If you like boating, then you will no doubt find outboard motors very useful. They are super versatile as they can be attached to the back of lots of different vessels including canoes, sailboats and everything in between.
If you are considering buying an outboard motor, you need to decide which length to buy.
Choosing the right shaft length for your outboard motor will depend on your vessel and your needs. But how do you measure outboard shaft length? Keep reading to find out.
Measuring Outboard Shaft Length
To measure outboard shaft length, you will need a measuring tape. Start at the transom where the mounting bracket begins and measure to the top of the cavitation plate which is a horizontal surface that can be found just above the propeller.
The cavitation plate is also called the anti-ventilation plate.
What Are The Different Shaft Lengths?
Shaft lengths tend to fit into three different categories – standard (also known as short), long, and extra long. Standard is 15 inches, long is 20 inches and extra long is 25 inches but can go up as high as 30 inches.
However, this is not universally agreed upon. You should always check the exact length of the shaft. A lot of small engines have a shaft length of 17.5inches which is somewhere between short and long.
Which Shaft Length Is Best?
There are different opinions when it comes to whether a long or a short shaft length is best. One rule of thumb is that the cavitation plate should be flush with the bottom of the boat, or around half an inch above it.
However, slower boats do better when the cavitation plate is lower than the bottom of the boat – perhaps 2 to 3 inches – to avoid aeration.
Having the cavitation plate too low can cause drag and reduce your speed, so it is important to choose the right shaft length.
The smaller the engineer, the shorter the shaft, so it will depend on what size engine you are looking for. It is only high powered boats that tend to have outboard motors with extra long shafts.
The faster your boat, the more important it is to select the right shaft length.
If you have a jet ski or a boat with a central console then you will need to carefully consider the height of the transom and the shape of the back of the boat to make sure that you choose a shaft length that allows the cavitation plate to sit in the ideal position.
You will probably need a shorter shaft. If you have a slower boat, a work boat, a sailboat, or a boat with a displaced hull then you will most likely opt for a longer shaft.
Check With The Manufacturer
If you aren’t sure which outboard shaft length would be best for your boat, you could check with the manufacturer. Most boat manufacturers are happy to offer advice, especially if the boat you have is still in production.
A lot of modern boats are designed with a particular outboard motor in size.
You can either call the manufacturer and speak to someone directly, or you can check online for information about the specific model that you have. Always take the exact measurements of your own boat first, just to be certain.
Brackets & Jack Plates
The above guidance on choosing the right shaft length is based on the bottom of the hull, not the waterline.
If you are using a mounting bracket or a jack plate then this means you are not necessarily restricted by the height of the transom when choosing your shaft length.
Vessels that go further away from the shore can use large mounting brackets which means that the transom height can remain at the maximum. If you have a smaller boat that is designed to go onto open seas, you can use an adjustable jack plate.
This means that you can use pretty much any outboard motor and use the jack plate to change how deep it sits in the water. You can also use jack plates for boats that do not have a flat transom.
If you want to mount an outboard that has a shorter staff that is not necessarily ideal for your boat size or transom height, this is when a jack plate can come in really handy.
Some sailboats also use jack plates to mount an outboard motor for use as a backup in case the main motor fails.
How To Measure Your Boat
There is a simple way to measure your boat to help you decide which length outboard shaft you will need.
Single Outboard Motor
This is the method to use if you are planning to install a single outboard motor. You will need to measure along the centerline of the hull. Start from the clamping plate on the transom, and measure down to the lowest point on the hull.
This is usually directly under your drain plug.
Twin Outboard Motors
Some boaters go for a twin outboard motor set up for more power. Decide where you will be mounting the motors – usually equi-distant from the centerline of the hull.
At the installation points, measure the length from the clamping plate down to the bottom of the hull. This length is usually shorter than the one for a single outboard motor.
If you are using a bracket to install your outboard motor, you will need to keep this in mind when taking your measurements. You should measure from the bracket rather than the clamping plate, still finishing at the lowest point of the hull.
It is important to choose the right outboard motor shaft length for your boat. You can use a longer shaft than is recommended.
The only downside is that you will experience some drag which will reduce your speed. It is generally safer to use a longer shaft than a shorter one.