I've got a 30 lb. thrust on a 17' aluminum semi-V duck hunting/crappie fishing boat, it's mounted to the stern and works very well. I use it for sneaking up on ducks back in the bays, pockets, and rivers. Normal load is 4 guys, a couple dozen decoys, and sometimes a dog.

For an 8' boat, I would think a 30 lbs. thrust would be all you need.