ROAR 6-cell or 4-cell Open?

By: Jay Turner

Building a competitive 8 or 12-cell race boat can take a bunch of money spent on batteries, exotic motors and electronics, and this is more than many modelers are willing to spend. This is why the 6-cell stock classes are becoming so popular, with these classes making up 25-30% of the total race entries at many National events. But the 6-cell stock class isn’t without problems.

The use of ROAR-legal motors opens up the specter of "cheating", and this is difficult to control at local events. At the National events handout motors or complete tear-downs can be performed, but this isn’t practical at smaller races. A racer with a very fast ROAR-powered boat will be accused of cheating by some racers, and this is hardly fair to either the winning racer or to the other competitors.

One way around this is to start a 4-cell open motor class. The Anchorage Model Boat Club has run this class for a season with generally very good results. We learned a lot, and I’ll relate some of that below, but the most important thing to realize is that 4-cell boats are NOT SLOW! This is the automatic reaction of most boaters who hear the idea for the first time, but we have to think about it for a minute. First off, the 4-cell boat will weight 4 ounces less than a 6-cell boat because it has two fewer cells; lighter means faster. Second, an 8 to 10-turn open motor has from 5% to 15% more power running on 4 cells than a good ROAR motor does when running on 6-cells. The result is more power and less weight, which translates into a faster boat. Most experienced racers who see my Key West run on 4 cells can’t believe it’s so fast.

The boats we’ve run the most are catamarans, the Graupner Key West hull being the race boat of choice. Setups are either direct or gear drives, and in the tight pool course the gear drive seems to work best. On the longer outdoor course the direct drive will likely work as well.

Here are some setup tips:

Motors: 8 to 11-turn mod motors with hard serrated brushes and heavy springs seem best, with around 1/8" to " timing advance. Use 8- to 10-turn motors in gear drives and 9- to 11-turns in direct drives. With the low cell count, motors don’t get as hot as they do with 6 or 8 cells, so they last longer. Unlike ROAR motors, these open motors can be rebuilt easily for even longer life.

Gears Drives: The Hughey dual drive has been used with only one motor to keep the height low in the hull. The new Hughey 24-tooth pinion gear allows the 1.5:1 to 1.6:1 ratios which seem to work so well.

ESCs: This is where we learned that not all ESCs are created equal. Some will not run well on only 4 cells – the Astro 212s will overheat and give poor performance for example. The Tekin 432 on the other hand works great. The various car ESCs should work fine too since they are designed for the popular 4-cell car classes.

Props: Direct drive props will be similar to those used on 6-cell ROAR boats. In surface drives, the Octura x435 and y535 will do well with 9 to 11-turn motors, but the smaller x432 may be needed with hulls which are not really free of the water. In my XL Cat, a P735 worked fine with a 10-turn motor in direct drive. For gear drives, the x438 and x637 seemed well suited on the short indoor course, giving strong acceleration. As with all boats, experimentation will be needed to determine the best prop for the particular application.

Hulls: This is where I break from the recommendations of some other boaters experimenting with 4-cell racing. Because these models are potentially faster than the 6-cell ROAR boats, I believe that the best hulls are similar to the current designs like the Key West hydro and the Warehouse Hobbies Bagshot mono. These are easy to drive, and are large enough to run on semi-rough water. While a few racers have tried to design very small boats for 4 cell use, I think that these tiny hulls limit the water on which the 4 cell boats can run. While I haven’t tried it yet, I believe that the Graupner Tornado is also a natural for 4-cell use, and I will be setting one up as soon as the ponds thaw in May.

In the typical kit boat like the Kyosho Viper the 4 cell concept works, but no better than the standard 6-cell stock setup does. These hulls are too large and have too much wetted area to take advantage of the lower weight of the 4-cell layout.

Race boats have best luck with surface drive setups, and this is also true with 4 cell models. To really take advantage of the 4-cell setup, boaters need to maximize the concept’s strong points, which are high power and low weight. A free-running 6-cell ROAR model will run even faster on 4 cells as drag is reduced and acceleration increased.

Batteries: The 4-cell motors will draw more amps than a ROAR motor will, and this will result in shorter run times. Set up to draw 40 to 50 amps gives exciting performance, but like the larger cell classes, RC1700 or RC2000 batteries are preferred for racing. For sport use any pack will work. The 4-cell boater will have to assemble his own packs, but those packs will only cost 66% of a 6-cell pack.


Four cell boats may never replace the 6-cell ROAR classes, but they are a viable alternative. Advantages are lower cost ( fewer cells, motors last longer ), higher speeds, no chance to cheat, and no motor teching needed at races. If you have a good running 6-cell stock boat, try a 4-cell pack an a low-turn motor and see how it runs!

This Article was written by Jay Turner Exclusively for Rum Runner Racing. Use of this article is prohibited without written permission from Rum Runner Racing.


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