Tugboat – design and construction – part 4

The system I chose, to allow my tug to hook into the seaplane turned out to be simple; solder a number of fishing hooks together (and sand off the sharp points), to form a 3 or 4 point grappling hook. The hook will come up or down (thanks to the remote controlled winch) and should allow me to easily hook the Catalina, or anything else for that matter. The RC circuit will use two channels; one to turn the winch on or off, and another that will trigger a relay to reverse the direction. For those interested in doing something like this on a larger scale, last month’s (August 2010) issue of Popular Science has a tip: strip the circuit out of a servo, swap the motor for a windshield wiper motor (with additional circuitry to power it), and substitute the potentiometer to a standard 5k unit.

A couple more mistakes. In order for the deck covers to form a good seal, I needed a level surface. This classic tugboat design has a deck that has a lovely bow to it, as well as a step. I thought I’d simply add a heavy layer of epoxy, and build up a contour around the edge of the openings, then it hit me that the proper thing to do, is rely on more wooden material, so I had to sand off the messy attempt of an epoxy buildup. Using cheap 1/4″ hobby sticks, and marking them black, I can build up a reliable structure, against which to form a good level seal.

My other mistake, was trying to build a deck lid, that was seriously over sized. I wanted to make sure that nothing was going to flex, when I screw down the lid, and used 1/4″ square braces in an “X” pattern, then wrapped it with 1/8″ aircraft plywood. It looked weird, and out of place, so I tried the 1/8″ ply all by itself; that looked a lot better, and worked just fine. All the deck lids will be held in place with 6-32 bolts.

The LiPo/A123 charger I bought from an eBay seller in China, finally arrived. I can now charge any 2 or 3 cell A123 battery pack (regardless of the capacity). I gambled that it would do well, as I’ve found no reviews of it anywhere, but I wasn’t disappointed. The 323B charger is supposed to be accurate to 15 mV, but I found a 0.2 volt difference between the charged cells; I’m not sure why yet, but I can always keep an eye on the charger’s function, and end the charge cycle before it enters its balancing phase.

The A123 batteries I purchased from an eBay seller in China, also arrived, and here is my first (of two), home-made A123 high capacity battery pack. It can deliver 2.3 amps (at 9.9 volts) for one hour, and more importantly, up to 160 amps for one minute. The most amount of current that I’ve pushed to the motors so far is 2 amps, to turn the prop at about 6’000 rpm, but once this tugboat  is ready for a real water test, I should be able to push more power to them. Each motor’s theoretical maximum amperage is about 100 amps, according to the manufacturer, but that would be for a very short period of time, as the internal wiring would simply burn up. The power draw also depends on the load put on the motor, which is very difficult to predict with a boat.

Once the tugboat’s weight is balanced so that the propellers are fully submerged, the first test I need to do, is to figure out what pulling power I can get, out of this tugboat. This is where the design of a tugboat gets into mysterious aspects; a rough estimate shows me that each propeller will displace a gallon of water per minute, but how this translates into pulling power, I have no idea.

The experience that I’m getting from building this tugboat has inspired me to plan building a real racer boat. Being a fan of the TV show Whale Wars, I spotted a fast wave-piercing trimaran that became famous as the Earthrace, by circumnavigating the globe in a little under 61 days, setting a world record. Not to be outdone, as always, I intend to make this the fastest RC racing boat ever built, which means that it’ll easily exceed 100 mph. I’ll be taking advantage of the same A123 (LiFe) battery technology, but using a pair of more efficient brushless motors. The model should be around 3 feet long, and will need a pair of fancy metal propellers. Given the cost of this project, it’ll have to sit on the backburner, until I can get enough funds for it.

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Tugboat – design and construction – part 3

The deck is completed. I chose to use 1/4″ oak, and used epoxy to waterproof it. The three openings are for the wheelhouse, the winch, and rudder. I’m building the covers out of 1/8″ model aircraft grade plywood, and they’ll be secured with one or more 6-32 bolt, and sealed with neoprene. The covers will receive a light coat of epoxy, and then painted white.

The winch is complete, as is the circuit to run it. I’ll be able to run the winch forward and backward, remotely. The beam in the picture above, is taped on, just to keep the boat level, while I work on the hull. Next up will be the crane.

The propellers and ducts are all set. I’m not 100% confident on the structure that supports the ducts, but I can make it stronger later, when I run some tests.

