Eric's R/C BOAT RACING

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by Eric Perez

Well, folks here is the big article about building this "small" boat. I've had a good time designing, building, and testing this boat. Even though this is the first prototype, I found that with some minor modifications the boat ran quite well. This was my first "Micro" boat experience and I've found it to be very rewarding. Total project cost was a little more expensive that what I had envisioned the "Micro" concept to be. The only good news was that all the items that I had to buy for the project can be carried over to my future boat projects.

Basic Design Notes for the Micro Aggressor 1.0

Based on my previous experience with other Cat hulls primarily Jay Turners XL CAT I found that my cat needed to be more stable in rough water. This was my primary design goal. On top of that a micro hull is quite small and light so anything other than glass water will seem like chop in small boats. Cats seem to handle varied water conditions quite well and with the proper adjustments to center of gravity and prop angle you can adjust the hull to run in different water/wind conditions with good success. That's why I chose a catamaran hull for my Micro boat.

Hull Design Basics 1.0

So with the fact that I wanted down force with the least possible amount of drag (Low profile) came the "aggressive" profile of my boat. The hull has a 2:1 ratio the hull is 15" long (less hardware) and it's 7.5" wide. The Tunnel depth and area are proportional to Jay Turner's XL Cat. I did a lot of research for this project and I borrowed on things that I knew would work and on other hulls that I had purchased. I also studied the Fast Electric/Nitro Boat literature ( Paul Williams, Jay Turner and John Finch). Included in the fray where a hand full of photos that I got on the net and from articles in RCBM.

So armed with all this knowledge I started coming up with all these hull concepts. Some of the sketches where simple to the point, like where the motor was going to be. Others where more elaborate like wire routing, water cooling routing and sponson construction. Slowly everything came together and I got the basic layout in CAD software. The Motor was going to be mounted between two rails in the center of the tub. alongside the rails where two channels where the batteries where placed. The channels would allow me to slide the batteries back and forth to fine tune CG placement. Really only after I had purchased some of the components, speed 480 motors and the Micro Servo would I be able to go to the next step.

Once the CAD plans looked good I printed the templates on my Ink Jet printer and went to work. The first tub was merely a skeleton for laying out the guts of the boat. I also used this to set the CG of the boat and to determine where the motor was going to be. After everything fit I made more changes to the CAD plans and made a foam concept boat.

This was my first time making my own wood over foam parts from scratch. The truth is I wish I'd done this sooner. WOF is a great tool for any boat builder, parts are more uniform and a lot more repeatable than with the bulkhead and plank building technique. The fun part was that the templates for cutting the foam where latter used as part of the boat structure so there really was no wasted materials in the process. 

Boat Construction

Overview: The boat is built out of three basic sections: The Tub, the left sponson and the right sponson. The parts are individually covered with ply and pre-finished and latter are epoxied together. The result is a straight and very strong hull.

Building the sponson:

The sponsons are cut from 2" styro foam (White Foam) using the inner left and right sponson templates as guides. Make sure the two plates match (Hold them together with a clamp and sand them together until they are identical twins. Now take a square piece of 2" X 2" thick styro foam that is at least 16" long and place it between the templates (1/8 hard balsa). Use a straight block or piece of square aluminum stock to align the bottom of the templates to the foam block. Now use a hot wire cutter and cut the top section of the sponson. Rotate the clamps around and cut the bottom of the sponsons. Repeat this with the other piece of foam.  

After you're done you should have two identical sponsons with a flat inside (tub side) and outsides. The Dihedral will be cut using the dihedral template. It's basically a smaller version of the templates that you used for making the sponsons. Now take a small ruler and place the dihedral template on the side of the sponson follow the dimensions on the plans. The critical dimension is on the bottom because it will determine what the dihedral will be set at. The current plan call out for either a 10 deg or a 10 front /5 rear degree dihedral angle. If you are running a 400 or 480 run the 10/5. If you are running a cobalt or brushless motor then 10 might work best, I didn't get real good performance with the heavier high capacity AA NiMH batteries needed to run high power systems -But I never had a real good motor or nice NiCad batteries to try them with. Let me know how it works out for you. At this stage you will now have a left and right sponson (don't get them confussed)

Now you have three of the four cuts needed to finish the sponsons. We will go over the last step after we have installed three skins on them. The first piece to epoxy to the foam is the left template( 1/8 Balsa) on the left sponson. 5 min epoxy works best for this, spread it thin with a 1/32 1" wide piece of scrap ply-wood on the inner side of the sponson (side that faces the center hull). After the epoxy sets 10-15 minutes, you lightly sand the top and bottom of the sponson to make the template and the foam blend into each other. We want to avoid rough transitions from the foam to the edge of the template or the 1/32 ply sheeting will have gaps in the edges.

