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  1. #1

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    Cobra plans underway

    I posted a while back that I was thinking about doing an electric version of the Cobra RC (free plans online). Well, I finally got up the courage to start building. Here is my old post: A/C engined hydroplane

    I work in the north (Iqaluit Nunavut) 3 weeks at a time, and for the most part there isn't much to do after work besides watch movies and drink rum, so I decided I would actually do something constructive. I started bringing model car kits up, but that got kinda boring. I found these free plans online for a unusual looking airboat, and thought I would give it a try. Every time I fly up to work, I have to overnight in Ottawa, so this gives me an opportunity to go to the hobby shop there to buy some balsa a bit at a time. I am really hoping to have it finished by next spring. This is what I have completed so far.
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  2. #2
    Good to see you back vintage So have you decided on the means of propulsion, as I recall there was some debate? I happen to live near Ottawa, there really isn't a lot available in the hobby shops these days leaving one having to order a lot of stuff that isn't available off the shelf. Looks like a good start as long as you leave the rum alone lol, we'll know how a warp got in the hull should there be one! Cheers

  3. #3
    Neat work Vintage, it'll be good to see one of these going together.

    Your plan seems to have jigsawed together really well, the copy I had wouldn't marry up properly

  4. #4

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    Thanks Jeremy! The plans I have aren't the best either, I think the guy who made it just traced the parts he built.. It took a little bit of fudging, but they mated up pretty good! The plans are in Italian for the most part. A few pieces might end up being a bit of trial and error, but I'm in no rush! Arcdude, I am pretty sure I'm going electric.. Turnigy makes some nice motors.. The one I am looking at is an equivalent of a 0.46 glow engine. I actually went to Michaels on Bank St to get the balsa (I stay at the southway hotel on my way up).. Does anyone know if Hobby King ships to Canada?
    I'll make sure to keep the rum drinking to a minimum!

  5. #5
    Can't help you out in regards to Hobby King but, an option for you may be to set up an account over at Great Hobbies located at the Trainyards not too far for you to make a pick up on your stop over in Ottawa. You can set up an account with basic info and just call in your order no credit card needed, pretty good folks to deal with. That's new to me that Michaels carries balsa, go figure. Another consideration is to build the hull so that you can switch to a nitro engine at a later date and electric for the great white north.

  6. #6

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    That's not a bad idea! Basically all I'd need to do is make the engine pod removable and build 2 of them! I'll have to give great hobbies a call! The wort thing is I only get into ottawa at 7pm..

  7. #7
    Sounds like a plan there vintage! Always nice to have alternative propulsion methods, I currently have two gas boats to build this winter that I may experiment with air engines Great Hobbies are open daily til 8 p.m. if I'm not mistaken or you could always have a package delivered to the hotel from the hobby shop or by taxi they'll pick up and deliver with short notice.

  8. #8

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    It's been a month now and I'm back to work, so here is a quick update on where I am with this boat. I have most of the frame completed now, just need to complete the coverings. I am still not sure what I am going to use to seal it... I've read that a 5 min epoxy and alcohol mix would work.. Any suggestions on what type of paint would work?
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  9. #9
    Good to see you back vintage. Good progress so far! From my understanding it's best not to thin out the epoxy, I'm sure someone else will offer their suggestion before long. What I've been able to ascertain is to just warm up the epoxy with a hair dryer or the like and this will allow it to flow quite nicely. Might I suggest installing whatever sheeting possible so that it will allow access to the all interior parts and at the same time it will be sealed and attached to the framing.

  10. #10

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    Quote Originally Posted by arcdude View Post
    Good to see you back vintage. Good progress so far! From my understanding it's best not to thin out the epoxy, I'm sure someone else will offer their suggestion before long. What I've been able to ascertain is to just warm up the epoxy with a hair dryer or the like and this will allow it to flow quite nicely. Might I suggest installing whatever sheeting possible so that it will allow access to the all interior parts and at the same time it will be sealed and attached to the framing.
    So, it's not really good to thin out the 5 min epoxy? I am kind of limited on what I can bring up here since I fly in.. I think they might frown on me bringing a can of sealent or a couple cans of spray paint! I do have access to some stuff up here though.. I can experiment with prop paint (only black white or grey). It's really tough paint but its not the best finish.. I'll see what I can do with sanding..
    The sheeting is going to be removable in the middle of the body (the cockpit part) and possibly the engine pod section if I do decide to make 2 pods. I think my biggest challenge is getting the C of G in the right spot!

  11. #11
    If all you have available is the 5 min epoxy your only option is going to be thinning it and considering your only sealing the wood and not using it as a finish surface you should be fine. What I'd suggest is a mix of no more than 10-1 with paint thinner or acetone if available or denatured alcohol at about 15% which should be adequate to seal the wood. When using epoxy as a fastener or for finishing by mixing it with other chemicals it alters the chemical base and will weaken the finish or cause the epoxy not to harden fully leaving the surface somewhat sticky. Your only sealing the wood so you'll be fine in this instance and only mix a little at a time. You could try placing a fan if available to help cure the sealant once applied, it may help dissipate the mix chemical quicker allowing the epoxy to dry. Cheers

  12. #12
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    I guess what I would suggest is to put sheeting on in a manor that will allow access to most of the innards so you can seal them with thinned epoxy. The sheeting that is left will need to be sealed separately on the underside and laid out in the rib chine pattern so you know where to put the glue.

