How to Measure an Outboard Motor's Lengthby Mary Lougee
Outboard motors mount on the transom, or back wall of a boat. There are four sizes of motor lengths that are used on different types of boats. The shortest length fits canoes and inflatable boats. A bass boat requires a longer shaft and pontoon boats need a shaft size greater than a bass boat. The larger shaft lengths fit large sailboats and barges to propel them through the water with the propeller at the correct height. Measuring you motor's length and matching it you boat transom length assures that the motor will operate at the correct height in the water.
Motor Shaft Length
Place the tip of a yardstick on the top of the mounting clamp bracket that secures it to the boat.
Read the measurement at the bottom of the cavitation plate. The cavitation plate is the flat area that is horizontal above the propeller.
Write the shaft length down in inches on paper with a pen.
Boat Transom Length
Place the tip of a yardstick on the top of the boat transom in the center. The transom is the back wall where an outboard motor attaches in the center.
Read the measurement at the bottom of the transom. Boat transoms are shaped differently depending on the make and model. Measure each type of hull to the bottom center of the transom on each type of boat.
Write the transom length down in inches on paper with a pen.
- The outboard motor shaft length and transom length need to match in order for the motor to be at the right height while it is operating in the water.
- Short shafts are 15 to 19 inches long. Long shafts are 20 to 24 inches long. Extra long shafts are 25 to 29 inches long and extra-extra long shafts are 30 inches and above.
Items you will need
- Pen and paper
- Fiberglass Boat image by Wimbledon from Fotolia.com