my trolling motor is a motorguide 54 lb. thrust but even on high speed does not move the boat ( 17' )very well. the 3 blade prop looks kind of small but was on the boat when i got it. thinking of changing over to a minnkota. any suggestions on size?

You might want to try a two blade prop and check the battery. That 54 should move the 17' boat fine.

think i'll check out a different prop first, a lot cheaper than a new motor. tried 2 different batteries with equal results.

The three-blade prop isn't all that fast.  When I replaced my trolling motor last year, it came with a three-blade prop.  My top speed decreased a great deal.

A 54 lb. thrust TM should move a tin boat quite fast and a 17' glass boat pretty well too.  I have a 43 lb. thrust MG on a 16' glass boat and it will run 2 MPH on 5.  It will also almost throw me out of the boat if I'm not paying attention.