A home-made Kontiki built from scratch

The only way to fish
By Steve Jeffares
[published in The Shed December 2014]

How it works
The motorised kontiki (sometimes called a torpedo) takes a fishing line out through the surf to deeper water. The motor timer is set by passing a magnet over the hull, activating the reed switches for a programmed time the motor will run, indicated by the flashing LED lights (see Inputs and Outputs). The motor is then switched on by a relay. Once the motor is going and the rudder direction set, the kontiki in the water drags the long line off the freewheeling winch and heads offshore. As the line runs out the fishing sequence is quickly clipped on: first a weight at least 50 metres away from the kontiki which ensures the torpedo cannot be dragged under, followed by 25 baited hooks at four-metre intervals and lastly another weight. The flag indicates where the kontiki comes to rest and floats more than a kilometre from shore. After an hour or so, time to rev up the winch motor and haul the line and kontiki (and presumably some snapper) back into the beach. In case of a shark biting off a snapper and the line, the hull has on it home phone numbers to call when a friendly beach walker finds the lost torpedo.

Whakatane kontiki launch.

Over the years I’ve tried sails, kites, giant bags, kayaks and surfboards to get hooks out where the fish are. After watching torpedos on the beach, I find it is now obvious that there really is only one way and it requires 12 volts and a motor.
Of course every challenge is only really about what you can learn in the process, so I set about building a kontiki torpedo and winch from scratch for as little as I could. I had to enlarge my capabilities especially in aluminium casting, plastics forming and in electronics.
The kontiki was divided into three parts:
• the hull including the motor guard, tow point and rudder;
• the motor assembly; and
• the power supply and electronics.

The standard 7.2Ah battery required 150 mm PVC pipe and while this profile offered the standard fittings, it presented challenges in having to fit everything to a round. I settled on a one-metre length of pipe to allow the motor to be set back from the front to make room for a front rudder. The front rudder is necessary when there is a strong current or a crosswind.
The overall length is 1250 mm including the nose cone and end cap (the “hatch”). I did not finalise the overall length until I had the other components so I could make sure I could achieve a balance in the water.

Ready for the beach, with winch, traceboard and kontiki on home-built holder.

Nose cone
The nose cone presented a challenge. Nose cones are commercially available but I was on a mission to do everything. I turned up a mould from laminated MDF and formed the plastic from 2 mm PVC on a vacuum former. During vacuum forming of larger shapes, PVC tends to gather unevenly around the mould causing “webbing.” To avoid this uneven stretching, a ring “follower” is slid over top of the mould as the air is extracted to ensure an even distribution of the PVC. Using PVC also meant I could solvent-glue it to the other fittings. I used a 150 mm end-socket as a jointer to get a strong join and filled the nose cone with expanding foam. It will go nose first into the sand at some point.

The rudder has a ball-bearing stop-lock.

Tow strop bracket
The tail end is just a threaded inspection cap joint which provides two thicknesses of PVC to bolt the tow bracket to. Using a wooden mould, I sand-casted this while I was doing the cast motor-guard. The bracket has several holes laterally to allow for the sea current and to compensate for motor torque and any slight misalignment of the motor.

Nose cone mould on the lathe and…

…with 2 mm PVC.

Motor guard
Not having the capacity to weld aluminium, I find, is very limiting in anything marine. But I did, however, have access to a crucible for casting aluminium. Forming a motor guard from 3 mm MDF and making provision for a motor fin connection, I set the mould in casting sand. It took a few goes to get it right.
• the sand was too wet and I had gas pockets,
• too dry and the sand crumbled, or
• I had inadequate venting.
• But I did finally get a tough, single-piece motor guard exactly the right size, contoured to the 150 mm pipe diameter.

Aluminium cast motorguard

Sand casting ready to pour.

Motorguard mould in sand.

Motorguard casting mould.


The rudder had to be tough enough to hit the sand yet be adjustable. I settled on PTFE (polytetrafluoroethylene) non-stick block which is inexpensive and easy to shape. A simple, ball-bearing stop-lock means I can set it on the beach. Waterproofing the 8 mm stainless bolts proved difficult. I set stainless nyloc nuts inside the hull and capped them as a precaution. The rudder assembly is then bolted from outside the hull.

