Introduction: A Better Rocket Boat
For the past decade my brother and I have kept alive a tradition started by my cousins and I when we were younger. This tradition involves fastening model rocket motors to a simple wooden boat and launching it across a lake. Although these boats started off as quite simple affairs, over the years we have been developing better boats to overcome the limitations of their successors.
Last year I produced the most complex boat yet hoping we could get a great run across the water. However, things did not go quite as planned and the boat ended up flying as easily as it floated. When I posted my experiences here on Instructables and on Youtube, I ended up receiving tons of comments on ways to improve my boat.
Step 1: The Design
As I read through all the comments several themes emerged, which I thought made a lot of sense. On my previous boat I had attempted to keep the boat on the water through the use of small angled planes under the water. While this kept the boat down, it also introduced a lot of drag. The alternative suggested numerous times was to use a wing (airfoil) at the front of the boat, which would help press the boat onto the water without significant drag.
The second issue I needed to solve was that of keeping the boat running straight. At least one person suggested placing a rudder a bit rearward of engine. This rudder would introduce a drag point, which would keep the boat pointed forward.
My initial idea was to completely build a new boat, but as the launch date was quickly approaching I realized I could simply modify my existing boat. This would save considerable time and some money too. This is the reason why for this project I will be picking up where I left off last year. If you want to see how I built the original boat you can check out that Instructable here.
As with the original boat, I designed my updates in Sketchup. In addition to the adjustable wing and rudder, the updated boat was designed to fit my homemade sugar rocket engines instead of D or E Estes-type engines. Since the sugar rockets have a slightly larger outside diameter, I determined that I would need to replace the original engine tube. I decided to go with a steel tube as I thought this would help protect the boat in the event of a catastrophic engine failure.
Step 2: Removing the Old Hardware
The first step of this re-build was to remove all of the old hardware from the boat. This included the two forward keels and the engine tube. After removing the engine tube, I could see that the ejection charge from the motors had actually burned through a portion of the hull beneath the tube. I fixed this hole by coating it with a standard two-part epoxy.
Step 3: The New Steel Parts
There were a total of five (six if you include the engine tube) new parts which needed to be made for the boat. I elected to make these parts from 16 gauge steel sheet. Steel will hold up better in the rare circumstance where the parts and engine exhaust happen to meet. However, steel does have its drawbacks - primarily in the additional weight over aluminum. To help minimize weight, I decided to cut away as much of the metal as possible, which lead to lots of internal slots and holes in the finished parts.
I designed the steel pieces in CorelDraw, printed them out, and traced around the paper templates with a Sharpie to transfer the designs to the steel sheet.
Step 4: Cutting the Parts
After the part shapes were transferred to the sheet, I cut them out using a combination of a hacksaw and jigsaw with a metal cutting blade. The jigsaw ended up being the easiest method for making the somewhat complicated cuts and it also allowed me to make some of the internal cuts by dropping the blade through a hole drilled in the sheet.
Step 5: The Rudder
The rudder (or rudder assembly) ended up being a 3-dimensional part due to two 90° bends made in the finished part. After smoothing the edges of the rudder with a file, the rudder was placed in a vise and the first bend was made. This bend creates the perpendicular part of the rudder assembly, which connects the rudder to the adjustment bolt on the opposite side of the boat's hull. The second 90° bend was to create the vertical tab, which connects to the adjustment bolt.
Step 6: The Rear Plate
The rear plate is a complicated little piece with a critical function. It mounts to the rear of the hull and provides support for the rear of the tube, the rudder hinge, and the rudder adjustment bolt. After cutting out the rectangular shape of the plate, I proceeded to drill numerous small holes where the lightening (as in "to make lighter") slots would be located. These holes were connected to form the slots using a carbide cutter in my Dremel. With the slots cut, I proceeded to cut out the concave section to fit the engine tube. After using my angle grinder and a cutoff wheel to get the basic shape, I worked the steel with a file until the tube fit snuggly. The metal tab remaining in the center of the concave cutout was bent to 90° to provide the support for the tube. Finally, the lightening slots were smoothed to their final shape using some small needle files.
Step 7: The Front Bulkhead
At the front of the engine tube is a round bulkhead, which serves two purposes. First, it provides a solid, durable surface for the engine to push against. Second, it serves as the forward support, or mount, for the engine tube. This part was primarily cut out using the angle grinder. After cutting, the tab was bent to 90° and drilled to accept the tube mounting bolt.
Step 8: The Wing Supports
Although the forward-mounted wing supports are sufficiently far from the engine and its hot exhaust gases, I decided to continue with my use of steel to help the boat have a better forward/back weight distribution. I cut the lower edge of these supports slightly oversize as I wanted to match their bottom edges perfectly to the bottom edges of the outrigger hulls.
After cutting the wing supports, they were temporarily mounted to the outriggers and I traced along the edge of the hulls to transfer the curvature of the hulls to the supports. Next, the supports were sanded down to this line, which gave the supports the same shape as the hull.
