I'm building a mini boat! - Motor Research

I have decided I want to try building a mini speed boat. After watching Paul Eakin’s video on his mini ocean vessel and not having worked on a big project for a while, I decided to jump in. I’ve also always wanted to improve my engine and electrical knowledge so I thought this would give me a good excuse to do so. I’ll start the construction of the boat soon, but first I want to research boat motors and which type might be best for my little bathtub. (I've sprinkled a few awesome vintage maritime photos throughout the article to try and make this very technical post a bit more interesting! :))

Purpose of Article:
Research both petrol and battery powered boat motors
Come to a decision as to which one I will use for my DIY project.

My Boat Specs 

Once finished, the boat should weigh around 14lb. If I use a motor similar to the one Paul Elkins used in his boat, that will add around another 15lb. (If I choose to go with an electric motor, that will add around 30lb). The boat should be able to hold around 187lb of extra weight. 

So how big do I need my motor to be?

Trolling and Gasoline motor power are measured differently.  

Trolling motor power is measured in pounds of thrust, while gasoline powered engines are measured in horsepower. There is a common misconception among boaters that pounds of thrust is directly related to horsepower, unfortunately, it’s not that simple. Horsepower and pounds of thrust are entirely different ways to measure force. 

How to convert thrust lbs to Hp.

Since a trolling motor and a gasoline motor’s power output are measured in different ways, you might still want to compare their respective power output. To estimate the horsepower of an electric trolling motor, you need to know the watts used by the motor at its fastest speed. Most manufactures provide you with the amps a motor uses at full speed. We can convert this into watts by multiplying amps drawn with how many volts your battery produces (12V for one battery, 24 for two, etc.). 

Example:

The NV 55lb thrust motor draws 52 amps at full speed and uses one 12V battery. (52 amps x 12V = 624 watts). This gives us 624 watts. Take the watts you calculated and divide it by 746 Watts (1 HP): (624 watts / 746 watts = .83 HP). So, our NV 55lb draws 624 watts at full speed, divided by 746 watts, gives us .83. Therefore, the estimated output of our NV 55lb is .83 horsepower.

So, how big do I need my motor to be?

Trolling Motor:

For trolling motors it is advised you need at least 2lbs. of thrust for every 100 lbs. of boat weight. Applying this to my boat which will weigh around 215lb, means I would only need a motor with about 4.3lbs of thrust (215/100 = 2.15 2.15*2 = 4.3). This number seems a little low however and I feel the formula doesn’t work so well with boats which are as light as mine. Instead, (and having researched what other people are using for a similar sized boat) a motor with around 20/30lb of thrust seems adequate.

Petrol Motor: 

Most boats should feature a capacity plate to tell you the safe and appropriate horsepower and total weight limit for your boat. Looking at examples, a motor with around 1.2 horsepower seems adequate for my boat.

  

A deeper look at Electric Trolling Motors and Petrol Motors.

Trolling Motor

What is a Trolling Motor?

A trolling motor is a self-contained unit that includes an electric motor, propeller and controls, and is affixed to an angler's boat, either at the bow or stern. Trolling motors are used quite a lot for powering mini boats as they create little noise and generally provide enough propulsion to power the lighter boats at a fair speed.

Who invented the electric trolling motor?

The electric trolling motor was invented by O.G.Schmidt in 1934 in Fargo, North Dakota, when he took a starter motor from a Ford Model A, added a flexible shaft, and a propeller. Because his manufacturing company was near the Minnesota/North Dakota border, he decided to call the new company Minn Kota. The company still is a major manufacturer of trolling motors.

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How do they work?

Modern electric trolling motors are designed around a 12-volt, 24-volt or 36-volt brushed DC electric motor, to take advantage of the availability of 12-volt deep cycle batteries designed specifically for marine use.

The motor itself is sealed inside a watertight compartment at the end of the shaft. It is submerged during operation, which prevents overheating. 

Things to look for when buying a trolling motor:

Thrust. 

Boat weight is key when determining how much thrust you need. General rule of thumb: you need at least 2 lbs. of thrust for every 100 lbs. of fully-loaded boat weight (people and gear included). If things like wind or current are major factors, you’ll want a little extra thrust.

As mentioned before Trolling Motors usually have a brushed motor inside rather than a brushless motor. You may be wondering what the difference between the two is. Let’s get into it:

  

What is a DC Brush Motor?

