That guide does not always work. Depends on the trolling motor. I have a 92 Champion 201. It is 20" from the deck to the water in calm water with two people on the boat, one on each end of the boat. Guide say I need a 42-45" shaft.
I bought an Ulterra 45" and the prop will come out of the water in any kind of moderate chop. Reason is because there is really only 37" of usable shaft length on an Ulterra. Minn Kota should let you know that before you spend $2K on the motor, BUT, I should have checked it out before I dropped the $2K. I should have bought the 60". Love the motor otherwise. It does not come in a 52" for some stupid reason.
Wayne
Yep - it's just a guide. Always better to measure first.
With that said - I have a 1988 184 Champion. For over 20 yrs I ran it with a 36 inch shaft Motor Guide Brute. Fished several big Tx lakes with 2 fishermen in the boat with it and never recall really wishing it had a longer trolling motor shaft. But - as a rule I don't fish in 3+ ft rollers either :-)
I replaced the old MG with a MK Terrova and then an Ultrex - both 45 inch shafts. I considered a 52 in. shaft for the Ultrex but went with the 45. No regrets and no problems fishing two in the boat. A 52 would have really stuck out a lot past the edge of the gunnel when TM laid down - the 45 in does some but not much.