Cardboard Boat

  The 4th Annual Edmond Cardboard Boat Regatta has come and gone. It was a blast! I will be back next year with something different. Here are the pictures and details I promised a while back.  I hope some of this helps you avoid my mistakes and build a seaworthy boat for next year. See you there!

Model Boats
Models ranging from 1:16 scale to 1:8

I first did a lot of research on naval architecture. I wanted to participate in the speed category and the demolition derby, but I knew I would end up with a boat that wouldn’t excel in either area. I decided to go for speed, because I have a need for speed. I also wanted to win best engineering design, so I dove head first into heavy R&D.  Here are some of the models that resulted from my initial concepts.

 The general thought was to minimize wetted area (the parts that get wet) and maximize the fineness ratio (how pointy it is). This is a delicate balance because as something gets longer, its wetted area will increase. I ended up with a boat that had a fineness ratio of 12, which is fairly high, but nowhere near as high as a racing shell is. It was the highest ratio by far at the competition. I had no idea if this was adequate though, so I designed a crude test rig to compute the drag coefficient.

  The test rig consisted of a vertical tube with pulleys at the top and at the waterline. It was suspended over the side of the pool. A 6′ ruler was hung against the back. This would allow speed data to be gathered by viewing the video and taking position data off the ruler and time data from the video frames’ timestamp. I ran a string from the model boat, around the tower’s bottom pulley, and up around the top pulley. A calibrated weight was then attached to the string at the top of the tower to provide the force to pull the model. Video rolled, and the weight dropped. Each run lasted less than 2 seconds. I was surprised to discover that the drag coefficient was actually less than I predicted by interpolating standard Cd data!

  I’d love to tell you that it was success all the way, but it wasn’t. Nothing ever is. The only way to keep winning is to fail a lot! My first proof of concept was just to get my hands dirty building something. I started back in June. It floated nicely, but buckled instantly once I got in it. That was expected, but I invested only $8 and about 3 hours. I learned a lot and began designing boat 2.

1:12 Model draft and max weight test in sink

I did an enormous amount of design on these boats. I easily spent over a hundred hours on design of all my boats. I was trying to find out how long my Excel design sheet had been open, but I don’t know if it tracks that. I wanted to know how my boat would behave before I actually built it. A lot like engineers do with everything nowadays. Specifically, I needed to know the draft, so I could find wetted area, center of buoyancy, and stability index. It involved deriving all sorts of equations for the shape of the hulls, but the draft calculation came out almost perfect, which was a real surprise!


Bow & Stern 2
Boat 2 Bow & Stern assembled and ready for varnishing


Boat 2 Float & Stability Test

  Boat 2 was based exactly on my models. It had a nice semi-circular cross section and a traditional canoe appearance that brought wetted area to an absolute minimum. The tradeoff was the worst possible stability. It’s also difficult to curve cardboard in either direction and have it look nice. I didn’t bother painting this boat, because I had no intention of racing it. If it worked, I’d build another one. I varnished it twice and it floated nicely and only weighed 7.8 lbs! It was extremely hard to balance in as predicted, but man was it FAST! I knew I was on to something.   Its high fineness ratio made cornering almost impossible, so I was hoping I’d be allowed to simply turn around and row the other way. Luckily, that was what ultimately happened. It could turn, but it took about 10 seconds, and I thought that was unacceptable. Its length made cornering tests very difficult in the pool because of the pool’s narrow width, but oh well. I knew that if I could improve stability and keep water out, I would almost certainly win. I decided to take it to the lake to see just how fast I could go; attempting to confirm my design calculations. It received a small “chink” in the side during my last test, and this tiny flaw caused the boat to buckle as soon as I got in at the lake. Disappointed, I packed up, and began design on a third boat.

  The third boat was exactly the same length, but it was a “stealthier” design, being composed of only flat faces. 😉 I didn’t want to attempt curves again because it was difficult and not very good looking. I went with a wave piercing design because research showed that it was the most efficient. I designed the cockpit section first to fit me like a glove. I made a few sketches of how the flat panels would come together at the bow and stern. I made a model in Solidworks to aid in assembly and make all sorts of performance calculations easier.

   I planned to tape the seams with duct tape, but found that Liquid Nails worked well, so I elected not to use tape. I didn’t think tape looked very good anyway. After much gluing, clamping, and waiting, I had a very smooth and extremely light and rigid hull. What I didn’t yet know was how much the varnish and paint would weigh.

Painted #3 with hood and trunk


In the end, I ended up with a boat that weighed 14.8 lbs. Only half of that was structural. I wanted to make sure it was absolutely watertight, so I did go a little overboard on paint and varnish, but oh well. I also put a hood and trunk on it to keep splashes out and create “watertight” compartments just in case. They came in handy.

Screen Capture from Boat 3 Test with Outrigger

   Initial testing proved as expected. Roll stability without an outrigger was horrible, but the boat was a speed demon. I added a detachable outrigger for peace of mind. Good thing too. The additional wetted area was negligible compared to the additional stability gained. What’s all that plastic? I wrapped the entire boat just in case I turned it over. This would buy me a few seconds to flip it back before water entered the boat. I tried to ensure all seams were watertight, but I couldn’t be sure, so I wrapped it all up. It didn’t actually get wet until the race. As it turned out, it was totally waterproof, but I didn’t want to risk it. I had originally planned on building a 4th boat, but after testing this one, I figured this would be the one I’d eventually race.


     I arrived early, not sure of what to expect. I was a rookie, and had no idea what others might build. I was all alone, everyone else had large crews and supporters. My initial recon revealed that most of the other boats were not built for speed. My primary worry was that I hadn’t trained hard enough to stay the course. Of course, I wasn’t 100% sure if my boat would work as planned. I walked around the park checking other boats out and visualizing the race.

   The first race was lining up. I was in the third race, and my father-in-law had not yet arrived. He was in charge of photography and pit crew if necessary. I took my boat down to the water by myself, and picked out my pit area. I was relieved when I saw him walking toward me just before the youth race started. Saturday morning is just too early to ask friends to come out and help. One of the boats in the second race, No eXcuses, was also going to be in my race. I was really surprised how fast their flat-bottomed surfboard was. I knew they would be my chief competition. Now it was my turn.

   I attached the outrigger and Daddio helped me get it to the water. I had to figure out how to get in, and I decided to sit on my knees because it was the most powerful position to row in. I hadn’t sat that high before, but I was confident the outrigger was up to the task. Big Mistake!

(Right: Climbing back in)

  The horn sounded. I took off really quickly and pulled ahead. About 3/4 of the way to the turn, I fell out on the left side (port side for you serious boat people) of the boat. I hadn’t practiced getting back in (I couldn’t risk damage), so I scrambled as fast as I could and got back in on the second attempt. By this time I had been overtaken by No eXcuses. I wasn’t sitting comfortably, but I didn’t care. I rowed to the turn around as fast as I could, and I turned around. Just me, not the boat. That is one advantage to building a boat with identical ends. I paddled as hard as I felt comfortable with. I did not want to over do it and fall out again. With adrenaline gushing, I caught up with No eXcuses and overtook them to take first place!

   It was so much fun. I encourage you to participate next year. I certainly will. This probably ranks as my favorite competition, and one of the best feelings of all time.

   Here are some pictures after the victory!     

Best Engineering Design

Me & My 1st Place Paddle!
Me & My 1st Place Paddle!

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