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.40 Size Airboat Build : With Questions....

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.40 Size Airboat Build : With Questions....

Old 03-28-2016, 04:42 PM
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NEW222
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Default .40 Size Airboat Build : With Questions....

Hi there. This is my first 'big' airboat, and I would like to do a build thread, along with questions should they arise along the way. Well, I will be honest, there will be a question near the end of this post. Anyways, the build is going to be out of 1/8" Baltic Birch Plywood, as that is all I could find around here with a reasonable price. Anyways, I cut out and sanded all of the parts so far, with the exception of the upper deck. In the pictures, it may not be visible, but the uprights at the engine mount have been doubled up, grain at 90 degrees from each other, and clamped and glued with 15 minute epoxy. There was a cutout on the plans on the rear edge, but I had seen no reason for me to cut it out, so I did not cut it out. If someone here knows why it was there, or more importantly why it is there, please let me know and I can still cut it out. I have not yet glued anything, and the pictures show only a test fit of the parts. Upon test fitting, I tried to bend the plywood up along the keels, but could not do so, as it was VERY stiff. The main hull is also with the grain running from front to back. So without success, I decided to clamp it at the point it starts to bend with a 2 x 6 under the bow a ways, then added a damp facecloth to hopefully soften it. If it works out, I will move it a bit more forward tomorrow to get a nice curve to the plywood. If anyone here also has a comment, or suggestions of how to do this, please post it as this is new to me. Thank you and enjoy. The build will be sporadic this week as I do have things going on followed by holidays where I will keep trucking on this.
Old 03-28-2016, 04:55 PM
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Now I'll try to add the pictures... After all that typing, I did not want to lose it by trying to add pictures.

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Old 03-30-2016, 02:33 PM
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Ok. Time for a little update. I have just finished bending my plywood. First attempt yielded me to make a second bend. There is a slight wave in the plywood as I had to go more forward for the second bend, but it is minor. Anyways, things did go according to plan for the rolling of the plywood. So, here it is in a mocked up configuration. Nothing is yet glued, and may attempt to start tonight. The only problem with the bend/radius, is that in the bottom of the curve, there is a slight gap, and it does not make 100% contact throughout. I am hoping this will not be a problem. I will use epoxy, and fill the void with it. I will also apply some fiberglass, or something to strengthen the area anyways. As I am having a hard time swallowing the price of the epoxy coatings, I will in turn use Minwax Polyurethane, although I am a fair ways away from that stage. Anyways, a few updated pictures for your enjoyment.

P.S. Would the cut-outs in the engine pylon be for air to flow through to the rudders? I will cut them out anyways, as they are on the plan.
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Old 03-31-2016, 05:34 AM
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Great work!! I'll follow your build.
Old 03-31-2016, 01:03 PM
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Thanks. I will be at a stand still till later next week as I am low in epoxy and won't be getting more till next Monday or Tuesday. I will truck along with what I have left, but it won't be much.
Old 04-12-2016, 12:20 PM
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I guess it has been a while since my last update, but not too much has been done as I was away on holidays. But, the hull for the most part is completed in the rough. It needs the edges sanded/ground, followed by installing my engine and rudder pushrods and the upper deck being cut, fitted, and installed. The weather back home here has been snowy, cold, and windy, which put a hold on things for me. But, I am going to attempt to fire up my airboats engine tomorrow, followed by doing a test fit on the boat itself. On such an airboat, could the fuel tank go below the engine, or is it best to use a smaller tank and install it behind the engine. Thank you.
Old 04-12-2016, 12:20 PM
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I will try and get a picture up later this evening.
Old 04-12-2016, 04:25 PM
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As promised, pictures and not much done. I had run out of epoxy just before leaving on holidays to the US, so I did glue the side pieces on with Gorilla Glue Wood Glue. It seems to hold very well with no tendency to separate and is strong. I do hope this is alright. I am now at the point that when the weather warms up and my dad can get his tablesaw out I can cut and test fit the top deck. I will however not glue it on until I seal the inside and install my pushrods and test fit my electronics. This is my first airboat and now have a couple more questions.

