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How To Get Into Hobby RC: Testing and Upgrading an Airboat

By Terry Dunn

Rather than a submerged water propeller, airboats have a large airscrew like you would find on a Cessna. One or more large rudders are stuck right in the propwash to provide turning authority. This configuration allows an airboat to ignore most vegetation on the water. It just skims right over it all.

A few months ago, we took a look at the RC boating hobby by dissecting two small, electric setups from AquaCraft: the beginner-friendly Reef Racer II and the speedy Minimono. Both boats are still going strong and my family continues to enjoy them. In fact, I decided that I wanted to bring along at least one RC boat on our summer trip to Florida.

As I was mentally justifying the cargo space for toy boats and thinking of the different lakes we could visit, I remembered fishing at many of those same lakes as a kid. I recalled that most of them had grass, lily pads, reeds, and even cypress knees all along the shoreline. While all of that aquatic flora is what I miss most about living in Florida, it would cause nothing but headaches with the submerged propellers of my RC boats. I decided that I needed a boat that was designed to traverse this kind of environment…an airboat, to be exact.

If you’re not familiar with the basic design of an airboat, I’ll elaborate. They utilize a wide, flat-bottomed hull. Rather than a submerged water propeller, airboats have a large airscrew like you would find on a Cessna. One or more large rudders are stuck right in the propwash to provide turning authority. This configuration allows an airboat to ignore most vegetation on the water. It just skims right over it all. Many can even claw their way across dry land. In short, airboats are loud, obnoxious, and extremely useful machines.

In my last-minute search for an airboat, I found that there are several wood kits that are available, as well as varied plans to DIY. But I was in a hurry and needed something off the shelf. I realized that there aren’t many hobby-quality RC airboats available as turnkey packages. In fact, I could find only two: the Alligator Tours and Mini Alligator Tours. Both are also AquaCraft products. The larger version of the Alligator Tours is powered by a fuel-burning motor, while the mini version is electric. I chose the electric version.

Inside the Mini Alligator Tours

The first thing I noticed about the Mini Alligator Tours (MAT) is that it looks a lot like the airboats that I used to see all around the St. Johns River as a kid. It has the boxy hull with a protective cage around the prop and a high-mounted seat for the driver (to help them see and avoid the stuff that an airboat CAN’T glide over). Too bad a driver figure isn’t included to complete the illusion. I supposed you could add your own if you don’t mind the weight and balance penalty. Truth be told, I cut the seat off of mine so that I could mount my Mobius video camera on board.

The Mini Alligator Tours is a ready-to-run electric-powered airboat. It shares many design elements with full-scale airboats.

The MAT’s hull components are made of plastic. All of the assembly is completed at the factory…even the decals are applied. Personally, I’d rather have some freedom with decal placement. But I won’t split more hairs over cosmetic stuff. To get started, all you have to do is add a few AA cells to the radio transmitter and charge the included onboard battery. Per my usual, however, I invested some extra time up front to improve the MAT’s long-term outlook.

Taking a look inside the MAT, it is obvious that this boat is a not-too-distant cousin of the Reef Racer 2. They share the same pistol-grip transmitter as well as the black box which houses the radio receiver and Electronic Speed Control (ESC). Like the Reef Racer, the MAT also includes a 6-cell, 1100mAh NiMh battery and peak detection charger. The MAT battery’s 2x3 form factor is slightly different, as are the power connectors. I’d never seen this type of connector before. But no matter, since I convert all of my stuff to Dean’s Ultra Plugs anyway. Having a mixed fleet of battery connectors invites misery. Homogenize.

Like AquaCraft’s Reef Racer 2, the Mini Alligator Tours includes a 6-cell NiMh battery and peak charger.

The battery and radio gear is housed in a small compartment under the driver’s seat. The lid is held in place with a small lip and four magnets. Inside you will find the steering servo, receiver/ESC, and a small spot for the battery. You actually have to wedge the battery in place a little to make it fit. None of the components are exposed to air or water for cooling. Be mindful of that and let things cool down for a few minutes between runs.

Since the MAT includes a 27MHz radio, the receiver has the requisite long antenna that must be routed and managed properly for good reception. There is an antenna mast (a plastic tube) mounted on the top of the radio bay hatch. Out of the box, the end of the antenna was positioned at the top of the mast and several inches of spare antenna wire was loosely bundled atop the receiver. A little antenna slack is necessary to be able to remove the hatch easily, but this setup was begging for bad reception and/or a damaged antenna.

