How much do proper mechanics affect speed?

You're still growing, so that has a BIG effect on your question, and everyone is different, what works for one may not work for another.

This is very debateable. I have seen many guys with poor mechanics who throw 90+ and lots withy great mechanics who throw much less. What it will do is help you keep your arm healthy and have better control. It also could help velocity but it depends on what you are doing wrong.

I think my problem is that I use only my arm when I pitch. I tried to take a longer stride but I still feel like I'm not using my body enough.

It is important that you get proper instuction. That instruction should use the towel dril that will help you get thye most out of your mechanics. The drill also has to be done properly.

Do you think that it is realistically possible for me to increase my speed by at least 5 MPH by doing the towel drill?

Considering the fact that no one's seen your mechanics that's impossible to say

Do coaches usually want a lot of pitchers on their team?

quote:

Originally posted by ro1249:
Do you think that it is realistically possible for me to increase my speed by at least 5 MPH by doing the towel drill?

No. You need to get qualified pitching instruction to perfect your mechanics. Throw 5-6x week, (long-tossing at least 2x week as part of that) to build arm strength, arm speed, and to promote arm health. Plyometrics 2-3x week, cardio work 4-5x week, band work 3x week, weight training 3x week, core work 4x week should also be part of your training. There's a lot, but it can be done if you work at it. Plan it out on a spreadsheet. Do it.