MICRO BOAT CLASSES & RULES
The simplest way to limit the size of boats

By Doug Forrester
(Copyright 2001)

**Additional Excel files that accompany this article: Motor Specs  Battery Chart  Boat Weight Specs

NOTE : You will no doubt soon notice that all the units of measurement used herein are metric - 99% of the world and the 21st. century and all that. It is hoped that this discussion and rules have application around the world and to keep converting units back and forth would have been too complicated. Trent has the conversion factors on MicroHydros homepage for any that need it.

DEFINING A RACING CLASS

Letês look at the second parameter first À "defining the race task". There seems to be a universal acceptance of the race or heat runtime of four (4) minutes. This parameter is a crucial one in defining the "micro boat" so if everyone is decided on a runtime of 4 minutes, we can move on from there.

The race course is quite another matter, however. World-wide, the course used varies from the Naviga triangle to the "M" course to the regular electric oval. The course used, however, has a much lesser effect upon the class rules and specs once the runtime is affirmed, so I think we can leave the racecourse configuration as a local club-level decision.

Defining the specifications of the race vehicle is then, our main task in establishing the micro-boat class as a viable racing entity. It is also the most difficult thing to do À finding a consensus of opinion.

I would hope that there would be unanimous agreement that the best set of class rules would consist of the least number of rules and it is paramount that they be easily enforceable. Complex motor rules are a bore and difficult to tech. Measuring hulls is equally boring and could stifle creativity of design.

The simplest way to limit the size of boats and the easiest to tech is to establish a maximum weight for any given class. Maximum RTR hull weight has been used in Europe and England as a class definer for many years. I can see absolutely no rational reason for creating a minimum weight limit for any class. You should be able to see from my projected figures that once you have decided to run for 4 minutes that this automatically establishes your maximum current draw for any given cell capacity. If you couple with this an overall maximum boat weight, then this limits the number of amp/minutes of battery capacity that you can carry.

This elegant simplicity of class specification, 4 minute runtime and maximum RTR boat weight, guarantees the small size of this class of boat. Nothing bigger will fit! With the exception of one particular set-up, a 4 cell, 05 can motored hull, only small to medium cell carrying and 400 to 480 motored hulls will fit within my proposed maximum weight. The four cell boat is a special case and must be dealt with in another manner. Even having to run for 4 minutes, four 2400 cells, allowing for an average current draw of 32-33 amps would provide a power loading within the maximum weight that would be hard to beat by even a Hacker powered micro. The easiest way to plug this potential loophole is to require a battery pack voltage minimum of 7 volts (nominal). I say pack voltage (nominal) rather than number of cells because we are likely going to see cell technology in the future that provides a nominal cell voltage of more than 1.2v. They may be 1.5v. or even 2v. per cell. By defining our classes by number of cells could be a trap in the future. By specifying minimum pack nominal voltage of 7v., this would effectively limit packs to 6-7 cells and rule out the 4-5 high capacity cell boats that can just squeeze under my proposed weight maximum, which no doubt you have all been dying to learn is 700 grams.

I know some of you will say À "what about us unskilled builders, we arenêt able to build ultra-light hulls". There may be some validity in your concern, but the downside of having a higher weight limit (even 750 gm.) possibly opens the door to some other combinations which may be counterproductive to keeping the "micro" in "micro hydros"!

If you look at the projected weight chart you will see that the average hull weight (not including mini Eco hulls) is around 145 gm. If we assume an overall boat weight of 630 gm. Then the hull represents only about 23% of the gross. If by utilizing sophisticated building materials and techniques someone is able to extract 30 gm. from the hull alone (a major accomplishment, I might add), then the 115 gm hull would represent about 19% of the gross weight. This overall reduction would only be a 5% change. The total weight of all the components is so much greater than the weight of the hull itself, that I just donêt think that the hull weight is such a major factor in performance unless itês so high that it puts you over the weight limit. Even a "lead-sled" hull of 200 gm (7 oz) can still be equipped to fall within 700 gm (see chart) I think that 700 gram can work as a micro boat class maximum weight, not stifle variation in usable equipment and set-ups (in other words, keep it interesting). All we can do is to try it. Any unforeseen loopholes or runaway advantages will soon be apparent À any competitive racer looking for "an honest advantage" will turn them up, pronto.

