To check the shaft length of a conventional gasoline outboard, measure vertically from the underside of the mounting bracket where it rests on the transom down to the anti-ventilation plate above the propeller. Then confirm the shaft designation in the specifications for your exact model.
Boat transom height is a separate measurement. Record both before choosing a motor or an extension kit. This guide covers conventional gasoline outboards; some electric motors use different reference points and size labels.
How to Measure Outboard Shaft Length
Have a tape measure ready. Keep the engine off and the boat or motor securely supported. Do not loosen the mounting bolts or lift a heavy outboard just to take a measurement.
Step 1. Position the motor upright so the measurement is vertical.
Step 2. Find the bracket’s transom seating surface: the underside of the bracket that would sit on the top edge of the transom. Do not start at the top of the engine cover.
Step 3. Locate the anti-ventilation plate, often called the cavitation plate. It is the broad horizontal plate above the propeller.
Step 4. Measure straight down from the bracket seating surface to that plate. Do not measure to the propeller center or the bottom of the skeg.
Step 5. Record the distance and check it against your model’s specifications. If the bracket reference is inaccessible, use the manufacturer’s dimension drawing or ask a dealer to identify it.
The motor and boat reference points are illustrated below. Tohatsu’s shaft-length guidance also distinguishes the bracket-to-plate measurement from the boat’s transom measurement.
Outboard Shaft Length Chart
These are common nominal shaft categories for gasoline outboards. They are size designations, not a promise that every tape measurement will match exactly.
| Nominal shaft length | Common name |
|---|---|
| 15 inches | Short shaft |
| 20 inches | Long shaft |
| 25 inches | Extra-long shaft |
| 30 inches | Extra-extra-long shaft |
Terminology and actual dimensions vary by manufacturer and model. A measurement between these sizes needs a model check, not rounding to the nearest category. For electric motors, follow the maker’s own chart: ePropulsion explains why its measurement and labels differ.
How to Measure Boat Transom Height
At the intended motor mounting position, measure vertically from the top of the transom to the hull bottom at the transom. For a conventional single-engine V-bottom boat, this is the centerline/keel reference. Do not follow the slope of an angled transom with the tape. See our guide to the boat transom if you need help identifying it.
Boats with a bracket, jack plate, pontoon motor pod, or unusual hull need their manufacturer’s mounting guidance. A transom measurement alone does not establish the final installation height.
Shaft Length and Mounting Height Are Different
Shaft length identifies the motor configuration. Mounting height describes where that motor sits relative to the hull. Do not set the anti-ventilation plate by the waterline while the boat is sitting still.
Follow the installation specifications for the particular boat, motor, and propeller. Plate position relative to the hull can vary with the setup; there is no universal “always flush with the hull” rule. Mercury Racing’s mounting-height example illustrates how the propeller and boat configuration affect the correct height.
Frequently Asked Questions
Is a 20-inch outboard a long shaft?
Yes, 20 inches is commonly the long-shaft designation for gasoline outboards. Confirm the exact model before ordering parts or comparing it with an electric motor.
What if my measurement is between standard sizes?
Check the reference points, then consult the model specification. Actual bracket-to-plate dimensions can differ from the nominal size. Keep the model and serial number with your measurement rather than guessing the shaft category.
Can I use an extension kit if my shaft is too short?
An extension kit may be an option for a compatible motor. Before selecting one, gather the brand, model, horsepower, serial number, current shaft designation, and required extension. Contact Bay Manufacturing with those details and photos of the mounting arrangement so we can help identify the appropriate kit.
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