Part 4 - Building an RC-Boat

After forming the search plan, the main question was how to search underwater. We were eager to start searching, but we knew that it couldn’t be done before winter sets in. During winter, the waters in Finland are covered with thick ice while during spring and early summer, there are floods, which make the waters extra murky. This meant that searching could only start in late June or early July. There would be plenty of time to come up with a solution.

We had scuba diving experience, so we had first-hand experience how difficult it is to find anything in the dark Finnish waters. I had heard that there was a device called a side scan sonar which is used in underwater searches. I’d never seen one, but people said that it produces detailed underwater imaging. Back in the early 2000’s, when we scuba dived actively, the cost of the device was so high it was used only by professionals.

Searching the Internet revealed a guide called ‘An Illustrated Guide to Low-cost Side Scan Sonar Habitat Mapping’. It was an eye-opener. It was created by U.S. researchers to give a detailed description on how to map large bodies of water using an affordable, consumer grade side scan sonar. Exactly what I was looking for!

The sonar images in the guide were encouraging. If the devices were able to produce such accurate images years ago, what could the current models do? The guide made it clear that this was the way to go.

A quick research taught me that modern models could record the side scan sonar data, not just the screen shots. This meant that the laborious process described in the document was unnecessary.

One morning I stumbled upon a sale and struck gold. There was a huge discount on one of the sonar models which I’d been looking at. There was only one item left, and it happened to be in the store just a few kilometers from where I lived. I drove to the shop and came back as a newly minted side scan sonar owner.

Sonar Sonar waiting for unpacking.

Now the big question was, how to transport the sonar over the water. Sonars are meant to be attached to a boat but I couldn’t get one as I didn’t have space to store it. An inflatable rubber boat was an obvious choice but I had a hunch that we would need to search several spots. Inflating and deflating a boat sounded like too much work.

I started to dabble with the idea of a remote controlled boat (RC-boat). Off-the-shelf models were out of the question as they couldn’t carry enough payload, so I went down a rabbit hole of constructing a remote controlled boat and DIY drone stuff. There was one design that really caught my attention. It was called Kenterprise.

Harald Molle’s blog was another good source of information. Molle’s boat was originally developed to create depth maps for scuba divers, so it had lots of elements I was interested in. Harald had also written the story in his blog from the beginning and he openly shared the problems of building his own RC-boat.

Both of these RC-boats were made from styrofoam. I didn’t have the space to do styrofoam work so I needed something else. I stumbled across a solution using sewer pipe to build a Catamaran. After some inspiration from Youtube, the plan started to take shape. The RC-boat would be a catamaran made from sewer pipe. There would be an electronics box on the top of the hull, which would house the boat’s required electronics and the sonar. The RC-boat would be powered by a Li-Po battery.

What about the propulsion? Should the propellers be underwater or up in the air? After reading a Kenterprise’s builder’s comment that weeds easily stuck to the underwater propellers, I decided to use drone propellers.

Having the sonar and a rough plan of the RC-boat ready, the planning could really start. I decided to use drone parts as much as possible, because they were cheap, somewhat reliable and easy to source.

The brainbox would be the Ardupilot Rover. The RC-boat would have four engines and propellers: two in the front and two in the back. The idea was to double the power line to increase reliability. I didn’t want to end up swimming as many times as Harald Molle.

The RC-boat would be controlled with a proper radio controller. The box to enclose the electronics caused a lot of headaches. IP65 rated boxes were heavy and expensive. Even the AliExpress stuff easily cost north of 200 euros, and they weighed several kilos. While walking in a hardware store, I stumbled on to a storage box with an IP45 rating. Checking the price tag, 9 euros, sealed the deal. It would be good enough.

On a paper, the construction seemed simple. But to actually build it, you also need tools: a soldering iron, a heat gun, a cordless drill, and all kinds of odds and ends. The shopping list seemed endless.

It took a few weeks to finalize the plans and order the parts. The first thing was to build a rough prototype mounted on 2” by 4”s with wheels on the bottom to test the concept.

RC-Boat version 0.1 A modest beginning.

The first prototype didn’t look like much but the mental effect was enormous. Now there was at least something to show and improve.

Having something in place, the real work could begin. After lots of learning and hard work, the electronics were in place. Next, the power line was ready, and just like that it was moving. Having never actually built anything before, it felt incredible and also confirmed that I might be able to pull this off.

There were lots of tasks I hadn’t anticipated: remote control transmitters and receivers; sizing of propellers; finding the right engines; finding the correct version of Ardupilot and configuring it; flashing the motors; and fitting and mounting all the stuff.

The Christmas holidays came at a good time, as I needed to construct the pontoons and didn’t have the space to do it. The plan was to create the pontoons from a 160mm (about 8 inch) sewage pipe. This was probably going to be overkill but it’s always better to have too much than not enough. My father happened to have some left over pipes which could be used. They just needed water tight end caps. So, during the holidays, I studied how to weld plastic from YouTube, and spent a large portion of the holidays in a cold garage welding the pipes.

Over the first days of January 2021 it had its maiden voyage on the Gulf of Finland. It was a triumph.

Later on, the propellors were changed to bigger 13” ones and the pontoons got hydrodynamic caps.

RC-Boat in the livingroom The RC-Boat in in it’s final form.

RC-Boat in the livingroom The transducer is stowed for transport.

Remote control The remote control.

While I was driving back home, I started to think that having a sonar image was not enough. What if we come up with a fuzzy sonar image and no one believed us. We would have to have underwater video footage.