1. I do not have epoxy to seal the interior with, and due to using 15 minute epoxy on the guts, I cannot use my polyester resin. I am not sure of using polyester resin by the wood glue though. Anyways, I do have Minwax Polyurethane that I was going to put a few coats on inside to seal it. Good, or bad?

2. I do now have 30 minute epoxy so when gluing on the top deck, should I use that, or is my Gorilla Glue Wood Glue good enough? I will however seal the inside compartment of electronics with epoxy for added safety.

Thank you.
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Old 04-12-2016, 04:27 PM
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Don't mind the burnt wood. I used a Dremel cut-off tool to trim the excess off. When it warms up outside, I am going to use my grinder to CAREFULLY grind it close to smooth then finish up by using my sanding block.
Old 04-14-2016, 07:15 PM
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I'd use a sanding block and rough grit sandpaper if it were me. Like 50 or 60 grit. I do natural finish on mine, so I avoid sanding through the veneer if I can help it. If you're going to paint it, then it doesn't matter. Using a grinder is risky business IMO.
Old 04-14-2016, 08:41 PM
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Thanks. I may try something coarse, but I think the grinder will be fine. It is my favorite air tool and use it daily for a living and have complete control over it. I should specify that it is an air powered angle grinder with 3" pad and a 36 or 50 grit disc. I can control the speed very well. I will however give the sanding block a try first and see how it goes.
Old 04-14-2016, 08:45 PM
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Didn't want to paint it after seeing how nice yours turned out, but this is my first and will paint it. My next one when I start will be done more carefully and hopefully a natural wood finish. For the next one I will try to find an old house door or closed door for paneling for the hull as I think it may be easier to bend. Do the Selph Inflicted boats you built use 1/8" or 1/16" for the bottom. I found the 1/8" Baltic Birch tough to bend to shape carefully, and think if I had something a bit thinner it would help in the bending process.
Old 04-17-2016, 04:55 PM
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Hi there. Just a little update. Yesterday evening I had time to seal up the sides to lower sheeting joint. I mixed up epoxy and did this. This afternoon, I had coated the whole inside with Minwax Polyurethane Varnish as a sealer for the wood. I will give another coat tomorrow. Weather and time permitting, I will install my pushrod tube for the rudder, and then decide on what to do for the throttle. I am debating on buying a cable for throttle, or putting a servo on the engine pylon and running a servo extension. Nothing major so no new pictures.
Old 04-18-2016, 04:21 AM
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For the SI Tapered Flatty I used 1/16" ply for the outer sheeting. All of the framework is 1/8" liteply. The plans spec for 1/4" ply for the main runners/engine stand/rudders, but I had 3/16" on hand so I used it.

The doors I took apart to use the plywood from had 1/8" birch ply on them. I used a floor sander (my trade for the last 16 years is wood flooring) to sand one layer of the ply down so it was thin enough to bend but still retained some rigidity.

Use a throttle cable from your servo to the engine. This way you can keep your electronics in a sealed compartment.

