I have an inflatable canoe, which I talked about before in the design of a Rudder. This is a good boat that I've enjoyed many good days with. When I added a fixed rudder it started to perform much better. The control of the boat was very easy and it would go much faster. Here is a picture of the boat:

One disadvantage of all inflatable boats is their soft body compared to boats made of fiber glass or other hard material. Whenever a persons sits in an inflatable boat its formation changes and this makes it more resistive against water. In contrast hard body boats maintain their efficient and hydrodynamic bodies resulting in a much faster speed with less effort needed to move it forward. Sometimes I would feel the need for a higher speed, especially when friends would pass my boat and leave me behind. Therefore I started to think of a way to increase the speed.

Of course the best way is to add an engine to the boat. But what kind of engine is suitable for such boats? It is obvious that combustion engines provide the highest thrust. What is thrust you say? Thrust is the force provided by a source of power that moves something forward. Horses, jet engines or boat engines all provide a thrust that drags a body behind them. The higher the thrust is, the more acceleration and speed is provided. 

Although a higher thrust seems to be more fun, this boat cannot handle such a heavy engine. Also a great power would tear apart such an inflatable boat. So what came in my mind was an engine with less power and weight, an electrical engine. Now that was something I could relate with! There are some battery operated engines called trolling motors. Such motors use something similar to a car battery (12V) and are mostly used along with high power engines on fishing boats. Fishermen use such motors to control the boat easily, slowly and quietly in a fishing area. They provide a small thrust that can be suitable to run a very light boat. I found the following motor from Minn Kota as an example. 

There are many different forms and kinds of such motors. But as you can see from the figure above, they are usually intended to be mounted on a solid part of the boat. I could design a fixture for my boat the mount the motor on. But then for better efficiency and control the engine must be mounted at the middle-rear of the boat. This is suitable for boats with flat backs, but not for my boat. Also it would be hard to grab the control stick while the engine is right behind of the person's body. So I used these excuses to design my own electrical engine suitable for my boat. 

First of all I had to think of specifications of the engine, how it would be controlled, how it would look like and etc. I wanted the control to be under the driver's hand, easy and fun. Boat engines usually have sticks such as above and steering wheels in other cases. For my boat, it would take a complex mechanical structure to transfer the control to the driver. Therefore I decided not to make it mechanically complex. I brought the problem in my own field, electronics. I decided to use a computer joystick to control the board, one shown in the figure below:

This would make it much more fun. Now, how would it control the engine? I was reluctant to turn the engine to control the direction, although it was the best way to do it. Because again I needed a complex mechanical structure driven with another engine to turn the propeller with a joy-stick. This would result in some more control issues and electrical complexities as well. So to be more cost efficient, I decided to go with another scheme. Instead of using one motor, I would use two motors each on one side of the boat on a wing at the boat rear. Figure below shows what I mean with this:

The advantage of using such structure is that having two motors, it would provide more thrust and also it wouldn't need extra mechanical structure to turn. But how is that possible? The solution is to use the motors the same way as using paddles to turn. When one wants to turn to one side, he/she paddles on the opposite side to turn, as also explained in the design of the Rudder. To turn to the right side, the right motor shuts down and the thrust of the left engine turns the boat right and vise-versa. This fact is shown in the figure below:

Some complex submarines that are usually used for researches also use the same way to turn, especially when they want to turn around in the same spot. Although this method works good, the boat loses its speed when it turns, because one motor is off. Yet I decided that it was the easiest way I could go with. Another issue was that how to make the engine. Of course, by connecting a propeller to a DC electrical motor that runs with 12V. It sounds easy, but let's see some of the challenges:

	Propeller is under water and therefore if the motor is directly connected to the propeller, it will be under water as well. A very well sealed chamber for electrical motor is required, otherwise it would burn in smoke and take the entire circuitry down with it when water leaks into the motor.
	A stand alone high power DC motor is quite expensive, especially if it comes with a gear box to increase its power.

It is possible to make a sealed motor chamber, as trolling motor producers do. But in my case, I wasn't sure that I could seal the chamber perfectly and therefore I decided not to risk it. I would place the electrical motor out of water and would transfer the power to propeller. The way I designed it, the power is transferred to the propeller similar to a bicycle where the pedal power transfers to the rear wheel, using gears and chain. I used two small gears with equal number of teeth. Such small gears are used usually on a rear gear shifter of a bike. 

For the DC motor, I got a more cost effective Idea. I have a Black & Decker cordless 12V drill, one of those which has drill, jig saw and sander heads. It is one of my favorite tools and I've used it to make many things, such as the Rudder I made for my boat and also the fixtures for this electrical motor. A picture of the drill is shown below:

I found out that I could get a whole set of this drill for a very low price of $45 (CDN) as refurbished. It would come with two 12V batteries as well. A new battery alone is more than $60 and I wanted to get one for a long time. So it was my chance to get two sets of drills with four batteries and yet the cost was less than one DC motor. In my design, I modified the drill to fit to my requirements. A sketch of  my design is shown in the figure below along with the final design:

           

It is a beauty, isn't it?! I used two of these, one on each side of a fixture that looked like a wing, as shown below:

In my first revision, the wing was mounted under the boat. But I realized that it would also result in a big friction against water that would slow down the boat dramatically. Therefore I redesigned the wing and put it on top of the boat out of water. Let's take a closer look to the parts and discuss the design further. Below is a close-up of the motor:

As you can see, I cut the excess of the drill and took the wires from the motor out to my control board. Using this drill has great advantages. First the drill head has a gear box that divides the rotation speed of motor by about 11 and therefore increases the power by 11 times. Second, it has torque adjustment capability. When ever the pressure over the drill is higher than the adjusted torque, the device disconnects the motor from the load causing it to skip. This feature is used to fasten screws when you don't want to fasten them harder than some amount. But the advantage of this in my design is that if branches or weed get stuck in the propeller and put pressure on the motor, it would skip avoiding the damage to the motor or to the circuit.

