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A/C engined hydroplane

Old 08-04-2014, 05:01 AM
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vintageshack
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Default A/C engined hydroplane

I recently downloaded plans for this propeller powered hydroplane. It is roughly 3 feet long and it calls for a 9.5 inch propeller and a glow engine (030?). I am looking to go electric instead, and I am pretty new to hobby r/cs. What size electric motor should I use? Do I need a reduction gearbox? What kind of prop should I get? I'm sure I'll have more questions before all is said and done, but I need a base to go on. The reason I am deciding to go electric is that I am building this up in Iqaluit, Nunavut which is on Baffin Island so it is difficult to get nitro fuel up here! Also electric is much cleaner and lower maintenance. Sorry for the sideways pic! I'm doing this from my phone..
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Old 08-05-2014, 03:48 AM
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Jeremy_H
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The Cobra is a well known plan, as it's one of very few available FOC on the net, but seeing them actually built seems rare. I don't want to put a downer on the idea but word is they are very sensitive to trim, and with that in mind I'd be cautious about going electric, because the weight of the motor and battery might be difficult to configure in a position to meet the balance needs of the boat. As such it may need some re-designing to allow access to batteries etc. Dooable though.

A .30 motor is essentially a 5cc, others will advise on electrics as I know little about them, but it'll be no problem at all to match the expected power, even exceed without extra weight.

'Lower maintenance' you said, I can't agree with that. From what I've seen the failure rate of electrics is scary. A 'Sports' quality IC engine will outlast the need for replacing batteries by a very long way. 'Cleaner' they certainly are, at least locally to you. If you're driven by ecology issues then it's worth a closer look, the manufacture of the battery packs and their disposal is very questionable. Many 'eco' initiatives are driven by 'use' requirements for pollution reduction when a product is being used, folk gaily ignore the bigger picture of how you make and get rid of these things after use because the requirement boxes that need to be ticked are not there. Mucho BS in the ecology world. Even the noise issue is BS if there's more than a few of these things running, electrics' incessant whining and high pitched squeals grates me more than most engines imho.

Sorry to sound so negative, but I think you've chosen a complex build boat with fine setup requirements which may not be the ideal entry to our interest in airboats. Tbh I'd recommend a simple flatty if you want versatility in varying conditions, or an SI3 hydroplane if you want something far more exciting at the expense of some limits on water conditions, they are also well within reach of a new builder from a technical and abilities perspective. It's thirty bucks for the plans for a part foam electric version, but you could fit a motor to a larger all ply version for the same plan price, plenty of folk have done that. You'll end up with pretty much the same expense but a far superior and accessible design.

This is the only vid I've come across for the Cobra:

http://www.youtube.com/watch?v=PZ43uv8h7ck

Last edited by Jeremy_H; 08-05-2014 at 03:52 AM.
Old 08-05-2014, 04:41 AM
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vintageshack
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I had a feeling it will be hard to trim, but I am mostly building because it is quite different than the rc boats out there! What I meant by cleaner is just there is no exhaust messing up the paint. I would use a Nitro engine if it wasn't so hard to get Nitro fuel here.. It would be expensive for me to ship Nitro fuel up here because it is considered dangerous goods. I could go for a gas engine, but they are pretty expensive. So, back to my original question, what would be my best choice for electric engine? I have a small Nitro engine I might bring home (newfoundland) and build a simple airboat to practice with.
Old 08-05-2014, 06:12 AM
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A .15 size electric may do the trick. Just run 4 cells if the motor is rated for it. Likely something with a 2,500 mah battery. If you mix your own glow fuel, it may be possible to get the methanol at a Home Hardware store, and the oil could be shipped. Skip the nitro. It will be a bit more tricky to get a good run on the motor, but removing a head shim will usually work. The is always the weedwacker motor option on a slightly bigger boat because of your remote location. Can't get much more remote than that.
Old 08-05-2014, 06:23 AM
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vintageshack
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Ha ha a weedwhacker motor would be a good idea, but no grass grows here! I did attempt to mix my own fuel with methanol and a very thin oil, it almost worked with my .049 engine.. I probably didn't have the proper mix.. Should I use a certain type of oil? I used bp2380 which is used in turbine engines (full size ones, I'm an aircraft mechanic), the motor kicked over a couple times but didn't stay going. Maybe I will try removing a shim..
Old 08-05-2014, 06:44 AM
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arcdude
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For a first boat that seems a bit of a challenge, the stability is somewhat questionable and may be fraught with grief in water getting into the front end. On the other hand if like myself and you have your heart set and overlook the short comings of the design in favor of personal satisfaction don't be swayed. My suggestion would be to downsize the plan and go to an .049 size glow and that way shipping a litre of glow fuel would be minimized and last quite a while. Good luck
Old 08-09-2014, 03:36 PM
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https://www.youtube.com/watch?v=FGI8WsGbinE https://www.youtube.com/watch?v=xT_Gn2mK404 then
I believe this topic should help considerably.
The builder of the first video Cobra s modified to use DC motor speed 600 :

http://www.ondalunga-forum.com/index.php?showtopic=6984



http://www.rcuniverse.com/forum/rc-a...free-plan.html
Old 08-09-2014, 03:50 PM
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vintageshack
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Wow! Those things fly! It's a little hard understanding that forum with translate, but I think I have the gist of it.. I will definitely have to go lipo to keep the weight down.. Starting to look more expensive! I might take another Crack at mixing my own fuel before deciding whether or not to go electric. Thanks for the info!
Old 08-09-2014, 04:53 PM
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I knew of one fellow that used diesel, not sure what modifications he made to use it but it may be something to look into, Cheers

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