Design Process Details
I had no idea what I was going to do, but I decided that I wanted to involve my nieces and nephew if I could, so that I could enter more races. I knew this would require a bigger boat, but that’s it. The first thing was to calculate the required displacement, which I did using estimates of everyone’s weights.
I still remembered a lot of that naval architecture stuff from the previous race, and I decided on using a similar design as before since it performed so well. Obviously, the new boat would require some scaling, so I started off with minimum internal dimensions for 1 small adult and 2 other small kids.
With the minimum internal dimensions set, I set about calculating displacement equations and such so that I could use OpenSolver in Excel to iterate the optimal for me given a set of constraints. Coming up with the equations for each of the boat’s facets is a real headache, but satisfying once completed.
Since I was going to have almost triple the weight as last time, I decided to make a honeycomb structure to increase rigidity without driving up the weight. Now that I’m done with it all, I’d say it wasn’t worth all the time that went into it. I probably won’t ever make something so large with a homemade honeycomb core.
But Wait! How do you know how thick your honeycomb needs to be, and just how strong is cardboard really? Well… I’m glad you asked. This year, I set about finding out just what the modulus of elasticity is for corrugated cardboard. My test rig is shown here. I figured that this picture doesn’t really give anything away.
So, what I have is a 2 point/centrally loaded bending problem. I used that black scale to measure the weight of each of those little steel rods so I knew what the force on the beam was. Now, bear in mind that I use the term “beam” loosely. It’s just a single sheet of cardboard for the most accurate test. I set the “beam” on supports 24″ apart then loaded the center section with increasing amounts of weight. I then measured the displacement with my calipers for each test. I used an average of 7 measurements for each set of weight to help eliminate the error in any one measurement. With the beam deflection formula, I then solved for E.
In the picture you can see that I’m measuring the displacement of one of the Sandwich Beams I built. The one in the picture is 1.5″ deep and 3″ wide.
So, once I knew a bit more about the cardboard, I was able to minimize the amount of cardboard I would end up using.
The next step was to make a 1:6 scale model of the boat and verify that its draft and everything was what I’d calculated in Excel. Luckily, everything worked out, and I’ll say that I could actually put 445 lbs in the boat before I had less than 3″ of freeboard remaining. (Freeboard is the distance from the water to the deck level, and I set 3″ as the absolute minimum I was willing to accept.) So I knew we were all nowhere near 445 lbs, so we had an extra margin of safety there.
With the basic shape and everything set, it was time to begin construction. I decided to build jigs this year because I had to construct the boat in sections then assemble it later. This was because I’d have to do all the work in my office during lunch. I really didn’t have the space at home, and I wanted to spend all my time at home with my family, and not building boats.
I built a jig to form the inner skin and then built a “honeycomb” grid on each face. I then glued another skin on top of this grid, and that gave it immense rigidity. The total time spent on this center cockpit section was about 3 weeks. But… OOPS!
I accidentally built an inner skin inside my original inner skin.
Now the cockpit was 3″ too narrow for me to fit!
So, after about 4 minutes of disbelief, I started over, with a wider design than before, because I wanted to make sure my crewmates would be comfortable. I also wanted it to be slightly more stable, so it worked out well anyway. Luckily, all I really had to do was change all the numbers in my excel sheet. The formulas didn’t change, which was nice, because setting up formulas is the hardest part. I had the new cockpit section completed within a week.
I had to build new jigs for the new cockpit, of course, but it sure makes things easy. I then had to build each bow section, and those were slightly more complex because they are not symmetrical like the center section. Lots of cutting, gluing, measuring, checking, cutting, rechecking followed over the next several weeks. I finally got all three sections completed on 6/11 and took them home for final assembly. (My office is only 11 feet long, and the boat is 12. The glue also smells pretty bad, and I didn’t want to irritate my coworkers too much more)
Here are some pictures of each bow section during assembly.
Oh, I should mention that I began building a second boat on 5/9. I thought it would
be a good idea to have a second boat just in case my first one was destroyed during one of the races with the kids. Oddly enough, I finished this boat before the first because of its much simpler design. There are no folds on this boat; just a bunch of flat or gently curved pieces. This one was named Swift. ->
Swift is a honeycomb, like the first, but much thicker, and has no interior space to sit. It’s essentially a very long board, with outriggers of course. It was 12′ long and 1′ wide at the midsection. The outriggers are 1/4 scaled versions of the main hull. This boat looks something like a huge trident. I finally got it wet on 6/14, and man it’s FAST. It was the fastest boat I’d built yet, and it looked downright mean. Since there was no interior space, I had to engineer a way to not fall off.
I stole a bit of sand from our sandbox and mixed it with paint. I then painted a separate sheet of cardboard with this mixture and glued it to the top of the boat. This stuff was probably 50 grit. It’s extremely rough, and holds on to anything. It’s kind of painful on bare skin though.
I never tested the second boat, which the kids named “Bandit”. I was confident that it would work since it’s similar to my last boat. I did do a dry run the week before the race with the kids to make sure we could all fit and turn around.
Paint was primarily red because red is the color of speed. I put a few whites and blues on it because there was a “most patriotic” category, and I thought it couldn’t hurt to try.
7 days to RACE DAY!