    Im going to go ahead and say, test a piece of material using the thinned epoxy as a finish coat....just to see what it does. If you have spray equipment, you can spray thinned epoxy onto a super clean surface but if you for see problems, use fish eye eliminator in the mix,
    After all is said and done, more is said than done.
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  13. #13
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    Epoxy is best thinned with alcohol. Next best is acetone. Do not use petroleum based thinner or mineral spirits. 5min epoxy may not give you enough open time and thinning it too far will require several coats. I use a 60min finishing resin personally. It works well for sealing if heated up which thins it considerably. Finishing resin is at least 3 times thinner in viscosity than any of the assembly type epoxies that are thicker than honey.
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  14. #14
    crispyspa's Avatar
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    Quote Originally Posted by 1QwkSport2.5r View Post
    Epoxy is best thinned with alcohol. Next best is acetone. Do not use petroleum based thinner or mineral spirits. 5min epoxy may not give you enough open time and thinning it too far will require several coats. I use a 60min finishing resin personally. It works well for sealing if heated up which thins it considerably. Finishing resin is at least 3 times thinner in viscosity than any of the assembly type epoxies that are thicker than honey.
    He is limited as to what resources he has. I think he only has the 5 minute with him.
    I am kind of limited on what I can bring up here since I fly in.. I think they might frown on me bringing a can of sealent or a couple cans of spray paint
    I tried the "skim cote" method for the finish a couple of times, but I never got the hang of it. Mix up a very small amount of epoxy (a little goes a long way) and use an old credit card or a gift card to literally scrape the uncured epoxy into the wood. sand lightly between coats and put on many coats.
    After all is said and done, more is said than done.
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  15. #15
    Sorry m8 forgot about the paint. Considering your going electric, pretty much any paint will do, not sure what prop paint is? You can sand the paint with 1500-2000 grit wet paper and get a nice finish with auto buffing compound. I recently used Krylon spray paint for a nitro boat and it has worked well and is available just about anywhere, unfortunately that appears to be out of your grasp in light of your work situation. Depending on paint used I would strongly recommend thinning it to get it to flow out better and apply a few light coats to a thickness that it doesn't run.

  16. #16

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    Thanks for all the ideas guys! I do have access to krylon spray paint here.. Prop paint is an enamel paint I use for the propellers on the aircraft I work on (I'm an aircraft mech). It is extremely durable. I think if I do light coats and sand in between, that'll work.. I might try the skimming method of sealing as well.. Sounds promising..
    Last edited by vintageshack; 11-19-2014 at 10:46 PM.

  17. #17

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    Well, I got bored at work last night, and I realized that there aren't any pictures (that I could find) of a complete cobra! Sure, there are a couple videos, and part of a pic at the beginning of the plans, but no complete pics or drawings. So I took the cutout drawing, pieced it together, and erased some things to get this:
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    You can see from the pic that the prop is a little off, but thats what the plans call for, although maybe not quite that much.
    I am thinking about going white with a dark red for the decals, but it looks a little plain on the side view.. Anyone got any ideas?
    Last edited by vintageshack; 11-22-2014 at 07:52 PM.

  18. #18
    Hey vintage, for some reason the system didn't highlight your new post, at least not for me. Looks like a good start on the theme, you could always leave it as is considering your using decals then when an idea comes to mind cut out paper templates and tape them on to see how it'd look. How about some exhaust pipes out the side of the hood, a V12 should about do lol? Things always come out a little different looking when completed than when they are drawn on paper. How's the progress coming on the build so far, any internal sealing done, sheeting? pics are always nice to see how things are coming along.

  19. #19

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    I like the v 12 exhaust stacks idea! Sadly though, work has halted for another 3 weeks! I have some sheeting cut out but not attached.. The frame is complete now though.

  20. #20

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    Quote Originally Posted by vintageshack View Post
    I like the v 12 exhaust stacks idea! Sadly though, work has halted for another 3 weeks! I have some sheeting cut out but not attached.. The frame is complete now though.

    Some progress in the construction?
    Be happy is to stop being a victim and becoming an author of the history itself.

  21. #21

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    Not a great lot.. It's been pretty busy at work, but I have been able to cover the bottom with 3 mm plywood.. I will get a few pics of the progress once I go back to work next week!

  22. #22

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    What a cool ....
    I will wait for new pics
    Ever thought about the engine?
    Be happy is to stop being a victim and becoming an author of the history itself.

  23. #23

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    I am definitely going with an electric brushless motor, but I'm not sure what one yet.. Any suggestions for a good motor to use with a 10 inch prop?

  24. #24

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    Quote Originally Posted by vintageshack View Post
    I am definitely going with an electric brushless motor, but I'm not sure what one yet.. Any suggestions for a good motor to use with a 10 inch prop?

    Look: http://www.rcgroups.com/forums/showthread.php?t=830269
    Using this formula for a variety of common glow motor sizes gives the following results:

    .049 or .051 = 100 watts

    0.10 = 200 watts

    0.15 = 300 watts

    0.25 = 500 watts

    0.32 = 640 watts

    0.40 = 800 watts

    0.45 = 900 watts

    0.51 = 1020 watts

    0.61 = 1220 watts

    0.75 = 1500 watts

    0.91 = 1820 watts

    idea to discuss what would be the most appropriate

    This site has several indications: http://www.headsuphobby.com/Motors-By-Watts_c360.htm
    http://www.headsuphobby.com/Power-Up...otor-E-590.htm
    http://www.headsuphobby.com/Emax-BL2...otor-F-292.htm
    Be happy is to stop being a victim and becoming an author of the history itself.

  25. #25

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    Thanks, that seems like a helpful site! And they ship to Canada too! Well, except for batteries.. Not sure what I'm going to do for a battery yet..


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