Now cut the bottom cover of the sponson with 1/32 ply. Make the part longer than you need to help you position it.You can use 1/64 if you want to keep weight down to a minimum. There will be a natural tendency for the wood to slant right because of the bend. This is okay just square up the back and rear inside and let the front go where it wants -we will trim the overhang latter.

After 15 minutes or after the epoxy sets take a sharp hobby knive and score the inside edge of the overhang. Either fold the edge over or continue to score until you remove the overhang. Sand the edge flat against the template with a sanding block.

Now you're going to cut the front overhang at an angle to match the tip of the foam. Leave a 1/16 to 1/8 of wood past the end of the foam so that you can join the front together. Use the template for the top piece to cut out the outer curve. Leave the nose flat, we will trim it after the glue has set.  Place the 1/64 ply top plate (square) on the top and square the rear.

Now score the inside over hang and the sand flush with the template. Now cut the excess wood on the tip and sand the edges till they are smooth.

Okay we now have a sponson with three pieces of wood and some ugly foam sticking out the end... You guessed it, we now take the hot wire cutter and we use the top and bottom plates as cutting guides. The picture shows the part that we cut off. Sand the edge smooth and glue on the side 1/64 piece. Score and cut the overhang -and sand flush...

Cut and sand the rear of the sponson flush with the foam, and epoxy the rear plate. Cut, sand till it's flush...

The main hull (TUB)

The main hull is constructed out of 1/8 hard balsa sides with 1/32 ply bottom and 1/64 ply top. The front nose is styro foam. All the bulkheads are made from 3/32 balsa.

We cut a small piece of foam 3 7/8" wide about 3" long and 2" thick. We now use the front portion of the tub sides as templates for cutting the foam. We use the same technique as with the sponsons.

Note: If you want to mold your own Micro Aggressor hatch then cut out an extra foam center hull. Take a foam block that's 3 7/8" wide about 14" long and 2" thick. It will only take a couple of minutes. Use the templates for the center tub and a hot wire to cut out the foam.

After the foam nose is cut we cut the 3/32 balsa bulkheads. Mark the tube sides 2 1/2" from the rear and make a straight line with a square. Measure the height of the tub in this location value (X1) and cut a piece of 3/32 balsa the same height as the tub (X1) make the piece of balsa 3 7/8" long. Now repeat this measurement for the rear of the tub sides, lets call this (X2). Cut a piece of 3/32 balsa (X2) tall 3 7/8" wide. Now cut 2 3/8X3/8 hard balsa sticks (X1) long and 2 more that are (X2) long.

Assembly of the center hull skeleton:

Take the center bulkhead and glue it to one side of the hull side (1/8 balsa) Use the piece of blasa stick to help keep it straight and to give it strength. Glue the transom on the other side of the hull. Now we will glue the balsa sticks, use scrap pieces of 3/32 balsa to line everything up. Now put both pieces of the hull together. The bulkheads and the balsa sticks make a jig of sorts to help keep everything straight. Make sure the hull side pieces are completely vertical and that the bulkheads are joining up with the square balsa pieces. Now glue the assembly.

The next step is to epoxy the nose to the skeleton center hull. Make sure everything is lined up and let the epoxy set.

The next step is to attach the 1/64 ply on the front bulkhead. Measure the piece and cut it slightly undersized so that it won't extend past the top or bottom of the foam nose. Use 5 min epoxy for this step. After the epoxy sets cut a piece of 1/32 ply slightly longer than the tub and about 4 1/4 wide. Leave 1/2" over hang at the front and rear of the tub. Use 5 min epoxy for the bottom of the tub.

