Rocket Boat

Every year in October, some friends of my parents have gotten together at a cabin next to a small pond. As a kid it was one of my favorite times of the year, as it meant I got to hang out with my two favorite cousins and one other friend. One year we got the idea to build a simple wooded boat and attach some model rocket engines to it. We built the boat and launched it from the dock. That boat only went about 6 feet, but an annual tradition was born. Over the next few years we built several more boats - most were dismal failures. Eventually, my brother decided to have a go at building a boat and his boats were considerably more successful. We gradually increased the rocket thrust and even added a second rocket stage. Our most successful run made it around half way down the pond before the boat lifted off the water and tumbled out of control. This year I decided to build my first boat in quite some time - this is the story of that boat.

As our boats increased in speed over the years so did the problems we encountered in controlling them. We did find some key design elements that worked well though. First, we found that a flat bottomed boat was much faster than a deep hulled boat as the water drag was reduced in this configuration. We also found that the boat needed to be sufficiently long so that the rockets did not push the front of the boat down under the water. The boat was also found to have stability issues at speed if it had a large, flat, upward sloping front. As such a boat would reach maximum speed, the air flowing under the angled front of the boat would lift the front of the hull, tumbling the boat in a spectacular backflip.

On Youtube I found videos of high-speed outrigger-type RC boats, like that shown above. Impressed by the stability of these boats at speed, and realizing that they fit all of our key design elements, I drew up a simple design for my rocket boat. I printed out paper templates of the side profiles of the front of the hull as well as the outriggers.

My boat was constructed of 3/32" basswood and 1/32" balsa. The basswood was used for the major structural components (sides of hulls, internal and external bulkheads), while the balsa was used for the top and bottom surfaces of the hull and outriggers.

I temporarily glued two pieces of basswood together with Loctite-brand spray adhesive (blue type). The template for the front of the hull was affixed on top of these with spray adhesive. Next the shape of the hull sides was cut using a scroll saw. The scroll saw doesn't always make the straightest cuts, but basswood sands easily so any small imperfections in the cut can be easily erased. Once the shape of the hull sides was cut, I drilled 5/16" holes in them using the paper template as a guide. Finally, the edges of the sides were cleaned up with some light sanding before peeling to two pieces of basswood apart. I really liked this technique of temporarily gluing together pieces of wood as I could easily create identical parts.

The sides of the outriggers were prepared in much the same way as the sides of the hulls. However, this time I temporarily glued up 4 layers of basswood.

9" long pieces of a 5/16" dowel were cut to create the rods, which would connect the hull with the outriggers. These rods were pressed through the holes in sides of the hull and outriggers.

With the rods pressed though the sides of the hull, the bottoms of the hull sides were glued to a piece of 1/32" balsa. I made sure to liberally apply glue along the entire length of the hull. Since the finished boat would have a closed hull, I wanted to minimize the chance of it having a leak along any glue joints. Once the glue was applied, I placed a board under the front of the hull to keep pressure on the wood along the curved portion of the hull. Books were added on top of the hull to "clamp" everything together as the glue dried.

I decided to use either D or E rocket engines, which have an outside diameter of slightly less than 1". A 1" ID polyethylene tube was cut to 7.5" long to serve as a holder for these engines.

Mounting the engine tube to the boat proved to be the most complicated part of the build. Since the boat has a closed hull, it is important that the outside surface of the hull wrap around the tube. In other words, the tube does not form part of the hull of the boat, but rather rides on top (or inside) of the hull. I began by cutting 4 bulkheads, which would fit inside the rear of the hull. I traced the outside edge of the tube onto these bulkheads, with the center of the tube located around 1/4" down from their top edge. This curve was cut from the 4 bulkheads and enlarged by using a sanding drum and sandpaper. Next, a piece of 1/32" balsa slightly longer than the tube was cut. This balsa was easily wrapped around the tube as its grain was perpendicular to the direction of the bend. The cut bulkheads were fitted over the tube with balsa wrapped around it and were glued into place. Finally, this entire tube holder assembly was glued into the rear of the hull.

The 5/16" rods through the hull were glued into place by sliding them slightly off center, applying glue, and then sliding them back into a centered position, while simultaneously rotating them.