The motors are now mounted. I was able to run a dry test, and everything runs just fine. I’ll be sealing the hole in the hull for the propeller tubes, then I can test for leaks, and make some preliminary test runs.

After that, comes the task of finishing the hull; I’ll be using a fiberglass resin gel to coat, sand it to shape, then prime and paint. This is where I can fix the issue with the jagged edges of the deck, by filling the gaps, then painting a 1/4″ black line around the edge.

I picked up an A123 battery pack on eBay dirt cheap. The man selling it couldn’t fit this 3 cell, side-by-side pack into his RC helicopter. It’s not the newer high capacity kind of cell (1100 mAh, instead of 2300 mAh), so it’s just temporary, but it’ll let me run this tug exactly the same way, until I put together the full capacity battery packs, which will have a total of 6 cells, for roughly 4 times the total capacity. That’ll give me enough run time to cruise around for about 2 hours, instead of 30 minutes. The nice thing is that whatever charger I get, will work with any A123 pack. Note here that this type of pack has two connectors; one for power, and one to charge each cell individually (balanced charging is needed for these types of cells).

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Tugboat – design and construction – part 2

I always end up learning from my mistakes.

As it turns out, my shortcut was a touch more complicated than I expected; there were a number of holes in the resulting hull. I went wrong in one aspect, where I tried to use a single piece of fiberglass cloth, to do the whole hull, instead of layering a number of strips. The second mistake I made, was not building the deck first, which left me with an uneven deck, jagged edges, and an asymmetric flat top.

But that’s ok, because I wasn’t making a precision vessel here; it’s just something that I’m putting together real quick. I still haven’t thrown out the idea of re-doing the whole tugboat at twice the size, if needed.

I draw a lot of diagrams on paper with pencil. I should be using some kind of CAD program, but I find it too restrictive; my doodles are more art than science. Lately, I’ve drafted the basic concept for the crane, the winch, the wheelhouse, and the lights. I’ve also figured out how to control the winch; one switch to turn it on and off, and another switch to reverse the direction.

Tricky, these lights. In Navigation, all boats (called vessels) are required to display the proper lights. One red light pointing forward and left at the bow, a green one pointing forward and right, also at the bow, one white light at the stern, and one white light at the top of the mast (you might not have that last one on a sail boat, unless it’s not sailing). With tugboats, there’s a few extras; you must also have a yellow light at the stern, then you must have a second white light at the top of the mast, unless your tow is unusually long, in which case you might be required to have three of four white lights. If the tugboat is pushing (instead of pulling) a vessel, then it has to have an extra set of side lights. These international regulations go on and on: http://www.bosunsmate.org/seamanship/lights.php . …and that doesn’t include the deck lights, so one can see where one is walking! One thing’s for sure, if you see 3 green lights on top of a mast, you’re in the wrong place…

LED lights are very popular, because they’re now cheap and durable, but small and miniature lightbulbs are still available. There’s also a unique color to a lightbulb that an LED has yet to be able to reproduce. It’s closer to a bright orange or yellow, than what the LED has revealed to us what white is (and still incorrect). An LED is designed to emit a light within a very narrow range of the whole spectrum, which is why we often see them in specific colors. Red and green were the first, because the chemistry involved is very simple. Only recently have we seen “white” and blue LEDs.

This chemistry has also made it a bit more complicated to use LEDs, for those that are electronically challenged. The old classic green or red LEDs were simple; they run on 0.7 volts and draw 20 milliamperes. To use them, you simply have to figure out what size resistor to use, between the power supply and the LED. The current flowing through the resistor remains the same; 20 mA, and the voltage is simply 0.7 volts below the supply. Using V = R * I (where V = voltage, R = resistance, and I = current), one can calculate any resistor value. But these new LEDs have different voltages; it could be 1.6 volts or more, and it could draw more or less power, usually as low as 10 mA, and as high as 2 full amps (for the fancy high power, super bright ones). The super bright, high power ones are already in use on cars today, on brake lights, and it’s just a matter of time before they replace headlights. The price is still too restrictive, but the gap is closing fast; wouldn’t it be nice to have a headlight that would outlive the car?

I have a set of super bright, power LEDs on hand, for the plane; 2 whites for the headlights, and three colored ones, for signal lights, that point down from under the left wing. The signal lights were used for planes to communicate with each other, while maintain radio silence, or communicate to a ground crew. I plan to use them to give me a status on the battery condition, while in flight. The signal lights will only blink shortly, but the landing lights will be fully on. I’m not sure if it’s wise to use so much power at the end of a flight, but I’ll figure out some kind of safe mechanism. Either way, it’ll be spectacular.

