Introduction: Airscrew Driven RC Boat

Picture of Airscrew Driven RC Boat

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.

It has taken a lot of tweaking, but I think the boat is finally working well enough to share here.

I have taken the wonky old scanned plans and completely recreated them in 3D CAD. The plans I have attached are ready for laser-cutting or CNC-routing (from 3mm ply - the bounding box is 1200x600mm).

Parts List - Mechanical

1200x600x3mm Plywood (Marine grade would have been nicer, but I went cheap)

1mm Imbuia/Oak Veneer/Ply (or 2mm if you can get it)

3mm Balsa sheets

Glue (I used various, but waterproof and relatively quick-setting are beneficial)

Paint (waterproof)

Parts List - Propulsion

I would seriously do some of your own research, since my choices were based on what I could get cheaply/freely. I feel like this boat could perform a lot better with a better motor/battery/ESC combo.

10x7 APC Prop (Aliexpress/Amazon)

2s 2800mAh LiPo

1200kV 3530 Brushless Motor (Aliexpress - What I ordered/Amazon - Similar Search)

ESC (Mine was an unknown Amperage, make sure your's meets your motor's requirements)

Parts List - Steering

1mm Stainless Steel Cable

Bicycle Brake Cable Sheath

Waterproof RC Servo (Hobby King - What I used/Amazon - similar search)

Parts List - Control

Radio Control Transmitter

Radio Control Receiver (I used a 5 channel, but only 2 or 3 are needed, depending whether you want to tilt the motor or not)

Step 1: Vintage Design, Modern Methods

Picture of Vintage Design, Modern Methods

I found the original plans here. I tried to contact the site owner to see if he had ever seen one built, or knew where the plans came from, but so far have received no response.

I spent far more hours than I would like to admit on digitizing the old plans. I have never attempted a model with so many strange lofted curves, and Solidworks 2007's surface modelling is sketchy, if you'll excuse the pun. Rumour has it that the recent versions have some rather fancy tools for unfolding surfaces, but I don't have access to those. In the end I decided to model the whole boat as a solid, and then use a multitude pf planes and carefully shaped surfaces to cut it into profiles. The "sheet metal" tool was able to unbend one or two curved surfaces, but I anticipate skinning the whole boat with hand-cut 1mm ply, so I am not going to fight with it to do the trickier ones.

Step 2: Cutting the Profiles

Picture of Cutting the Profiles

While you could cut these profiles by hand on a jigsaw, I specifically designed them to be cut on a CNC machine of sorts, they have been laid out to fit on a 1200x600mm piece of 3mm plywood.

If you need to dimension the drawing, then scale it so that the slots match the thickness of the wood you are using, I designed the slots for 3mm plywood.

I was fortunate in that a friend runs a CNC router at his furniture factory and he was good enough to guide me through using it to cut these profiles.

This design is actually far better suited to laser cutting, what with all the tight inside diameters, but when you have a generous friend who will do it on a CNC router for the price of takeaways, that's the way you're gonna go!

Step 3: Assemble and Glue the Profiles

Picture of Assemble and Glue the Profiles

Dry Fit

Before touching the glue I strongly recommend assembling it dry, make sure that everything fits and that you know where it all goes.

I marked the various pieces with numbers and alignment lines with a pencil, you don't want to have to do any thinking when the glue is involved.

Right Angles and Parallel Lines

It is imperative that you keep the whole assembly straight, the profiles must all make a right angle with the two long profiles that run stern-aft.

In the photos you will see that I used two long straight beams to clamp the profiles to which helped keep everything in line.

Stringers

This is also a good time to install the stringers, these are 12x12mm wooden beams that fit at an angle down the length of the boat, they will help keep things true.

These beams also form the support for the lid/fuselage.

Step 4: Assemble and Glue the Fuselage

Picture of Assemble and Glue the Fuselage

The "fuselage" for lack of a better term (really it is kind of a lid), is the same as everything else, simply slot the profiles together and start gluing.

I cut some extra notches in the horseshoe shaped pieces so that I could run some long strips of wood (about 3x6mm in profile) from the front to the back. I also glue some extra scraps of balsa to the vertical profiles, in order to give the skin more surface area to bond to.

Zip ties were used to hold it together while the glue set.

When it came to gluing the skin I used 3mm balsa, because it gave some "meat" to sand into nice curves.

Step 5: Bend and Glue the Sides

Picture of Bend and Glue the Sides

Up until this point I had glued the side profiles on as far as I could before significant bending was required.