An extra thick bulkhead (2 layers of basswood) was added at the front of the engine tube. This bulkhead would directly receive the majority of the trust from the engines, which is why it was made extra thick. In addition, 1/4" x 1/4" rods were cut to fit between the front of this bulkhead and the rear dowel. These rods were notched so that they would securely rest on the dowel. The idea was that these rods would help support the thick bulkhead by directly connecting it to the most rigid section of the the boat. This is why I called them pushrods - as they help transmit any force acting on the thick bulkhead directly to the rear dowel.

Additional basswood bulkheads were cut and glued into place along the length of the hull. These were placed at around 2" intervals as this provided sufficient support for the very thin balsa top and bottom surfaces of the hull.

The nice thing about the 1/32" balsa is that it is super easy to cut. An X-acto knife works very well for either cutting or trimming it. This meant that I could glue the balsa to the boat with a slight (or in the case of the bottom - very large) overhang and simply trim it down with the X-acto.

Once the hull bulkheads were complete, the sides of the outriggers were glued to the dowels. I used a similar method as that for gluing the dowels into the hull. The sides were pushed slightly away from their final position so that glue could be applied. They were then carefully slid back into position. It was critical that the outriggers be parallel with the hull and consistent in width along their length.

Two 1.5" wide pieces of 1/32" balsa were cut and glued to the bottoms of the outriggers. As when gluing the bottom of the hull, the front edges of the outriggers were supported, which allowed the bottoms to be firmly "pressed" around the curvature of the outrigger bottoms.

An angled bulkhead was added to the rear of the outriggers. I cut the top edge of this bulkhead with the scroll saw table set at 30°. This ensured that a small gap would not form along the top edge of this bulkhead. Three additional bulkheads were glued into the outriggers at regular intervals along their length. Some small pieces of basswood were used to connect the outrigger sides at their leading edges.

Once the glue for the bulkheads dried, the balsa bottoms were trimmed along the front edges of the outriggers. The tops of the outriggers and the hull were then lightly sanded to ensure all the surfaces were flush with one another.

Top surfaces were cut from 1/32" balsa for both the hull and outriggers. A section of the top of the hull was carefully cut away to allow for the engine tube at the rear of the boat. All of these top surfaces were glued in place, weighed with books, and allowed to dry.

With the top surfaces secured, small 1/4" x 1/4" blocks were glued to the leading edges of both the hull and the outriggers. Painters tape was used to secure these blocks while the glue dried. These blocks were added to form a hard, strong leading edge for the boat.

Wood filler was used to fill gaps around the engine tube, leading edge blocks, and the rear bulkheads of the hull and outriggers.

After letting the wood filler dry for a day, the leading edge blocks were sanded to form rounded "points" using a sanding drum. After sanding the wood filler and other surfaces of the boat, the boat was ready for paint.

I applied numerous coats of primer to ensure that the hull of the boat was as sealed as possible. Any moisture getting through to the wood could cause it to warp or result other undesirable events (the hull filling with water for example). Two coats of red paint were applied over the primer - finishing the work on the main structure of the boat.

Several aluminum parts were added to the painted boat. Two small keels were screwed to the sides of the outriggers. It was hoped that these keels would help keep the boat running straight at speed. At the front of the engine tube, a round plate was screwed to the thick bulkhead to protect the boat from the ejection charge of the rocket. In addition, a small tab on the top of this plate was used to mount the front of the tube to the boat. I designed the boat to use either two D or E model rocket engines. Since E engines are longer than D engines, a stop plate was added across the tube so that two D engines would perfectly fit in the distance between the stop plate and rear of the tube.

It's always exciting testing a rocket boat for the first time! You really don't know how it will behave until you light those engines. When we lit up the boat for the first run it launched out of the tube, bounced briefly on the water, and almost immediately lifted into the air. It spun around on the engine for a bit and ended up upside-down in the water.

It was determined that the design of the boat would need to be changed to allow it to hold itself down onto the water. In particular, the front of the boat needed to be held to the water when at speed. To accomplish this, the bottoms of the two keels were bent to form small planes, which would pull the front of the boat down. As the speed of the boat would increase, the downwards force exerted by the planes would also increase. Since we also had an issue with the rear of the engine tube lifting out of the boat, we added a small holdown clamp to the rear of the tube.

These modifications finally allowed the boat to stay on the water for the entire run! We ran it three more times and continued to make modifications. If you want to see how we made out, check out the video above. Overall, I'm very pleased with how the boat came out - although given some of the performance results I can't wait to build next year's boat!