Sorry for my late reply. Life is a little hairy right now.
Old 04-18-2016, 04:29 PM
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Thanks for the reply. I will probably be at a stand still for a couple weeks till I can get to the hobby shop to pick up a throttle cable. Oh well. I was kind of thinking exposed electronics may be a problem.
Old 04-18-2016, 04:32 PM
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Ok. When I get to the SI Tapered Flatty, I will try to source out something thinner for building then. It makes perfect sense. It was tricky to forming the 1/8", but I managed, albeit a bit heavy. I guess I will not have a speed machine, but just being able to drive something I built will be fun enough. Tomorrow it looks like I will cut some foam to put inside the hollow cavities.
Old 04-18-2016, 04:34 PM
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Stupid me. I did add the last good coat of Polyurethane today as well as rough cut the top deck, cut the rudders (5 x 3), and am in process of laminating my engine mount as I type the epoxy is drying. I assume the balance point should be roughly 1/3 total length forward of the transom?
Old 04-18-2016, 05:43 PM
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Originally Posted by NEW222
Stupid me. I did add the last good coat of Polyurethane today as well as rough cut the top deck, cut the rudders (5 x 3), and am in process of laminating my engine mount as I type the epoxy is drying. I assume the balance point should be roughly 1/3 total length forward of the transom?
1/3 is a good starting point. Plan on being able to move your engine fore and aft to get it just right. Some boats run great as setup during the build and others need a lot of tweaking. It depends on a number of variables. But yea, start with 1/3 hull length from the transom.
Old 05-13-2016, 04:02 PM
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Sorry for the lack of updates, but things have been very busy here lately withe yardowrk, maintenance, and spring cleaning. Top that off with the better half requiring attention after a small surgery. Follow that by losing a computer and all on it. So back to the topic at hand. I have the rudders attached in mock-up fashion, control rods were salvaged last weekend. I had to add 1/4" to the engine mount plate to raise it to clear the deck by approximately 3/8". I now have the tools to make channels for the pushrods, and if all goes well and I get some free time, my goal this weekend it to have the deck glued in place. I will be trying tonight to mount the electronics. Sorry for the slow build, but things are as they are.
Old 05-13-2016, 05:16 PM
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Is all good dude. Just keep up the good work and post pics when you can. I'll be cleaning my hobby room and getting some projects finished and finish repairing some of my other boats.
Old 05-15-2016, 10:36 AM
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NEW222 - Sorry to hear about your computer - I am looking forward to your updates.
Old 05-16-2016, 04:41 PM
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No major updates, but some stuff got done. On the weekend, I got the servo rails installed. Then this afternoon after work, I got the top deck 3/4 epoxied down. Reason for it not all getting glued down is I used up all my clamps and weights on the edges and center deck leaving the rear center un-clampable. I used BSI 15 minute for this. I am letting it dry good overnight clamped and weighted down, then tomorrow afternoon, I will glue down the rear center section from the front of the engine pylons back and wait till the following day. With a busy week this week, I will try to get the hull ready for glassing, but realistically next week it will happen.
Old 05-18-2016, 06:10 PM
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Ok, just a little more to report on. Along the edges of the top deck, there were a few small (less than 1/16") gaps between the hull sides and top deck. Yesterday, I used up some of my 5 minute epoxy and filled those holes along the sides, as well as the front edge of the deck where it joins the hull. My next step to do is going to be to grind away all the excess plywood from the rear, and side edges of this thing, then give it a quick sanding. This will be proceeded by cutting out my servo/radio access from the top deck and installing my pushrods and engine plate on top of the pylons. Then the fun part. Finishing.
Old 05-18-2016, 06:18 PM
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And now a question regarding the finishing. I do not have any fine fiberglass cloth, so was wondering what would be best to use from what I have on hand. Please keep in mind that this will not be something crazy, but a run or two every couple of weekends. My options that I have available to me are:
Minwax Polyurethane Clear
Polyester Fiberglass Resin
Also, how many coats of each would be required for finishing? Thank you.

As of right now, the total weight is at 5lbs 1oz. This is the bare wood hull with 2 servos, 2 control rods, 4 cell NiCd battery pack, engine, and the excess wood to yet be removed. Is this an acceptable figure, or will this be a dog? Thanks again.
Old 05-19-2016, 10:18 AM
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If you're using Fiberglass cloth, then use the polyester resin. If you're not going to use cloth, then use the polyurethane.

5# is reasonable... If you can trim some weight, it will run better. If using a .40, I'd use something like a 10x4 or 10x5 prop max. I don't think a 9x5-9x6 is gonna be enough thrust.

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