Airscrew Driven RC Boat

A modern rebuild of a vintage boat design

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I came across some some horribly low-res old plans, in a language I don't understand, to build a radio-controlled boat that uses a gas airplane motor and propeller for propulsion. I don't have a gas motor either and have never built a boat, so naturally I couldn't get the project out of my head.

I recently stripped some laser printers for parts and came across a whole selection of brushless DC motors, with control circuitry on the board, some of which seemed like they might have the oomph to drive a RC vehicle of sorts. Time to find out (Spoiler Alert: Nope).

Since I had to scale the original plans I decided to redraw them in CAD, with the intention of CNC cutting them if possible.

If the boat ends up working nicely, or at all, I may consider popping in a gyro to help control the yaw which I expect to be pretty skittish (Also not an issue so far, unless I can get my hands on MORE POWER).

The relevant design files will be published here as I go along:

  • 1 × 1200x600x3mm Plywood Plywood for the main structure. Marine grade would have been nicer, but I went cheap.
  • 1 × 1mm Imbuia/Oak Venner
  • 1 × 7x5 Prop
  • 1 × 2s 2800mAh LiPo
  • 1 × 5 Channel RC Tx/Rx Only 2 channels in use so far

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  • Wrap Up Video

    Ossum02/09/2016 at 08:21 0 comments

    After getting a new motor and prop (10x7 now), I finally got the boat up to planing speed. It is still a long way off fast, but I think it is successful enough that I can put the project to rest.

    If I had the dough for a decent ESC/Motor combo that could handle 4S then I am sure it would be quite a bit more exciting, nevertheless, here is some video for all y'all as it stands.

  • It Floated!

    Ossum09/21/2015 at 20:13 0 comments

    It feels like it has been ages since I started this project, but three months after find the plans it finally hit the water. My dad was good enough to come along and take some photos. The wind was blowing a bit, so we headed for the most sheltered section of the lake.

    First a final functional check to test that the was moving and the motor was responding. Apparently I am not NASA material, because I didn't notice that the rudder was moving backwards, fortunately I was able to flick the reverse switch in the back of the remote easily enough.

    You can probably see the problems in this photo already, but at least it's floating!

    The boat sits a little low, but that shouldn't be a problem when it gets up to planing speed. Unfortunately, it just can't get there. There were a handful of reasons:

    • The prop is pretty small (I am going to look for a 3-bladed 7x5, or something even more aggressive than the current 7x5
    • The prop is nicking the water, so I don't think it is getting up to full RPM. I am going to try and raise the motor.
    • The motor could be more powerful I guess, but I have no idea what this one's ratings are, and I don't have any others to try.
    • I could run this motor on 3S (it is currently on 2S), but the ESC in its waterproof box has zero air-flow, so I would need to figure out cooling. Also, I don't have any 3S batteries.
    • The water was quite choppy, which will probably always be an issue for this design. As you can see in the photo below, if the prop digs in, the boat suddenly tries to become a submarine.

    On the upside, the balance looks pretty good and the rudder worked really well, despite the low speed

    Nevertheless, it was nice to finally see it puttering around. If I had just made it look like a tugboat then I wouldn't have minded that it performed like one ;-)

  • Last Step: Electronics and Steering

    Ossum09/16/2015 at 21:53 0 comments

    I found the smallest snap-shut "tupperware" tub that I could find. Unfortunately, it is just too tall.

    So I made it less watertight

    And then I made it more watertight again, using a bit of scrap plastic and some hot glue.

    It just fits the monster old receiver and the tiny ESC, let's hope they don't interfere with each other. The battery obviously sits outside the box, as does the rudder servo, which is waterproof-enough (I have run the same one in my RC buggy in some pretty grotty conditions, it should be fine)

    The rudder servo sits between two blocks of wood, hot-glued into the boat. Each of the wood block has a M3 hex standoff epoxied into a hole drilled in it, once everything is finalised I will be able to screw down a little cross-bar to hold the servo in place.

    The rudder cable passes through two tubes (bicycle brake cable sheath, but I replaced the actual cable with some thinner, more flexible, stainless steel cable) out to the back. The pull-pull arrangement means that I can use thin flexible cable, since it doesn't have to apply any pushing force.

