Design Guide - Boats

by Wayne Young

Model Solar Boat Guide 20070722.pdf

Motors for Model Solar Competition

There are many small, low voltage, direct current motors available, but not all are suitable for use in the model solar competition. The dilemma is which motors are suitable and which is the best for your vehicle. The data below will give a starting point.

Advanced boats, like for the student designed cars, in this event there is no limitation placed on the motor used. It is entirely up to the team.

Within the competition regulations you can choose from a limited number of motors. There are some basic motor features that limits your choice.

  • Motor size and weight. As in all motor sports power to weight ratio of the vehicle is critical to performance. This is particularly so in the boats.

  • Motor power. The motor must be capable of converting the electrical power available from the solar array into mechanical power. It is no use having 6 watts available from the solar array and a motor that can only produce 1 watt. Things are just as bad in the opposite direction if you have a 30 watt motor a significant portion of the solar array output will be used in just running the motor. The additional weight of this higher powered motor will adversely influence the power to weight ratio.

  • Motor rated voltage. This rating must be compatible with the output from the solar array. But do note that in many cases the motors used in the student designed car competition are rated at 6 volts but are being operated at up to 20 volts. Operation at over voltage increases motor RPM and the power it can deliver but will shorten the motor life.

  • Motor efficiency. Obviously the highest efficiency possible is desirable. Generally high efficiency is only available in proper industrial type motors not the typical toy motors available at the hobby shops. High efficiency usually comes with a high price.

  • Motor RPM. This does not matter much in the student designed car event as a gear reduction is required between the motor and drive wheel. The gear ratio is chosen to suit the motor RPM. It is very important in the boats as matching motor RPM to propeller characteristics is critical in obtaining maximum power transfer from solar array to propeller. In the advanced boats a gear reduction is permitted so it can be selected to obtain maximum power transfer. Many advanced boats use a direct drive, junior boats must use direct drive which makes motor RPM critical.

Motor type is another variable, typically permanent magnet DC motors are used. There have been many suggestions that a brush-less DC motor such as those used in model aircraft and drones would be better. So far no one has come up with such a motor that is suitable for the model solar competition. The main issues seem to be RPM and efficiency and possibly more importantly the characteristics of the solar array being so different to the batteries normally used to power these motors.

Keep looking. Just because no one has found a suitable motor yet does not mean there is not one out there somewhere! You never know what will appear in the future.

What motors do most competitors use? This is a good place to start.

We scrutineer the student designed cars each year, so have a very good idea of what motors are being used.

In over 90% of cars we see the Faulhaber 2232 6 volt motor. Maxon motors make up most of the remainder.

The Faulhaber and Maxon motors are both proper industrial motors with efficiencies around 86%. From dynamometer tests conducted on both these brands there is nothing in their performance that would suggest one is better than the other. They both perform to their advertised specifications. The range of motors available from Maxon is wider than from Faulhaber. It is most likely that the main reason for the Faulhaber being so prevalent is its increased availability for competitors to acquire with relative ease.

Tests have been performed on motors from two other manufacturers with claimed performance equivalent to the Faulhaber and Maxon. Neither of these motors performed to the quoted specifications one was over 20% below the claimed performance, while the other was quoted to have a no load current the same as the Faulhaber but when tests actually had 4 times this no load current and was so poorly constructed that it exhibited serious vibration at high speed along with low efficiency. Be careful when selecting motors from relatively unknown manufacturers.

Advanced boats have no motor restrictions imposed on them in the regulations. Many use the Faulhaber motor with or without a gear reduction. It has been stated that some boats have used a motor and gearbox intended for model aircraft. Again tests performed on one of these units and it was found that its no load current to be equivalent to 40% of the solar array output. This means that the boat would not even run below this Sun level. Unfortunately, no information on the manufacturer of this unit is available but use its performance test results as a warning to test any motors you intend to use to verify they perform to specification or at least to a level that gives reasonable performance.