Step 9: The Engine Tube
My favorite part of this boat was the new engine tube. For this part I used a 7.5" long section of 1.084" ID, 1.25" OD steel tube, which I ordered from onlinemetals.com. Since this part ended up being the heaviest of them all, I drilled evenly-spaced 1/2" holes along the top centerline of the tube. Two additional rows of 3/8" holes were set at an angle on both sides of the 1/2" holes. I made sure to only place these holes where they would "vent" to the environment and not to the hull of the boat.
For mounting the front of the tube, a hole was drilled and tapped to fit the bolt passing vertically through the tab on the front bulkhead. At the rear of the tube, a shallow slot was cut using a carbide cutter. This slot was also recessed on the inside of the tube to allow the 90° tab on the rear plate to fit into it.
Step 10: The Wing
I was initially unsure how I would make the airfoil to serve as the wing. However, I decided I would simply try to sand a piece of pine to shape using my belt sander. Surprisingly, this worked much better than I had hoped. The belt removed wood quite rapidly and the curved edge of the belt worked well for creating the concave portion of the wing. After shaping the wing, I drilled a hole in each end and screwed threaded brass machine screw inserts into them. Since I was worried about the pine cracking as I threaded in the inserts, I lightly clamped the wood to give it some extra support.
Step 11: Hull Extensions
As I neared completion on the boat, I realized that the extra weight of the steel parts in the rear would cause the stern of the boat to ride quite low in the water. To give the boat more buoyancy in the stern, I decided to add two small hulls to the sides of the rearward portion of the main hull. I made these hulls from some 3/4" thick pine I had laying around. Their forward edges were cut using a combination of a bevel and miter on my miter saw. I apologize for not having a good explanation on how I did this. I'll just say it was a complicated procedure to get two identical hulls with identical, mirrored, compound cuts. After sanding away the paint on the hull, I glued these hull extensions to the main hull using exterior wood glue.
Step 12: The Rudder Adjustment Bolt
Before finishing the boat, I added the rudder adjustment bolt. This bolt passed through the rear plate and was epoxied into the hull of the boat. During finishing of the boat, I was glad for this bolt as it gave me a convenient point to hang the boat from as I painted it.
Step 13: Coating the Boat
Since last year I had some issues with the boat leaking, I wanted to better seal the hull. After doing some research, I decided to use Goop brand Coat It, which is a two-part coating designed for boat hulls. In retrospect, I should have just used some sort of epoxy as this coating was not very friendly to work with. After mixing the two parts together, the Coat It spread like molasses and it was nearly impossible to get a nice smooth coat. This meant that after everything had dried, I spent several hours trying to smooth everything out with a file and my orbital sander. Still, I was eventually able to get it acceptably smooth.
Step 14: Paint It
During my boat design livestream it was suggested that I paint the boat yellow. I wasn't sure about this color at first, but then I decided it was a great color for maximum contrast with the dark water of the lake. All of the steel pieces (with the exception of the wing supports) were painted a dark shade of gray.
Step 15: Assembly
With everything painted, the boat was assembled. Note that the rudder is mounted to the rear plate using a small brass hinge from the hardware store. Any of the screws piercing the hull were coated with a silicon RTV sealant before threading into the boat to prevent water leaking around them.
Step 16: It's Finished
Getting the boat together was a great feeling and I was super pleased with how everything came out. As I mentioned before, I was particularly proud of the lightened engine tube.
Step 17: First Tests
I decided to try building my own rocket engines this year using a combination of potassium nitrate and sugar. (I'll have an Instructable on this before long). During some previous engine tests, I had issues with some of my engines exploding due to engine pressures, which exceeded the holding pressure of the engine's nozzle. Since I wanted to avoid this happening in the boat, I prepared 4 engines with 15% baking soda by weight. The baking soda helps slow the burn rate, which deceases engine pressure (and power).
This decrease in power proved to be quite substantial for two of these first four tests. Test 1 refused to leave the launching platform and test 4 burbled around the lake for an entire minute until the fuel was exhausted. Tests 2 and 3 must have had slightly less baking soda in the engines as they took off well across the lake. The boat ran fast and straight without incident - proving its design.
Step 18: RIP Rocket Boat
With the success of tests 2 and 3, I decided to make two more engines without any baking soda to see how the boat would handle under higher power. Unfortunately, this was a big mistake. No sooner had we lit the engine and the boat began to move than everything exploded. Despite my best efforts to ensure the survival of the boat during such an event, it was obvious the forces involved were well beyond what the boat could handle.
Looking at the footage of the explosion, we could see that the nozzle of the engine failed, which pushed the engine housing forward so forcefully that it literally split the boat right down the middle. After splitting the boat, the PVC engine tube continued halfway across the lake before hitting the water!
Obviously it was disappointing to see all my hard work disintegrate before my eyes. However, it was encouraging to know that the boat performed well and that the basic design was sound. Now I just need to perfect those engines for the next boat.