First invented in the 1800s, DC brush motors are one of the simplest types of motors. They were the first type widely used, since they could be powered from early direct-current lighting power distribution systems. You will find this type of motor all over the place. DC motors have more torque than brushless DC motors. DC brushed electric motors turn electricity into motion by exploiting electromagnetic induction. The motor features a permanent magnet (called the stator because it’s fixed in place) and a turning coil of wire called an armature (or rotor, because it rotates). The armature, carrying current provided by the battery, is an electromagnet, because a current-carrying wire generates a magnetic field. The key to producing motion is positioning the electromagnet within the magnetic field of the permanent magnet (its field runs from its north to south poles). This interplay of magnetic fields and moving charged particles (the electrons in the current) results in the torque (depicted by the green arrows) that makes the armature spin. A single, 180-degree turn is all you would get out of this motor if it weren't for the split-ring commutator — the circular metal device split into halves (shown here in red and blue) that connects the armature to the circuit. Electricity flows from the positive terminal of the battery through the circuit, passes through a copper brush to the commutator, then to the armature. But this flow is reversed midway through every full rotation, thanks to the two gaps in the commutator. This is a clever trick: For the first half of every rotation, current flows into the armature via the blue portion of the commutator, causing current to flow in a specific direction. For the second half of the rotation, though, electricity enters through the red half of the commutator, causing current to flow into and through the armature in the opposite direction. This constant reversal essentially turns the battery's DC power supply into alternating current, allowing the armature to experience torque in the right direction at the right time to keep it spinning.

What is a Brushless DC Motor?

Due to the physical contact the brushes have with the commutator in a brushed motor, the parts wear down quite quickly which is why brushless DC motors that use electrical sensors to change the current flow and require no contact have become more popular. With brushed motors, an electronic sensor turns the magnetic fields (permanent magnets) on and off in succession, driving the rotating magnets (mounted on the rotor) to line up with the next electromagnetic field, creating a continuous turning motion. 

Brushless motors are more efficient because they use sensors and not brushes which create friction. 

Pros and Cons of each:

Brushed

• Easy to Control. Simply apply a DC voltage to make the motor turn. A higher voltage (or higher PWM duty cycle) will make the motor run faster. Reversing the polarity reverses the direction of rotation. Brushed DC motors don’t even need a microcontroller to use, you can make them run just by connecting one to a battery.
• High Starting Torque. Brushed DC motors output high torque at low speeds. This is important because this high starting torque allows the motor to get up to speed quickly, even if there is a load on the motor.

• Wears Quickly. Because the brushes physically rub against the commutator as the motor operates, those brushes wear out over time. Therefore, compared with other types of motors, brushed DC motors wear out more rapidly.
• Lots of Electrical Noise. Inside a brushed DC motor, electricity arcs between the brushes and the commutator. This causes a lot of electrical noise which is not the best for microcontrollers or sensors working in the same system.
• Limited Top Speed. The physical contact between the brushes and the commutator during operation means there is friction between these two parts. Where there is friction there is heat. Brushed DC motors have a limited maximum speed because excessive speed would cause damaging levels of heat.

Brushless 

• Low Wear. The only physical interface between the rotating outside of the motor case and the stationary windings on the interior are ball bearings, meaning that brushless DC motors wear very slowly.
• High Speed .Brushless motors have much less friction than brushed DC motors so they can operate at much higher speeds.
• High Efficiency. Compared with other types of motors, brushless motors have a very efficient operation, meaning lower power consumption at the same power output compared with brushed DC motors.

• Very Complicated to Control. Brushless DC motors require specialized controllers and complicated control algorithms to operate correctly.
• Expensive. The cost of the motors themselves is not overly high, but when the cost of the controller is added as well, the overall cost of including a brushless DC motor into a project is relatively high.

Good video about the differences between Brushed and Brushless DC motor

Good video about how a brushed DC motor works 

Good video about electricity 

Trolling Motor Batteries 

When it comes to selecting trolling motor batteries, there are a few things you will want to consider: Battery type, battery amperage hour rating and budget.

To begin, let’s have a quick electricity fundamentals refresher:

Electricity is the flow of electrons from one place to another. Electrons can flow through any material, but does so more easily in some than in others. How easily it flows is called resistance. The resistance of a material is measured in Ohms.

Matter can be broken down into:

• Conductors: electrons flow easily. Low resistance.
• Semi-conductors: electrons can be made to flow under certain circumstances. Variable resistance according to formulation and circuit conditions.
• Insulator: electrons flow with great difficulty. High resistance.

Since electrons are very small, as a practical matter they are usually measured in very large numbers. A Coulomb is 6.24 x 1018 electrons. However, electricians are mostly interested in electrons in motion. The flow of electrons is called current, and is measured in AMPS. One amp is equal to a flow of one coulomb per second through a wire.

Making electrons flow through a resistance requires an attractive force to pull them. This force, called Electro-Motive Force or EMF, is measured in volts. A Volt is the force required to push 1 Amp through 1 Ohm of resistance.

As electrons flow through a resistance, it performs a certain amount of work. It may be in the form of heat or a magnetic field or motion, but it does something. This work is called Power, and is measured in Watts. One Watt is equal to the work performed by 1 Amp pushed by 1 Volt through a resistance.