Now lets work on the center and rear bulkheads. Take a piece of 1/32 ply and laminate the inside of the transom use thick CA or epoxy. Now laminate the rear of the transom with a piece of 1/16 ply cut it out 4 1/8 so that you can tie in the side plates along with the 1/32 tub bottom together for strength. Use 5 min epoxy or thick CA to laminate the balsa. Lets trim the overhang of the bottom tub so it's flush to the transom. Also trim the sides and leave about 1/16-1/8 on the front tip beyond the foam.

Now we strengthen the tub side by gluing a 1/8 X 1/8 strip of bass wood to the top edges of the tub. You can use 1/8 balsa strips hardened with thin CA as well. Run from the front bulkhead to the center bulkhead. Also run some 1/8 bass wood strips along the inside of the radio box. This will strengthen the tub and will give better support to the 1/64 ply top. Sand the top of the hull, and make sure all the 1/8 wooden strips are flush with the top. See picture above.

We are getting ready to cover the top nose of the tub. You may need to place a few 3/8 wide 1/8 balsa strips on top of the tub after the foam ends. These pieces of wood will help keep the wood from bowing inwards when you bend it over the top of the tub. The strips of wood should be flush with the top of the tub. You can use thin CA to harded the strips of wood to help keep them from bowing. Have the 1/64 nose ply section over lap the last support piece by about 3/8 of an inch so that we may glue a 1/32 strip to support the hatch. Leave some overhang on the nose area and epoxy the top 1/64 ply to the foam and supporting structure. You can use rubber bands to keep pressure on the plywood while the epoxy sets.

Trim the wood around the top nose piece and sand flush.

Hardware Planning stage:

At this point you basically have a blank canvas for hardware installation. I prefer to layout the drive-line and install the radio at this point. It's so much easier to work when everything is exposed especially with such a small radio box.

It's here that you're going to start to either build or purchase the hardware set. Keep everything as small and light as possible, go with aluminum as much as you can. I choose common 1/8 stub shaft .098 flex cable drive. It's light, simple and parts are readily available from a multitude of sources. I get most of my parts from Octura Marine Supply. Most hobby shops don't cater to the Micro Boat enthusiast so your best bet is to go on the net and check some of the bigger sites for links. Make sure you mention that the hardware is for a micro boat.

Here is a pic of the hardware set used for my project

They sell laser cut speed 400 motor mounts in the slow-flier airplane section of your hobby shop, it's probably the easiest way of making a motor mount. The pictures are self explanatory. My mount is made with 1/8 ply (overkill) but I only support it at the bottom. If you go with lighter ply you'll have to make a triangular mount to support the bottom and sides of the motor mounting plate.

If I could re-design this mount I would go with a sandwich pinch mount so that I could slide the motor out of the hull just by removing one screw. Another alternative is to use a slotted 1/16 aluminum plate, like in the Thunder CAT boat.

Cut a 3/8 X 3/8 X 3 7/8 balsa stick. Glue it against the bottom of the middle bulkhead, this will help support the stuffing tube.

When you make the rails that go along side the engine mount if you connect the front bulkhead to the center bulkhead with them you basically bulletproof your drive train. It will make the center tub very strong. You may even use 1/64 ply for the bottom of the tub because the motor mount will be supporting most of the load.

Radio Installation:

For my steering chores I used a Mighty Micro Hi-Tech servo. It's overkill for a 15" hull, but it can see other uses for other boat projects, even non-micro boats. If you want the best performance you need to put the boat on a diet. A mini or sub-micro servo should do the job just as well.   

For control I chose a JR receiver. It weighs in at .9 ounces, remove the plastic case and epoxy the board (water protection) and the receiver is twice as small and only weighs .4 ounces. You can do this on most receivers even the large looking ones, and come up with a decent size unit for Micro boats.

For speed regulation I chose a Castle Creations Pegasus ESC. It can deliver 35A with proper cooling. Slightly overkill for a speed 400 boat -but it only weighs .8 ounces with 13AWG wires/connectors and a water proof coating. To cut down a bit on the weight I would install lighter gauge wires, but still .8 is not bad. Smaller 20 Amp ESC will weigh slightly less but will generate more heat, thus you loose efficiency. Another thing to remember about Castle Creations ESC are that unless it specifies as being (Marine) on the packaging then it's going to be a pain in the butt to set it up on your radio because they are programmed as airplane ESC. I was lucky that my computer radio lets me do stuff like 125% travel and other hocus pocus stuff so I'm able to make it work.

