Having trashed my Alpha 40 trainer airplane, I decided to use it’s EVO .46 engine with a 10/6 prop in an airboat just to design and build something different. I looked at a lot of pictures of RC airboats and found almost as many different designs, including a foam airboat that I liked, so I borrowed some ideas from that design. As I started working on it the week ending with the July 4th holiday weekend, I decided on red, white and blue as my colors. As of this posting, I haven't yet tested it to see how it performs. As soon as I do, I'll edit this posting. If you were at this web page before 7/21/08, you will see that some of the photos were changed as I rebuilt my engine mount and edited this page.
To get right to it, this is the finished airboat.
The overall dimensions are 12" wide, 22" long, hull thickness 2 1/2", cabin height 1 1/2". As I could only find the Dow Blue Foam insulation in 1/2” thick sheets I made the hull from five layers of the 1/2” foam. Having the 1/2” sheets turned out to be a benefit as my design had cut out areas of varying depths, so I could make cutouts in the individual sheets as needed before laminating them together. The top two pieces have an area cut out for both the electronics compartment and an open area to mount the engine stand an inch lower than the top of the boat, thus lowering the center of gravity. Just aft of the electronics compartment a piece of 1/4” plywood inset into the bottom of the 2nd piece to be used to anchor the engine mount.
The bottom layer is three pieces of foam with an electronics compartment cutout in the top two of the three pieces of foam that make up that layer. I hadn’t cut out the areas for the pieces of plywood that hold the blind nuts for the engine mount, so I drew them in.
Before joining both layers, I had to get my engine placement correct (I hoped). I glued a couple of pieces of plywood to the bottom of my plywood sheet to hold the blind nuts so that the top ply only has the small holes for the screws. Then I drilled holes for the 8/32 screws and installed 8/32 blind nuts on the bottom of the plywood pieces before gluing the plywood sheet into slots cut in the bottom of layer 2. This was after I had glued the three bottom pieces together. I had to cut out sections for the new plywood pieces to fit into (drawn in the previous photo). The plans will show those cutout in layer 3 before the bottom sheets of foam are glued together. This is the underside of the plywood sheet glued to the bottom of the top layer.
For gluing the layers together, I used Titebond II wood glue to glue the two-piece top layer together and the three-piece bottom layer together. I used epoxy to glue in the lite plywood engine mount decking, and I used gorilla glue to glue the top and bottom part together. I put on the glue very thin and weighted the two joined parts with a couple of concrete blocks to avoid having them pushed apart when the glue expanded.. This is after joining the two layers for a 5-piece layered hull (before and after shaping and sanding).
I decided that it would be less complicated to mount the engine to pull rather than push like a typical “real” airboat. The majority of RCer’s airboats that I’ve seen photos of are “Pullers”. I decided to make my engine stand from two 1/4” lite plywood upright pieces that also support the rudders. I attached two pieces of aluminum angle to the uprights to attach the uprights to the plywood deck with #8/32 round head bolts screwed into the blind nuts. as the engine mount. To mount the engine, I cut out a platform of 1/4" lite ply on which the engine is bolted. The platform is attached to two pieces of 3/4" x 3/4" x 3" blocks of wood that are attached to the side supports.
The fuel tank is fastened behind the engine on the engine platform with lock ties. It may not look professional, but it works for me. I had an 11 ounce tank, but decided to buy a smaller one, an 8 ounce Dubro.
I used platic wire holders to fasten my linkage and antenna tubing to the deck and uprights. The connectors are a little large, so I put small pieces of fuel line on the tubing to take up the slack and fastened the connectors around the fuel line to the plywood with small round head screws.
I made the “cockpit” or “cabin” as the cover to the electronics compartment. It’s held in place by a single #6/32 x 1 1/2” bolt in the center that screws into a blind nut on some lite ply in the compartment. The entire electronics compartment has a 1/2” high “lip” of 1/8” lite plywood around it and the cabin foam fits tightly around this lip to keep water out. Hopefully, the only way that water can find it’s way in to the compartment is if the boat gets upside down in the water, and then, not much can seep in.
In the electronics compartment, I made a servo tray that screws into a small block of balsa glued to either side so that I can remove either a single servo, or the en tire tray. The holes for the linkage and antenna tubes are sealed with silicone caulking. I used a switch with a charge port attached that was in my now defunct RC Alpha 40 airplane and the two Hitec servos and 4-AA battery pack that came with my Hitec Ranger 2N radio. I tucked all of the excess wiring under the servo tray.
I made a cover for the battery and RX to keep them from coming out of their recessed areas. The cabin cover will hold this piece in place.
The foam and few wood parts were covered with two coats of Envirotex 2-part bar-top epoxy that I had on hand. This stuff is well over 10 years old, but had never been opened, and it still works…UNBELIEVABLE?…but true. There are some tricks to using this stuff and I have a post on the RCUniverse Boat forum about it. It’s well worth reading about my experience with it. http://www.rcuniverse.com/forum/m_7707234/tm.htm.
The epoxy can be sanded some to smooth it, but the smoothness comes from being really careful when applying it and avoiding runs (hard to do, but can be done with patience). I used whatever brand of spray paint I had on hand, from Rustoleum, to Krylon, to Dollar General spray paint.
I used some 1/16” twisted cable I had for the flexible throttle linkage and installing that was probably the most difficult and tedious part of the whole project. I used some crimp tubing to solder the cable to pieces of linkage wire…threaded for a qwik link connector at the throttle and a fixed piece to connect to the servo. As the crimp tubing wouldn’t go through the plastic tubing, I had to solder on the servo end, then measure and clamp the tubing to bend up and forward to the throttle linkage. Then I had to measure and cut the cable to fit, remove the plastic tubing from the engine mount, bend it down and solder on the kwik link connector, then reattach it to the engine mount before connecting the qwik link connector to the throttle. Whew, what a job that was.
I didn’t have enough room in the servo compartment for a straight piece to the servo arm, so I had to run the wire linkage piece forward of the servo and bend it back. You can see this in the photo above of the electronics compartment. The rudder linkage was much simpler with just a piece of linkage wire through some plastic tubing with a qwik link connector at the rudder horn.
If anyone wants a set of simple drawings of my design to use as plans to build one, just email me at cracker39@yahoo.com and I'll email them to you as jpg images.
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