WC if you were anchored you may do better fishing the 25 to 30' depths but that doesn't mean you can't catch big crappie nightstalking in 17' depths. The bigger fish tend to stay out of the light. Try putting a lighted float on a couple of lines and throwing them away from your boat and away from the 'light ring' that your under water light makes. Use a adjustable float and set the bait a little above the depths the smaller fish are biting at. If the small fish are feeding at 15' depth set the float at 8' to 10'. I have had nights where the rods with the floats will out catch the tight lines in the light. Good luck and let us know if this helps you.
Stump.
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Ascend 133X 13' - MotorGuide Xi3 & Mercury 4