Boats 2011 – Death and Taxes

Now that I’m back from San Diego, I finally get to post about my boat.

This boat features a drive system known as the Voith Schneider propeller (VSP), which is those blades you see sticking out of the bottom of the ship. Wikipedia has a more detailed explanation of how the system works , but in short the system works by rotating the blades in a circle while changing their angle, in effect “scooping” water in a particular direction when the blades are not tangential to the circle they rotate in. The direction of thrust can be changed by moving a control rod that controls the center of tangency (?) of the blades (the blue dot in this diagram.)

Here is a closeup of one of the drive units.

(I would include a Solidworks screenshot here as well, but I can’t connect to the license server right now.)

The nuts and white fiberglass arms in the center of the ring connect to the blades . The pin in the middle of the slot controls the “center of tangency” of the blades and is connected to both servos, which allow me to position the pin within the center of the ring. Underneath the rim of the ring is a toothed pulley, which connects via drive belt to the motor. All the parts were made on a laser cutter. Here’s the shot of the whole thing exploded, showing all the parts that go into it:

As D said in this post, the boat was built, but not quite functional, on the 13th. Since the VSP requires you cut a large hole in the bottom of the ship, I had to stop the thing from leaking, which isn’t easy when you have a hojillion rotating pieces all nested inside each other. I tried Vaseline-ing all the gaps, but that just lead the the whole thing seizing up and not turning very fast. We took it out to the lake, where it didn’t leak, but didn’t run either. I was going to call the boat Pushme-Pullyu, but due to its relative unreliability the name Death and Taxes stuck, so that “there would at least be something for certain about this boat”.

Fast forward to the week after that, where I raised the drive units above the waterline with some spacers, wiped all the Vaseline off, and stuck some fresh(er) batteries into it. Success!

(click here to watch on youtube, because imbedding isn’t working for some reason)

Since the two drive units don’t turn at quite the same speed, driving the boat requires holding the control sticks at funny angles. Improvements for next year: independent motor controllers, a better power supply, and probably some electronics to take the guesswork out of driving this thing.

Also seen in the video: D’s Lucky Channel (foreground), and another boat, Cipher (upper-right, 0:43 to 0:58), which we hacked together quickly just before the trip to the park with the lake so that my cousins would have something to drive. It’s based on a hull I used two years ago in another failed boating attempt (due to bad homebrew electronics). Cipher‘s best feature is probably the logos on the back:

There’s another boat that didn’t make it into the shot, and that is a paddle-wheel boat named Gerboat.

(Please ignore the name S.S. Charmer, which was presumably doodled on the hull by a 4th-grade D.)

Gerboat is based around a pair of Tamiya drive units connected to an old circuit board from an RC blimp. I think the Tamiya motors are probably a bit much for the board, or else the circuit board got fried somehow during soldering, since the controls were glitchy; we wound up not taking it out on the water. The paddles were also made on the laser cutter. Since the hull is so small, we were afraid it would capsize, hence the foam blocks for emergency buoyancy.

The name, by the way, comes from a portmanteau of “gerbil” and “boat”: “gerbiling” is the behavior where a hamster or gerbil runs on its wheel and then stops suddenly, but is carried along by the remaining momentum of the wheel. In essence, we were afraid that the paddles would stay where they were and the hull would flip over, which was a really legitimate fear when you consider version 1 of the paddlewheels:

I still have the big paddles, so maybe we’ll get to use them in another project next year.


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