Switch and fuse

As it turned out, I had bought a 36 lb-thrust 12 volt motor that was not well set up for the vertical connection to the hull. I had to borrow a large BSP die and turn an outside thread to accept a brass fitting. Once I had that, though, the rest was just ½” BSP threaded brass. The stem went right up through the hull to take the flag on top, and it means it can handle getting dumped by big waves. The wiring exits through a hole inside and there are shaped spacers under each of the four brass flanges that secure the stem to the hull. Again, the spacers are PTFE for ease.

Timer breadboard testing using a 3-pin download cable.

Once I had the main elements, including the batteries, I made a small see-saw and balanced the whole lot in approximately the right place. This gave me the main assembly points. By allowing 100 mm either side of the batteries, I had the ability to finalise the balance once the battery enclosure was in place later on.

Motor angle
Because the tow load pulls the stern down, the motor is canted upwards towards the stern as an offset. Using both homework and guesswork, I decided to place the motor about 2 degrees offset to the line of the hull, 20 mm across 600 mm. This meant the vertical stem was also tilted, so finding the entry and exit points was crucial. Attaching the motor and guard had to be done in one operation, even with the prop removed. But after a few dry runs (no sealant) the motor was in and the wire was accessible from the stern. I did not put the nose cone on until I had the motor in place, and then filled the hull with polystyrene. Even if I have a disaster and hit an iceberg, it will still float.

Battery enclosure.
Square PVC downpipe is slightly too small for the batteries so I hot-folded 2 mm PVC into a rectangular tube. The tube is constructed over-length for battery movement. I hung the whole kontiki from the shed trusses and slid the batteries to achieve balance. Once they were positioned, I packed the whole hull with polystyrene pieces to lock the batteries in place and to maximise floatation.

LEDs flash as timer is set. A 20-minute timer set drags the line out about 1.2 kilometres.

Electronic timer
This part of the project was perhaps the most challenging for a builder. The simple aim is to use a cheap microprocessor to activate a 12 volt marine motor from outside the hull using only a magnet. It sounds simple enough but it requires a range of run times and has to have an interrupt to shut it down at any time. My ideal was to run the timer off the same 12 volt supply as the motor but the sensitive nature of microprocessors meant that electromagnetic interference from the motor and relay was an issue that I could not resolve. I trialled and settled on an isolated and switchable lithium CR2032 3v battery to power the microprocessor. This is housed in a screw-off lid near the rear hatch for easy access.

Kiwi Patch boards are a dream to solder.

Kiwi Patch board
This clever, prototyping printed circuit board (PCB) has been devised in New Zealand by Andrew Hornblow and QSM Ltd Electronics to allow designs on a plastic breadboard to be easily converted to a real soldered board. It is set up to accommodate the PICAXE or PIC microprocessor.
At this point I must confess that the timer here is up to the Mark IV version. All previous versions of the timer suffered from current supply problems and electromagnetic interference. Mark I functioned very well until the batteries ran down a little and left the timer resetting. Mark II was a simple start/run/stop with no interrupt.
Mark III again suffered current supply problems and or interference and the Mark IV finally resolved all of these using the 3v isolated supply. Previously, when the motor came under a heavy load—either from drag or wind—the PICAXE microprocessor would reset. The marine motor can draw more than 20 amps and because the PICAXE shared the supply voltage, it seems available current was too low and the timer would cut out, shutting down the motor, or the interference in the system just messed with the chip.
The Kiwi Patch prototyping board is set up for a regulated voltage, stepping down from 12 volts to 5 volts. In the supply are capacitors that should provide a buffer for the supply, filling and discharging to maintain an even 5 volts.
This was not the case with my earlier versions, although I tried larger capacitors to maintain supply, protection diodes to prevent current flow from reversing and snubbers which prevent induced electro-magnetic interference from disturbing the supply.
With each timer, I had to test it by installing, waterproofing and sending it out to sea to ensure I was getting the load required.