PROJECTED WEIGHT CHART
Download - Boat Weight Specs

In order to study the role that the mass of various components play in contributing to the RTR gross weight of a micro class racing boat, I have prepared a chart. In looking at the marketplace it was possible to achieve a median weight for each family of components likely, such as receiver, rudder servo, speed control, driveline hardware, hull, motor, battery pack, etc. In certain instances, I have specified lightweight components used or " lead sled" hulls used. If you have components that differ significantly from these examples, simply substitute them for your own applications. By projecting these weight totals, we are able to see whatês achievable and what is unlikely and come up with a boat maximum weight (700 gm.) that accomplishes what we want in defining the micro boat and keep out what we donêt want.

BATTERY PACKS FOR MICRO BOAT RACING
Download - Battery Chart

The seven cell battery pack in the micro boat is the single largest weight component in the boat. The amount of stored energy in this pack dictates completely, how long your race boat will run for and the speed which it achieves. Since we have already decided unanimously that the voltage to be used will be 8.4 volts (nominal) or seven cells, however remember what I proposed about specifying that the pack voltage must be between 7 À 9 volts (nominal), we must look to the size (weight and capacity) of the cell that we can use to build this seven cell pack.

DISCHARGE CURRENT LEVELS

The average discharge current level of a boat in a race heat is dictated by several factors.

- Voltage constant of the motor (a measure of the electrical characteristics, resistance, of the armature wind.) This is usually stated as the number of RPM per volt applied.
- Capacity of the cells used in the pack.
- Mechanical load applied to the motor (prop used and driveline friction)
- Runtime of the heat

If the boat is to still be running at speed at the end of the specified heat time then the average discharge current allowable is automatically laid down by the capacity of the cells used. In order to fully understand this, letês look at the discharge characteristics of some of the cells that we are likely to be using.

The cellês capacity or amount of energy stored within is rated in milliamperehours. For our application, this rating is best converted to ampminutes.

Example: 1300 mahr À this means that the cell will deliver a current of 1300 milliamps for an hour, or since there are 1000 ma in an amp, 1.3 A. for an hour is the same thing. To convert Ahrs to Ampminutes simply multiply by 60 À 1.3 A/hr is = 78 A/min. (1.3 X 60). Therefore this 1300 mahr cell will theoretically deliver 78 Amps for a minute or 39 A. for 2 minutes or 19.5 A for 4 minutes. Simple arithmetic relationship!

However, this assumes that the cell operates at 100% efficiency, which ,of course, it cannot. There are always some losses. The higher the current level of the discharge cycle, the greater the losses and the lower the capacity of the cell. The nominal rating of the cellês capacity (by the manufacturer) is usually established at quite a low amp draw, much lower than would be of any use to us in a boat. Consequently at the currents used in fast electric boats, we never see the advertised cell capacity unless the manufacturer has under-rated the cell intentionally. The new CP 1300 scr cell, at a discharge rate of 20 amps (tested by an independent supplier) has a "real world" capacity of about 1100 mahr. This converts to 66 Amin.

The other effect of cell efficiency at high currents is to decrease the average cell voltage delivered during discharge. The 1.2 v. (nominal) is only maintained at very low amp currents (less than 1 amp.) The higher the discharge current, the lower the average cell voltage. Generally the higher capacity cells will maintain a higher voltage at high rate

discharge than a smaller cell. At the currents that we commonly use in FE boats, we rarely see much over 1 volt per cell during discharge.

Example:
CP 1300 cell À averages about 1.06 v. per cell at 20 amp.
The 1300 cell at 10 amp would probably deliver 1.1 v. per cell.
CP 1700 cell À averages about 1.1 v. per cell at 20 amp.

THE FORRESTER RULE OF THUMB FOR RACING NICAD CELL SELECTION

*This unscientific observation was made after participating in many years of nicad cell abuse.

THE LARGER THE CELL DIAMETER, THE LOWER THE CELL INTERNAL RESISTANCE.

