Do Submarines Need Fins

[Loriet]

I am doing this for my science project. We have built the sub as instructed on the website, but the body continues to spin regardless of where the ruler is situated. We have tried using a large bottle (33 oz) and a standard water bottle. Could our propeller be overpowering the fin? Any helpful hints for the bottle/propeller would be greatly appreciated.

[deleted-249560]

I haven't done this one myself, but maybe we can figure this out. The instructions call for using a standard ruler as the fin. I'd imagine that you don't want one with any hole in it, and it shouldn't be flexible. Did you find a nice rigid ruler without holes? Have you tried something wider? It may not last long in the water, but perhaps a piece of corrugated cardboard just to see if a wider fin fixes your problem?

When you say the body is rotating, it's rotating fin and all? Or are the rubber bands holding the fin to the body slipping? If they weren't tight enough I could imagine that might be an issue. If you can describe what you did and what you're seeing in a bit more detail that would help.

Howard

[bradleyshanrock-solberg]

Without seeing what you built it is hard to give specific advice, but I might have an idea of what is going on.

There are two ways you can get the observed behavior (bottle spins). One way could be something wrong with the fin...not attached right, not big enough, similar. But in that case I'd expect more that the "submarine" would move, but just not in a straight line. (the force exerted by the propeller isn't perfectly straight back...because it rotates it'll naturally cause a tendency to "pull" based on the rotation, and the fin on a submarine is supposed to counter that. This force is relatively small compared to the forward-force on the submarine though, so lack of a fin shouldn't cause it to just spin in place without forward progress)

There is another way to see this effect though, and one that would be a very strong effect, one a fin could not counter no matter how large or well attached.

The propeller might be spinning the entire bottle as if it was the propeller shaft. Normally I'd rule this out, but it depends on your propeller and how it is attached to the shaft. If only the propeller "blades" spin when the rubber-band unwinds, then this is unlikely, but if the propeller is rigidly attached to the propeller shaft, it is possible that the spinning shaft is "gripping" your bottle and transferring all or some of its force to the bottle. Essentially if there is not some kind of grease, ball bearing or other mechanism to allow the propeller blades to spin without being "attached" to the bottle, the bottle will want to spin at the same speed as the propeller. And if the grease/bearing/whatever is merely inefficient it might provide enough force on the bottle to make it spin anyway, even if the propeller is also spinning.

This failure mode is something you should be able to tell very easily - if your experiment passes the following test than this theory is likely wrong:

1. Grip the bottle firmly in your hand, and wind up the propeller all the way.
2. Stick the propeller in water and release it
3a If you feel the bottle trying to twist in your hand, the propeller is "attached" too strongly to the bottle. You need to do something to let it spin more freely
3b If the bottle instead is mostly "pulling" straight out of your hand, the problem is likely with the fin, as mentioned in the prior post.

[deleted-315599]

how do i graph the data/results in this experiment?