Rocket Boat

Intro: 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.

STEP 1: Design

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.

STEP 2: Prepare Sides of Hull

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.

STEP 3: Prepare Sides of Outriggers

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.

STEP 4: Connecting the Outriggers

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.

STEP 5: Glue on Bottom of Hull

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.

STEP 6: The Engine Tube

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.

STEP 7: The Tube Mount

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.

STEP 8: Glue Dowels Into 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.

STEP 9: Thrust Management

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.

STEP 10: Hull Bulkheads

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.

STEP 11: Trim Balsa

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.

STEP 12: Glue Outrigger Sides

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.

STEP 13: Glue on Bottoms of Outriggers

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.

STEP 14: Outrigger Bulkheads

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.

STEP 15: Clean Up Outriggers

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.

STEP 16: Hull and Outrigger Tops

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.

STEP 17: Leading Edge Blocks

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.

STEP 18: Wood Filler

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

STEP 19: Final Sanding

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.

STEP 20: Painting

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.

STEP 21: Additional Details

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.

STEP 22: First Test Run

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.

STEP 23: Modifications to Boat

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.

STEP 24: It Finally Worked!

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!

Comments

[MetinŞ]

I need a project

[BigMouth380cal]

Are the Paper Template files available to print? Sure would make repeating your process much easier, rather than guessing at lengths and widths of things.

[vikingdemon]

Look into how the jet boats are designed. Very little is in the water. All of their controls and whatnot are on top. Fins and spoilers. They are built very much like a race car.

[JoshXarles]

Thanks for the feedback. This is my plan going forward. I'll be sure to show you guys what I eventually come up with.

[vikingdemon]

WOOT

[zencuke]

I noticed you mention keels in the video. On a sailboat keels are in the center because their job is to keep the boat from sliding sideways without changing the direction the boat is pointing. Different problem. Especially when sails are pretty flat (going up wind) they push sideways as well as forewords. The keels job is to fight that. Without it a sailboat would point one way and slide another. On the other hand the rudder's job is to control direction so it is all the way in the back. To keep your boat going straight you want a rudder that points straight. A rudder is not just for turning.

[JoshXarles]

Yea, I've been thinking of a "fixed" adjustable rudder that would counteract any turning inherent to the boat. Basically a trim adjustment. I never really thought about the function of sailboat rudders that much, but your explanation makes a lot of sense.

[zencuke]

Hydrofoils would be fun but I suspect they wouldn't get you much. The point of hydrofoils is to get a fat old displacement hull up out of the water instead of pushing through it. A flat bottomed planing hull like yours pretty much skips on the surface already. Most of the displacement drag is gone. It would sure look cool though. Just the idea of a rocket powered hydrofoil gives me chills.

[JoshXarles]

Haha. Your last comment made me laugh. I think you're probably right on there not being much improvement - and actually I could see the foils slowing it down a bit. I think it would take some tuning to make sure it lifted out of the water, but not too far out of the water at speed.

[zencuke]

OK sorry for all the traffic but you've got me excited about the design challange. Here are some other suggestions, all worth exactly what you pay for them.

1) Add a really BIG fin in the back; maybe two, one on each side of the center hull if that is easier. As a start bigger is better. This is really my rudder idea said a different way. This will keep it pointing straight. You can shrink the rudder later when the other problems are fixed but solve one problem at a time.

2) If the engine forces the bow up fix the engine instead of the boat. Tilt it up slightly. Make it adjustible for experiments. Use a strap down engine mount and bring a bunch of shims you can stack under the back or front before you strap it down. Adjust the number of shims until you like the angle.

3) For an adjustable engine mount a couple of those rods you used to connect the outriggers made into pegs beside the engine and you could strap it down with a really long rubber bands wrapped back and forth over the engine and around the pegs. You can get rubber band strips that are many yards long. Slingshot rubber would probably work as well.

4) The outriggers should be in the back. Out front they probably cause most of the instability. Think arrow and rocket design again.

5) Catamaran? I'm making one of these Buckminster Fuller rowing catamarans. Wait for my Instructable, hopefully in the spring. ;-) Isn't it pretty? http://www.solwaygallery.com/images/ARIIST_PAGES/Buckminster%20Fuller/Buckminster_Fuller_rower_2.jpg

6) Big powerful engine in back? Keep thinking arrow and rocket as your prime design models. It is really only a boat for a second or two until it gets on top of the water. Don't fix the problem in the water by pulling it down. That adds drag and even if it works slows it down. Make it fly better instead. Maybe add spoilers like a race car. Race cars will fly if the designers aren't careful. That's why spoilers were invented.