    The cable passes through a hole in the rudder and is held in place one each side with a doodad who's name I can't remember.

    All in all, the boat is ready to go! Unfortunately I loaned out my lone RC battery, so until I get that back the maiden voyage will have to wait. As they say, control without power is boring. Or something.

    The next project log will either be "It Sank!" or "It Floated!"

  • Paint!

    Ossum09/06/2015 at 21:40 0 comments

    This project log has been an awfully long time in coming, since I was waiting for the boat to get painted. I asked one of the friendly maintenance guys at our office park to spray the boat with "anything waterproof left in the gun after a job", but he really went above and beyond, hitting it first with a few coats of waterproof primer, then some military green-brown, followed by a couple of coats of clear.

    Here it is before the clear coat.

    In the previous picture you can see that I also made a little engine-mount/cowl, which was just a few scraps of wood, stuck together and sanded into shape (the backing is some 3mm hardwood of sorts):

    The idea behind this cowl/mount was that I could swap the whole thing out if I later decide to run a motor with completely different dimensions. I also just wanted to disguise the little motor a bit, because it looks kind of silly and "toy-like" to me.

    Here is a decent shot of the "cargo" space, you can see how the motor stays behind when the cover comes off. I am really trying to avoid having to detach cables when opening up the boat.

  • Decking and Touchups

    Ossum08/18/2015 at 09:10 0 comments

    I cut some strips of balsa at 45° to use as fillets along the fuselage.

    There is a bit of a flaw in my design when it comes to waterproofing the interior, it is almost impossible to get under the stringers. There are twp options, one would be to put little walls in, essentially dropping the stringers straight down, and narrowing the "cargo" space. I didn't want to do that, so I used a sponge to squeeze acrylic paint in everywhere that I could. Not ideal, but it's probably ok. I will fix this in the next design.

    The decking is going to be done with 1mm veneer. I tried to make a 2mm ply out of it, but it ended up being too rigid to conform to the curves. If I built this again I would just use 3mm balsa, but I was insistent on using the veneer that was kindly given to me.

    After pegging it to the stringer I made a rough outline from the bottom, so that I could cut it to shape (with a box cutter).

    The gluing process involved lots of weights (to keep the middle down), as well as pegs (to hold the edges to the stringers) and electrical tape (to hold the outside edges down) and zip ties (because I ran out of tape!).

    The unwrapped (and trimmed) result is starting to look pretty good, if I say so myself! It looks like something batman would be proud of (once painted black, or very dark gray of course).

  • Taking Shape

    Ossum08/15/2015 at 06:46 0 comments

    Added a bunch of balsa bits along the edges, and then sanded them flush, to give some more surface for the cladding to adhere to.

    Gluing 3mm blasa to the bottom of the boat, since I didn't have any suitable clamps I had to get creative with conforming it to the curves.

    On the fuselage I did the same thing, anticipating that I was going to cover it in the 1 or 2 mm plywood.

    It turned out that the corner radii were just too tight for even the 1mm veneer, and it kept splitting, so I decided to clad the fuselage in 3mm balsa instead. The zip-tie holes finally came in handy here.

    Lots of test fitting to make sure the fuselage isn't warping

    I am stubbornly insisting on cladding the top of the boat with the 1mm veneer, since it was given to me, so I put in some diagonal balsa cross-braces to support it as well as give more gluing area. It would probably be a lot easier to clad the top of the boat in balsa too.

  • Assembly Continues

    Ossum08/06/2015 at 14:29 0 comments

    Assembly has been plodding along. It is a slow process, since the glue takes a while to dry between each step. Among other things I have been:

    Using cable ties to start bending the curves into the plywood (I considered wetting the plywood, but since its not marine-grade I wasn't sure if that was a good idea or not). I left it like this for a few days and it seems to be working.

    Making 2mm plywood from 1mm oak veneer

    Gluing down the first sheet of oak plywood "skin".

    Measuring and installing the 12x12mm beams that form the supports for the lid/fuselage

  • First Assembly

    Ossum08/03/2015 at 08:39 0 comments

    Finally we have some real physical progress!

    Cutting out such fiddly pieces on a machine designed for cutting big furtniture panels requires an alternative method, so the 3mm ply was screwed down (in many many places) to the wasteboard, which could be held down with the suction cups.