The radio gear is housed under the driver’s seat. It’s a tight fit, but there is room for everything.

I tried numerous alternative approaches to routing the antenna. I ultimately drilled a small hole in the side of the radio box and passed the antenna through it. I bought a longer mast for a few cents (it’s a standard hobby shop item) that I ziptied vertically to the prop cage. I have not had any reception issues with this arrangement. As a bonus, the hatch is no longer tethered to the antenna and there is a little less wire clutter inside. I was slightly worried about water seeping through the antenna exit hole and into the radio bay, but that has not materialized.

On the Water

My first outing with the MAT was in a backyard pool. This thing sure is nimble. Like the Reef Racer, it can turn in an instant. The difference, I quickly found, is that the MAT doesn’t carve through turns like the Reef Racer--it slides around them like a drift car. But that’s what an airboat is supposed to do. Take a few minutes to get used to that before you cut loose. That advice comes from a guy who sent his MAT sailing sideways into the pool wall more than once. No damage was done.

After a few minutes of acclimation, I had no trouble driving all around the pool at full throttle. Not that full throttle is all that fast, but image racing your car through a large parking garage…it’s like that. I had no qualms passing the transmitter to anyone who wanted to try it. Several people, young and old, took the wheel and had a good time without breaking anything.

The stock boat is a well-suited to pool use. It held up well to many different drivers and countless bumps with the wall.

As with the Reef Racer, I was able to use my odd collection of 2-cell 1100mAh A123 and Lipo battery packs with no discernible differences in performance from the stock battery. Keep in mind that the MAT’s ESC will not warn you when the battery voltage gets low. So it is up to you to prevent overdischarge (and death) of any lithium-based batteries. I head to shore as soon as I notice a reduction in speed. If this paragraph doesn’t make sense to you, stick with the stock batteries for now.

I later took the MAT to a few of those lush lakes that I mentioned. The MAT doesn’t have enough power to slide over land, but it does well in the water. I purposely took it through grassy and over grown areas and it handled all of those things just fine. I found out, however, that lily pads are surprisingly sticky!

Out on the lake, the Mini Alligator Tours can unwind a bit more. It isn’t a fast boat, but it is responsive and fun.

In the open water of the lakes, I was able to explore the top speed of the MAT. Again, it isn’t very fast. Yet, it doesn’t plod along like a tug boat either. It’s fun to get it going at a full gallop and crank in hard rudder to make it spin out on the water. With a gentle touch, you can pilot the MAT along a surprisingly precise path. If you get ham-fisted, you’ll spawn the spin outs mentioned above.

Most of the time, I was lucky to have the calm waters that the MAT favors. You have to be careful when there is any chop to the surface. If the hull catches an edge while sliding sideways, the boat stops in a hurry! I haven’t caused the boat flip over yet, but I have had some close calls. When the first flip does happen, I don’t expect that it will do any damage.

Areas like this are where the airboat really shines. This kind of vegetation would anchor a boat using a water propeller.

Stepping Up

Each time I drove the MAT at full power, it seemed that it was on the verge on planing. Scientifically speaking, a boat is planing when the hydrodynamic force created by moving through the water is the primary force keeping the boat afloat, rather than buoyancy. Think in terms of water skis. At rest, the buoyancy of the skis does very little for the skier. At speed, hydrodynamic forces kick in, the skis get “on plane”, and the fun starts.

I felt that if I could find a little extra power to get the MAT on-plane, I would see a significant jump in top speed. I’ve seen videos of MATs that have been really tricked out with high-power brushless motor systems. Indeed those boats are fast, but those videos also confirmed my hunch that there is a practical upper limit to how much power the MAT can handle.

I could have installed a mild brushless motor system and it would have worked out just fine. Recall, however, that the radio receiver is integrated with an ESC for the stock brushed motor. Going brushless would have also required changing out the receiver and installing a brushless ESC. I decided to explore options for coaxing more power out of the stock system.