You've already got the key part completely right. Paul MacReady, who finally won the Kremer prize for the first successful human powered airplane, said his real secret was making a plane that could survive many experiments. MIT would bring a sophisticated highly engineered but delicate design that would last one crash so they had to fold until next year. MacReady brought one plane with a basically decent design and lots of spare parts. He kept hacking away until it worked.

Keep us posted.

[JoshXarles]

Thanks for all the ideas and that's no problem with the "traffic". I like the catamaran btw. It looks really neat, fun, and fast! I've been a bit hesitant to have the engine adjustable, but you're making me reconsider it. Actually, given all your comments I'm tempted to make a more robust boat that has adjustments all over it for different things - wings, rudders, engine, etc. Then I can really tune things to get it just right. Once again, thanks for all the ideas and we will do this again, but it might have to wait for next year. The lakes will ice over before too long - although that would be another interesting application for a rocket "boat".

[zencuke]

Beautiful job. It encompasses two of my favorires, boats and rockets. I love the family tradition part as well. Minor suggestion: if you are looking for straight line stability from fins they should be as far back as possible, behind the center if drag for the rest of the boat. That is why rocket fins are way at the back, often mounted at an angle so they can trail behind the body. Think about arrows. The feathers are at the back. If fins are too far forward they can make it more unstable instead of less. Of course in your design the fins are now dual purpose so that complicates things. The hold-down force needs to be up front. I never needed fins on my boats but I never tried to rocket power them either. ;-) I put propeller engines on a rear mounted pylon, like a Florida swamp buggy. Mounting the engine up in the air helped keep the bow down as well. I'd be afraid to try that with those powerful rockets engines though. Something to think about.

[JoshXarles]

Point well taken on the fins. I'm planning on a rudder-type structure back there. I too am hesitant to put the engines up in the air as I know that too much hold-down force is negative as well. We've pushed many boats under the water.

[Wokie-777]

where can you buy the rocket engine

[JoshXarles]

Most hobby shops will have rocket engines up to a size E. Stores like Walmart will typically have up to a size C. I purchased size D12-0 like these: http://amzn.to/2eiunwX from a local hobby shop. Have fun! :-)

[Swansong]

Fantastic instructable, I like that you added the info on what didn't work and how to tweak it. This looks like so much fun!

[JoshXarles]

Thanks! It is fun and it has always been a learning process making these things.

[Galt]

Awesome job on your "ible". I build kayaks, so probably none of what I know is of any value to you, but here's some random thoughts for mental floss anyway. Looking at the Navy hydrofoil it occurred to me that as already said the forward position of thrust would aid stability, but how about adjusting weights? Maybe the added weight would help maintain directional stability by limiting the effect that every little ripple or puff of wind might have. Also the added weight might make it simpler to try and balance using actual foils to raise the entire craft out of the water while still keeping the underwater wings submerged? Also kept thinking about the manta ray looking device that they sell to affix to the lower unit of outboards to help acceleration attitude from a rear propulsion approach. That wing looks a lot like the foils of the hydrofoil. Just noodlin'. We have a nearby mill pond where people race RC nitro-methane boats. Might have to build one of these just to blow their minds one day. :)

https://en.wikipedia.org/wiki/Hydrofoil

[JoshXarles]

Thank you! It would be amazing if I could actually get the boat to ride on hydrofoils - and I agree that weight would probably help with getting that setup correctly. I was just thinking of adding foils to keep the boat on the water - an inverted foil, but now you have me thinking about using an actual hydrofoil! Check back in the future to see what I end up doing :-)

[Galt]

Like I said - just mental floss to add to your already creative view. Quite the engineering challenge all told, especially since I can't imagine the thrust delivery from the Estes engines is all that steady. Used to launch and lose them all the time as a kid.

Nothing full scale really equates due to the obscene ratio of thrust to size and weight that you're dealing with, other than maybe an air-to-air missile. Reminds me of the urban legend/Darwin Award about the guy who strapped a surplus jet engine to the roof of his station wagon out in the desert. Supposedly nothing found of him except the rubber from him standing on the brakes just before the car left the road and went airborne, ultimately becoming a black greasy spot on the side of some butte. :)