a note before we get in to it: I've been telling you the fan-like thing that pushes a boat is supposed to be called a screw. actually, i've been corrected. it can actually be called a propeller as well. turns out old school guys call it a screw but its technically a propeller as well, so the both terms work. i feel bad for teaching you guys (all one of you) wrong.
on to today's events! we set up the motor to run the prop (through the foam board we bought). this was a bit of a task because we needed a long drill bit to get through the foam. the hole for the prop shaft had to be at a fairly shallow angle so the screw didn't point downward and drive the nose of the boat under. however, small diameter drill bits are quite short. heres how we solved the problem.
I'm not sure you noticed. look at whats attached to the yellow drill. a FOOT long drill bit. this thing was MONSTROCIOUS! (insert twss joke) anyways, that took care of the motor assembly (at least at that point) heres the prop shaft and screw sticking out of the back of the hull
the rudder assembly was a bit more troublesome. we epoxied the rudder together but had trouble because of the quickness it dried. it turned out ugggggglY! its sort of hard to be proud of what it looks like but i if it works i'll get over that right away. with that premise, heres a photo of the rudder we made.
dang, it is pretty scrappy. oh well. prototype.
when we had all that taken care of,we filled up the pool to see how she flew... er... glided? whatever. you can see the pool wasn't ideal
that was ok though, we were just testing that all pieces worked. turns out they didn't. two problems surfaced.
firstly, the force of the screw driving forward wasn't actually pushing the ship, only the prop shaft. if you're having trouble imagining this, pretty much that means the boat stayed still while the screw and shaft moved forward. very bad. forturnately we were testing at low speeds and caught it very early on. it actually turned out to be an easy fix: we put a spacer in between the screw and the prop sheath and we think the problem got solved. don't know for sure though because a new problem came to light
the second issue (here comes a crappy pun) sunk us. figuratively of course, but it was still frustrating. the issue is the golden colored surgical tubing that you see coming off the prop shaft (at the intersection of the black lines).
the servo is blocking most of it. now you know where you're looking, find it in this pic and you can see it a little better
the problem is that this tubing acts as a universal joint connecting the motor to the prop shaft (ultimately translating the motor's motion to the screw), hence it turns at the speed of the motor. when you get what is basically a string (and here come the physics!) rotating at that rpm, it starts vibrating and seems to be acting like a wave. the vibration/pandaemonium actually dislodges the tubing from the prop shaft. so, even at the lowest of speeds, the motor gets disconnected from the prop shaft.
this can be solved a couple of ways. first, slowing the rpm of the motor will likely fix the problem. this is what the hobby shop guy told me to do for a different reason and is the ideal fix. it would give the boat way more torque and protect the motor and electronic speed controller (ESC). however, this is a bunch a bunch of work, would take gearing calculation/ordering/prep and all sorts of time, and being that its a $10 motor and cheap ESC, i think we're going to skip this possible fix
the second way to fix the issue is to cut down the length of the tubing. the tube experiences harmonic motion because there is maybe 3 inches of tubing between the two nodes (physics again, bear with me again briefly). to eliminate the extent of the amplitude of the wave, physics tells us to shorten the wavelength, shorten the length of tube! easy right? well, yes, and sadly no. shortening the tube is easy. remounting everything on the boat will take about an hour. still way way better than the fix mentioned above, but unfortunately we didn't have enough time left today. SO, we do it tomorrow.
tomorrow we cut the u-joint (tubing), remount the hardware, use the kiddy pool to make sure the boat propels itself forward and then go in search of a big pool to test it for real. hopefully we get our hands on a video camera for that. you'd think 2 infocus guys could, but i don't know, we'll see. certainly i'll take still shots. hopefully it won't do the boat justice though ;)
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