Lining up the LEDs.

The 08M2 is an advanced microprocessor with enough inputs and outputs, one that is easy to programme (it is used at year 7-8 level in schools) and inexpensive. No doubt there are other options worth investigating for the electronics enthusiast.
The PICAXE does require the investment in a download cable. And if you don’t have a serial port (which few of us do any more) a USB adapter is needed. The software is free from the PICAXE website and there is an abundance of internet support and guidance. Prototyping onto a breadboard is essential to make sure everything goes as intended. A friend tells me that electronics is binary—it goes or it doesn’t. There is no middle ground and fault-finding can be frustrating to within an inch of the skip. Perseverance and research answered most of my questions and blown components answered the rest.

Timer assembly showing automotive relay with snubber (top left) and 3 v lithium battery with switch (top right).

The inputs were a question of reed switches vs. Hall effect sensors. I had success with the glass reed switches which I thought may be too fragile, but they handled a few dumpings on the sand no problems. A Hall effect sensor is an alternative analogue magnetic sensor requiring a “debug” or the establishment of the sensors thresholds. This was doable for setting the timer but I could not easily activate a reliable interrupt to kill the motor any time I wanted.
I settled on the reed switch for its on/off state; ON when a magnet is over it and OFF otherwise. I have been using a $3 12 mm neodymium magnet which activates the reeds up to 2 cm away, more than enough for the hull thickness. The reed switches require a capacitor to take some of the bounce out of the reed and a pull-down resistor to avoid a “floating” state. One reed switch interrupts and resets anytime and the other moves the programme through three timer settings and back to the start.

Header pins make solid lead.

Timer is sealed in waterproof plastic housing.

Holding the magnet over the reed moves into a Timerready A stage for the first timer and the LED flashes at half-second intervals. If it is removed, Timer A activates. If it is kept, Timerready B stage begins and the LED flashes at longer intervals and so on back to the start. Timer A outputs current through leg C.2 to the transistor which opens the gate for the 12 volt current to the automotive relay which latches and connects the negative 12 volt supply to the negative wire of the motor.
There is a power diode at the transistor to prevent reverse current (current backflow from the relay coil as it rests) and an interference “snubber,” a resistor and capacitor in series across (in parallel with) the relay. The snubber provides an alternative path so any inductive current generated can be dispersed without interfering with or damaging the rest of the circuit

Battery and switch in screw-off lid for easy access. Timer assembly in plastic case in the background.

For waterproofing, the timer has to be sealed in a plastic housing with the LED protruding. A hole is drilled in the hull and the LED is set in silicone below flush. I super-glued a layer of 1 mm clear plastic over it, just in case.

Master switching
Inside the hull, a 12-volt 25 amp toggle switch and a 20 amp automotive activates the feed to the relay and the motor.

Not being able to weld aluminium, I also had to cast the winch components. This project was mostly about what I could get hold of rather than what was ideal. So I cast two flanges from an MDF mould and turned them down with a stub either side: one for the shaft grub screw and another to press-fit and rivet a piece of 50 mm aluminium pipe. The shaft stands were also cast and bolted to aluminium U section (actually old pallet bearers). A donated 20 mm stainless steel shaft through 20 mm stainless bearings was coupled to a 24 volt wheelchair motor, providing loads of pull. Connecting the winch shaft to the motor shaft required a sleeve connection that could accommodate a shear pin. The wheelchair motor is very powerful and the shear pin give some snag insurance. The winch drum accommodates two kilometres of 400 lb line, and a 20-minute timer set drags it out about 1.2 kilometres.
Am I getting my quota? To date we have fed the family on many occasions, though I haven’t met even the new quotas for snapper. We are averaging two good size snapper per set but mostly in the change of light hours. It’s just really nice to have a one-person operation on the most beautiful coastline anywhere and to be catching fresh fish on a machine that I made myself.

The winch is wound in with a 24 volt wheelchair motor

Steve Jeffares with friend Paul about to launch the kontiki. Phone numbers are for “lost and found” calls if the machine strays.