Shaft length is just that, length of shaft from bottom of the head to the top of the motor
When measuring the shaft length of a motor do you measure from base of head to prop shaft? And do you measure in the stowed position or do you let the motor out all the way and measure?
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Shaft length is just that, length of shaft from bottom of the head to the top of the motor
How deep does the prop need to be in the water?
According to that chart ...... about 2ft give or take an inch or two.
But I've also heard that you want the prop as shallow as possible, but still deep enough that you are not pushing water INTO the bow of the boat ... but, UNDER the bottom of the boat. You also don't want it so shallow that waves will lift it out of the water. With that in mind, it might depend on the size & style of the boat & how deep it sits in the water, and how buoyant that boat is ...
If you walk down the walkway of a mooring dock, and look at the trolling motor shafts on various different boats, you'll usually see a discoloration or scum line on those shafts. They will likely be at various points on those shafts, depending on the length of the shaft & size/style of each boat. I would venture to guess that many of them were never moved up/down from the position they were initially mounted (either by the factory or dealer) ... and probably even those that were mounted replacements by the boat owners.
This is what my minn kota manual calls for but like crappiepappy said you will just have to fine tune it to your needs.
Be safe and good luck fishing