    Here you can see the four billion screws and the first cuts (for the 3mm slots)

    An action photo without any visible action!

    The parts need a bit of cleanup, which is mostly since the plywood is horrible and splintery, nothing that a few minutes with some sandpaper couldn't solve.

    First glue-less assembly looks good, this design goes together very easily.

    However, once I assembled with glue I discovered a problem, which is probably pretty evident in the next photo. Despite my best efforts, the whole thing kinda "paralleogrammed" and I ended up with a boat which would be suitable for NASCAR and not much else. This is my own stupid fault, I should have designed a piece which slotted in the third axis, to prevent this kind of skewing.

    Some force and a craft knife got me back to square one.

  • Prop Mount

    Ossum07/26/2015 at 19:59 0 comments

    Not much progress to report, but I did manage to get hold of a collet (something like this) from my local hobby shop to attach a prop to a 6mm shaft. The prop was salvaged, along with a collection of others, from my late granddad's garage. I have fond childhood memories of the cool RC planes that he and my uncle built from scratch, he would have loved this project.

    The prop seems to generate an appreciable amount of thrust, pulling roughly 600mA at 24V (14.4 W), but I am still unsure whether it will be enough.

    My reading on electric plane motor choice (since no one seems to make boats with props and talk about their motor choice) says that 60W per pound is bare minimum. Even if an airboat only needs a quarter of the power of a plane (that sounds boring), I don't see my boat coming in at half a kilo. If anyone has some actual numbers for me to work with, I'd appreciate it.

    Along with the prop and things I also got the radio I think I will be using; with "1991 Style" technology and five channels it is light years more advanced than any of the transmitters I am using for my RC cars.

  • Motor Control

    Ossum07/23/2015 at 23:56 0 comments

    Real life got in the way for my friend doing the CNC-cutting, so there are still no parts. In the meantime I decided to get stuck into the motor control.

    I pulled these motors from an HP Color LaserJet 3000 series printer a few months back. After some digging I discovered that googling "HP Printer <number> service manual" invariably brings up some pretty decent documentation.

    Since I really wasn't sure whether the motor would even have enough power for my purposes, I whipped up a quick prototype with an Arduino Nano clone to test it out. If I end up using the motor I will build a little board and port the code to an ATTINY85 or similar.

    The control signals are pretty simple (I am using 5V logic, which it seems happy with)

    • /ACC requires a PWM for speed control
    • /DEC is a "brake" (I think of it as an enable pin)
    • REV controls direction
    • FG is an output which can be used for closed-loop control

    The biggest nuisance is that the motors require 24V DC, not a great voltage for standard RC batteries.

    I have it running off a bench supply at 24V at the moment, the Arduino is sitting on an old board left over from another project, which works out well, since it has a 5V switch-mode supply on it, to feed the Nano and the RC receiver.

    The Nano listens for standard RC servo signals from the receiver (this post was handy) and sets the pins for the motor accordingly. I had to increase the PWM frequency of the Arduino and found this TimerOne library made it easy-peasy.

    I have some video of the motor running, but my internet is being ridiculously slow, I will upload it when possible, so that you can all bask in its glorious whine. I can't say for sure yet whether the motor will work for the boat. It spins with enough torque that the 6mm shaft burns my fingers when I try to stop it, but my finger-heat to thrust calculations are a bit rusty. Once I figure out how to mount a prop I will know more (or just lose my fingers all together).

    UPDATE: here is the video, as filmed with a shaky low-resolution potato

    The code is attached below, in case anyone is interested

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silvio biasiol wrote 04/14/2016 at 18:20 point

That's a very nice project! I'm impressed! I'm trying to build a boat as well but it will not probably come out nice as yours lol

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Ixbidie wrote 07/14/2015 at 12:01 point

This project makes me remember this

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Ossum wrote 07/14/2015 at 12:20 point

Those are pretty great! I have my doubts I will have enough power-to-weight for such shenanigans... but I like the elevators and twin tail, that gives me ideas.

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Joni wrote 07/14/2015 at 08:45 point

If you get going fast enough it will take off and then you might have some pitch problems. The strut between the two hulls just ahead of the prop will act as an elevator.  Maybe you should consider including a tab on the strut that you can tweak?  Ekranoplan typically have a large T tail to help them stay stable.