There are probably better ways to bend the plywood, I have read about methods that use steam, others that use ammonia (particularly window cleaning spray), but I just went low-tech. I used cable ties to pull the wood in a little bit more each day that I worked on the boat. Once the sides had been sufficiently bent I drilled 1.5mm holes and used copper wire to hold them in place while they glued.

Step 6: Skin, Top and Bottom

Picture of Skin, Top and Bottom

Material Choice

I used a variety of materials, because I was using what I had available and experimenting as I went along.

3mm balsa (for the bottom of the two hulls)

2mm oak ply (for the large surface of the tunnel between the hulls)

1mm oak/imbuia ply/veneer

Thin Ply

I used a lot of 1mm and 2mm ply to do the decking and the tunnel, it is nice and flexible which makes it conform to the curves relatively well (although the Oak was much more prone to cracking along the grain than the tight-grained Imbuia). The 1mm ply can be easily cut with a craft knife.

As you can see from the photos, I use electrical tape and oodles of weights to hold it down while the glue set. Apparently bags of rice or sand also would have worked really well.

Balsa Supports

I 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.

I stubbornly insited 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 3mm balsa instead.

Step 7: Painting/Waterproofing

Picture of Painting/Waterproofing

This part is up to you, so long as the paint is waterproof it should be ok.

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.

If I was doing this myself without access to a spraygun, I would either go for a rattle-can, or even just use acrylics and a foam roller, since they cure to a fairly robust waterproof finish.

Step 8: Rudder

Picture of Rudder

As you can see from the design, my original plan was to use a plywood rudder, it's shape is still included if you would like to try it. In the end I decided to use a slightly larger piece, made from aluminium.

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, which is used to fasten the servo down.

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 (in the hobby shop you will find used for holding wheels onto RC plane's landing gear axles).

Step 9: Mount the Motor and Propellor

Picture of Mount the Motor and Propellor

I tried a number of different mounts and have attached photos of all of them which should be self explanatory.

Version 1: All Form, No Function

Aesthetically the small motor mounted directly on the nose was the nicest by far (and was where I had always designed it to go), but the 7 inch prop I had designed it around simply wasn't big enough.

Version 2: Adjustable Tilt

The second attempt included a hinge assembly and a second servo to adjust the up/down tilt of the motor. This worked and was fun to play with, but not really necessary (however, with a hotter motor, or more battery cells, it may become more important to be able to tilt down when the boat begins to take off).

Version 3: Simple and Effective

The final mount was a simple L-shaped piece of aluminium, bent with about 10 degrees of up-tilt and high enough to mount a 10 inch prop. Simple and effective, but rather ugly. If I built the boat again I would adjust the fuselage in order to fit a larger prop on the nose.

Propeller

The size and pitch will be determined by your motor, ESC and battery voltage. Most motor manufacturers recommend a specific range of prop sizes per motor, so I would start with one of those. My experience was that I needed all the thrust I could get, but I was only running a 2S battery which was limiting. I ended up with a 10x8 propeller on a 1200kV 3035 motor.

If anyone can find a 7 inch 3-bladed propeller with a fairly high pitch I would love to hear about it, but I couldn't find one myself.

Step 10: Electronics

Picture of Electronics

I won't go into the details of the connections, since it is a bog-standard RC receiver, servo, esc setup, like any RC car or boat. If you need some help there are plenty of tutorials, here is one on this very site.

Watertight Container

I found the smallest snap-shut "tupperware" tub that I could find. Unfortunately, it is just too tall to fit in the boat, so I just cut a slot in it (with a hand saw) and then resealed it using hot glue and scrap plastic.

This container will house at least the RC receiver, but depending on your setup, it could house the ESC too.

Heat Dissipation

Depending which of your electronics are waterproof, you may want to change the layout. In my first build, as you can see in the picture, I put the receiver and the ESC in the waterproof box, which worked, but you will need to consider heat dissipation. If you are pushing the ESC anywhere near its current limits, then you are going to need to get rid of significant heat, which can't happen inside a plastic box.

In my second iteration I sandwiched the ESC between two old heatsinks, making sure to waterproof the whole lot liberally with hot glue. You can see this setup in the photos and the video..

Step 11: Fasten the Fuselage

Picture of Fasten the Fuselage

The fuselage already fits rather tightly, but since all of the thrust is applied to the hull via it, it needs to be firmly attached.

It would be easy to use nuts and bolts, but I wanted it to be easy to detach without tools, so I came up with a solution using steel pins (actually screws) that pass through the hull and into brass tubes glued into the fuselage, the tubes have magnets at the end that stop the screws from falling out.

The pictures tell a better story than words, so have a look (and read the annotations).

Step 12: Go Boating

Picture of Go Boating

Some tips for your first test run:

  • Be ready to swim, according to Murphy's law it reduces your chances of needing to.
  • Make sure that the rudder works (and isn't reversed)
  • Make sure the motor is turning in the right direction before putting the boat in the water.
  • Put a thin strip of electrical tape down the joints between the hull and fuselage, in order to keep splashes out.
  • Take a camera, you never know if the maiden voyage will be titanicesque or not, but it would be sad if it was and you couldn't post it to Youtube.

Comments

ThirdEarthDesign (author)2016-04-14

Thanks for sharing, it has given me some inspiration for a future project, keep up the good work.

ossum (author)ThirdEarthDesign2016-04-14

Glad to hear it, thanks for commenting :-) Hope to see your project once it is done!

Pindelcarrugiolungo (author)2016-04-14

cool

ThomasK19 (author)2016-04-09

I noticed that you pull the rudder directly with a steel cable. I think that using horns on both sides would give a more accurate steering. And probably you would need less power wasted in the servo. As experimental improvement you could mount the motor so you can tilt it opposite to the rudder (just a bit) giving some sort of vector steering. That would of course need some extra clearance in the hull.

ossum (author)ThomasK192016-04-09

You are absolutely right. In fact, I originally had horns on the rudder, but I had stupidly placed the cable-sleeve holes in the hull too close to the center, so I couldn't get the travel I needed.

The way it is now actually works with surprisingly little resistance though, because the steel cable follows a nice tangent, out of the one cable sleeve, through the rudder and back into the other cable sleeve.

I also considered vector steering, but as you say, hull clearance is the issue. I think the answer would be to run a higher voltage so that you could use a smaller prop. If I can get my hands on another ESC I wan't to try with 4S and see how it works.

ThomasK19 (author)ossum2016-04-10

Maybe you could launch a Cobra V2?

ossum (author)ThomasK192016-04-10

I think I shall have to one day, since I never quite achieved "ridiculously fast" with V1. My current project is a completely custom RC car, which I need to get completed while I have momentum, but once I have shared that one I think I will be about ready to look at the Cobra again :-)

ThomasK19 (author)ossum2016-04-10

The original plans just look like a plane which got some catamaran mods. To get it really speedy you probably need ways to get more air under the foils. The current version blows most of it over them.

DIYWEAPONS (author)2016-03-13

You have my vote 150% !

ossum (author)DIYWEAPONS2016-03-13

Then you have my thanks 200% :-D

DIYWEAPONS (author)ossum2016-03-16

:-D It is a seriously cool design mate

warriorethos2 (author)2016-03-06

ossum, your airscrew driven RC boat is nice. Looks pretty fast with the size of motor that you have. Did you mount two cameras on the boat or did you change the view by flip-flopping the camera when you brought the boat in? I like that you can see as the boat was moving as if you were on the boat itself. With a live feed you could easily drive the boat using the camera. Your instructions, photos, and videos were easy to follow and well organized. Your suggests from mistakes and just lessons learned are also well annotated. Great job on your boat and good luck in the contest.

ossum (author)warriorethos22016-03-07

Hi warriorethos, thank you for the kind words on my instructable, it is nice to know that someone read it.

Regarding the camera, it was just one, mounted on the top of the boat and flip-flopped it for various shots (in fact it was the Sony Action Cam that I won in the Intel IOT Challenge - it beats my old GoPro for onboard RC stuff, because the lens isn't offset).

A live feed for first-person video (FPV) is something I am dying to get for my RC builds, but I just can't justify the expense yet. The video goggles and little cameras are really coming down in price though. One day when I'm all grown up I'm sure I'll get one ;-)

warriorethos2 (author)ossum2016-03-07

Thanks for the reply and I will look into the Sony Action Cam. Good luck in the contest.

FlorinJ (author)2016-01-29

I wonder how this would work built at a scale which would allow a person to sit inside, and pedals-powered.

ossum (author)FlorinJ2016-01-29

As cool as that would be if it worked, I have my doubts. Comparing the speed that people get out of models with water props versus air props (with equivalent motors) I would have to say the airscrew is way less efficient. I'd love to be proved wrong though, because it would be a heck of a fun thing to ride!

FlorinJ (author)ossum2016-01-30

Found this: http://www.gizmag.com/seahorse-human-powered-airbo...

Not quite the same, but still, the idea of a human-powered boat using an air propeller seems feasible.

ossum (author)FlorinJ2016-01-30

That is great, I am surprised how well it seems to work!

arpruss (author)2016-01-26

Did you do anything to waterproof the airscrew motor? I'm thinking of converting a Harbor Freight dual prop RC plane to an airscrew RC boat, but I'm worried about splashing on the motors.

ossum (author)arpruss2016-01-26

I didn't, brushless motors especially, but brushed too really, can actually run under water (apparently RC racers sometimes run new brushed motors underwater to seat the brushes or something). I think the bearings will probably take a beating though, they aren't designed to get full of moisture.

If you make it I'd love to see it.

arpruss (author)ossum2016-01-30

Where did you put the battery? I can't see it in the pictures. Is it in another waterproof container, or is it in the same one (heat would be my worry if it's in the same one)?

I've pulled apart the plane I want to base our airscrew boat on, taped the fuselage to a block of styrofoam, and it moves very nicely in the tub. :-)

Next, need to cut the styrofoam to a better shape, find waterproof containers, and replace the 600mah battery pack with 4xAA NiMH.

ossum (author)arpruss2016-01-30

I like where it's heading, thanks for the update. I didn't waterproof my batteries, since I normally don't on RC cars either, but I did notice corrosion on the inside of the battery connector afterwards (I use XT60 connectors on all my LiPos).

I'm not sure sure if it is in any of the pics actually, but the battery sits between the waterproof box and the rudder servo.

arpruss (author)ossum2016-01-26

I didn't know that. I wonder if it would help to lubricate the bearings more? If so, with what?

I assume the motors we've got are brushed.

ossum (author)arpruss2016-01-26

In general, for hobby RC motors, 2 wires mean it is brushed and 3 wires is brushless.

Have a look on youtube, there are a whole lot of videos of motors working underwater, it's kinda cool. I think that if one oiled the bearings after each run there probably wouldn't be too much of a problem.

PhilippeG1 (author)2016-01-29

plan de l'ONU Detaille ici:

http://www.john-tom.com/RCPowerBoats/PowerBoatPlan...

forum:

http://www.rcuniverse.com/forum/rc-airboats/10392868-cobra-s-free-plan-print.html

ossum (author)PhilippeG12016-01-29

I couldn't translate your comment, but at a guess, yes, those are the old plans that sparked my redesign. The original plans were for a larger gas powered boat and spread across 48 pages, so I recreated them in CAD and modified them for my own use.

Thanks for the forum link, I see people were asking for plans for the boat, if anyone is a member on that forum and wants to share mine, please go ahead http://www.rcuniverse.com/forum/rc-airboats/10392868-cobra-s-free-plan-print.html

3366carlos (author)2016-01-28

awesome

MichaelV57 (author)2016-01-27

The profile is reminiscent of the old Ferrari hydro. Jon-tom has a lot more sets of plans on his site worthy of consideration including the famed Aphrodite.

ossum (author)MichaelV572016-01-28

The Ferrari Hydro is beautiful. I may end up adapting another of Jon-Tom's designs in the future, because I want to build a robotic boat, and this one really isn't suited to long efficient running. What is the reason you singled out the Aphrodite, does it have a history?

MichaelV57 (author)ossum2016-01-28

Aphrodite is one of the few surviving Hudson river commuters. These were powerful yachts whose purpose was to ferry their owners to their offices in NY from their upriver estates.. She was found in a state of near total decay on shore and restored at great expense to her current reincarnation. Check her out here: https://www.google.com/#q=commuter+yacht+aphrodite

ossum (author)MichaelV572016-01-28

I have learned something new, thanks :-)

serfixalot (author)2016-01-27

You have give many good details for building this model and I would love to build this one but I don't see any size requirements or is that left up to each builder to come up with his own dimension? Thanks for a great project it must have taken you a huge amount of time to build and post the information.

serfixalot

ossum (author)serfixalot2016-01-28

Thanks for the kind words! You are absolutely right about dimensions, I wasn't terribly clear, and I also only uploaded a PDF (whoops!). I have now added the DXF of the profiles, as well as the DXF with the toolpaths that we generated for CNC cutting (but this would depend a lot on your routing bits etc, it probably isn't terribly useful).

You can scale it however you like, but the slots will of course change with it. Mine was scaled for 3mm slots and fits in a 1200x600mm area.

Turf1975 (author)2016-01-27

Hi, you could use a ramoser vario prop from germany which can be bought in various versions 2,3,4 and 5 blades.
regards,

maarten

mariners (author)2016-01-26

It's an Italian project of the 60' still in sale today by Olympic.

