If you own an inflatable boat or are looking to purchase one, getting an outboard motor or trolling motor for your boat has crossed your mind. Gas outboards are ideal for most dinghy owners that are looking for speed and power. Trolling motors are also a great alternative for owners looking for more green solutions with less of a price tag. If you are new to boating, it can be tough to know which one will best fit your inflatable dinghy. Getting the right shaft length is vital to the performance of the inflatable and ensuring your motor doesn’t bottom out. In this article, we will discuss the best sizing options for inflatable boats and how to measure a motor to ensure it will work for your setup.

Determining Shaft Length For Your Boat

Let’s assume you have just purchased a new dinghy and you don’t know what size motor to pair with it. In order to choose the correct size gas outboard or trolling motor, you will need to make two different measurements on your boat. Here we will go over the two different measurements needed.

Gas Outboards

The most important information is figuring out what size shaft you need for your inflatable boat. This is a quick and easy measurement that anyone can do with a ruler. Simply measure from the top of the transom to the bottom of the hull. On most inflatable dinghies and Newport Vessels inflatable boats, this will measure to be between the range of 15-17”. This measurement dictates that your boat requires a “short” shaft gas outboard motor. If the measurement is 20-22”, you will need to consider a “long” shaft motor. It is very unlikely, but any transom to hull measurement larger than 22” will require an “ultra long” shaft for optimal performance.

Trolling Motors

Trolling motors are slightly different in the measurements required to make a correct purchase. Trolling motors do not need to line up with the bottom of the hull to operate at top performance. In order for a trolling motor to perform properly, the motor compartment needs to be submerged 10”-12” below the surface of the water. This measurement will come in handy later.

You will still need to start your measurement at the top of the transom like earlier, but you only have to measure down to the waterline. This measurement will vary depending on weight, sea conditions and the type of boat. Add 10”, that is needed below the water, to this measurement to get the recommended minimum shaft length. Add 12” to the waterline measurement to get a more comfortable fit. If the length is in between sizes or equal to your measurement, move to the larger length.

Measuring Motor Shaft Length

If you own a motor and you are looking for an inflatable boat to mount it onto, it is important to know the specifications of your motor, especially the length. Getting the wrong size motor could lead to underperformance or worse, damage to the motor.

Gas Outboards

To measure the shaft length on an outboard, you will need to measure from the top of the mounting clamp bracket on the outboard, down to the anti-ventilation plate (commonly referred to as the cavitation plate). This measurement will vary between 15-17” for a short shaft motor, 20-22” for a long shaft motor and 25-27” for ultra long shafts. While these sizes may vary, you will want to check that the anti-ventilation plate lines up with the hull of the inflatable boat to achieve optimal performance.

Trolling Motors

To measure the shaft length on a trolling motor, you will need to measure from the bottom of the motor head to the top of the propeller housing. This length may vary by an inch or two, but round to the nearest even number to get the motor’s rated shaft length.

Knowing how to correctly measure motor shaft length and how to measure the correct length for your inflatable boat is extremely useful information. Not only is it important for new dinghy owners, but also current owners that are looking to upgrade.