You can also see from the figure above that the chain passes over the gear, which has its shaft simply fastened in the drill head. Also two stainless steel washers are placed on the sides of the gear. They help a lot as they eliminate the possibility of the chain falling off the gear. The chain then passes through plastic pipes to the propeller under water. The pipes isolate the chain from the weed and branches inside the water.

Another thing I experimented on, was the drill head gear box. I modified the gearbox and increased its speed by three times. This would provide a much larger thrust, but also would put the motor under a big load that was damaging to the motor and the electrical circuit. So I left the gear in its original condition. But it would be great if I could have more speed! Yet it would make the design more expensive. Now let's take a look at the propeller side:

As you can also see from the picture of the sketch design above, this part shown in the figure above basically contains a shaft with bull-bearings that propeller is mounted on. The chain runs through the pipes into the chamber and drives a gear mounted of the shaft. The chamber is simply made of a water pipe with a diameter very close to the propeller. The head of the pipe is modified into a shape much less resistive to the water flow. The black area you see on the head is the modified pipe, which is bent to form the proper shape. Then it is covered with epoxy glue and a black fabric resulting in a hard surface. The chamber is not isolated against water, but it keeps the debris out and the parts cleaner. But cares were taken to choose parts that wouldn't rust in water, made of stainless steel. 

Through the design of this motor I learned a lot about propellers as well. I put the knowledge I learned in the Propellers section. It is good to read this page if you like to know how propellers function in general. Next, I designed an electrical controller circuit using a micro-controller as the brain of the device and other electrical drivers. The board is shown in the figure below:

 

The motors and joystick are connected to this board. Having a micro-controller is very convenient, because it is possible to choose any convenient method of controlling the motors with the joystick. The joystick has two useful buttons, one under the index finger and one under the thumb. I made use of both as described below:

	Index Finger Button: No matter what the propeller turning direction is, pressing this button each time increases the speed one step at a time, a total of 15 steps to the maximum speed. Holing the button continuously results in continuous increase in speed until the maximum speed.
	Thumb Finger Button: No matter what the propeller turning direction is, pressing this button each time decreases the speed one step at a time, a total of 15 steps to the propeller stop condition. Holing the button continuously results in continuous decrease in speed until the propeller stops.

The joystick can be pressed to six different positions: right, middle and left front and right, middle and left rear. Table below shows how motors act to each position for a given turning speed. The resulting boat movement direction is the same as how the joystick is pressed. 

The power source for the device is a marine battery from Nautilus. A marine battery is very similar to a car battery. But it has a much larger capacity, in my case it is a 75 Amp-Hours battery. With the current that my motors take from it, it can work up to 225 minutes or 3.75 hours. Also a marine battery can be deep discharged where a car battery will be damaged if it is discharged completely a number of time. 

Lets take a look at how they are mounted on the boat. I used the same tough elastic bands I used to mount the rudder on the boat:

Battery and control board go under the storage section of the boat, under the cover behind the rear seat. 

Now you can see how the entire motor is mounted on the boat in the following picture:

And below you see the motor in action:

Yep, I think I covered this design properly! Now lets check the performance of the motors. In the final test of the motors, the device performed very well as it was supposed to. I achieved a good speed for the boat, not very fast though. It runs faster than paddling alone and you don't need to paddle anymore. The speed is somewhere around 3.6km per hour or 1 meter per second, between running and fast walking, not bad for a floating object. Overall I call it a successful design and I learned a lot through the design.  

One final thing, let's see after all this, what things I would do the same and what things I would do differently or improve if I was to make it again. First, the things that I would do the same:

	I would go with the same topology for the motor, I mean the wing with two motors at the ends. Because this is the easiest way to mount a motor on such canoe-like boats in my opinion, and also having two motors generates a higher thrust and therefore a higher speed.
	I would keep the control system, meaning that I would use a joystick and a control board as it is much more convenient and fun compared to a mechanical structure connected to the steering device.

Here is what I would do differently or improve:

I would consider steering the boat by a moving rudder or turning motors instead of shutting down one motor. Because this would increase the performance of the navigation to twice in turns. Also sometimes, especially when the wind blows from the boat side, a slight steering is required to keep the boat in the correct direction. But with the design I used, the boat either goes straight or turns and there is no small steering condition to correct the direction. One should switch between the straight and turn conditions to steer the boat for a slight correction.

Although it was fun and educational for me to design the motor mechanics myself, there are disadvantages to this design such as: Drill in general is a noisy device. Having two running at the same time would be even more noisy. Although I got good deals on parts, still motors ended to be expensive and making them was time consuming.

My motors were still less expensive than ready trolling motors. But if there is a good deal, I would buy motors already designed, such as the motor and propeller part of the Minn Kota Engine Mount motor shown in the figure below:

I could mount two of these on the wing I designed and control them with my control board and joystick. Such motors are sealed properly and go under water. Therefore there is much less noise while you get a good thrust. Yet, choosing to go with these available motors comes down to their price. 

Please also read:

• Propellers