This means that the larger the diameter of the cell, the higher the cell efficiency at high current draws, and higher average voltage during discharge. If you have to choose between two cells of different diameters but the same rated capacity and weight À go with the larger diameter cell. Generally, cell manufacturers group their cells by diameter and then add length to increase capacity. When the cell gets too long for its diameter, they jump up to the next larger series diameter. Have a look at the battery chart included to see the cell dimension pattern. This is why the CP 1300 cell, for the same weight as the old 800 AR, has much greater capacity (35% more) and will deliver a lot more amps at a higher average voltage.

THE EFFECT OF CELL CAPACITY ON POWER AND POWER LOADING

To be able to compare the power output of several motor/batter combinations we must first understand the power rating used and how it is calculated. The power of an electric motor is calculated by multiplying volts X amps. The unit of power is watts (actually the metric unit of power is joules, but I think we will stick with watts). To understand this relationship better, we see that 746 watts = 1 horsepower. This power rating of volts X amps is input power to the motor. In order to arrive at output power we would have to multiply the result by the efficiency of the motor and factor in the frictional losses of the drivetrain. This would then be the power to the prop. In the interest of keeping the arithmetic relatively simple for our comparative usage, we will stick to input power comparisons.

POWER LOADING À A MEASURE OF A BOATêS POTENTIAL PERFORMANCE

I think that we all can agree that a 200 watt boat should be faster than a 100 watt boat. What about two 100 watt boats where one weighs 500 gm. all up and the other one weighs 1000 gm. (1 kilo). I think there is a very strong likelihood that the lighter boat with the same power would be faster. The concept of power loading then combines power and weight and rates the power per unit of gross weight. In automotive parlance this is called the "power to weight ratio". Iêll refer to it as "power loading". This rating then allows us to compare different boat setups and to have some measure of predicting performance. The units of measurement that Iêll use will be watts per kilogram. Donêt worry about the use of the unit of weight as kilogram since we are only using this rating as a comparative between different boats, besides, as Iêve stated before, you better get used to using the metric system.

MAXIMUM CURRENT POSSIBLE WITH MOTOR/BATTERY COMBINATIONS

All calculations are based on a heat runtime of four (4) minutes.

Here are five combination to illustrate "real world" currents possible with different motors and cell.

COMPARING SOME EXAMPLES OF POWER LOADING

*** remember that these watt ratings represent input power to the motor ***

All the examples cited will use 7 cells - I have estimated average discharge cell voltage depending on the current and the size of the cell. These examples are used for comparative purposes only and as the phrase goes À "actual results may vary" Also, all examples are based on a heat runtime of 4 minutes.

PROJECTED WEIGHT CHART

The included weight chart contains estimated weights of different boats and motor/battery combinations. Remember that these weights are only estimates and if you can significantly differ from these simply substitute the value to come up with your own estimates. I think, however that these estimates allow us to see what is realistically possible with microboat all-up weight and where to draw the line for a class maximum, in order to maintain the small size. Remember, however that we have to accommodate from the lowest power sp400 motors up to the open spec brushless systems and that they will never have to compete against each other.

ROUND-UP TIME

Well there you have it, a hell of a lot more than I started out to say! I found that the deeper I went, the more I felt that to be explained to newcomers. Please forgive me if all of this is old hat to you. I really do feel that maximum RTR weight is the best way to define the size of the boats after determining the pack voltage and runtime. The lower powered boats (sp 400 motor Ecos) will naturally use smaller cells because their motors will not handle much over 12 Amps and there is no reason to carry around more battery capacity than you can use in 4 minutes. Smaller batteries will also allow for smaller, lighter hulls in these classes. Open motor hydro classes will have to carry as much battery capacity that the class maximum weight will allow. This will mean larger heavier hulls for these classes up to the maximum weight. What should the maximum weight be? I hope that this article has made my case for establishing this weight to be 700 grams. Any higher and it can permit some things that we donêt want (bigger, higher powered boats) and any lower and it could severely limit creativity and exciting speeds.

Already I have been hearing the first rumblings of "Boy, it sure goes a lot faster if I run it for less time!" If we are satisfied with shorter heat times, yes, the boats can be made to go faster. Do we really want to run them for four minutes or is three minutes enough (or 2 minutes)? All you have to do is change all my calculations based on a shorter runtime. Itês a whole new ball game! Anyway thatês my $1.98 (inflation).