Power plants:

For the hull I purchased a Graupner speed 480 7.2V Race motor. It has a different mounting dimensions as the more popular speed 400 and speed 480 sealed can motors. So you might as well drill out the mounting plate for both type of motors. powering the motor is an 8-cell pack of 500AR NiCad batteries. I've tried the NIMH AA batteries and they just don't have the punch that the 500AR have, they also weigh more. I'm going to try some high capacity NiCad AA cells to see if performance is still good. I choose the Octura plastic X431 prop. It's inexpensive and easy to cut down if you need to.

You can also use inexpensive 600AE cells

Making a second skeleton hull actually helps a bunch in this area. It will let you do CG calculations and will let you layout all the components so you have a clear picture of what needs to go where to balance out the hull.

Now that you have a good idea where everything needs to be take out the hardware and lay it out on the bench. Start by gluing the motor mount/ chassis stiffener to the center tub. it helps if you make pencil lines on the tub floor to help you line up everything. Now install the motor on the mount along with the flex adaptor. cut a piece of .098 cable and stuff it into the flex adaptor. You want the piece of flex to be long enough to reach the center bulkhead. Mark the point where the stuffing tube needs to be and drill a 1/8 hole at an angle to match the angle of the motor. Take a 1/8 drill bit and push it through the center bulkhead and mark where the drill bit hits the radio box floor. Now take a dremel and make a slot along side the 1/8 hole in the radio box. This will allow for room to position the radio box.

Don't feel bad if everything does not line up perfectly, the holes will be enlarged latter to allow for the epoxy to flow around the stuffing tube. If you don't have a bunch of experience installing hardware, I would suggest you run teflon tube inside the stuffing tube. It will make adjustments easier -but it wont be as strong as a brass tube. You can heat the brass tube with a torch an let it air cool to make it easier to bend.

At this point we are ready to install the strut bracket on the transom. I made my own brackets out of aluminum angle, you can make them out of 1/16 angle to, but they will have to be a bit wider than mine. Drill clearance holes for the screws on the bracket, three or four will do the job. Drill out 3/32 holes on the wood and screw in 1/2 4-40 cap head screws. Remove the screws add a drop of thin CA to the hole and re-thread the screw. 

I usually mount my strut on the center line on the right side of the bracket. This way if you need left rudder to have the boat track straight you could just swap the strut to the left side of the bracket to make the prop offset to the left. This will help point the boat forward.

Now remove the stuffing tube and strut and lets work on the rudder.

Install the servo as shown make some type of hardwood mount for the servo ears. It's real tight in there but you could use cap head screws to fasten the servo. Servo tape or a thin bead of silicone adhesive will also work. I made a small wooden box around my servo to help keep it in one spot. The wooden box also acts to reinforce the tub floor to prevent flex from the 1/32 plywood from affecting steering response.

Now lets follow the same procedure as before to install the rudder bracket. You may need to move the bracket up or down a bit to help align the servo arm to the rudder arm. You can always bend the linkage a bit to make up for  some misalignments.

Now take a piece of control rod and place it in the rudder control arm clevis, mark the position the rod must pass through the transom. Make a 1/8 hole (may need to be enlarged depending on the linkage used) and connect it to the servo horn. I used a clevis lock on the servo horn and a quick link on the rudder arm. Try to make it as simple as possible. Remove the strut and rudder brackets from the boat.

We will finish the drive line installation after the top of the radio box has been installed.

Radio box top:

Take a couple of 1/2 strips of 1/32 ply and arrange them so that they cover the perimeter of the radio box. cut an oversized radio box top out of 1/64 ply, leave 1/2 of overhang over the center bulkhead. Cut the radio box opening in the center of the 1/64 ply piece. Take into consideration that you have 1/2 of . Glue the 1/32 strips to the underside of the 1/64 ply to creating a small ledge for the cover. After the glue sets, install the top of the radio box on the tub. Use thick CA or epoxy to glue them to the boat. Trim the excess from the sides and on the transom and sand flush. Cut a piece of 1/2 wide 1/32 ply and install it under the lip of the radio box top to create a ledge in the center tub. 