Amps: is the amount of electricity 

Volts: is the push, not the amount

Ohms: slows the flow

Watts: is how much gets done 

Ohm’s Law

The term Ohm’s law refers to one of the fundamental relationships found in electronic circuits: that, for a given resistance, current is directly proportional to voltage. In other words, if you increase the voltage through a circuit whose resistance is fixed, the current goes up. If you decrease the voltage, the current goes down.

Ohm's law may sound a bit confusing when written in words, but it can be described by the simple formula:

where I = current in amps, V = voltage in volts, and R = resistance in ohms

This same formula can be also be written in order to calculate for the voltage or the resistance:

Here’s an example of how to calculate voltage in a circuit with a lamp powered by the two AA cells. Suppose you already know that the resistance of the lamp is 12 Ω, and the current flowing through the lamp is 250 mA, which is the same as 0.25 A. Then, you can calculate the voltage as follows: 0.25*12 = 3. Therefore there are 3 volts in the circuit. 

If you ever need help in remembering the different equations for Ohm's law and solving for each variable (V, I, R) you can use the triangle below

As you can see from the triangle and the equations above, voltage equals I times R, current (I) equals V over R, and resistance equals V over I.

Let's also take a look at how a battery works:

How do batteries work?

Electricity, as you probably already know, is the flow of electrons through a conductive path like a wire. This path is called a circuit. Batteries have three parts, an anode (-), a cathode (+), and the electrolyte. The cathode and anode (the positive and negative sides at either end of a traditional battery) are hooked up to an electrical circuit.

The chemical reactions in the battery cause a buildup of electrons at the anode. This results in an electrical difference between the anode and the cathode. You can think of this difference as an unstable build-up of the electrons. The electrons want to rearrange themselves to get rid of this difference. But they do this in a certain way. Electrons repel each other and try to go to a place with fewer electrons. In a battery, the only place to go is to the cathode. But, the electrolyte keeps the electrons from going straight from the anode to the cathode within the battery. When the circuit is closed (a wire connects the cathode and the anode) the electrons will be able to get to the cathode. 

Battery Capacity

Amp hour is the rating used to tell consumers how much amperage a battery can provide for exactly one hour. In small batteries such as those used in personal vaporizers, or standard AA sized batteries, the amp hour rating is usually given in milli-amp hours, or (mAh). For large batteries, the rating is abbreviated as Ah. Most deep cycle marine batteries (a deep cycle battery is a battery designed to be regularly deeply discharged using most of its capacity) will tell you the Ah rating at multiple C ratings. The C rating tells you how many amp hours the battery can provide for a very specific period of time. For instance, at C/5 a battery might safely provide 26.8 amp hours. This means that it supplies 26.8 amps in the duration of 5 hours without dropping off. Meanwhile, the same battery may safely provide 36 amp hours for a period of 100 hours. Depending on the amount of use you intend to get out of your battery (daily versus sporadically), you will want to compare amp hours for different C ratings. 

Is there a danger from electrucion using an electric trolling motor?

To answer this question we first need to answer this question - Voltage or Amperage, what is more deadly?

We often think that 10,000 volts would be deadlier than 100 volts. This is, however, only partially true. Electrocutions are often implemented using household voltages of 110 Volts, or in some instances, as low as 42 Volts. However, we shouldn’t discard voltage entirely. Without voltage or a potential difference, there’d be no current at all. This is the reason why hanging on a power line wouldn’t electrocute you unless you touch the ground. Hanging from the wire forms an equipotential with the wire, whereas touching the ground immediately creates a potential difference, which draws a huge current through us.

Regardless of the voltage, the real cause of death is the current that is forced through the body.

Although the physics are complicated, some experts use an analogy of a flowing river to explain the principles of electricity. In this analogy, voltage is equated with the steepness, or pitch, of the river, while amperage is equated with the volume of water in the river. An electrical current with high voltage but very low amperage can be seen as a very narrow, small river flowing nearly vertical, like a tiny trickle of a waterfall. It would have little potential to really hurt you. But a large river with lots of water (amperage) can drown you even if the speed of flow (voltage) is relatively slow. Of the two, amperage is what really creates the risk of death, which becomes clear when you understand just how little amperage is necessary to kill. Different amounts of amperage affect the human body in different ways. The following list explains some of the most common effects of electrical shock at various amperage levels.

To understand the amounts involved, a milliampere (mA) is one-thousandth of an ampere or amp. A standard household circuit that supplies your outlets and switches carries 15 or 20 amps (15,000 or 20,000 mA). 

1 to 10 mA: Little or no electrical shock is felt.

10 to 20 mA: Painful shock, but muscle control is not lost.

