Schnellboot $100 (S-100) – Part 1 Planning

I had this planning post drafted up to help me in ordering the parts for a 1/144 Schnellboot remote control model, but a couple of sick days last week meant I went on impulse shopping online and all my parts have arrived before this is even posted! Sick days are bad for my wallet. Very bad for my wallet.

Requirements

One of my colleagues always nags about requirements and I always set the bar low for myself so I can’t fail…

Must float (What an absolute failure if it doesn’t…)
Should float even if ‘sunk’ by ball bearing fire (RC Warships) but ‘decks awash’
Should achieve a top speed of 44 knots (equivalent scale speed)
Target a cost per boat not including shipping/postage of $100USD. (Hard one…)

Concept

In addition to just the boat, I’ll also have to consider the remote control aspect. I could just buy almost everything ready to go (probably at great expense), but that’ll fail one of the requirements but most importantly defeat my purpose of ‘learning’ from this building experience. Yup this will be an exercise in self education in the fields of electronics, 3D modelling, micro-controllers and RF technologies.

The plan is to buy commercial off the shelf components, motor controllers, microcontrollers, RF modules etc and ‘integrate’ them together to form a working system. A kind of mid-way point between making everything yourself or buying everything off the shelf.

Straight onto the parts list and the rationale behind some component selections…

Parts List

Wireless

Two Nordic Semiconductor based 2.4HHz RF modules, one for the remote control and one for the boat. I wanted to get lower frequency modules (2.4GHz is the resonant frequency of water and don’t work to well ‘under-water’). Still the price and small dimensions of these units won me over.

I opted for a small chip antenna on the boat as to be inconspicuous

Controller – Transceiver nRF24L01+ Module with RP-SMA ($19.95USD)
Boat – Transceiver nRF24L01+ Module with Chip Antenna ($19.95USD)

Micro-controllers & Motor Controllers

Two very similar thus equally awesome controllers from Pololu with on-board H-bridges! The on-board H-bridges support 1.2A continuous current and make the electronics side on the boat a-lot simpler. To make things simple and have common components I opted to use the same module (actually with a slightly larger flash memory AVR main chip) for the controller.

Controller – Baby Orangutan B-328 Robot Controller ($29.95USD)
Boat – Baby Orangutan B-48 Robot Controller ($29.95USD)

Propulsion

This is probably the unexpectedly expensive bit of the design as I originally intended using cheap ($2) motors but ended up getting these cute 10:1 metal geared motors. The boat will operate on differential drive without a rudder so two motors with two props… (doubling the expense!). Haven’t figured out what I’ll do for the prop yet…

2 × 10:1 Micro Metal Gearmotor HP (2 x $15.95)

Hull

There were several options I considered, namely :-

Option 1 – Laser cut PETG, folded then weld-on #3.
Option 2 – Fibreglass moulded by someone else ($25AUD)
Option 3 – Vacuum formed PETG/Styrene

But for now, I’m going with the lazy way of Option 2 and ordering it from someone with the appropriate cut-outs etc according to the AusBG model warship rules :).

Controller

Ooops almost forgot this. This is going to be a custom job using hacked parts from Ebay sourced USB gamepads and the other bits above… Hopefully I’ll be able to design a 3D printed contoured single hand control…

Wrap-up

Nothing to see here…until tomorrow where I’ll post photos of the bits that’ve arrived and a bit more detail planning on how I’ll fit everything together.