All that is left now is to install thin strips of 1/64 ply to the top of the tub in the motor area. This will connect the front top (nose piece) to the radio box top. This completes the building of the center hull section.

Now we need to make a jig to connect the sponsons to the center hull. Use 5 minute epoxy and line up everything. It helps if you support the center hull section with a square balsa jig that has the same height as the tunnel depth in the plans. It will aid in keeping everything square. Now epoxy the other sponson. Let the glue set.

At this stage I would finish the hull with thinned finishing epoxy. Use 1 part 20 min finishing epoxy and 1 part denatured alcohol. Brush it inside and outside the hull. If you want your boat to have color you can give it a light coat of spray enamel before you use epoxy or color the epoxy with RIT clothes dye. The epoxy is not just cosmetic it will soak into the wood giving it more strength and it will bring up the grain in the wood. If you decide to pre-paint the boat let it dry for a day or two before you cover with epoxy. Usually after the first epoxy coat is already set( but not cured) brush on a second layer to touch up on thin spots or to add more shine. This is also a good time to inspect the hull/sponson joint area. Make sure that this is sealed all the way around. You may need to add some more epoxy or thick CA glue to seal the gaps.

After the epoxy is set re-install the hardware. Lets make the connection from the stuffing box to the rear of the strut, with either Teflon tube or brass tube. Once you have the cable and the strut in place and everything is lined up epoxy the stuffing tub to the hull with 5 min epoxy and CA. At this point you should also install the rubber boot on the rudder linkage to keep water out. a little Vaseline on the control rod will keep the tip of boot from tearing.

There is still some more things that need to be done, water cooling, wiring, hatches and setting the CG.

CG adjustments

At this point you should install the flex cable and grease it up. After this sets install the servo, receiver and the motor. Now lets set the CG of the hull. Make sure you have everything in the hull, batteries, motor, receiver, servo and ESC. The hatches are quite light and will not affect the CG much. All you need to do is to slide the batteries back and forth until the hull balances at the desired location. Use pieces of foam to keep the batteries from moving in the channel.

Note: Make sure the motor is fully broken in before you set off to break speed records on the local pond. These small 400 size motors are stressed to a high degree in our boats, you need all the brush contact you can get. The motors are only 60-70% efficient all the rest goes into heat that's ready to melt the brushes and burn the motors commutator.

Since micro boats run more than the 1-2 minutes like the sprint boats, then we need to keep the motor cool. Make a coiling  coil out of 1/8 or 5/32 aluminum tube. Wind it around the area of the motor without the torque ring. Now slide it over the torque ring. Aluminum tubing is soft, so you won't get a very tight fit, but try to get it as snug as you can. You can smear white heat sink gel between the coil and the motor to help heat transfer. Make a hole on the leftmost side of the radio box and route another tube in there to catch the prop rooster tail. I made the water exit through the left sponson, you can dump the water anyway you want, but the shortest route will be the best. Connect the aluminum tubes with silicone fuel tubing.

The hatches are cut from thin plastic lexan sheets. You can use any other thin flexible plastic. The hatches fit into the recessed portions of the hatch openings. Secure the hatch with radio box tape or electrical tape. glue a short 1/4 piece of wooden dowel on the radio box hatch and drill a hole for the antenna and wire to come up through.

Carbon Fiber Hatch article will be available soon!

Initial setup:

Set the prop shaft slightly above the bottom of the rear sponson and kick it back a bit to help lift the nose of the boat. Charge up the batteries!  At this point you call all your friends so they can witness the bathtub float! check for leaks around the stuff tube exit. Make sure the prop is spinning the right way.

Pond testing:

Set the strut angle and depth so you can keep your speed on the turns and keep the boat at a positive angle of attack. If the boat flops up and down on the front and wants to blow over reduce strut angle. If the strut is parallel with the bottom of the tub and still is porpising then move the CG forward.

Have fun!

Hope you enjoyed building the Micro Aggressor.

Eric Perez

Team NitroRC.com