20 to 75 mA: Serious shock, including a painful jolt and loss of muscle control; the victim cannot let go of wire or another source of shock.

75 to 100 mA: Ventricular fibrillation (uncoordinated twitching of ventricles) of the heart can occur.

100-200 mA: Ventricular fibrillation occurs, often resulting in death.

Over 200 mA: Severe burns and severe muscle contractions occur. Internal organs can be damaged. The heart can stop due to chest muscles applying pressure to the heart, but this clamping effect can prevent ventricular fibrillation, greatly improving the chances of survival if the victim is removed from the electrical circuit.

However for this kind of current to be sent through the body, the voltage would need to be high enough, and resistance low enough to allow it to pass through and thus cause electrocution. 

Let's look at a personal example:

Let’s say I use a 12v 22Ah battery to power my motor. A 22Ah battery is able to abundantly provide the killing amperage, however the resistance of a human body will not allow a 12V potential to produce such a current (in most cases).

Regardless, investing in a A marine battery box would be a good idea.

A marine battery box is intended to keep a trolling motor battery enclosed and secured, prolonging the useful life of the battery and protecting it during transport and use. Exposure to fresh or saltwater can cause corrosion to the trolling motor batteries. To prolong the usable life of your trolling motor battery, a battery box power center can be used to protect the battery from corrosive elements such as water. A marine battery box prevents the battery terminals from making contact with other metal objects, which can cause short circuits and pose risks to the user. The battery box gives us the assurance that the battery connections are secure by keeping it sealed and exposed; thus, reducing the chances of the passengers getting shocked. Without the battery box, high powered batteries can generate dangerous voltage that may cause electrocution. Furthermore, when batteries are open, the electrolyte it produces is toxic and dangerous when it comes into contact with the skin and eyes.

After that Far-Too-Long look at Trolling Motors, let’s take a brief look at Petrol Powered Outboard Motors:

Most outboard motors are petrol-driven, two-stroke engines although four-stroke engines are becoming more common. Outboard motors may have from one to eight cylinders.

A Diagram of a Standard Petrol Motor:

Small outboard motors, up to 15 horsepower or so are easily portable. They are affixed to the boat via clamps and thus easily moved from boat to boat. These motors typically use a manual start system, with throttle and gear shift controls mounted on the body of the motor, and a tiller for steering. The smallest of these weigh as little as 12 kilograms (26 lb), have integral fuel tanks, and provide sufficient power to move a small dinghy at around 8 knots (15 km/h; 9.2 mph).

General Things to keep in mind when buying  a motor:

Choose the Right Shaft Length

At the back of your boat, measure the hull from top-to-bottom in the center. This will be your transom height. Your outboard motor will be mounted at the top of the transom, and the shaft of your motor will extend to the bottom of your transom. The prop should extend just below the bottom center of your hull, allowing it to pull water from beneath the boat and push it behind you. Outboard motors come with different shaft lengths, and you'll want to make sure that the one you choose has a shaft equal to your transom.

Trim Angle

By changing the outboard motor’s drive angle, the vessel’s bow can be made to rise or fall. The performance and stability of a vessel depends a great deal on correctly trimming the outboard. The correct trim angle depends on the vessel’s handling characteristics, the size of the outboard, the sea and loading conditions. Care must therefore be taken to ensure the outboard is trimmed correctly under different sea and loading conditions. On smaller outboards, the trim angle is adjusted manually by moving an adjusting rod to different holes in the mounting bracket. The bigger outboards usually have a Trim Switch fitted on the remote control lever.

So having done quite a lot of research at this point, I think I have decided to wait until I’ve finished building the boat to decide which one I’ll buy. I plan to buy a kayak paddle so I can take it out onto the water and gauge how well it floats ect. This will hopefully help me choose what is best to go for. I’m also considering building my own using a cordless drill attached to a propeller, but this will require some more research. For now, I’ll summarise what I believe are the pros and cons for each type of motor (for my particular situation).

Trolling Motor:

• Much quieter than a petrol motor
• Cheaper
• Some trolling motors are very small, providing only around 10lbs of force. This could be quite good for my boat as due to its small size as it does not need much power. 

• The motor has to be powered by a battery. If the boat were to sink, this could pose a threat in terms of battery acid leakage into water as well as electrocution.
• Motor run time is quite limited due to battery.

Petrol Motor:

• Lots of power, though due to the size of my boat, this is not necessarily needed.
• No danger of electric or battery acid leakage
• Longer run time (depending on size of motor)

• Very expensive. There are some 2HP motors which are around £150, but they do not look particularly relabble
• Very noisy! The cheaper ones are especially loud

I can’t seem to source any motors below 2HP. Around 1.2HP is all I need. The high quality 2Hp motors cost around £400. Way too much.

All sites used and referenced:
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Now for some more epic maritime imagery: