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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.

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

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.

<p>Thanks for sharing, it has given me some inspiration for a future project, keep up the good work.</p>
Glad to hear it, thanks for commenting :-) Hope to see your project once it is done!
<p>cool</p>
<p>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.</p>
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.<br><br>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. <br><br>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.
<p>Maybe you could launch a Cobra V2?</p>
<p>I think I shall have to one day, since I never quite achieved &quot;ridiculously fast&quot; 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 :-)</p>
<p>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.</p>
<p>You have my vote 150% !</p>
<p>Then you have my thanks 200% :-D</p>
:-D It is a seriously cool design mate
<p>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.</p>
Hi warriorethos, thank you for the kind words on my instructable, it is nice to know that someone read it.<br> <br> 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 <a href="http://amzn.to/1QBhYUF" rel="nofollow">Sony Action Cam</a> that I won in the <a href="https://www.instructables.com/contest/intel/" rel="nofollow">Intel IOT Challenge</a>&nbsp;- it beats my old GoPro for onboard RC stuff, because the lens isn't offset).<br> <br> 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 ;-)
Thanks for the reply and I will look into the Sony Action Cam. Good luck in the contest.
<p>I wonder how this would work built at a scale which would allow a person to sit inside, and pedals-powered.</p>
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!
<p>Found this: <a href="http://www.gizmag.com/seahorse-human-powered-airboat/35009/" rel="nofollow">http://www.gizmag.com/seahorse-human-powered-airbo...</a><br></p><p>Not quite the same, but still, the idea of a human-powered boat using an air propeller seems feasible.</p>
<p>That is great, I am surprised how well it seems to work!</p>
<p>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.</p>
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. <br><br>If you make it I'd love to see it.
<p>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)?</p><p>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. :-) </p><p>Next, need to cut the styrofoam to a better shape, find waterproof containers, and replace the 600mah battery pack with 4xAA NiMH.</p>
<p>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).</p><p>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.</p>
I didn't know that. I wonder if it would help to lubricate the bearings more? If so, with what?<br><br>I assume the motors we've got are brushed.
<p>In general, for hobby RC motors, 2 wires mean it is brushed and 3 wires is brushless.</p><p>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. </p>
<p><font><font><font><font>plan de l'ONU Detaille ici:</font></font></font></font></p><p><font><font> </font></font><font><font><a href="http://www.john-tom.com/RCPowerBoats/PowerBoatPlans/Cobra/cobraS.pdf">http://www.john-tom.com/RCPowerBoats/PowerBoatPlan...</a></font></font></p><p><font><font><font>forum:</font></font></font></p><p>http://www.rcuniverse.com/forum/rc-airboats/10392868-cobra-s-free-plan-print.html</p>
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.<br><br>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
<p>awesome</p>
<p>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.</p>
<p>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?</p>
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
<p>I have learned something new, thanks :-)</p>
<p>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.</p><p>serfixalot</p>
<p>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).</p><p>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.</p>
Hi, you could use a ramoser vario prop from germany which can be bought in various versions 2,3,4 and 5 blades. <br>regards, <br><br>maarten
<p>It's an Italian project of the 60' still in sale today by Olympic.</p><p>I've seen moore than one since I was a child, ideal propusion was an OS max40, but is very sensitive to lateral wind</p>
<p>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.</p>
https://www.youtube.com/watch?v=FGI8WsGbinE&amp;index=216&amp;list=FLvhi4FBBK1-yeE1tZpv_bQg<br><br>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.<br>enjoy<br>Piero
<p>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.</p><p>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).</p>
What radio transmitter/receiver you used? And what is it's range? Nice project!
I used my granddad's old 5 channel tx/rx &quot;challenger 550&quot; which was overkill for the project, I am not really sure what its range is, but I didn't have any issues.<br><br>You could really use any tx/rx, a car one would be fine since you only need 2 channels (unless you include motor tilt)
<p>Mega awesome! Seems like a very nice project to attempt with my 5 year old. Thanks for sharing!</p>
I bet he'd love it! My little guy is 2.5yrs and is a big fan of mine.
<p>Here is a 7x6 3 blade prop:</p><p>http://www.hobbyking.com/hobbyking/store/__16711__Replacement_3_Blade_Propeller_7x6_.html</p>
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.
<p>You could always design a prop to your liking and 3D print it! In fact, you could 3D print the entire boat!!!</p>
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.<br><br>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.
<p>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!</p>
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.
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. <br><br>I was wondering though, how prone to flipping is that hull design? It looks like it would be very stable in turns.
<p>That is great, thanks for sharing! What size motor and prop do you have on there?</p><p>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. </p><p>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 &quot;taking off&quot;, 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.</p><p>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.</p>

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Bio: Electrical Engineer by trade, tinkerer by heart.
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