Outboard shaft length calculator

How to measure transom height

Measure vertically from the top of the transom, where the motor clamps sit, straight down to the lowest point of the hull directly below — the keel line at the back of the boat. Measure along the centreline, follow the angle of the transom, and do not measure to the waterline or include a bracket you are not fitting.

Matching shaft to transom

Outboards come in fixed shaft lengths, and the shaft is measured from the mounting surface to the anti-ventilation plate — the flat plate just above the propeller. Fitted correctly, that plate sits roughly level with, or up to an inch below, the bottom of the hull. Manufacturers label the lengths S, L, XL and XXL, and the letter usually appears in the model code: an "L" in a Yamaha or Suzuki model number normally means a 20in shaft.

What goes wrong with the wrong length

Too short and the gearcase runs too high: the propeller draws air, the motor over-revs, it loses grip in turns and the water pump can lose its supply. Too long and the extra gearcase drags through the water, costing speed and fuel, and the motor sits low enough to catch obstructions and take water over the cowl in a following sea.

Borderline measurements

If your transom measures between two sizes, tell us the exact figure and the boat before you order. Some hulls run best a little higher than the textbook position, and a jack plate or setback bracket changes the answer entirely.