Shaft length is one of the most important factors to consider when purchasing an outboard motor. A motor with the wrong shaft length causes performance and handling problems, so it is worth measuring accurately before you order. This guide walks you through measuring your transom so you get the right size motor for your boat; it pairs with our broader guide on how to choose an outboard motor.
Consequences of the Wrong Shaft Length
- Short shaft on a high transom: the propeller may not stay fully submerged, so it ventilates and loses power, and the water intakes can run dry and overheat the engine.
- Long shaft on a low transom: the lower unit hangs too deep, adding drag and spray, costing speed and fuel, and exposing the skeg to grounding.
Step 1: Gather the Required Tools
You'll need a few tools before you start measuring:
- Tape measure (transom heights run 15–30 inches, so any tape will do)
- Level
- Straightedge
Measure with the boat sitting level (check with the level on the deck or gunwale) and hold the tape straight down the transom, not along its slope. Lay the straightedge flat under the hull so you can read the bottom edge accurately.
Step 2: Calculate the Proper Shaft Length
- The anti-ventilation plate (the flat plate just above the propeller) should sit level with, or up to about 1 inch (25 mm) below, the bottom of the hull when the motor is fully down; Suzuki's owner's manuals give 0–25 mm below the hull bottom. That is the height that keeps the prop in clean water without dragging the lower unit; our outboard mounting guide shows how to set it once the motor is on the boat.
- Measure from the top of the transom to the bottom of the hull at the centerline (not to the waterline). Suzuki and Honda size their shafts to that transom height: short shaft (S, 15 in.) for transoms of roughly 15–17 in., long shaft (L, 20 in.) for roughly 18–22 in., and extra-long (X, 25 in.) for roughly 23–27 in. Round to the nearest size; if you are between sizes, choose the longer one for rough water.
- On a Suzuki the shaft length is in the model code: S, L or X after the horsepower (a DF9.9BS is short shaft, a DF9.9BL long). Our Suzuki model code guide decodes the rest, and the specifications chart lists which shaft lengths each model is offered in.
Step 3: Check the Boat Builder's Recommendation
- Check the boat builder's recommendation for your specific boat model. Some hulls, brackets and sailboat transoms call for a different size than the general rule of thumb, and the builder's instructions take precedence.
Step 4: Final Check
- Before buying an outboard motor, double-check the measurement and the shaft length it points to. If you are unsure, send us your transom height and boat model and we will confirm the size before you order.