The motors Scorpio Technology sell (SM 403) for use in the junior boat competition are used by many competitors. These motors are the result of several years testing motors from many suppliers trying to find a motor that performs as best as possible for the junior boats. Unable to find a suitable motor from the then current units available Scorpio Technology has had motors specially manufactured for them. These motors have characteristics that match Scorpio’s solar panels and propellers.

Solar Panels

In order to get the best out of your vehicle it is important to know what power your panel is delivering. For the student designed cars there is strict limits, the regulations stipulate that the panel power produced at standardised AIM 1.5 sun level must not exceed 10 Watts. And when the car is presented for race scrutineering the panel is checked using calibrated equipment.

While this may seem a complicated process your can perform a Power Test using some relatively simple equipment and still estimate the power your solar panel is generating to within less than 10% of what a calibrated system would measure.

If you also use a Calibrated solar panel at the same time you test you can adjust your measure values to correct the reading to the AIM 1.5 standard to generate a more accurate maximum panel power value.

What is a Solar Panel?

A solar panel is one or more solar cells, or photovoltaic (PV) cells, that coverts the energy in light waves into electrical power.

These cells all share a same trait, they generate a voltage and current flow when a photon of energy in light is absorbed by the cell. This powers devices.

There are MANY different types of construction technique for creating a PV cell and they range from the relatively cheap and common Silicon based unit all the way to the very expensive and uncommon Gallium arsenide devices that are used to power satellites and even the International Space Station.

Calibrating the panel

Without a calibrated light box, it is difficult to achieve exact calibration, however the simple process described below detailing how calibration can be performed in Sunlight will give reasonable accuracy.

To calibrate the panel in Sunlight it is necessary to know the Sun level prevailing, this can be measured using a Scorpio No. 10 calibrated cell. For accuracy, it is essential to have the calibration cell and panel being measured in the same plane when measurements are being taken.

At a known Sun level measure the panel’s open circuit voltage (OCV) and short circuit current in amps (ISC). Make these measurements with the solar panel at 25 Degrees C and at a Sun level greater than 70% for best accuracy.

Approximate power can be calculated by multiplying voltage and current together then multiplying by 0.7 which is an average fill factor for these panels.

OCV × ISC × 0.7 = Power in watts

Having calculated the power at a specific Sun level as described above, ratio the power obtained up to the power expected at full Sun.

To calibrate the panel to produce 5.5 watts at full Sun, mask a portion of the panel with a light excluding covering to reduce the power produced to 5.5 watts. Note this covering should cover the same proportion of every cell in the panel. The approximate size of the mask can be calculated using the calculated full Sun power and the desired 5.5 watt maximum and using the ratio of these powers to reduce the panel surface area by the same ratio.

Always recheck the panel power after masking to ensure the correct size of masking has been applied.

(Special note: Not all Scorpio No.26 panels will produce 5.5 watts, some panels may only produce 5.2 watts. In past years Scorpio provided “standard panels” with a power output of from 5.2 to 5.8 watts at a lower cost than their “premium panels” which had power output above 5.8 watts.)

Testing your Vehicle

There is one re-occurring theme that you will encounter when looking through the technical resource information. It is a task that many first time competitor may be eager to skip, although they do so at their own peril. Yes it is testing.

Regardless of the division in which you are competing if you decide not to test and evaluate the performance of your constructed vehicle then you are running a very large risk that you will encounter major issue when attending the events. You mentors in the design and build phase of the model creation should have talked to about what your vehicle needs to be able to achieve. For a boat the ability to run straight along the guide without submerging. For the both the kit car and student designed it is the ability to run along the track with no loss of guidance events in all levels of sun. Whilst these seem obvious it is important that you perform tests to ensure that these needs are satisfied otherwise it makes no difference if you have created the fastest ever vehicle as it will never complete the race.

Once the basics have been covered then you can move onto testing for satisfy other requirements of your division, the most demanding is finishing first but you can never tell what the actual conditions of the race will present on the day.

The design guides for each division include more information on tests that you should perform and the expected results of those tests. You are encouraged to review that information and the current regulations to ensure that on race day you have as few unexpected occurrences as possible.