

• no need for support material or build plate adhesion
• standard speed
• 210-215 deg extrusor temperature

Specific parameters for the fins:

• solid printing (100% infill)
• 0.1 mm layer height

Specific parameters for the engine mount:

• support material
• honeycomb, low density

Building

Glue the two large tubes and the small one, inserting the coupling rings between each pair. I use Araldite style epoxy adhesive. Check that the tubes remain straight while the glue is curing. Glue the remaining coupler to the nose, then fit it into the body tube without glueing and let it cure while placing it straight.

Trim and drill the aft cone and the motor mount so that the screws fit easily. Glue the four fins into the recesses of the aft cone. Glue the brass tube to the aft cone. I use Poxipol style Epoxy mastic, which you can form clay-like and then fit into the cone to wrap the brass tube. Fit in place the motor, the full shaft, and the motor mount with the aft cone in order to properly align the brass tube and the shaft. Once everything has cured, glue the nuts to the inner part of the motor mount. I use Poxipol style mastic.

Prepare all the electronics and wires, allowing an access of length for easy manipulation. One everything is ready, and has been thoroughly checked, it is time to provide watertightness. Fill the four screw holes with vaseline, and cut a rubber ring between the aft cone and the engine mount. Then glue the engine mount to the body tube. Then fit the nose dome to the body tube without actually glueing it. I use duck tape to snap fit it firmly.

Operating

You may find the schematics and the programa for an Arduino board in GitHub:

https://github.com/humbertomb/torpedo_mk1

Program the Arduino board to power the motor for a short amount of time (2-10 s depending on the size of the pool you will be testing it). Use a Reed switch for magnetic activation of the program. Trim the torpedo a little nose up, and put enough ballast so that you get close to neutral buoyancy, but stay in the positive side.