Here I thought I had scored. I bought a 28 lb Thrust Minn Kota TM from a swapmeet for $10. It works,but now that I look closer,im not sure how worthy it will be in the water. The fin that goes on the bottom of the TM is broke off,the prop(clear) is still good. I didnt notice that broke off fin when I bought the unit. Does that fin really make a difference and is this TM still useable without it? What does that fin do?

it will still work. You just will not have any protection for the prop when you hit something. if you run a ground with it there is nothing to hit but the prop. But for $10 it will work.

Yes, that is called the "skeg" and is used as protection for the prop.  Don't use it in skinny water and you should be fine.  

There might be an issue of balance with the weight of the skeg missing but if not, you're good to go.  Heck, for $10, you're good to go anyway, just bring an oar.

Try asking at a local auto shop to see if someone can weld a skeg on there for you.  Take the coil out and have it put on, put it back together and there you go!!!

As LBH stated, it's only to protect the prop, just be carefull in shallow water.  I wouldn't try to have it welded, the motor would have to be taken completely apart and the magnets removed, just not worth it.  For $10 and it runs good, you got a good deal, keep it that way.