I've seen moore than one since I was a child, ideal propusion was an OS max40, but is very sensitive to lateral wind

ossum (author)mariners2016-01-26

It's really still on sale? That's cool. If you ever come across another one in the wild I would love to see a photo. I did quite a bit of googling and came up dry.

mariners (author)ossum2016-01-26

https://www.youtube.com/watch?v=FGI8WsGbinE&index=216&list=FLvhi4FBBK1-yeE1tZpv_bQg

french improved this model with front and rear engines and added a movable tailplane on the rear as an Aircraft. Someone try to make the front tailplane movable to modify the air inlet in the hull tunnel to avoid the frequent capsize. Most of the cobras i've seen are always underpowered and too heavy to have a decent run.
enjoy
Piero

ossum (author)mariners2016-01-27

Thanks for the video, that one sure is pretty! Realising that there are still people building these makes me want to try an improve my electric version. Weight and powered are definitely issues, as you say.

The one in the video definitely is more bouyant than mine, but I think that is because mine is about 60% of the original scale (and I didn't use the lightest materials around).

NetoC (author)2016-01-26

What radio transmitter/receiver you used? And what is it's range? Nice project!

ossum (author)NetoC2016-01-26

I used my granddad's old 5 channel tx/rx "challenger 550" which was overkill for the project, I am not really sure what its range is, but I didn't have any issues.

You could really use any tx/rx, a car one would be fine since you only need 2 channels (unless you include motor tilt)

avayan (author)2016-01-26

Mega awesome! Seems like a very nice project to attempt with my 5 year old. Thanks for sharing!

ossum (author)avayan2016-01-26

I bet he'd love it! My little guy is 2.5yrs and is a big fan of mine.

BikeHacker (author)2016-01-26

Here is a 7x6 3 blade prop:

http://www.hobbyking.com/hobbyking/store/__16711__Replacement_3_Blade_Propeller_7x6_.html

ossum (author)BikeHacker2016-01-26

Thanks! That's an ideal looking prop, especially with a slightly higher kV motor (don't quote me on that, i'm new to props). Not expensive either, although I may have to do a kickstarter to afford the shipping to Cape Town :-P I'll definitely try one if I find myself ordering anything from HK in the future.

Numbutz (author)2016-01-26

You could always design a prop to your liking and 3D print it! In fact, you could 3D print the entire boat!!!

ossum (author)Numbutz2016-01-26

if I had access to a 3d printer I would be printing a hull already ;-) unfortunately I think it's size would make it cost more than a real boat if I ordered it from Shapeways. I think vacuforming would be a good solution for a light tough hull.

3D printing a prop is an interesting idea, although, from what I have read, the rough surface of a 3d print wouldn't make a great prop. There must be ways around that though.

BikeHacker (author)2016-01-26

This is BRILLIANT! I've had a few standard rc boats and the props always get snagged on seaweed and stuff. I'll be building this this summer!

ossum (author)BikeHacker2016-01-26

That is definitely one of its biggest advantages. My current rudder design would snag in weeds, but I have considered putting the motor on a swivel for steering, which would mean that with enough power it could plow straight over sandbanks and grass. A dual air rudder might work too.

bobaluedooley (author)2016-01-25

Love the hull design, think my father and I may have to build one up. This is one we just finished.....again. Built the original hull when I was 7 or 8, I'm now 37. Originally had a tiny Cox gas engine. We just recently updated to electric, and went from single to dual rudder, with much better performance now. I also went a little crazy and scratch built the cage for the motor and prop, and the railings. They are styrene stock tube of various sizes and the netting came from a fruit bag.

I was wondering though, how prone to flipping is that hull design? It looks like it would be very stable in turns.

ossum (author)bobaluedooley2016-01-25

That is great, thanks for sharing! What size motor and prop do you have on there?

I love that you are doing it as a family thing with your dad. I actually started this project in order to use up some balsa and props that used to be my grandfather's from his airplane days, he would have loved this project.

The design I used has been incredibly stable so far, but it is not quite as overpowered as I would like (yet). I initially expected to have problems with it "taking off", and was going to build in some elevators on the front wing or reintroduce the motor tilt, but for now it just isn't neccessary.

If you check out the video you can see it does some pretty tight on-the-spot turns without threatening to tip. However, I did make one mistake, I threw the boat into the water off the jetty while the prop was turning at full speed and totally forgot about the torque it would exert, the boat landed upside down and I had to go swimming ;-) I guess that effect could be a problem if the boat was powerful enough to be getting air.

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