A model boat design

Hi, for my science fair project I'm going to be working with thrust for a model boat; however, I ran into a few problems. The design of the model boat itself I find to be quite challenging as well because the propeller itself I don't know how to submerge it in water with the motor and how to design it for my purposes. Additionally what type of equipment would I need to measure thrust? Thanks a lot!

 

I don't know much about model boats, but I think I can get you started in the right direction. For your prop problem, I would suggest keeping the motor above the waterline, and connect a little pully to the shaft. Then run a plastic timing chain down to the prop which is connected to another small shaft with another timing pulley. Just make sure you mount the motor to something solid, as well as the prop shaft. I'm guessing at the scale, the small motor, and being for a science fair and not having to last long, you wouldn't even need to worry abou bearings anywhere. If its real small, you might be able to mount the prop on a coffee stir straw and use a tight rubber band as a belt to connect the motor shaft to the prop shaft. IDK, maybe theres a better way to do it, but knowing nothing about model boats, thats what I'd do.

As for measure thrust, depending on the amount, you could use something called a fish scale. Basically, its the same thing the weight of fruits and veggies at the grocery store (that is, if they're not all digital now). One you would need would have a hook on each end. Connect one end to the side of the take (you can use string probably), and connect the other end to the boat. When the boat pulls on it, it will stretch and tell you with how much force its pulling. If you can't find one of those small enough (or if you're trying to not have to buy anything), I would suggest making your own simple version out of a rubber band. All you have to do to calibrate is hang a rubber band of a hook, and using the rubber band, suspend a small weight from it so the band stretches. Use as little weight as possible just to get the rubber band straight. Then, add a known weight to it and measure how far it stretches. Do that one more time with a little more weight. Then, plot those points on a graph (strech on the X axis, force on the Y axis) and connect the dots with a straight line. The slope of that line is called the spring constant of the rubber band. Then, you can hook the rubber band up to your model boat and measure how far it stretches. When you know how far it stretches, you can look at your plot and know how much force corresponds to that amount of stretch. Just keep in mind that all rubber bands will be slightly different.

Depending on how bad you wanted an A on your project, you could due the calibrating on a whole bag of rubber bands, plot all the points, and figure out the average spring constant. That way, if your rubber band breaks, you can just grab a new one without having to recalibrate. Thats what I would do.

Hope this gets you started in the right direction. Good luck!