Trolling motor shaft length calculator

Load the boat the way you fish it, then measure vertically from the surface the motor bolts to down to the waterline. On a bow mount that is the bow deck, and on a transom mount it is the top of the transom. Add 16 inches for sheltered water or 20 for rough, then round up to a stocked shaft length.

The prop breaks the surface when the bow lifts, which is called ventilating. The motor loses bite, the pitch changes audibly and the noise pushes fish away. It happens exactly when you need control most, in wind and chop, and no adjustment fixes it. This is the single most common complaint about an otherwise correctly sized motor.

Mildly. A longer shaft sits deeper, which costs a small amount of efficiency and makes the motor more likely to find bottom in very shallow water. Against that, it stows higher and may foul the deck. Between two stocked lengths, taking the longer one is the safer error because the shorter one cannot be corrected.

Sixteen is the figure for sheltered water where the bow stays put, and twenty is for anywhere the hull moves in chop or where you fish standing on a raised deck. Both are convention repeated across the industry rather than a published standard. Where your motor maker publishes its own table, that figure governs instead.

The measurement is the same but the answer is much shorter, and the constraint is different. Kayak specific motors come with 24 to 36 inch shafts, which is a genuinely different product rather than a boat motor cut down. A boat shaft on a kayak stows badly and puts the head of the motor somewhere you cannot reach from the seat.

Not directly, but it decides whether you get the thrust you paid for. A ventilating prop is delivering a fraction of its rating regardless of the number on the box, so shaft length and thrust are two halves of the same decision. Size the thrust from your loaded boat weight and the shaft from your loaded bow height.