It seems that a fellow blogger has done a lot of work, about gears on RC boats:

All Geared Up

If it turns out that I need to use a gear, I’ll be sure to use his advice.

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The tugboat – design and construction

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.

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Recovery tugboat

Got the charger, and I’ve tested all the radio gear.

In the mean time, I had to give some thought about some kind of recovery system: what if the plane looses power, and makes a water landing, in the middle of a pond or lake?

I already have an old RC boat that I built over 10 years ago, with an outboard electric motor that I salvaged from a toy, but it’s not going to be powerful enough to push a 20 – 25 lb, 10 foot wide plane to the shore. Also, the radio system is out of date: it still works, but would not be allowed at any field, because it would cause too much interference.

So what better for the task, than a tugboat? Doing a little research (i.e. googling), I found that there are several types of tugboats. Some tugboats are designed to be sea-going, and others are restricted to either a river or a harbor, to move a barge or ship. The Navy and USCG (United States Coast Guard) used them, and since I’m trying to stick to a USCG theme here, this makes the choice easy.

This is the kind of RC project that I really like: build a hull, put in a drive and steering system, decorate to taste, and go. No precision required. It’s not optimized for extreme speed, loaded up with an expensive power system, or any of that, which seems to be the focus of most people (on the internet) involved in RC boating.

So I start off with a pair of propellers that I came across on eBay. The most difficult part of RC boating, is figuring out a good way to attach a motor to a propeller, and finding the supplies to do it. Since the propellers I chose come with a metric threaded connector, and the motors I picked have a 1/8″ shaft, getting the two to connect becomes an unnecessarily complicated task of figuring and finding the necessary pieces.

Since I’m still in the design phase for my plane, building this tugboat will give me some real hands-on work to do, and will let me experiment with the battery system that I’ll be using.

By the way, if you want a good source for supplies and information on how to build an RC boat, I recommend http://www.offshoreelectrics.com . Hobby-Lobby.com also carries just about every part you’d need.

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Move / update

Today I moved this project from a Facebook “Note” page, to a proper blog.

The PBY “Catalina” actually came in a number of different versions, as WW2 progressed:

XP3Y-1 : original prototype

X-PBY-1 : modified prototype

PBY-1 : first production (60 made)

PBY-2 : minor modifications (50 made)

PBY-3 : better engines (66 made)

PBY-4 : better engines, new propeller spinner (32 made)

PBY-5 : better engines, new spinner, waist gun blisters (684 made)

PBY-5A : retractable landing gear (802 made)

PBY-6A : new tail design, radome over cockpit (175 made)

GST (“Gydro Samoliot Transportnyj”, aka PBN-1, aka “Nomad”) : Russian variant

PB2B: Boeing built

OA-10/OA-10A : US Army Air Forces (USAAF) version

(the above is a rough reference, I’ll try to update it in more precise details)

For the past couple of weeks, I’ve been looking at the airfoil (cross section of the wing) for this plane, and I found that while every reference I came across lists it as a standard NACA 0021, Stanford university thinks it’s a Davis airfoil (for version 5 and later).

As it turns out, while all the historic references and engioneering papers I’ve come across say otherwise,  Stanford University might be right: the Davis airfoil was used on the model 31 (experimental/prototype XP4Y “Corregidor”) as well as the model 32 (B-24 “Liberator”).

http://en.wikipedia.org/wiki/XP4Y_Corregidor

http://en.wikipedia.org/wiki/Davis_wing

That being said… I came across a picture, going through the National Archives website (www.nara.gov) that shows a bunch of workers for the Consolidated Aircraft Corporation, having lunch, and sitting against a bunch of prefabricated PBY wings. The problem is, some of those wings  have a symmetrical airfoil and some don’t. NACA0021 is symmetrical, and Davis’ airfoil isn’t.

To top it off, I also came across information that the Davis airfoil is actually a lot more efficient than the NACA 21 airfoil, for the real plane, as well as a model. Since the design of the wing is going to be critical to my model, I may have to use a Davis airfoil anyways.

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The radio

Received the Radio transmitter: a JR XP8103A. Now all I need, is a charger for it 😦

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