I usually avoid using someone else’s plans, because I get more satisfaction from doing something all by myself, but in this case, I made an exception: this is not an important project, it just needs to work. ( http://www.mo-na-ko.net/images8/LodPuerto.jpg )
Given that the plane is built at a scale of 1/10, at a length of 18 inches, this tugboat does not scale easily: it would be only 15 feet long in real life (most tugboats are 60 to 100 feet long). So I’ve opted to use the above as a guide, a USCG utility tug, which conveniently has a crane, very much like the one I will need.
I start off by scaling and printing the plan, so that I end up with an 18 inch long tugboat. I then use a spray adhesive to stick it to foam core boards, then cut out all of the shapes.
The way that this type of construction is normally done, one would then trim pieces of balsa (or other) wood, to the perfect shape and angles, so that they all interlock and fit perfectly, and “plank” the whole boat, after which, the whole thing would be covered in fiberglass cloth and resin.
I decided to skip that step: I have no interest in either using wood, or taking all that time to shape it, carve it, and fasten it all together, to form a perfect fit. Instead, I just jump straight to the fiberglass part. I know it can be done, because that’s how I built my first RC boat. It’s a bit of a trade-off; the fiberglass alone is lighter, but the extra filler material that I’ll need to add to fill the imperfections, will add some weight.
I do use a piece of 1/4″ square poplar wood to form the keel, including a short piece of brass tubing, for the front of the tugboat (better to use a bit of brass that I can bend, instead of trying to piece together something). The rest is covered with an upholstery string, to hold the fiberglass in shape, while it dries.
The real challenge here, is figuring out what size motors to use. In an RC car, it’s pretty straightforward; top speed is (essentially) dictated by the speed of the motor (and gears, if any), and the acceleration depends on the mass of the car (or truck).
In RC boats, it’s completely different, and the steering system can add another element of complexity. First, you face the fact that contrary to a car’s wheel, the propeller is not 100% efficient (in fact, you’ll be lucky to get 80% efficiency) because part of the water that it moves, just churns around and doesn’t actually propel the boat. This efficiency also becomes relevant later, for the selection of the motor. I searched high and low for any kind of information about how much pulling power I could expect to get out of this tugboat, and the bottom line is that it comes down to simple physics: how much water does the propeller move efficiently, over a specific amount of time.
The second difficulty, which turned out to be simpler than I thought, was the top speed: it’s a simple combination of the pitch (or blade angle and shape) of the propeller, and the rotation speed. As it turns out, the 4 bladed propellers I got, the only ones I ever found (except for outrageously prices brass ones) come with a rating (as they all do, just like airplane propellers), that help me calculate the maximum speed I can expect to achieve. In this case, that speed is 5 mph (the propeller’s “pitch” is 20 mm, which means that for every revolution, the propeller would move the boat forward by 20 mm, assuming 100% efficiency). Given that the propeller has a diameter of 45 mm, I can now calculate how much water is moved by one rotation, and factoring in the efficiency, the kind of pulling power that I’ll have.
Ok, so 5 mph isn’t impressive, and I wish it could go faster, but I really want to stick to the 4 bladed propellers because it’s appropriate for a tugboat. All of the RC racers out there mostly use 2 bladed propellers, or 3 bladed (very rarely), in the few I’ve seen. There are a few reasons why real tugboats use 4 bladed (or more) propellers, but the most important is that having 4 blades allows to push more water, more efficiently. Racing boats are built for speed, not for their ability to pull a heavy load. The closest analogy I could use here, is comparing a car in first gear, with a car in its top gear: the first can pull any heavy load at low speed, while the latter wouldn’t allow you to even start moving, and would damage the car at high speed as the transmission would overheat. Either way, I’m not spending 100$ a piece, for a fancy 4 bladed propeller made of brass, which I would likely have to modify.
The next complexity is related to the speed, and ties in the steering. At 5 mph, any rudder is not likely to provide enough control, that I would need to be able to position a hook onto my plane. This is solved by having two separate propellers with individual speed controls: steering is performed by applying more or less power to either propeller. This is how they do it in real life. Rudders are still used, but they only add to the control; they are not the main way that a tugboat like this is steered. It’s important to realize here, that a tugboat will usually find itself in one of two situations; it’s either merrily cruising along (where the rudder works well), or is trying to push (or pull) a heavy load, where the rudder is just caught in the turbulent water.
This is where we get into something that is unique to tugboats; In order to get more efficiency out of those 4 bladed propellers, there is a tube built around them, called a Kort nozzle (or ducted propeller), to help drive the moving water in the right direction. These were in use during WW2, but later developments resulted in the complete elimination of the rudders, by having the entire propeller-nozzle assembly rotate into place (steering and power comes from the top of the mechanism). I found that using a wheel rim from an RC car, and cutting out the middle portion, makes a very good Kort nozzle.
I should mention something about cavitation, which is a problem, if you want to get more efficiency out of a propeller. Cavitation is the effect by which water is turned into vapor (or bubbles), and is caused by low pressure (or extremely fast movements of the propeller). Racers aren’t concerned by this, because they run their propellers so fast, that there is simply no way to avoid it (in other words, they ignore the problem).