Before I explain what I did, I’ll mention the obvious: making these changes could very well destroy your boat. I think the ESC is pushed the hardest and is most likely to release the magic smoke. Since it is integrated with the receiver, an ESC crash would be a double whammy--even worse if there is collateral damage. Bottom line: If you’re not ready to lose your entire boat in a glorious Viking funeral, stay stock or take another upgrade route.

There are two basic ways to get more power to a prop driven by an electric motor: Increase the prop size, or feed the motor more voltage.

There are two basic ways to get more power to a prop driven by an electric motor: Increase the prop size (diameter and/or pitch), or feed the motor more voltage. Because of the prop cage and the motor’s proximity to the boat deck, a larger prop was not an option. The stock 5.5” diameter (unknown pitch) was already the max I was comfortable with. Also, I didn’t think that altering pitch alone would give the desired results.

The only option left was to bump up the voltage. Piling on more voltage is something that must be done in a very calculated way. With all other things being equal, higher input voltage will make the motor spin the prop faster…that’s what we want. At the same time, it will also increase the current draw of the motor (amps). Rather than a linear relationship, this usually follows a more exponential path. We have to be sure that by increasing voltage that we do not cause the amperage of the system to exceed the limit of any link in the chain: battery, motor, ESC.

Because of the limited space in the radio bay, I knew that I would need to use a 3-cell lipo battery. A higher voltage battery (more cells) of any other chemistry would be physically too large. I had a few Great Planes 3-cell 850mAh batteries with me that fit well and could dump out plenty of amps. One component down, two to go.

The MAT’s motor is a 380-size brushed can motor. These were once very popular for RC airplanes. From my experience with them, I knew that 12 amps is a reasonable upper limit on these guys. Even that will likely yield a short service life. So I set 12 amps as my upper limit for the system.

I didn’t have any specs for the ESC, but I took a calculated risk that it would be good for the higher voltage and current that I planned. My biggest concern was that the higher voltage would fry the Battery Eliminator Circuit (BEC) component of the ESC. The BEC takes the input voltage of the primary battery and knocks it down to a 5 volt feed for the radio gear. Upping the input voltage forces the BEC to dissipate more excess electrical energy. Once the BEC is saturated, it’s meltdown time. Another calculated risk was in order.

The last element was to configure the system to stay under my 12 amp limit. This was done by sizing the prop to fit the situation. The stock prop would cause the system to pull way too much current at the higher voltage. From my experience, I knew that I would need somewhere around a 4.5” diameter prop with a 4” pitch (4.5 x 4 in prop sizing nomenclature) to hit my target. I removed the stock prop and replaced it with one measuring 4.9 x 4.3.

A smaller prop, a different battery, and a little gambling paid off with a significant performance boost for the airboat. Time will tell how long it lasts.

Bingo! That first prop hit 12 amps right on the money. I took the boat back out to the lake to see how it would perform. It was like a whole different boat! The top speed was much, much faster. It still could not trudge across dry land, but it sure could scoot across the water. With the increased power, I had to be more careful about those full speed spinouts. Actually, it required more attentive driving across the board. The tradeoff was a more powerful and speedy ship.

With all of the components being pushed much harder, the cool down periods have become more vital. The same is true for self-managing the battery voltage. After a half dozen cycles on the new set-up, I decided that I could scale back a little. I switched to a 4.5 x 4.1 prop, which causes the system to pull about 10 amps. I’m still experimenting, but this prop seems like a happy medium of performance and equipment risk. After a further half dozen cycles with the smaller prop, these are no obvious signs of failure. The long term results will tell the whole story.

Summary

I’m glad that I looked into an airboat for my Florida trip. Having the Mini Alligator Tours rather than the Reef Racer or Minimono definitely made the inescapable water vegetation much easier to deal with. Choosing the right boat for the situation can make all the difference between a carefree outing and a frustrating misadventure.

My fun with the stock airboat, and then the souped-up version inspired me to create my own airboat model from scratch. I’ll post details about it in this separate post next week. Hopefully what you see here will entice you to buy, build, or create an airboat of your own.

My fun with the Mini Alligator Tours prompted me to build another airboat from scratch.

Now that we have covered many of the basics of RC in this column, we’ll begin exploring a few specialized skills and DIY projects. Next month, I’ll fill you in on the basics of painting a body for an RC car or truck.

All photos courtesy of Lewis Dunn