@Hacker404 it sounds like you suffered from some gyroscopic precession.  Sopwith Camel famously had the same problem. 

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Ossum wrote 07/14/2015 at 09:03 point

The T tail is an interesting addition. I mentioned to Hacker404 below that I was considering some sort of elevator on that strut. I have a vague recollection of watching an documentry about those monster F1-style racing boats as a kid, the thing that stuck with me was one of the pilots saying how they had a pedal to control "elevators", which he had to stomp on as fast as possible if the boat got too much lift and started to fly.

The simple trim tab you suggest is actually a good first step though. The sizes of the control surfaces are complete guesswork so far (and influenced by those original plans of course). The rudder feels really far from the prop, but I guess that's how it is on a regular plane, so it should work. I also considered little rudders/elevators just behind the prop, on the fueslage, but I have never seen a design like that, so perhaps there is a reason for it.

I have never built a RC boat or RC plane before, so all this input is appreciated! I notice you are an alleged aerospace engineer, so I'll add a few percent extra weight to your suggestions ;-)

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Joni wrote 07/14/2015 at 12:04 point

On most aircraft having the tail far back works because you get a long moment arm from the center of gravity and center of pressure back to the tail so you only need small elevator movements to control aircraft pitch trim.  Small movements mean less drag.  I'm not doing any calcs here so history is a better guide I think.  Have you heard of hump drag?  Seaplane drag increases with velocity and then suddenly drops off as they lift out of the water. It's often associated with a shift in the center of pressure which can make your plane pitch up(and stall and crash) or down(and crash).  This can be compounded if the thrust axis is not through your center of gravity.  Its worth thinking about so you can watch for it in testing. 

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Ossum wrote 07/14/2015 at 12:17 point

Aha, the "leverage" of a small control surface further away from the COG makes sense.

I had not heard of hump drag outside of camel racing circles. Reading that doc makes a lot of sense, it explains exactly why things will go pear-shaped if my boat suddenly gets enough lift to clear the water (or if it hit a wave and launched out, the effect would be the same). Thanks!

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Ossum wrote 07/14/2015 at 06:47 point

Hi there, thanks for commenting! Sounds like you need to build a little recovery submarine  ;-)

I have also been wondering whether roll would be a problem, but I had never considered the centripetal force of the motor. When testing it in my hand I did notice it had some significant gyroscopic effects.

I have been toying with the idea of putting elevators (or just one flap, to control lift/downforce) on that "wing" between the sponsons, in theory they could manage the roll (especially if controlled with a gyro), but I think I will have to see how it performs before i tackle that.

I am actually beginning to doubt that the motor I have will produce enough thrust, but I want to give it a go anyways, since it was the whole impetus behind the project.

I am hoping that I am building in enough flotation that the boat won't sink if it flips, but time will tell. At least my lake is crock-free.

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Hacker404 wrote 07/14/2015 at 09:46 point

Yes indeed a recovery submarine would do the trick. With my track record I would have to make so that it continually spins (roll) under the water. Perhaps with a big water screw on the outer surface. When rolling fast it should go left and when rolling slower it should go right. lol It would probably end up in exactly the right place.

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Ossum wrote 07/14/2015 at 09:51 point

Or is exactly the left place if you get the calculations wrong ;-)

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Hacker404 wrote 07/14/2015 at 01:13 point

Hi, I made something vaguely similar to this as a kid. It died on the very first use. It flipped sideways (roll) into the water (inverted) and sank. The creak was and is crock infested so it will still be there today (at the bottom) some forty years later. 

I saw the size of the motor in your CAD drawings so I might mention what went wrong with mine all that time ago. 

The ethanol motor I was using would run erratic and had a tendency to stall. I found that I could fix this problem on the work bench by adding a small flywheel to the prop shaft.

On the water I found that full right input would have it turning slightly left and when I decreased right input it just flipped left and inverted due to the centripetal force of the flywheel. 

In your drawing it looks like the motor has a significant diameter and if it is a brush-less motor then it's most likely external rotor / internal stator. 

You may find that roll becomes more of a control problem than yaw.

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