For the drive train, I’ll be using a pair of electric motors (size 540, a standard size in RC cars). These are “brushed” motors (i.e., there is a graphite piece that rubs against the motor’s internal, to make the electrical connection). I considered using one of the newer brushless motors, which are more efficient, but they are still too expensive, at least for this project.
I picked up a couple of ESC (Electronic Speed Controllers) to control each motor, tested them out, and chose the Traxxas XL-5 ESC. It’s powerful enough for this size motor, provides enough fine speed control, is very efficient, and best of all, because all those RC car/truck guys out there are switching from brushed to brushless, they are readily available on eBay for a reasonable price. I also tested a ProBoat ESC, but it failed in efficiency, and fine control. While the ProBoat ESC does allow me to go into reverse simply, the Traxxas unit requires that I push the control in reverse once, before it will allow me to throw a motor in reverse (called a “double-tap” control); I don’t like it, but I can work with it. Not all ESCs will allow you to run a motor in reverse, but the XL-5 does, so I’ll be using a pair of them, one for each motor/propeller.
I spent an inordinate amount of time working out a gearing solution, only to put it aside. While the 540 electric motor is capable of up to 20’000 rpm, I’m not likely to need that kind of rotational speed, and everything I’ve read, pointed me to a gear reduction. Unfortunately, there is no cheap solution to this. I’ve put together a couple of reducers, using a differential from an RC truck, but it is unusually bulky, and angled at 90 degrees, which would force me to mount the electric motors vertically, throwing off the center of gravity of the tugboat, to a potentially dangerously high point (i.e. it could capsize more easily). The differential is also not locked (power is applied to the portion that offers less resistance), so I would have to modify this differential by “locking” it, or tying down one of the axles on it. I could make this modification temporary, if I ever decide to re-use these differentials another way elsewhere.
So without these differentials, I may end up with a motor that is too powerful, or not very efficient, which impacts the amount of time that I can keep it running, but I’ll have to wait until it is running, to decide if it’ll work well, without the reducing gear. I’ve run some tests with a temporary rig, and found that the motors draw 1 to 2 amperes at around 10 volts (that calculates to 10 to 20 watts) for about 6’000 RPM; while this might not seem like a lot, the motor’s electrical efficiency is below 50%, but I’m not likely to find an electrical motor, cheap, that meets my exact needs.
I am planning on using the new Lithium Iron phosphate rechargeable batteries, known as A123 or M1 batteries. Batteries have come a long way in the past 20 years. We started out with the old lead acid recipes, which were also available in a sealed package (gel cell), then came Nickel-Cadmium (Ni-Cad), then Nickel Metal Hydride (Ni-Mh), then lithium ion polymer (LiPo). The A123 is the latest advance. I originally came across these with Black and Decker in their VPX series tools, but the line was abandoned during the latest economic downturn. DeWalt still uses them. They are readily available on eBay, from merchants in China (but one must be careful and watch out for fakes).
The A123 is a safe system, contrary to its lithium polymer counterpart which can explode if handled incorrectly. It also offers a charge level that is similar, if not better than a LiPo battery pack, for the same weight. Each cell is rated to 3.3 volts each, and can hold a charge of 2.3 amp/hour (in it’s latest version). That means it can deliver 2.3 amps for an hour, but the unique thing about A123, is that they’re also designed to provide a lot of amperage, for short times; one could (theoretically) push a current of up to 70 amps out of a cell like this (but it would discharge the cell completely in about 2 minutes).
I have no plans to pull that much power out of these cells, but it’s nice to know that there’s just no way that I’m going to damage the cells if I push the motors to their maximum, and that these cells should last for a very, very long time. Since I will be running the motors at 10 volts, I’ll be using 3 cells (9.9v), and unless I know for sure that it’s not a problem, I’ll be using two of these 3-cell battery packs (one for each motor) this should give me a little over 2 hours of “pulling” time. I also plan to use this battery type in my plane.
The configuration by which one powers and steers a vehicle with two throttles, is called a Caterpillar drive. This is how any vehicle that runs on tracks operates, such as a tank (except for the most recent ones), and a bulldozer. Tying this to a Remote Control system, one has several options. The simplest is still, one ESC per motor, individually controlled. The disadvantage with the simpler solution, is that I need a controller with at least 3 control signals: port engine, starboard engine, and rudder. This rules out any of the dirt cheap, trigger styled, RC systems out there, that are common in RC cars. The fancier (but expensive) gizmos will cleverly read the signals from two inputs, and automatically apply the proper power to each motor, while running the rudder.
Lucky for me, I came across an excellent find on eBay, for a fair price, for a 6 channel remote control system. While it was originally designed to run a plane, it was converted by an authorized service shop to run on a frequency of 75 MHz, which makes it legal for me to use it, on any official field. As a bonus, it came with two receivers (JR R700), so I can use the same controller to run both my boats, and finally retire that old system that I’m not allowed to use anymore.
PBY Catalina RC model 
